Ca⁺⁺ inhibition of isoproterenol responses in mammalian skeletal muscle

In noncontracting mouse hemidiaphragms incubated in modified Krebs-Ringer-bicarbonate buffer with 10 mM Ca⁺⁺, isoproterenol-stimulated phosphorylase a formation, conversion of phosphorylase kinase to the activated form, elevation of cyclic AMP-dependent protein kinase activity ratios and increase in cyclic AMP concentrations were reduced 35 to 50% over the responses in buffer with 2.5 mM Ca⁺⁺. In buffer with 10 mM Ca⁺⁺, the initial rate of isoproterenol-stimulated cyclic AMP accumulation was 59% of that in buffer with 2.5 mM Ca⁺⁺. The inhibitory action of Ca⁺⁺ on cyclic AMP accumulation was antagonized by verapamil, but not by inhibitors of cyclic nucleotide phosphodiesterase activity. In buffer with 2.5 mM Ca⁺⁺, isoproterenol-stimulated cyclic AMP accumulation was inhibited by A23187 and caffeine, agents that can increase intracellular Ca⁺⁺ concentrations. In addition to Ca⁺⁺, high concentrations of Co⁺⁺, Ni⁺⁺, Mn⁺⁺ and, to a lesser extent, Sr⁺⁺ inhibited the isoproterenol response. The results of these studies indicate that high buffer Ca⁺⁺ concentrations inhibit the response of the glycogenolytic pathway to isoproterenol by an action on cyclic AMP formation. The authors propose that the site of the inhibitory action of Ca⁺⁺ is the divalent metal activator site associated with hormone-stimulated adenylate cyclase activity.