Comparison of renal function in a congenital hyperbilirubinaemic Southdown sheep and normal sheep

Renal function was studied in a six-year-old mutant Southdown ewe with congenital hyperbilirubinaemia, two normal Southdown ewes and four Merino ewes. Clearances of p-aminohippurie acid and phenol-sulphonphthalein were within normal ranges in the mutant Southdown, but inulin clearance was 1.18±0.05 ml/min per kg body weight compared with a mean value of 2.55±0.29 in control sheep. Urine dilution and concentration tests were carried out in all seven sheep. In the mutant Southdown minimum urine osmolality 3 h after water loading was hypertonic to plasma. In the six normal sheep it was hypotonic. After 36 h of water deprivation maximum urine osmolality in the mutant Southdown was 753 mOsm/kg and plasma osmolality was 295. However, during a graded water deprivation test the maximum urine osmolality rose to 1336 mOsm/kg and plasma osmolality rose sharply to 337 mOsm/kg. In the normal sheep urine osmolality rose in deprivation tests but there was little change in plasma osmolality. Vasopressin infusion in the mutant Southdown failed to raise the urine osmolality above the plasma osmolality. It did in the normal sheep. Renal histopathology in the mutant Southdown was consistent with that found in congenital hyperbilirubinaemia. The underlining functional impairment was a reduced ability to concentrate or dilute urine. That was probably due to the renal structural changes revealed by histopathology.