Stimulation of slow action potentials in guinea pig papillary muscle cells by intracellular injection of cAMP, Gpp(NH)p, and cholera toxin.

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Abstract
To test the hypothesis that intracellular cAMP has a regulatory role in cardiac slow channel function, intracellular pressure injections of cAMP and adenylate cyclase activators, Gpp(NH)p and cholera toxin, were carried out. Guinea pig papillary muscles were depolarized to about -45 mV by superfusion with 22 mM K+-Tyrode's solution to inactivate the fast Na+ channels. Induction of slow action potentials or enhancement of ongoing slow action potentials was observed in about 70% of all cells in which a successful intracellular injection of the testing compounds was obtained. The slow AP is highly dependent on slow inward current and is known to be enhanced by catecholamines. The effect of the injected cyclic nucleotides and related compounds occurred within 3 minutes after starting the injection, whereas superfusion with these compounds (dibutyryl cAMP was used in place of cAMP) required 10-30 minutes to show an effect. This difference is attributed to the intracellular injection of the compound. The effect on stimulating slow action potentials persisted (greater than 5 minutes) after termination of the application of either Gpp(NH)p or cholera toxin, indicating the long-lasting nature of their action. The effect of the cAMP injections decayed within 1 minute. Intracellular injection of 5'-AMP was without effect. These results support the view that a causal relationship exists between intracellular cAMP level and slow channel function. Phosphorylation of a protein constituent of the slow channel by a cAMP-dependent protein kinase may be involved.