Renal function in sea snakes: I. Glomerular filtration rate and water handling

Renal clearance studies were performed on anesthetized sea snakes (Aipysurus laevis) under various salt and volume loads. The median values for the control glomerular filtration rate (GFR) and urine flow rate (V̇) were 0.78 and 0.11 ml·kg^-1·h^-1, respectively. Acute salt loading with 1 mol/1 NaCl significantly increased V̇ to 0.68 ml·kg^-1·h^-1, whereas acute water loading significantly decreased GFR to 0.17 ml·kg^-1·h^-1. Acute loading with either isosmotic NaCl or seawater did not significantly alter either V̇ or GFR. Chronic loading with water or seawater by intraperitoneal injection significantly increased both GFR and V̇. No consistent relationship between plasma osmolality and GFR was observed. Instead a threshold-type relationship existed between GFR and plasma K concentration. Within each treatment group, high plasma K concentrations were correlated with low GFRs. There was an abrupt increase in GFR at low plasma K concentrations, the inflection point ranging from 2.5 to 4 mmol/l K depending on the experimental treatment. A curvilinear relationship existed between V̇ and GFR; as GFR increased, V̇ approached a maximal limiting value. At low GFRs (<0.2 ml·kg^-1·h^-1), V̇ often exceeded GFR, indicating net fluid secretion that was correlated with net Na movement. In addition, there was an inverse relationship between urine-to-plasma osmolality ratio and V̇. At low values of V̇, urine osmolalities modestly exceeded those of the plasma. Overall, renal function of A. laevis appears to be adapted to conserve water.