125 I-Glucagon Binding and Adenylate Cyclase Activation in the Fetal Rat Heart 1

Abstract

Summary

Hearts from 13- and 18-day rat fetuses were shown to specifically bind ¹²⁵I-glucagon and to have a glucagon-sensitive adenylate cyclase. ¹²⁵I-glucagon binding was observed over the concentration range 5 × 10^-8 M-5 × 10^-6 M. Glucagon activation of adenylate cyclase occurred over a concentration range 10^-7 M-10^-5 M. Similar results for both ¹²⁵I-glucagon binding and adenylate cyclase activation were obtained in adult rat hearts. In addition, the data in the present study demonstrate that glucagon activation of adenylate cyclase in the fetal rat occurs on day 18, when physiological and biochemical studies have shown insignificant inotropic and chronotropic responses to catecholamines, activation of cardiac adenylate cyclase by catecholamines, but no significant inotropic or chronotropic responses to glucagon. Several explanations for this discrepancy are considered including the possibility that cyclic AMP may not mediate the inotropic and chronotropic responses to glucagon in the fetal rat heart.