A Comparison of the Responses of the Amphibian Kidney to Mesotocin, Isotocin, and Oxytocin

Abstract

Renal clearance techniques were used to assess the effects of several neurohypophyseal polypeptides on renal function in Ambystoma tigrinum larvae. Hypophysectomy decreased urine flow (V̇) from 249 ± 49 μl/10 g-h to 64 ± 12 μl/liter and glomerular filtration rate (GFR) from 336 ± 48 to 76 ± 7 μl/10 g-h. Isotocin (ISO) at 5 ng/g restored V̇ to 172 ± 34 and GFR to 235 ± 58 μl/10 g-h. Oxytocin (OXY) was without effect. A comparison of ISO with mesotocin (MST) at lower doses revealed that MST is active at 100 pg/g, while ISO is required at 1,000 pg/g to be effective. These results have interesting implications related to the effect of substitutions of amino acids at the number 4 and number 8 positions of the polypeptides. The presence of isoleucine in position 8 appears to be of paramount importance to the diuretic activity; however, glutamine in position 4 (MST) produces a diuresis at lower doses than does serine in this position (ISO).