Renal responses to mesotocin in Western painted turtles compared with the antidiuretic response to arginine vasotocin

We have examined the renal responses to the two neurohypophysial peptides mesotocin (MT) and arginine vasotocin (AVT) in Western painted turtles (Chrysemis picta). Ureteral urine was collected, after removal of the urinary bladder, to exclude modification by trans-bladder fluxes of ions and or water. Test doses of MT ranged from 5 to 500 ng kg−1 i.v. but only the uppermost pharmacological doses of 300 and 500 ng kg−1 were followed by even a small reduction in GFR, a renal tubular antidiuresis, and a decrease in percent relative free-water one word clearance. A much lower, physiological, dose of 5 ng kg−1 of AVT attenuated significantly the GFR for 30 min and decreased the percent relative free-water clearance until the end of a 150 min observation period. This is the first report of the renal response to MT in a reptile. It was concluded that MT is not a diuretic hormone, as it is in amphibians and that AVT may be the naturally occurring antidiuretic hormone in the Chelonians.