Emerging evidence in several species suggests that the use of low-dose ACTH stimulation testing may be important in diagnosing mild degrees of adrenocortical insufficiency and a new syndrome, critical illness—related corticosteroid insufficiency (previously known as relative adrenal insufficiency), which has been recognized in human and veterinary patients.1–5 Critical illness-related corticosteroid insufficiency is characterized by inadequate production of cortisol in relation to an increased demand during critical illness and is usually defined by an inadequate response to exogenous ACTH administration.6,7 Low-dose ACTH stimulation testing appears to be more sensitive than standard-dose testing for detecting critical illness—related corticosteroid insufficiency and reductions in adrenocortical reserve in humans.8
Adrenocorticotropic hormone stimulation testing is commonly used to evaluate adrenocortical function in veterinary patients. In cats, ACTH stimulation testing has been used primarily to test adrenocortical production of cortisol, progestins, and androgens.9–17 Although the primary regulators of aldosterone secretion are the renin-angiotensin system and extracellular potassium concentration, ACTH acts as an additional stimulant.18–22 Administration of exogenous ACTH to cats causes a reliable increase in aldosterone secretion and can be used for evaluation of adrenal mineralocorticoid function.18 Traditionally, a standard dose of cosyntropin (125 μg/cat, IV) has been used for ACTH stimulation testing. A previous study23 documented that lower doses of cosyntropin will stimulate maximal cortisol secretion in cats. However, the study used per cat dosing as opposed to per body weight dosing and did not evaluate the aldosterone response.
The purposes of the study reported here were to determine the lowest dose of cosyntropin on a per body weight basis that would produce maximal cortisol and aldosterone secretion and the ideal timing of blood sample collection after ACTH stimulation in healthy cats. Cats were administered 5 doses of cosyntropin and saline (0.9% NaCl) solution as a control treatment; serum cortisol and aldosterone concentrations were measured at multiple time points. The peak cortisol and aldosterone concentrations and kinetics of secretion were compared between different cosyntropin doses.
Materials and Methods
Animals—Seven adult, purpose-bred catsa were used. The cats were sexually intact males aged 1.5 years with a mean weight of 5.48 kg (12 lb) that ranged from 5.4 to 7 kg (11.9 to 15.4 lb). The animal-handling portion of this study was completed in its entirety at the University of Missouri. Animals were cared for according to the principles outlined in the NIH Guide for the Care and Use of Laboratory Animals, and the study was approved by the Animal Care and Use Committee at the University of Missouri. The cats were maintained on commercial adult cat food and water ad libitum. Cats were deemed healthy on the basis of physical examination and results of a CBC, serum biochemistry profile, and urinalysis. Cats had not received corticosteroid, etomidate, or ketoconazole at any time, nor did they have clinical evidence of adrenal disease. The cats were habituated to human handling and intermittent placement in a cat carrier for 1 month prior to initiation of this study.
Vascular access port placement—Cats were instrumented with subcutaneous implantable vascular access portsb 2 months prior to the study. Cats were anesthetized, intubated, and maintained on isoflurane and oxygen for the duration of the procedure. Each cat was aseptically prepared for surgery. A curvilinear skin incision was created, and a vascular access port (5F) was sutured to the deep fascia of the left lateral cervical musculature. A separate skin incision was created over the left jugular vein. A small subcutaneous tunnel was created that connected the 2 incisions, and the catheter was drawn through the tunnel. The left jugular vein was dissected from its attachments, and the introducer stylet was placed into the left jugular vein. The catheter tip was placed through the introducer into the vein to a predetermined length (to place the tip in the cranial vena cava immediately cranial to the right atrium). The catheter was connected to the port and secured with the provided boot. The subcutaneous tissues and skin of both incisions were closed in a routine fashion. Meloxicamc (0.1 mg/kg [0.045 mg/lb], SC or PO, q 24 h for 2 days) and amoxicillind (15 mg/kg [6.8 mg/lb], PO, q 12 h for 5 days) were administered postoperatively.
Cosyntropin preparation—Cosyntropine was supplied as 250 μg of lyophilized powder in 2-mL vials. Each vial of cosyntropin was reconstituted with 1 mL of sterile saline solution, in accordance with the manufacturer's directions. After reconstitution, cosyntropin was frozen in aliquots at −20°C for up to 2 weeks in a sealed syringe24 and used for subsequent study days. The dose for each cat was diluted to a final volume of 5 mL with sterile saline solution prior to administration.
Treatments and blood sample collection—Procedures were performed at the same time each study day. Cats were placed in individual cat carriers that were large enough to allow the cat to stand, turn around, and lie down comfortably but small enough to minimize movement. Vascular access was obtained by inserting a Huber needleb through the vascular access port diaphragm. The needle was attached to an extension set to allow blood sample collection and medication administration with minimal or no animal restraint. The heparinized saline solution was removed from the vascular access port, and the catheter was flushed with 5 mL of sterile saline solution. A table of random numbers was used to assign cats for treatments in a crossover block design. The cats received 5 doses (125 μg/cat and 10, 5, 2.5, and 1 μg/kg [4.54, 2.27, 1.14, and 0.45 μg/lb]) of cosyntropin and saline solution IV with a 2-week washout period between treatments. Each dose of cosyntropin or saline solution was administered 1 time in each cat. Saline solution was considered the negative control, and cosyntropin at the dose of 125 μg/cat was considered the positive control because this dose is routinely used for ACTH stimulation testing in cats.9,11,13–18 Blood samples for determination of serum cortisol and aldosterone concentrations were collected from the vascular access port before (baseline) and at 15, 30, 45, 60, 75, and 90 minutes after cosyntropin or saline solution administration. At each time point, 4 mL of blood was collected by use of a 3-syringe technique and placed in a standard serum clot tube. After clotting and centrifugation (2,500 × g for 7 minutes), the serum was removed, placed in a plastic tube, and frozen at-80°C until analysis.
Hormone assays—Serum cortisolf and aldosteroneg concentrations were measured by radioimmuno-assays previously validated for use in cats.25,26 Samples were assayed in multiple batches; however, samples from each cat were run in a single batch when possible. All samples were assayed in duplicate. The sensitivities of the assays are 14 nmol/L and 14 pmol/L for cortisol and aldosterone, respectively. Because a hormone concentration in a sample can not be accurately measured if the concentration is below the sensitivity of the assay, for statistical analysis, all such samples were assigned a value of 50% of the lowest standard.
Statistical analysis—Data analysis was completed by use of a commercial software program.h The mixed procedure was used with the repeated option. Dependencies were modeled by use of unstructured correlations with an autoregressive of order 1 for the repeated measures over time. To compare the mean response to doses at a fixed time, the least squares means method was used. To compare the peak serum cortisol and aldosterone concentration in response to various doses of cosyntropin, the mixed procedure was used considering the cat as a random effect and dose as a fixed effect. Outliers were defined as a datum point at least 1.5 times (outlier) or 3 times (extreme outlier) the interquartile range outside the first or third quartile, respectively. Outliers are noted in the results section but were not removed from the statistical analysis. Values of P < 0.05 were considered significant.
Results
Cortisol—Baseline cortisol concentrations did not differ significantly among treatment groups (ie, 5 doses of cosyntropin and saline solution). Serum cortisol concentration did not significantly change from baseline after saline solution (negative control) administration but did significantly increase after administration of cosyntropin at 125 μg/cat (positive control). Mean serum cortisol concentration was increased significantly, compared with baseline values, at all time points after administration of all cosyntropin doses (Figure 1).
Mean serum cortisol concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline (0.9% NaCl) solution (control). Mean baseline cortisol concentrations did not differ significantly between the 5 doses of cosyntropin and saline solution. Mean serum cortisol concentration did not significantly change after administration of saline solution. Mean serum cortisol concentration increased significantly (P < 0.05), compared with baseline values, at all time points after administration of all cosyntropin doses. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Mean serum cortisol concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline (0.9% NaCl) solution (control). Mean baseline cortisol concentrations did not differ significantly between the 5 doses of cosyntropin and saline solution. Mean serum cortisol concentration did not significantly change after administration of saline solution. Mean serum cortisol concentration increased significantly (P < 0.05), compared with baseline values, at all time points after administration of all cosyntropin doses. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Mean serum cortisol concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline (0.9% NaCl) solution (control). Mean baseline cortisol concentrations did not differ significantly between the 5 doses of cosyntropin and saline solution. Mean serum cortisol concentration did not significantly change after administration of saline solution. Mean serum cortisol concentration increased significantly (P < 0.05), compared with baseline values, at all time points after administration of all cosyntropin doses. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
To determine the lowest dose of cosyntropin that resulted in maximal secretion of cortisol, the peak serum cortisol concentrations were compared among treatment groups. The peak serum cortisol concentration after administration of the test doses of cosyntropin happened at earlier time points, compared with that after administration of the positive-control dose (cosyntropin at 125 μg/cat), with the lowest doses of cosyntropin resulting in the earliest peak response (Table 1). Although the peak serum cortisol concentration occurred at various time points, peak serum cortisol concentrations did not differ significantly in response to cosyntropin doses of 5 and 10 μg/kg and 125 μg/cat (Figure 2). Conversely, peak serum cortisol concentrations were significantly lower in response to cosyntropin doses of 1 and 2.5 μg/kg, compared with peak serum cortisol concentration following administration of cosyntropin at 125 μg/cat. Therefore, the lowest dose of cosyntropin that stimulated cortisol secretion equivalent to that of the positive-control dose was 5 μg/kg.
Peak serum cortisol concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Each box represents the 25th to 75th percentiles, the horizontal line within the box represents the median value, and the bars represent the range. The open circle represents an extreme outlier. Peak serum cortisol concentrations with the same letter (a or b) are significantly (P < 0.05) different from each other. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Peak serum cortisol concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Each box represents the 25th to 75th percentiles, the horizontal line within the box represents the median value, and the bars represent the range. The open circle represents an extreme outlier. Peak serum cortisol concentrations with the same letter (a or b) are significantly (P < 0.05) different from each other. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Peak serum cortisol concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Each box represents the 25th to 75th percentiles, the horizontal line within the box represents the median value, and the bars represent the range. The open circle represents an extreme outlier. Peak serum cortisol concentrations with the same letter (a or b) are significantly (P < 0.05) different from each other. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Mean ± SD serum cortisol (nmol/L) concentrations at various time points in response to 5 doses of cosyntropin and saline (0.9% NaCl) solution in 7 healthy cats.
| Treatment | 0 minutes | 15 minutes | 30 minutes | 45 minutes | 60 minutes | 75 minutes | 90 minutes |
|---|---|---|---|---|---|---|---|
| Saline | 51 ± 42 | 53 ± 36 | 43 ± 32 | 55 ± 51 | 48 ± 36 | 66 ± 53 | 62 ± 56 |
| Cos (1 μg/kg) | 39 ± 25* | 166 ± 30* | 196 ± 33 | 207 ± 35a | 182 ± 49* | 154 ± 68* | 134 ± 91* |
| Cos (2.5 μg/kg) | 68 ± 50* | 168 ± 30* | 193 ± 33 | 209 ± 47 | 210 ± 73a | 193 ± 77 | 176 ± 58 |
| Cos (5 μg/kg) | 74 ± 31* | 166 ± 16* | 205 ± 13* | 216 ± 12* | 237 ± 21b | 224 ± 34 | 210 ± 27* |
| Cos (10 μg/kg) | 68 ± 37* | 188 ± 12* | 213 ± 9* | 227 ± 10* | 240 ± 20 | 256 ± 20b | 225 ± 37* |
| Cos (125 μg/cat) | 64 ± 33* | 172 ± 30* | 200 ± 22* | 219 ± 35* | 238 ± 35 | 246 ± 42 | 251 ± 40b |
Peak serum hormone concentration in response to each dose of cosyntropin or saline solution is identified by a superscripted letter.
Within each row, significantly (P < 0.05) different mean serum hormone concentration, compared with the peak serum hormone concentration.
Peak serum hormone concentrations with different letters are significantly (P < 0.05) different from each other.
Cos = Cosyntropin. Saline = Saline solution (negative control).
The pattern of cortisol secretion was evaluated to determine the optimum sample collection time after each dose of cosyntropin. Administration of cosyntropin at 1, 2.5, 5, and 10 μg/kg and at 125 μg/cat resulted in peak serum cortisol concentrations at 45, 60, 60, 75, and 90 minutes, respectively. For each cosyntropin dose, the peak serum cortisol concentration was compared with the serum cortisol concentration at the other time points to determine whether sample collection at other time points would result in a significantly different result (Table 1). After administration of cosyntropin at 1 μg/kg, the peak serum cortisol concentration at 45 minutes was not significantly different from the serum cortisol concentration at 30 minutes but was significantly greater than at all other time points. Following administration of cosyntropin at 2.5 μg/kg, the peak serum cortisol concentration at 60 minutes was not significantly different from that at 30, 45, 75, or 90 minutes. However, peak serum cortisol concentration after administration of cosyntropin at 2.5 μg/kg was significantly greater than the serum cortisol concentration at baseline and at the 15-minute time point. After administration of cosyntropin at 5 μg/kg, the peak serum cortisol concentration at 60 minutes did not differ significantly from the 75-minute time point but was significantly greater than the serum cortisol concentration at all other time points. Following administration of cosyntropin at 10 μg/kg, the peak serum cortisol concentration at 75 minutes did not differ significantly from the 60-minute time point but was greater than the serum cortisol concentration at all other time points. Following administration of cosyntropin at 125 μg/cat, the peak serum cortisol concentration at 90 minutes was not significantly different from that of the 60- and 75-minute time points.
Aldosterone—Baseline aldosterone concentrations did not differ significantly among treatment groups (ie, 5 doses of cosyntropin and saline solution). Serum aldosterone concentration did not significantly change after saline solution (negative control) administration but did significantly change after administration of cosyntropin at 125 μg/cat (positive control). Mean serum aldosterone concentration increased significantly compared with baseline values, at all time points after administration of all cosyntropin doses except for the dose of 1 μg/kg. Administration of cosyntropin at 1 μg/kg resulted in significantly greater serum aldosterone concentrations, compared with baseline values, at all time points except at 90 minutes after administration of cosyntropin (Figure 3).
Mean serum aldosterone concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Mean baseline aldosterone concentrations did not differ significantly between the 5 doses of cosyntropin and saline solution. Mean serum aldosterone concentration did not significantly change after administration of saline solution. Mean serum aldosterone concentration increased significantly (P < 0.05), compared with baseline values, at all time points after administration of all cosyntropin doses except for the dose of 1 μg/kg (0.45 μg/lb). Cosyntropin at 1 μg/kg resulted in significantly (P < 0.05) greater serum aldosterone concentrations, compared with baseline values, at 15, 30, 45, 60, and 75 minutes only (not at 90 minutes). Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Mean serum aldosterone concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Mean baseline aldosterone concentrations did not differ significantly between the 5 doses of cosyntropin and saline solution. Mean serum aldosterone concentration did not significantly change after administration of saline solution. Mean serum aldosterone concentration increased significantly (P < 0.05), compared with baseline values, at all time points after administration of all cosyntropin doses except for the dose of 1 μg/kg (0.45 μg/lb). Cosyntropin at 1 μg/kg resulted in significantly (P < 0.05) greater serum aldosterone concentrations, compared with baseline values, at 15, 30, 45, 60, and 75 minutes only (not at 90 minutes). Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Mean serum aldosterone concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Mean baseline aldosterone concentrations did not differ significantly between the 5 doses of cosyntropin and saline solution. Mean serum aldosterone concentration did not significantly change after administration of saline solution. Mean serum aldosterone concentration increased significantly (P < 0.05), compared with baseline values, at all time points after administration of all cosyntropin doses except for the dose of 1 μg/kg (0.45 μg/lb). Cosyntropin at 1 μg/kg resulted in significantly (P < 0.05) greater serum aldosterone concentrations, compared with baseline values, at 15, 30, 45, 60, and 75 minutes only (not at 90 minutes). Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Peak serum aldosterone concentrations were compared among treatment groups to determine the lowest dose of cosyntropin that resulted in maximal secretion of aldosterone. The peak serum aldosterone concentration occurred at earlier time points for each of the cosyntropin doses tested, compared with that of the positive-control dose, with the lowest doses of cosyntropin resulting in the earliest peak response (Table 2). Although the peak serum aldosterone concentration occurred at various time points, peak serum aldosterone concentrations did not differ significantly in response to cosyntropin doses of 2.5, 5, and 10 μg/kg and 125 μg/cat (Figure 4). However, peak serum aldosterone concentration was significantly lower following a cosyntropin dose of 1 μg/kg, compared with that following administration of cosyntropin at 125 μg/cat. The lowest dose of cosyntropin that stimulated maximal aldosterone secretion was 2.5 μg/kg.
Peak serum aldosterone concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Each box represents the 25th to 75th percentiles, the horizontal line within the box represents the median value, and the bars represent the range. Peak serum aldosterone concentrations with the same letter (a) are significantly (P < 0.05) different from each other. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Peak serum aldosterone concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Each box represents the 25th to 75th percentiles, the horizontal line within the box represents the median value, and the bars represent the range. Peak serum aldosterone concentrations with the same letter (a) are significantly (P < 0.05) different from each other. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Peak serum aldosterone concentrations in 7 healthy cats after administration of 5 doses of cosyntropin and saline solution (control). Each box represents the 25th to 75th percentiles, the horizontal line within the box represents the median value, and the bars represent the range. Peak serum aldosterone concentrations with the same letter (a) are significantly (P < 0.05) different from each other. Each cat received each treatment once.
Citation: Journal of the American Veterinary Medical Association 238, 2; 10.2460/javma.238.2.176
Mean ± SD serum aldosterone (pmol/L) concentrations at various time points in response to 5 doses of cosyntropin and saline solution in 7 healthy cats.
| Treatment | 0 minutes | 15 minutes | 30 minutes | 45 minutes | 60 minutes | 75 minutes | 90 minutes |
|---|---|---|---|---|---|---|---|
| Saline | 119 ± 71 | 97 ± 56 | 97 ± 69 | 100 ± 69 | 99 ± 72 | 121 ± 93 | 128 ± 99 |
| Cos (1 μg/kg) | 57 ± 56* | 197 ± 65 | 238 ± 100a | 203 ± 96 | 161 ± 131* | 139 ± 128* | 116 ± 136* |
| Cos (2.5 μg/kg) | 91 ± 66* | 185 ± 100 | 230 ± 97 | 250 ± 107b | 215 ± 135 | 187 ± 131 | 152 ± 142* |
| Cos (5 μg/kg) | 90 ± 70* | 205 ± 94* | 241 ± 125 | 256 ± 115b | 227 ± 116 | 208 ± 111 | 186 ± 81* |
| Cos (10 μg/kg) | 81 ± 64* | 213 ± 87* | 271 ± 79 | 285 ± 82 | 296 ± 101b | 244 ± 113* | 212 ± 101* |
| Cos (125 μg/cat) | 97 ± 59* | 239 ± 77* | 290 ± 100 | 315 ± 85 | 341 ± 91b | 225 ± 96* | 294 ± 124 |
See Table 1 for key.
The pattern of aldosterone secretion was evaluated to determine the optimum sample collection time after each dose of cosyntropin. Administration of cosyntropin at doses of 1, 2.5, 5, and 10 μg/kg and 125 μg/cat resulted in peak serum aldosterone concentrations at 30, 45, 45, 60, and 60 minutes, respectively. For each cosyntropin dose, the peak serum aldosterone concentration was compared with serum aldosterone concentrations at the other time points to determine whether sample collection at other time points would result in a significantly different result (Table 2). After administration of cosyntropin at 1 μg/kg, the peak serum aldosterone concentration at 30 minutes was significantly greater than serum aldosterone concentrations at all other time points except for at the 15- and 45-minute sample collection times. Following administration of cosyntropin at 2.5 μg/kg, the peak serum aldosterone concentration at 45 minutes was not significantly different from serum aldosterone concentration at 15, 30, 60, and 75 minutes but was significantly greater than serum aldosterone concentration at baseline and at 90 minutes. After administration of cosyntropin at 5 μg/kg, the peak serum aldosterone concentration at 45 minutes did not differ significantly from that at 30, 60, and 75 minutes but was significantly greater than serum aldosterone concentrations at baseline and at 15 and 90 minutes. Cats administered cosyntropin at 10 μg/kg had a peak serum aldosterone concentration at 60 minutes that did not differ significantly from the serum aldosterone concentration at 30 and 45 minutes but was significantly greater than serum aldosterone concentrations at baseline and at 15, 75, and 90 minutes. Following administration of cosyntropin at 125 μg/cat, the peak serum aldosterone concentration at 60 minutes did not differ from serum aldosterone concentrations at 30, 45, and 90 minutes.
Discussion
This is the first study to evaluate cortisol and aldosterone response to various doses of cosyntropin on a per body weight basis in cats. Lower doses of cosyntropin resulted in an adrenocortical response that was equivalent to the traditional dose of 125 μg/cat. The lowest doses of cosyntropin that stimulated a maximal cortisol and aldosterone response were 5 and 2.5 μg/kg, respectively. Low-dose ACTH stimulation testing with administration of cosyntropin at 5 μg/kg and blood sample collection times at 60 to 75 minutes achieved serum cortisol and aldosterone concentrations that were equivalent to those achieved following administration of cosyntropin at 125 μg/cat.
The cosyntropin dose of 125 μg/cat was used as the positive control in this study because it is the dose of cosyntropin most commonly used for ACTH stimulation testing in cats9–18 and it has been shown to cause maximal adrenal response with respect to cortisol secretion.23,27 We compared the peak serum cortisol concentration achieved by adrenocortical stimulation with the positive-control dose to that achieved with several lower doses of cosyntropin. Peak serum cortisol concentrations after adrenocortical stimulation were equivalent when comparing cosyntropin doses of 5 and 10 μg/kg to that of 125 μg/cat. Although peak serum cortisol concentrations following ACTH stimulation with cosyntropin at 1 or 2.5 μg/kg were significantly greater than baseline, the peak serum cortisol concentration was significantly less than that obtained after administration of cosyntropin at 125 μg/cat. Therefore, the lowest dose of cosyntropin that stimulated cortisol secretion equivalent to that of the dose of 125 μg/cat was 5 μg/kg.
A lower dose of cosyntropin may be preferred over a standard dose because it may provide a more sensitive means to detect critical illness—related corticosteroid insufficiency and reductions in adrenocortical reserve.4,6,8,28,29 The results of the present study would appear to be useful in subsequent investigations to evaluate the hypothalamic-pituitary-adrenal axis in critically ill cats, specifically in the identification of cats with critical illness-related corticosteroid insufficiency. The standard dose of cosyntropin (125 μg/cat) currently used for ACTH stimulation testing is greater than that necessary to produce maximal adrenocortical stimulation in healthy cats. Therefore, the use of this dose may mask subtle decreases in adrenal gland reserve and hinder identification of cats with critical illness-related corticosteroid insufficiency. Low-dose (cosyntropin at 5 μg/kg) ACTH stimulation testing should be compared with standard-dose (125 μg/cat) ACTH stimulation testing in a population of critically ill cats (eg, cats with sepsis). If low-dose ACTH stimulation testing is found to be more sensitive for detecting insufficient adrenal gland function than standard-dose ACTH stimulation testing, diagnostic screening for critical illness—related corticosteroid insufficiency could potentially be enhanced and patient outcome may be improved if critically ill cats with illness-related corticosteroid insufficiency were promptly identified and administered low doses of glucocorticoids. However, at this time, the use of low-dose (cosyntropin at 5 μg/kg) ACTH stimulation testing for determining adrenocortical reserve has not been evaluated in cats with critical illness; direct application of these data to critically ill cats should be done with caution.
From a clinical perspective, it may be advantageous to have a shorter duration between IV administration of cosyntropin and peak response because it will shorten the testing interval. The peak serum cortisol concentration after administration of the test doses of cosyntropin happened at earlier time points, compared with that following the positive-control dose of 125 μg/cat, for all of the cosyntropin doses tested, with lower doses of cosyntropin resulting in an earlier peak response. Administration of cosyntropin at 125 μg/cat resulted in a peak cortisol response at 90 minutes, which is similar to what has been reported previously,30,31 compared with a peak cortisol response at 60 minutes following administration of 5 mg/kg, the lowest dose of cosyntropin that stimulated an equivalent cortisol response. However, the serum cortisol concentration at 75 minutes was not significantly different from that at 60 minutes; thus, when evaluating cortisol, blood sample collection at either 60 or 75 minutes after administration of cosyntropin at 5 μg/kg may be used.
Peterson and Kemppainen23 compared plasma cortisol response after administration of cosyntropin at 125 μg/cat with response after administration of 2 lower doses of cosyntropin: 12.5 and 1.25 μg/cat. Our results differed from that of Peterson and Kemppainen23 in 2 respects: peak cortisol concentrations and duration between IV administration of cosyntropin and peak cortisol response. Peterson and Kemppainen23 demonstrated that, although administration of cosyntropin at 125 μg/cat resulted in higher peak plasma cortisol concentrations within individual cats, there was no significant different between the standard dose of 125 μg/cat and the lower test doses of 1.25 and 12.5 μg/cat. Further, administration of cosyntropin at 1.25 or 12.5 μg/cat resulted in peak plasma cortisol concentrations at 30 minutes.23 Extrapolating from the reported weight range of the cats used in the study by Peterson and Kemppainen,23 1.25 and 12.5 μg/cat resulted in administration of cosyntropin of approximately 0.25 to 0.42 μg/kg (0.11 to 0.19 μg/lb) and 2.5 to 4.2 μg/kg (1.1 to 1.9 μg/lb), respectively, on a per body weight basis. Using our calculations approximating the per body weight dose, we expected that all of the doses used in our study (1, 2.5, 5, and 10 μg/kg) would have resulted in peak serum cortisol concentrations that were equivalent to that of the dose of 125 μg/cat. However, we found that doses of cosyntropin < 5 μg/kg resulted in peak serum cortisol concentrations that were significantly lower than that of the dose of 125 μg/cat. Additionally, peak serum cortisol concentrations were later (at 45 and 60 minutes for doses of 1 and 2.5 μg/kg, respectively) than what Peterson and Kemppainen23 reported. The differences in the results between these 2 studies may be related to differences in the cat population and investigational methods. Peterson and Kemppainen23 used older male and female cats, dosed cosyntropin on a per cat basis, and evaluated plasma cortisol concentrations every 30 minutes, while in our study, we used younger male cats, dosed cosyntropin on a per weight basis, and evaluated serum cortisol concentrations every 15 minutes. Additionally, because our study involved more cats, it was more adequately powered to detect a statistical difference.
Previously, a significant increase in serum aldosterone concentration was described at 30 and 60 minutes after administration of cosyntropin at 125 μg/cat.18 However, dose response, time to peak concentration, and duration of aldosterone secretion has not been evaluated in cats. The study reported here demonstrated that IV administration of cosyntropin resulted in aldosterone secretion within 15 minutes and that lasted 90 minutes after administration of cosyntropin, depending upon the dose. The peak and duration of aldosterone secretion were dose dependent with higher doses of cosyntropin resulting in a higher peak effect and longer duration of aldosterone secretion.
The interval between IV administration of cosyntropin and the peak serum aldosterone concentration followed a pattern similar to that of cortisol with lower doses of cosyntropin resulting in an earlier peak serum aldosterone response. Peak aldosterone response when cosyntropin was administered at 125 μg/cat occurred at 60 minutes. Administration of cosyntropin at both 2.5 and 5 μg/kg, the 2 lowest doses that simulated a serum aldosterone response analogous to that of the dose of 125 μg/cat, resulted in a peak aldosterone response at 45 minutes. Blood sample collection following cosyntropin administration at 2.5 μg/kg may be accomplished at 15, 30, 45, 60, or 75 minutes with no significant difference in serum aldosterone concentration. If cosyntropin is administered at 5 μg/kg, aldosterone may be evaluated at 30, 45, 60, or 75 minutes after treatment without a significant difference in serum aldosterone concentration. Integrating both cortisol and aldosterone response, the ideal time to collect blood samples for both cortisol and aldosterone determination after administration of cosyntropin at 5 μg/kg is at 60 or 75 minutes after treatment.
Evaluating dynamic hormonal response is inherently difficult. There are a multitude of factors that influence the hypothalamic-pituitary-adrenal axis that should be considered when interpreting these data. Sex plays a role in adrenocortical responsiveness in some species.32,33 All of the cats in this study were sexually intact males, and thus, sex may have influenced our results. However, a previous study34 demonstrated that sex and neuter status did not affect serum cortisol or aldosterone concentrations in cats. Thus, this bias may be limited. Age is another factor that alters adrenocortical responsiveness.22 The influence of age on adrenocortical responsiveness in cats has not been evaluated; therefore, our results should be interpreted with caution when applying this information to older cats. Additionally, route of administration of cosyntropin influences adrenocortical response. In cats, IV administration of cosyntropin at 125 μg/cat results in a significantly higher peak serum cortisol concentration than that following IM administration of cosyntropin.30 Thus, our results can only be applied to use of cosyntropin IV. Finally, cats rapidly secrete cortisol in response to stress.35 To circumvent this interference, vascular access ports were used in this study to minimize the stress of injections and repeated blood sample collection. Vascular access was obtained prior to baseline samples, and the Huber needle was left in place for the duration of the study day to avoid the stress of repeated needle sticks. With this method, the cats were allowed to stay in their carriers and had minimal handling from the investigators. In the authors' opinion, this system minimized the stress of these procedures for the cats. Additionally, there was no significant change in serum cortisol concentration after saline solution administration over the duration of the study, supporting the idea that the cats were minimally stressed.
In conclusion, serum cortisol and aldosterone concentrations were reliably increased in all cats after the administration of each dose of cosyntropin. Higher doses of cosyntropin resulted in more sustained cortisol and aldosterone production and a later time of peak response. This variability will be important if low-dose ACTH stimulation testing is performed in cats, as the current protocol of a blood sample collection at 1 hour after administration of cosyntropin will not adequately represent peak serum cortisol concentrations if a cosyntropin dose of 1 μg/kg is administered. At all other cosyntropin doses, blood sample collection at 1 hour after administration of cosyntropin is appropriate. Cosyntropin administered IV at 5 μg/kg produced maximal cortisol and aldosterone secretion in healthy cats.
Liberty Research, Waverly, NY.
Vascular access ports and Huber needles were donated for this investigation by Norfolk Vet Products, Skokie, Ill.
Metacam, Boehringer Ingelheim Vetmedica Inc, St Joseph, Mo.
AMOXI-DROP, Pfizer Animal Health, New York, NY.
Cortrosyn, Amphastar Pharmaceuticals Inc, Rancho Cucamonga, Calif.
Coat-a-Count cortisol assay, Siemens Medical Solutions Diagnostics, Los Angeles, Calif.
Coat-a-Count aldosterone assay, Siemens Medical Solutions Diagnostics, Los Angeles, Calif.
SAS, version 9.2, SAS Institute Inc, Cary, NC.
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