Neonatal calf diarrhea is a major source of economic loss to the cattle industry and the leading cause of calf deaths in most countries. Diarrhea is an important cause of morbidity and death in unweaned dairy heifer calves in the United States. In 2002, the USDA estimated that 6.5% of the annual calf crop died from diarrhea between birth and weaning.1 Diarrhea can lead to dehydration, strong ion (metabolic) acidosis, hyperkalemia, and impaired cardiovascular and renal function.2,3,4 Orally administered electrolyte solutions are practical and inexpensive for treating mild to moderate strong ion acidosis and dehydration in neonatal ruminants that have a suckle reflex. However, there is still uncertainty about the optimal electrolyte concentrations, type of buffer, and type and amount of energy source as well as the pH and osmolarity of the solution.3,5 It has been suggested that the optimal formulation for an OAES should include a sodium concentration from 90 to 130 mmol/L; a potassium concentration from 10 to 20 mmol/L; a chloride concentration from 40 to 80 mmol/L; 40 to 80 mmol/L of metabolizable (non bicarbonate) base, such as acetate or propionate; and glucose as a source of energy.3,5
Most OAESs that are commercially available in the United States use bicarbonate as an alkalinizing agent. There are at least 3 lines of evidence to indicate that bicarbonate-containing OAESs are not optimal for treating diarrheic calves. First, suckling a bicarbonate-containing OAES reduces weight gain in diarrheic calves, relative to suckling a non–bicarbonate-containing OAES. This effect is attributed to interference with normal clot formation in the abomasum of milk-fed calves.6 Bicarbonate- and citrate-containing OAESs markedly prolong milk clotting time when mixed 1:1 with whole milk; in contrast, acetate-containing OAESs do not alter milk clotting time.7,8 Second, healthy neonatal calves fed milk replacer and bicarbonate-containing OAESs have a significantly higher prevalence of diarrhea than calves receiving milk replacer and non–bicarbonate-containing OAESs.9 Third, OAESs that use acetate as the alkalinizing agent induce higher clinical recovery rates at 36 hours and 72 hours in calves with diarrhea than do other OAESs, including those that contain bicarbonate.10 Acetate and propionate have similar alkalinizing abilities as bicarbonate on an equimolar basis, with the advantages being that acetate and propionate are readily metabolized by peripheral tissues and do not alkalinize the abomasum and proximal portion of the small intestine, whereas bicarbonate can permit bacteria to proliferate in an alkalinized abomasum.2,11,12
The rate of abomasal emptying influences the rate at which ingesta is delivered to the small intestine. With an OAES, the small intestine is the major site of fluid absorption thus influencing the rate of rehydration of a sick calf. We hypothesized that isotonic solutions of sodium acetate, NaHCO3, and NaCl would be emptied rapidly from the abomasum but that suckling NaHCO3 would result in a sustained increase of abomasal luminal pH, compared with suckling sodium acetate or NaCl. The objectives of the study reported here were therefore to determine and compare the abomasal emptying rate following suckling of 3 common OAES components (sodium acetate, NaHCO3, and NaCl) via scintigraphy, acetaminophen absorption, and ultrasonography and to monitor the dynamics of abomasal luminal pH for 12 hours after OAES ingestion in calves.
Materials and Methods
Animals and instrumentation—Five healthy colostrum-fed male Holstein-Friesian calves were obtained from the university dairy farm within the first week of life. Body weights on arrival ranged from 33 to 36 kg. An abomasal cannulaa was surgically placed, as described,13,14 and calves were kept unrestrained in stalls bedded with wood shavings. Calves were fed twice a day (60 mL/kg of body weight) with an all-milk protein–medicated milk replacerb (crude protein minimum, 20%; crude fat minimum, 20%; crude fiber maximum, 0.15%; calcium minimum, 0.5%; calcium maximum, 1.0%; phosphorus minimum, 0.6%; decoquinate, 45.4 g/ton) and had access to fresh water at all times. Neither hay nor calf starter ration was fed. A 16-gauge catheter was placed aseptically in the jugular vein at least 10 hours before the study for collection of jugular venous blood. The study was approved by the institutional animal care and use committee.
Experimental protocol—Between days 5 and 34 of age, at least 2 days after surgical cannulation and at least 10 hours after the previous feeding of milk replacer, each calf was weighed and placed in a movable calf stall that allowed sternal recumbency and standing but prevented excessive lateral movement. A flexible miniature glass pH electrodec was calibrated and placed in the abomasal lumen via the cannula as described.13,14 The pH electrode was connected to a pH meterd and the analogue output digitizede at 1 Hz. Digitized data were stored and analyzed offline by use of commercially available softwaree on a personal computer.
Calves were administered 5 treatments in random order in a Latin square design, and a minimum washout period of at least 36 hours was used between treatments. Acetaminophenf (50 mg/kg) and 370 MBq of 99mTc-DTPA were mixed with 2 L of the test solution. The test solution (at 18° to 24°C) was then suckled by the calf from a bottle with a nipple. The 5 treatments were as follows: milk replacer,b milk replacerb with parenterally administered atropine (0.01 mg/kg, IV, then 0.02 mg/kg, SC, q 30 min), isotonic sodium acetate (150mM), isotonic NaHCO3 (150mM), and isotonic NaCl (150mM). Atropine was included in the study because we anticipated that atropine would delay abomasal emptying rate. Calves did not have access to water for the first 4 hours of the study but had access to water from 4 to 12 hours of the study.
Scintigraphic images were obtained via 30-second acquisitions at 0, 10, 20, 30, 45, 60, 90, 120, 150, 180, and 240 minutes after the start of suckling, with the calf in a standing position. The scintigraphic images were recordedg and processedh as described.15 Jugular venous blood samples for determination of plasma acetaminophen concentration were obtained at 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, 90, 120, 150, 180, and 240 minutes after the start of suckling isotonic sodium acetate, NaHCO3, and NaCl and at 0, 10, 20, 30, 45, 60, 90, 120, 150, 180, 240, 300, 360, and 720 minutes after the start of suckling milk replacer (with or without parenteral administration of atropine). The blood samples were processed and analyzed as described.15 Ultrasonographic measurements of abomasal dimensions (length, width, and height) were obtainedi as described16 before suckling; immediately after the end of suckling the test solution; and at 10, 20, 30, 45, 60, 90, 120, 150, 180, and 240 minutes after the start of suckling the test solution. Jugular venous blood was obtained immediately after scintigraphic measurements had been obtained but before ultrasonographic measurements were acquired. Abomasal luminal pH was recorded for 12 hours after suckling. At the end of the pH recording period, the pH electrode was removed from the calf and recalibrated, and the calf was fed its allotted volume of milk replacer. Calves were euthanatized at the end of the study via IV administration of an overdose of sodium pentobarbital (60 mg/kg), and the carcasses were incinerated.
Data analysis—Data from several sources were analyzed.
Scintigraphy
The Siegel modified power exponential equation17 was used to fit a modeled curve to the data, as follows:
where y(t) is the proportion of maximal radioactivity remaining at time t in minutes from the start of suckling, κs is the abomasal emptying rate constant (min−1), and βs represents the extrapolated y-intercept for the terminal portion of the curve and is the time in minutes when the second derivative of the function is zero. This model provided the best method for describing the scintigraphic abomasal emptying rate in suckling calves.15 By use of the values for κs and βs obtained from nonlinear regression, the scintigraphic abomasal half-emptying time (scintigraphic t1/2), which represented the time at which half of the administered radioactive dose had been emptied from the abomasum, was calculated.
Acetaminophen Absorption
Jugular venous plasma was thawed at 22°C and analyzed spectrophotometrically as described.15 The actual Cmax and Tmax were obtained from a plot of the plasma acetaminophen concentration-time data. The Maes first derivative of the Siegel modified power exponential formula was used to describe the plasma acetaminophen concentration-time relationship, as follows:
where C(t) is the acetaminophen concentration in plasma (Mg/mL) at time t in minutes, and m, ka, and Ba are constants; m is the total cumulative acetaminophen recovery when time is infinite.18 This model provided the best method for describing the acetaminophen absorption curve in suckling calves.15
Ultrasonography
Abomasal volume was calculated from the abomasal dimensions by use of the formula for the volume of an ellipsoid (volume = width × length × height × [P/6]). This method provides an accurate measurement of abomasal volume in suckling calves.16 A volume versus time curve was generated for each experiment, as follows:
by use of the modified power exponential formula of Siegel,17 where y = proportion of peak volume remaining, time is the time interval from start of suckling in minutes, ku is the abomasal emptying rate in min−1, βu is the extrapolated y-intercept for the terminal portion of the curve (βu > 1 indicates an initial delay in emptying; βu < 1 indicates an initial rapid emptying), and ** indicates raised to the power of βu.16 By using the values for ku and βu obtained from nonlinear regression, we calculated ultrasonographic t1/2 as t1/2 = (−1/ku)•log(1 − 2−1/Bu) as described.16
Abomasal Luminal PH
The lowest pH value for each 60-second interval was used as the pH value for that minute; this prevented the inclusion of transient high pH values caused by the pH electrode contacting the abomasal mucosa. The mean preprandial pH (from time = −15 minutes to 0 minutes), maximum pH after suckling, minimum pH after suckling, and mean postsuckling pH were determined. The lowest pH at which 10%, 25%, 50%, 75%, and 90% of the suckled volume had been emptied was calculated from the scintigraphic and pH data. The κs and βs values determined in the equation from the scintigraphic data were used to determine the time that 10%, 25%, 50%, 75%, and 90% of the suckled volume had been emptied (equivalent to a proportion of maximal activity remaining of 0.90, 0.75, 0.50, 0.25, and 0.10). The pH time curve was then used to determine the measured pH at the stated times for each calf and test solution.
The pH return time was calculated as the time required for luminal pH to return to within 1.0 pH unit of the mean preprandial pH value. A cut point of 1.0 pH units provided the best method for describing the abomasal emptying rate in suckling calves.j
Statistical analysis—Models to describe the scintigraphic, acetaminophen absorption, and ultrasonographic data were fit by use of nonlinear regression,k,l and the adequacy of model fit was assessed by use of visual examination of plots of observed versus predicted concentrations and examination of residual plots. Abomasal emptying rate indices and abomasal luminal pH indices for the different test solutions were compared by use of repeated-measures ANOVA.k,l Data were expressed as least square means and SEM, and P < 0.05 was considered significant. Statistical softwarek,l was used for all statistical analyses.
Results
Scintigraphy—Suckling 2 L of milk replacer resulted in a slightly exponential pattern of emptying with a mean half-emptying time of 143 minutes (Figure 1; Table 1). Suckling 2 L of an isotonic solution of sodium acetate, NaHCO3, or NaCl induced an exponential pattern of emptying. The half-emptying times for sodium acetate, NaHCO3, or NaCl were similar but markedly shorter than that for milk replacer. The pattern of abomasal emptying in calves administered atropine was not exponential and appeared to have a rapid early phase and a slow late phase. As a result of the nonexponential emptying pattern in calves administered atropine, the Siegel modified power exponential equation did not provide a good fit to the data, and values for t1/2 and lag phase duration were not calculated.
Results (least square means) of use of 4 methods for measuring abomasal emptying rate in 5 calves suckling 2 L of milk replacer; milk replacer with parenteral atropine administration (0.01 mg/kg, IV, then 0.02 mg/kg, SC, q 30 min); or 150mM solutions of sodium acetate, NaHCO3, or NaCl containing acetaminophen (50 mg/kg) and 370 MBq 99mTc-DTPA.
Method | 150mM Na-acetate | 150mM NaCl | 150mM NaHCO3 | Milk replacer and atropine | Milk replacer | SEM |
---|---|---|---|---|---|---|
Scintigraphy | ||||||
t1/2 (min) | 57a | 72a | 49a | ND | 143.0b | 15 |
Tlag (min) | 20a | 22a | 14a | ND | 51a | 12 |
β | 1.45a | 1.48a | 1.17a | ND | 1.21a | 0.19 |
Acetaminophen absorption | ||||||
Actual Cmax (μg/mL) | 41.5a | 35.4ab | 40.4a | 23.5b | 27.3a | 4.8 |
Actual Tmax (min) | 62a | 80a | 76a | 304b | 208c | 20 |
Model Cmax (μg/mL) | 38.6a | 33.5a | 36.2a | ND | 22.9b | 3.9 |
Model Tmax (min) | 92a | 96a | 88a | ND | 190b | 15 |
Ultrasonography | ||||||
Preprandial volume (mL) | 34a | 55a | 123a | 43a | 56a | 54 |
Maximum postsuckling volume (mL) | 2139a | 2157a | 2217a | 2025a | 2053a | 120 |
t1/2 (min) | 31a | 31a | 37a | ND | 86b | 10 |
Tlag (min) | 14a | 15a | 21a | ND | 17a | 5 |
β | 2.22a | 2.93a | 2.20a | ND | 1.19a | 0.70 |
Luminal pH | ||||||
Preprandial pH | 1.25a | 1.52a | 1.50a | 1.31a | 1.52a | 0.21 |
Maximum postsuckling pH | 6.59ac | 4.52b | 7.85c | 5.91ab | 5.84ab | 0.43 |
Minimum postsuckling pH | 0.80a | 1.16ab | 1.12ab | 1.48b | 1.09ab | 0.18 |
Mean postsuckling pH | 206ab | 1.72a | 2.60bd | 3.97c | 2.99d | 0.31 |
pH return time (min) | 128a | 52a | 136a | 468b | 335b | 36 |
ND = Not determined.
Within a row, values with different superscripts are significantly (P < 0.05) different.
Acetaminophen absorption—The actual Cmax for plasma acetaminophen concentration was the same for sodium acetate, NaCl, NaHCO3, and milk replacer (Figure 2; Table 2). However, the actual Tmax for milk replacer was approximately 3 times as long as that for the 3 isotonic test solutions. There were no significant differences in Tmax among sodium acetate, NaHCO3, and NaCl.
Lowest abomasal luminal pH (least square means) associated with various percentages of ingested volume in 5 calves suckling 2 L of milk replacer or 150mM solutions of sodium acetate, NaHCO3, or NaCl.
Percentage emptied from the abomasum | Lowest luminal pH | ||||
---|---|---|---|---|---|
150mM Na-acetate | 150mM NaCl | 150mM NaHCO3 | Milk replacer | SEM | |
10 | 6.40a | 4.12b | 7.43a | 5.50ab | 0.47 |
25 | 6.12ac | 3.58b | 7.55a | 5.41c | 0.41 |
50 | 5.76a | 2.05b | 7.67c | 5.12a | 0.22 |
75 | 4.87ab | 1.86a | 5.98b | 3.95b | 0.83 |
90 | 1.78a | 1.68a | 2.61a | 1.89a | 0.61 |
See Table 1 for key.
The pharmacokinetically calculated value for Cmax of milk replacer was less than that of sodium acetate, NaHCO3, and NaCl (Table 1). The calculated value for Tmax of milk replacer was approximately twice that of sodium acetate, NaHCO3, and NaCl. The calculated Cmax and Tmax values were similar for sodium acetate, NaHCO3, and NaCl.
Ultrasonography—We detected an exponential pattern of emptying when 2 L of milk replacer was suckled, with a half-emptying time of 86 minutes (Figure 3; Table 1). The same exponential pattern of emptying was seen when 2 L of an isotonic solution of sodium acetate, NaHCO3, or NaCl was suckled. The half-emptying times for sodium acetate, NaHCO3, and NaCl were similar but markedly shorter than that for milk replacer.
Abomasal luminal pH—Electrode drift during the 12-hour recording period was ± 0.06 (range, 0.00 to 0.15) for buffer pH 2.00 and pH 7.00. Raw pH values were used for statistical analysis because of the minimal drift. Abomasal luminal pH was similar for all groups before suckling (overall mean pH = 1.42), then rapidly increased after suckling of the test solutions (Figure 4).
Calves 4 and 5 had slower emptying rates for all solutions across all tests. As a result, their pH profiles when combined with those of the other calves created a second plateau in the pH curves as the test solution emptied (Figure 4). Sodium bicarbonate and sodium acetate had similar maximum postsuckling pH values, which were higher than the maximum pH for NaCl Table 2—Lowest abomasal luminal pH (least square means) associated with various percentages of ingested volume in 5 calves suckling 2 L of milk replacer or 150mM solutions of sodium acetate, NaHCO3, or NaCl. (Table 1). There was no difference in maximum luminal pH between sodium acetate and milk replacer test solutions.
The pH return time was similar following suckling of sodium acetate, NaHCO3, or NaCl but much longer after suckling milk replacer. The pH return time was the longest in calves suckling milk replacer that were administered atropine. However, overall mean postsuckling pH did not differ for NaHCO3 and sodium acetate (Table 1). Sodium bicarbonate had the highest mean luminal pH when half of the suckled test solution had been emptied from the abomasum. Isotonic NaCl had the lowest mean luminal pH when half of the suckled volume had been emptied, whereas similar values for the pH at 50% emptying were present for milk replacer and sodium acetate.
Discussion
The major findings of this study were that isotonic solutions of sodium acetate, NaCl, and NaHCO3 are emptied rapidly and at a similar rate from a calf's abomasum but vary in their ability to alkalinize the abomasum. Specifically, administration of isotonic NaHCO3 causes transiently greater alkalization of the abomasum of milk-fed calves, compared with an isotonic OAES containing sodium acetate or NaCl.
The 4 methods used to evaluate abomasal emptying rate, scintigraphy, acetaminophen absorption, ultrasonography, and pH return time detected similar relative rates of emptying when the test solutions were compared. There was, however, a methodologic difference in the respective numeric values for abomasal emptying-rate variables. This difference was attributed to the different aspects of abomasal emptying measured by each method. The 99mTc-DTPA is suspended in the fluid phase. It is not absorbed or adsorbed and therefore passes through the intestinal tract with the liquid phase.19 The change in radioactivity measured in the region of interest over time gives a measurement of the rate of decline of radioactivity in the abomasum associated with emptying of liquids but does not take into account the volume of abomasal secretions. Scintigraphy therefore provides an accurate measurement for emptying of a known suckled volume from the abomasum. In comparison, ultrasonography measures the change in stomach volume over time20; the abomasal volume at any time is the sum of the preprandial volume, suckled volume, and volume secreted by the salivary glands and abomasum minus the volume emptied.
Both ultrasonography and scintigraphy can be inaccurate when assessing the location and dimensions of the abomasum. The depth of penetration of the ultrasound probe can influence the accuracy of abomasal volume calculations, as can abomasal contractions. Movement of the calf during imaging and the presence of overlying intestines can interfere with the accuracy of region-of-interest calculations during scintigraphy. The presence of radioactivity in the small intestine overlying the abomasum is probably the greatest potential source of error in that the ultrasonographic emptying curves and luminal pH time curves for isotonic OAES essentially suggest complete emptying by 3 hours after suckling; assuming an exponential rate of emptying, > 97% of the ingested volume is emptied from the abomasum after 5 half-emptying times. For comparison, the scintigraphic emptying curve indicated incomplete emptying by 3 hours, with a plateau in the curve after that time. This means that 2-L isotonic solutions of sodium acetate, NaHCO3, and NaCl are effectively emptied from the abomasum by 3 hours (on the basis of ultrasonographic half-emptying time and pH return time). For comparison, 2 L of the all-milk protein milk replacer was effectively emptied from the abomasum within 7 hours.
Acetaminophen absorption is an indirect measurement of abomasal emptying because the rate of absorption is determined by the rate of acetaminophen delivery to the small intestine.21 The plasma concentration of acetaminophen is influenced by many factors, including the rate of passage through the proximal portion of the intestinal tract, mucosal contact time, intestinal absorption ability, and dilution by abomasal secretions. Despite these potential limitations, scintigraphic t1/2, acetaminophen absorption, Tmax, ultrasonographic t1/2, and pH return time yielded the same conclusion—that isotonic electrolyte solutions empty at similar rates that are at least twice as rapid as that for milk replacer (Table 1).
The mean half-emptying time for milk replacer in calves with a reentrant duodenal cannulae is approximately 2 hours22; this value was similar to that obtained in the study reported here (scintigraphic t1/2 = 143 minutes). However, the only other study23 that has used nuclear scintigraphy (99mTc-sulfur-colloid–containing fresh cow's milk) to assess emptying rate recorded longer half-emptying times of 210 minutes (lateral view) and 240 minutes (ventral view). The difference in emptying rate between the 2 studies23 may have been caused by acquiring images with the calves in dorsal and lateral recumbency in that manipulation and restraint may have delayed emptying by stressing the calves.24 We chose to use standing right lateral scintigraphic images of the abomasum to allow natural calf behavior during the postprandial period and to prevent disruption of the pH probe by manipulation of calf position.
Acidemia in diarrheic calves results from strong ion acidosis in response to hyponatremia, normochloremia to hyperchloremia, and hyper–D-lactatemia and nonvolatile buffer ion acidosis in response to increased plasma protein concentration.4 Acidemic calves with diarrhea should therefore be treated with an OAES containing sodium and a highly effective strong ion difference,3 although there is debate about what the most effective alkalinizing agent should be. We evaluated the 2 most common alkalinizing components in commercial OAESs, acetate and bicarbonate, with most of the commercially available OAES solutions in the United States containing bicarbonate. Sodium bicarbonate is a very effective alkalinizer that does not require metabolism to exert its effect. However, NaHCO3 has the disadvantage that alkalinization of the gastrointestinal tract can facilitate growth of pathogens.25 Acetate has similar alkalinizing ability to bicarbonate on an equimolar basis, with acetate having the advantage that it also provides energy but does not interfere with abomasal acidity or milk clotting.26 Acetate stimulates sodium and water absorption in the small intestine27 and is metabolized readily by peripheral tissues. There was no difference in the rate of abomasal emptying after suckling isotonic solutions of bicarbonate and acetate. Isotonic solutions are emptied at the same rate if they are isocaloric as sensed by receptors in the duodenum.11,28 On this basis, NaCl and NaHCO3 would be expected to be emptied at the same rate, but sodium acetate might be emptied a little slower. However, it would appear that acetate was not sensed as a caloric compound, and bicarbonate solutions alkalinized the abomasum to a much greater extent than acetate. This increase in pH may make survival of bacteria, including ETEC and Salmonella enterica subsp enterica serovars, more likely for calves fed bicarbonate-containing solutions.
The importance of high abomasal pH in promoting ETEC diarrhea has been clearly demonstrated by modifications to the standard ETEC diarrhea technique, which requires giving neonatal calves < 24 hours of age a large oral inoculum (typically 109 to 1011) of viable ETEC bacteria.29,30 The modified ETEC diarrhea technique, which is used in colostrum-fed calves older than 24 hours of age, requires oral administration of NaHCO3 (4 to 10 g in 60 to 150 mL of water) to alkalinize the abomasum and ameliorate the effect of so-called abomasal sterilization, followed immediately by oral administration of a large inoculum (109 to 1011 bacteria) of viable ETEC.31,32,33,34 The modified protocol induced diarrhea in 67% (100/150) of calves aged 5 to 10 days,31 53% (32/60) of calves older than 1 day of age,32 61% (31/51) of calves < 7 days of age,33 and 51% (38/75) of calves 7 to 14 days of age.34 The high success rate of the modified ETEC diarrhea technique raises concerns over the appropriateness of bicarbonate-based OAESs to treat diarrheic calves in that abomasal alkalinization may facilitate development of ETEC in calves older than 1 day of age.
High abomasal pH also appears to be important in facilitating salmonellosis in calves. Cattle develop an age-dependent resistance to salmonellosis that is associated with development of a functional rumen, diverse small intestinal bacterial population, and low abomasal pH.35,36 The optimum pH for growth of salmonellae is generally accepted to be from 6.5 to 7.5,31 and salmonellae are susceptible to destruction by exposure to low pH37,38,39 in that salmonellae isolated from a small number of cattle were killed at pH ≤ 3.4 and multiplied at pH ≥ 5.5.38 Abomasal acidity provides a natural barrier to ingested salmonellae,36 and maintaining a low abomasal pH will decrease the number of viable salmonellae that reach the small intestine, thereby increasing nonspecific resistance to intestinal colonization and decreasing infection and clinical disease. The presence of bacterial metabolic waste products such as short-chain fatty acids may also play a role in preventing salmonellosis in calves.37,40,41 Acetate, propionate, and butyrate inhibit the growth of salmonellae, even in concentrations as low as 20 mmol/L37,40 that are frequently found in oral electrolyte solutions administered to diarrheic calves.5
Oral electrolyte solution formulations affect abomasal luminal pH and the percentage volume of ingesta that is exposed to a pH that will kill bacteria or support bacterial growth. It is recognized that the rapid emptying of gastric contents minimizes the number of bacteria that are exposed to a low pH,42 thereby altering the effect of the so-called abomasal sterilizer. Accordingly, the abomasal emptying rate interacts with abomasal pH to determine the percentage of ingested bacteria that survive abomasal transit. Complete characterization of the potential effect of abomasal pH on bacterial survival requires simultaneous measurement of luminal pH and emptying rate. To our knowledge, this is the first study to concurrently measure abomasal emptying rate and luminal pH in suckling calves. The mean preprandial abomasal luminal pH recorded in this study (pH = 1.4) was similar to that found in other studies13,22 of suckling calves; this level of acidity will not support survival or growth of most bacteria. The mean maximum pH we obtained with milk replacer (5.8) was consistent with previous reports11,13,43 for calves suckling milk replacer. In comparison, suckling an isotonic NaHCO3 solution markedly increased the abomasal luminal pH to 7.9, and the lowest pH encountered by 50% of the NaHCO3 solution was 7.67. At this markedly alkaline pH, Escherichia coli production of K99 and STa enterotoxin, important virulence factors in neonatal calves with ETEC diarrhea, are promoted. This is because bovine ETEC strains produce K99 pili at pH > 6.544 and production of STa enterotoxin by bovine ETEC strains is greatly promoted at pH > 7.2.45
We used milk replacer instead of fresh cow's milk for comparison to the isotonic electrolyte solutions because milk replacer is commonly fed in the United States to dairy calves. Because it is important to provide a source of energy and nutrients for a sick calf,6,9 the continued feeding of milk to diarrheic calves is currently recommended. The sodium content of cow's milk is low, compared with calf plasma; sodium concentration in nonmastitic milk is approximately 35 mEq/L.46 This sodium concentration alone is insufficient to replace sodium lost in the feces in calves with diarrhea; thus, suckling a sodium-rich solution is always required for the initial rehydration of dehydrated diarrheic calves.
Atropine was administered to calves in this study in an attempt to delay abomasal emptying. Surprisingly, atropine administration caused a biphasic emptying pattern characterized by an initial rapid rate of emptying followed by a slower emptying rate. Atropine inhibition of gastric accommodation will facilitate passage of ingesta to the small intestine as a result of increased abomasal luminal pressure47,48; however, once the pressure gradient between the abomasum and duodenum equalizes, the rate of emptying is slowed because of vagal inhibition49 of abomasal and small intestinal motility. Atropine did not appear to interfere with the esophageal groove closure in these calves because the postprandial volume measured ultrasonographically was not different from that of the other treatments.
Isotonic solutions of sodium acetate, NaHCO3, and NaCl emptied rapidly from the abomasum of suckling calves. The ability of these solutions to alkalinize the abomasum varied. Suckling an isotonic solution of NaHCO3 alkalinized the gastrointestinal tract of suckling calves to a greater degree than a metabolizable base such as sodium acetate. Sodium bicarbonate–containing OAESs therefore have the potential to promote survival of bacterial enteric pathogens and increase the number of viable ETEC and salmonellae reaching the small intestine. Sodium bicarbonate–containing OAESs therefore could increase the frequency of infection or severity of clinical disease in diarrheic calves treated for dehydration.
ABBREVIATIONS
99mTc-DTPA | Technetium-99m-diethylenetriamine-pentaacetic acid |
Cmax | Maximum observed plasma concentration |
ETEC | Enterotoxigenic Escherichia coli |
OAES | Orally administered electrolyte solution |
T1/2 | Half life |
Tmax | Time of maximum observed plasma concentration |
Percutaneous endoscopic gastrostomy kit, MILA Instruments Inc, Florence, Ky.
Agri Master, Supreme All Milk, Blain Supply, Janesville, Wis.
M3 internal reference glass pH electrode, Medical Instruments Corp, Solothurn, Switzerland.
Cole-Parmer pH/mV/Rel mV/oC Bench top Meter, Cole-Parmer Instrument Co, Vernon Hills, Ill.
Windaq, DATAQ Instruments, Akron, Ohio.
Sigma-Aldrich Inc, St Louis, Mo.
Technicare Omega 500, Technicare Corp, Cleveland, Ohio.
Alfanuclear IM512P, MEDX, Arlington Heights, Ill.
Ultramark 4, Advanced Technology Laboratories, Tempe, Ariz.
Marshall T, Constable PD, Wittek T, et al. Ability of the abomasal luminal pH-time relationship to predict the abomasal emptying rate in Holstein bull calves (abstr), in Proceedings. 23rd World Buiatrics Cong 2004;34:112.
PROC NLIN, SAS 8e, SAS Institute, Cary, NC.
PROC MIXED, SAS 8e, SAS Institute, Cary, NC.
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