A role for adenylate cyclase in the subsensitivity to isoproterenol during ontogenesis of the embryonic chick ventricle

Abstract

Isoproterenol (ISO) increases contractility and cyclic AMP content in ventricles of embryonic and hatched (H) chicks. A transient decrease in beta agonist sensitivity for both effects (10-fold shift in ISO EC50) is seen in 18-day embryos (18 E) (Pappano and Biegon, 1982). Beta adrenoceptor-coupled adenylate cyclase (AC) and receptor binding were characterized in 14,000 X g ventricular particulates from 10 to 11E, 17 to 19E and 1-week-old chicks (5-6H). The concentration for half-maximal enzyme activation (Kact) for ISO [+100 microM 5'-guanylylimidodiphosphate (Gpp(NH)p)]-stimulated AC is greatest in the 17 to 19E (1.6 microM vs. 0.1-0.2 microM in the 5-6H and 10-11E). Maximal enzyme activity in the presence of ISO + Gpp(NH)p (picomoles of cyclic AMP/20 min X mg of protein), however, is greatest in the 10 to 11E (2018 +/- 80 vs. 1020 +/- 80 and 1120 +/- 60 in the 17 to 18E and 5 to 6H, respectively). The Kact values for NaF and Gpp(NH)p are the same for all three ages (2.4 mM and 8 microM, respectively), whereas maximal activity with these effectors of the regulatory and catalytic protein-dependent AC decreases with increasing age. Neither the Kact nor maximal activity of catalytic AC, assessed with MnCl2 and forskolin, changes with age. The respective Kact values for MnCl2 and forskolin are 0.3 mM and 4 microM.(ABSTRACT TRUNCATED AT 250 WORDS)