Vasopressin-Dependent Adenylate Cyclase Activities in the Rat Kidney Medulla: Evidence for Two Separate Sites of Action*

Abstract

This study demonstrates the existence of an adenylate cyclase sensitive to vasopressin in the medullary portion of the rat thick ascending limb. Maximal adenylate cyclase stimulations achieved in that segment (31-fold) were higher than those obtained in collecting tubules from the same rats (22-fold). From comparisons of absolute maximal responses it can be calculated that thick ascending limbs account for about 80% of the response to vasopressin of a kidney medulla homogenate. The apparent Km value of adenylate cyclase activation (from 10-9-2 .times. 10-8 M) in thick ascending limbs was higher in each experiment than that simultaneously measured in the collecting tubules from the same rats (2 .times. 10-10-3 .times. 10-9 M). Such a lower sensitivity is probably not due to a greater hormone degradation by the thick ascending limb samples. Experiments using structural analogs of the oxytocin series ([deamino-6-carba]oxytocin and vasotocin) did not give evidence for different vasopressin receptors in the thick ascending limb and the collecting tubule. A step beyond the hormone-receptor interaction, thus, must account for the different patterns of adenylate cyclase response to vasopressin of these 2 segments.