Salt intake and plasma renin activity in the freshwater turtle

Abstract

In mammals, a well-documented inverse relationship exists between oral salt intake and plasma renin activity (PRA). We carried out this study to determine if a similar relationship exists in the freshwater turtle Pseudemys scripta. Three groups of turtles ( N = 8 for each group) were fed different amounts of salt dissolved in distilled water (2 cc/kg). The high salt group received 580 mg (10 mEq) NaCl/kg twice per week, the medium salt group received 290 mg (5 mEq) NaCl/kg twice per week, and the low salt (control) group received 2 cc/kg distilled water twice per week with no salt. The animals were weighed weekly and blood was collected by cardiac puncture after 3 and 6 weeks of salt loading and again 3 weeks after termination of the salt loading. PRA and plasma electrolytes (P Na, P K, P Cl) were determined. The low salt (control) group showed no changes in any of the measured variables throughout the 9-week study. At 3 weeks the following mean values were obtained for the control group: P Na, 119.5 ± 7.2 mEq/liter; P K, 5.0 ± 1.0 mEq/liter; P Cl, 74.1 ± 2.6 mEq/liter; PRA, 1.0 ± 0.2 ng/ml/hr. In the medium salt group, no significant changes were seen until 3 weeks after termination of the salt loading when P Na and PRA were elevated to 137.9 ± 3.6 mEq/liter and 1.9 ± 0.2 ng/ml/hr, respectively ( P < 0.05 compared to control). In the high salt group, P Na (136.5 ± 6.5 mEq/liter) was elevated after 3 weeks of salt loading. At 6 weeks, P Na (136.8 ± 4.9 mEq/liter) remained elevated in the high salt group and P Cl (86.3 ± 1.3 mEq/liter) and PRA (1.7 ± 0.2 ng/ml/hr) also increased significantly ( P < 0.05 compared to control in each case). Three weeks after termination of salt loading, P Na (136.8 ± 3.3 mEq/liter), P Cl (88.0 ± 1.6 mEq/liter), and PRA (1.9 ± 0.2 ng/ml/hr) remained elevated ( P < 0.05 in each case) in the high salt group while P K dropped to 3.5 ± 1.6 mEq/liter ( P < 0.05). The results show that after 6 weeks of salt loading sufficient to raise P Na in the turtle, PRA was significantly elevated.