Photochemically produced intracellular concentration jumps of cAMP mimic the effects of catecholamines on excitation-contraction coupling in frog atrial fibers

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Abstract
Previously, we reported that concentration jumps of cAMP produced by light flashes in the presence of a photosensitive analogue of cAMP increase the amplitude of the slow inward current (Isi) in isolated bullfrog atrial trabeculae (Nargeot et al. 1983). Here, using newly designed photolabile cyclic nucleotides (Nerbonne et al. 1984a), we have examined the effects of intracellular concentration jumps of cAMP and cGMP on excitation-contraction coupling in frog heart. Concentration jumps of cAMP increase the amplitude and the duration of action potentials, increase Isi and twitch tension. Following single flashes, maximum responses are observed in 10–30 s and recovery times are 30–120 s. The time courses of the cAMP-induced increases in Isi and phasic tension amplitudes are parallel, implying a direct correlation between Ca2+ influx through the slow channels and the development of phasic tension. Although the amplitudes are increased severalfold, cAMP jumps do not measurably alter the kinetics or voltage dependences of the current or tension. cAMP concentration jumps increase the delayed K+ current (IK) and decrease tonic tension; relaxation of contraction is not, however, influenced by cAMP jumps. Concentration jumps of cGMP, on the other hand, have no measurable effects on the action potential, Isi, IK or tension in this preparation.