Salt intake and plasma renin activity in the freshwater turtle

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 (PNa, PK, PCl) 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: PNa, 119.5 +/- 7.2 mEq/liter; PK, 5.0 +/- 1.0 mEq/liter; PCl, 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 PNa and PRA were elevated to 137.9 +/- 3.6 mEq/liter and 1.9 +/- 0.2 ng/ml/hr, respectively (P less than 0.05 compared to control). In the high salt group, PNa (136.5 +/- 6.5 mEq/liter) was elevated after 3 weeks of salt loading. At 6 weeks, PNa (136.8 +/- 4.9 mEq/liter) remained elevated in the high salt group and PCl (86.3 +/- 1.3 mEq/liter) and PRA (1.7 +/- 0.2 ng/ml/hr) also increased significantly (P less than 0.05 compared to control in each case). Three weeks after termination of salt loading, PNa (136.8 +/- 3.3 mEq/liter). PCl (88.0 +/- 1.6 mEq/liter), and PRA (1.9 +/- 0.2 ng/ml/hr) remained elevated (P less than 0.05 in each case) in the high salt group while PK dropped to 3.5 +/- 1.6 mEq/liter (P less than 0.05). The results show that after 6 weeks of salt loading sufficient to raise PNa in the turtle, PRA was significantly elevated.