Materials and Methods

Animals—Medical records of dogs serving as kidney donors at the Auburn University veterinary teaching hospital between April 2004 and April 2005 were reviewed (n = 19). In addition, owners of the donor dogs were contacted and asked to complete a questionnaire about their dog. Owners were asked to have their veterinarian obtain and send blood and urine samples to the veterinary teaching hospital for a CBC, serum biochemical profile, complete urinalysis, and urine proteinto-creatinine ratio determination. This step was taken so that all samples would be processed by the same laboratory. All donor dogs with a complete medical record and appropriate sample submission were included in the study (n = 14). All results of clinical pathology tests were shared with the owners and their regular veterinarian. The study was approved by the Auburn University Institutional Animal Care and Use Committee, and owners consented to the use of their animals.

Imaging—All imaging was performed at the veterinary teaching hospital. However, because of the widespread geographic distribution of the donor dogs, the number of dogs available for postnephrectomy imaging was limited (n = 6). Of these 6 dogs, all had ultrasonographic evaluation of the remaining kidney and 4 of 6 had dynamic renal scintigraphy. Sedation for ultrasonographic examinations was performed by administration of butorphanol (0.4 mg/kg [0.18 mg/lb], IM or IV). Sedation for dynamic renal scintigraphy was performed by administration of butorphanol (0.2 to 0.4 mg/kg [0.09 to 0.18 mg/lb], IM) and medetomidine (5 μg/kg [2.3 μg/lb], IM). Reversal of sedation with atipamizole was performed at the discretion of the supervising veterinarian. Glomerular filtration rate was determined in 4 dogs by means of dynamic renal scintigraphy with 3 mCi of technetium Tc 99m pentetate.¹⁰ Six dogs underwent ultrasonographic evaluation of the retained kidney with calculation of renal volume by use of the prolate ellipsoid formula (renal length × width × height × P/6).^11–13

Renal histology—All 14 dogs underwent renal biopsy of the retained kidney at the time of nephrectomy for kidney donation. Renal biopsy specimens were obtained with a 14-gauge biopsy needle.^a In all but 1 dog, the left kidney was collected for transplantation. Follow-up kidney biopsy specimens were obtained from the 4 dogs that underwent nuclear scintigraphy determination of GFR. Biopsy specimens were collected via an exploratory laparotomy. All biopsy specimens were fixed in neutral-buffered 10% formalin solution and stained with H&E and periodic acid–Schiff stains.

Statistical analysis—Statistical analysis was performed with statistical software.^b Descriptive statistics and 2-sided P values were calculated for each variable. Differences in normally distributed pre- and postnephrectomy laboratory values were evaluated by use of a paired Student t test. Data that were not normally distributed were analyzed by use of the Wilcoxon test. Association between pairs of variables was evaluated by use of Pearson product moment correlation. Values of P ≤ 0.05 were considered significant; data are reported as mean ± SD.

Results

Determination of clinical status—The clinical status of all 14 dogs was considered to be normal. Mean age at time of donation was 24.8 ± 24.7 months, and mean time between donation and the reevaluation examination was 18.7 ± 5.1 months. Donor dogs represented various breeds and included 7 Greyhounds or Greyhound crosses, 2 German Shepherd Dogs, 1 Bull Terrier, 1 English Setter, 1 Labrador Retriever, and 2 mixed-breed dogs. Four dogs were castrated males, and 10 were spayed females.

Results of physical examination were normal in 11 dogs. Of the remaining 3 dogs, 1 dog was obese, 1 dog had chronic seborrheic dermatitis of unknown etiology, and 1 dog was pyrexic at the time of examination (rectal temperature, 40.4°C [104.7°F]); however, this was attributed to anxiety. No dog was reported to be polydipsic or polyuric. Mean systolic blood pressure was 136 ± 14 mm Hg (range, 117 to 155 mm Hg; n = 8), and median systolic pressure was 133 mm Hg. None of the dogs were receiving medications other than heart-worm and flea-tick preventatives. All owners reported that their dog's overall health was excellent.

Clinicopathologic findings—Results of serum biochemical analyses, CBC, and urinalysis were summarized (Table 1). There was no significant (P = 0.092) difference between mean pre- and postnephrectomy BUN concentrations. Mean postnephrectomy serum creatinine concentration was significantly (P < 0.001) increased, compared with prenephrectomy values, but both pre- and postnephrectomy values (1.22 ± 0.22 mg/ dL and 1.66 ± 0.27 mg/dL, respectively) were within reference range. Mean serum phosphorus concentration was significantly (P = 0.001) higher before nephrectomy than after nephrectomy (5.23 ± 1.56 mg/dL and 3.56 ± 0.71 mg/dL, respectively), but concentrations remained within reference range. There were no significant differences between pre- and postnephrectomy albumin concentrations (3.89 ± 0.38 g/dL and 3.77 ± 0.25 g/dL, respectively; P = 0.42) and pre- and postnephrectomy total protein concentrations (6.50 ± 1.11 g/dL and 6.46 ± 0.54 mg/dL, respectively; P = 0.868). All other biochemical values were within reference ranges.

Mean urine specific gravity was 1.038 ± 0.0091 before nephrectomy and 1.037 ± 0.0107 after nephrectomy; the difference that was not significant. Five prenephrectomy urine specimens had trace ketones, 3 had trace blood, and 1 had positive results of bacterial culture. Five postnephrectomy urine specimens had trace bilirubin, 7 had trace ketones, 1 had triple phosphate crystals, 1 had calcium oxalate crystals, and 1 had amorphous crystals. Mean urine protein-to-creatinine ratio was 0.104 ± 0.156 after nephrectomy. One dog had 3+ proteinuria and a protein-to-creatinine ratio of 0.64 (reference value, < 0.5).

Mean Hct values before and after nephrectomy were 47.42 ± 6.18% and 55.2 ± 5.83%, respectively. There was a significant (P = 0.016) increase in Hct after nephrectomy, but both values were within reference range. The RBC count was also significantly (P = 0.015) increased after nephrectomy (mean prenephrectomy value, 6.86 ± 0.81 × 10⁶ cells/μL; mean postnephrectomy value, 7.70 ± 0.65 × 10⁶ cells/μL). Mean corpuscular volume was significantly (P < 0.001) increased after nephrectomy (mean prenephrectomy value, 67.88 ± 1.67 fL; mean postnephrectomy value, 71.42 ± 2.56 fL). Mean corpuscular hemoglobin concentrations before and after nephrectomy were 34.96 ± 0.84 g/dL and 33.12 ± 0.84 g/dL, respectively, a difference that was significant (P = 0.001). Although the changes in pre- and postnephrectomy values for Hct, RBC count, mean corpuscular volume, and mean corpuscular hemoglobin concentration were significant, all values were within reference range.

Imaging—Six dogs underwent ultrasonographic evaluation to determine length, width, and height of the remaining kidney at the time of reevaluation; mean values were 7.64 ± 0.69 cm, 4.21 ± 0.75 cm, and 4.09 ± 0.47 cm, respectively. Mean calculated renal volume for all 6 dogs was 70.54 ± 24.62 mL. Mean renal volume of young dogs (< 1 year; n = 3) was significantly (P < 0.001) higher than that of older dogs (n = 3; 102.15 ± 4.27 mL and 54.72 ± 2.23 mL in younger and older dogs, respectively).

Glomerular filtration rate was determined in 4 dogs at the time of follow-up examination. Mean GFR in these dogs was 2.82 ± 1.12 mL/kg/min (1.28 ± 0.51 mL/lb/min; Table 2). A correlation between age at kidney donation and GFR could not be established on the basis of the number of dogs in which GFR was determined, but GFR was highest in dogs younger than 12 months of age at donation.

Renal histopathologic findings—All 14 dogs used for kidney donation had histologically normal kidneys, as determined by microscopic examination of prenephrectomy biopsy specimens. Follow-up biopsy specimens were obtained from 4 dogs. Two of those specimens were microscopically normal, and 2 had changes indicating minimal focal glomerulosclerosis (2 affected glomeruli/60 glomeruli counted and 2 affected glomeruli/52 glomeruli counted).

Discussion

In the 14 dogs reported here, BUN and serum creatinine concentrations remained within reference range after nephrectomy, although there was a significant increase in serum creatinine concentration. Renal concentrating ability was also considered normal on the basis of urine specific gravity values. The urine protein-to-creatinine ratio was within reference limits for 13 of 14 dogs, and the ratio in the remaining dog (0.6) was only slightly greater than the reference limit (< 0.5). That dog had 3+ proteinuria, but the urine sample was collected via free-catch rather than via cystocentesis, and it is possible that the protein originated from the lower portion of the urinary tract. Proteinuria is a common finding in human kidney donors but usually is neither progressive nor associated with decreased GFR or hypertension.^6,7,14 In a previous study in dogs,¹⁵ proteinuria developed after experimental unilateral nephrectomy, but the dogs did not develop hypertension and renal function remained stable.

One half of normal renal mass has been determined to be adequate to sustain normal erythropoiesis in dogs, but a decrease to 25% of normal renal mass may not be sufficient to maintain adequate erythropoietin synthesis.¹⁶ Although none of the donor dogs had a decrease in Hct, RBC count, or other hematologic variables, significant differences between pre- and postnephrectomy samples were found via CBC. The RBC count, mean corpuscular volume, and mean corpuscular hemoglobin concentration were significantly increased, compared with prenephrectomy values; however, all RBC indices were within reference ranges. Postnephrectomy hypochromasia and macrocytosis suggest accelerated RBC production, but the mechanism is unclear and further investigation was beyond the scope of this study.

Serum phosphorus concentration was lower after nephrectomy, compared with prenephrectomy values. Because several donor dogs were young Greyhound crosses, this finding was attributed to the higher phosphorus concentrations often detected in young, largebreed dogs; the lower serum phosphorus concentrations detected after surgery were believed to be indicative of skeletal maturity and normal renal function. An increase in phosphorus excretion has been reported in 30% of human kidney donors, but urine phosphorus concentration was not determined in dogs of the present study.^5,14 Therefore, the true reason for the decreased phosphorus concentration was not known and warrants further investigation.

Microscopic examination of biopsy specimens from the remaining kidney, collected at the time of kidney donation, indicated that kidneys in all dogs were histologically normal. Follow-up biopsy specimens were available for only 4 dogs. The specimens were normal in 2 dogs, and only minor histologic changes were detected in the other 2 dogs. It is not known whether these changes were natural, related to prior biopsy, or related to the increased workload in the remaining kidney after nephrectomy.

The ultrasonographic appearance of the retained kidneys was considered normal. Interestingly, the mean renal volume of the retained kidneys was normal or high, compared with weight-adjusted normal ranges. In dogs that were < 1 year of age at the time of nephrectomy, mean renal volume at the time of reevaluation was 102.15 ± 4.27 mL, which was significantly higher than that in the older dogs (54.72 ± 2.23 mL).

The GFR directly reflects the number of functioning nephrons. Four dogs were available for GFR measurement after kidney donation. The reference limit for GFR in dogs with 2 kidneys is > 3 mL/kg/min (1.35 mL/lb/min). However, dogs with unilateral dysfunction or a single kidney may have a GFR > 1.5 mL/kg/min (> 0.68 mL/lb/min) because of compensatory hypertrophy.¹⁰ The GFRs in the present study were within the reference range for dogs with a single kidney, although the degree of compensation varied substantially. On the basis of GFR measurements, younger dogs had greater compensation for the loss of renal mass. Although it would have been ideal to have performed prenephrectomy GFR studies, this imaging modality was not available at the time of nephrectomy. Remnant kidney GFRs of 70% to 75% of preoperative values have been reported^7–9 in human kidney donors, suggesting that the remaining kidney undergoes functional hypertrophy. This compensatory ability maintains normal kidney function in donors.^7–9

The capacity for and mechanisms underlying the compensatory renal hypertrophy and hyperplasia seen in young animals is not fully understood.¹⁷ Renal hypertophy is influenced by age, reduction in functional renal mass, and dietary protein intake. Because all dogs in the present study had a reduction of 50% in renal mass and none of the dogs were on protein-restricted diets, age at time of donation was the greatest variable affecting renal hypertrophy. The implication that younger dogs may have greater renal compensatory abilities deserves further study.

These results suggest that donor age as well as histocompatibility should be considered in the selection of canine kidney donors. Furthermore, the difficulty of finding related donors that are younger than 12 months of age underscores the need for alternative approaches to standard immunosuppressive protocols to induce renal allograft tolerance.^18–20 Living donor transplantation programs should be designed to minimize risks to the donor while maximizing the potential benefits to the recipient.