A comparative investigation of the volumes of plasma and extracellular fluids and the renal clearances of urea and creatinine in Merino sheep from flocks with different genetic capacities for wool growth

Abstract

Adult castrate male Merino sheep (n = 24; average liveweight 47 kg), from two flocks, differing in their genetic capacity for wool growth as a result of selective breeding for clean fleece weight, consumed one of two diets, differing in both quality (i.e. digestibility and nitrogen content) and quantity, termed L and H. The L (low) and H (high) diets supplied 0.42 and 0.81 kg digestible dry matter day-' and 9.5 and 26.0 g nitrogen day-I respectively. We compared the volumes of plasma and extracellular fluids, as well as the concentration of urea in plasma and the rate of clearance of urea by the kidneys of sheep from these two genetic groups, consuming the different diets. The average plasma volume of these sheep, estimated from the dilution of Evan's Blue in plasma, was 2.24 L (s.e. 0.08). The average volume of extracellular fluid, estimated from the dilution of 35S-sulfate, was 10.3 L (s.e. 0.4). These distributional volumes were similar in sheep from the two genetic groups and were not influenced by the dietary treatments. The sheep consuming the H diet maintained significantly greater concentrations of urea-nitrogen in their plasma (7.2 v. 2.8 mmol L-1: s.e. 0.3) and cleared both urea (37.1 v. 16.1 mL min-I : s.e. 5.3) and creatinine (87 v. 49 mL min-1 : s.e. 11) at significantly greater rates from the plasma. The sheep from the two genetic groups had similar average values for these three traits.