Modulation of Ca2+ transients by photorelease of caged nucleotides in frog skeletal muscle fibers.

Action potentials and intracellular Ca2+ transients were monitored in current-clamped segments of frog skeletal muscle fibers using the triple vaseline-gap technique. Calcium signals were measured with the fluorescent indicator rhod 2. Action potentials produced a transient increase in intracellular Ca2+ that was estimated, by deconvolution of the fluorescence signals, to range between 3 and 12 microM. The comparative effects of flash photolysis of caged adenosine 3',5'-cyclic monophosphate (cAMP) and caged ATP on action potentials and Ca signals in muscle were investigated. The photorelease of both nucleotides produced a reduction in the amplitude of the afterpotential that follows the spike. Photorelease of cAMP and ATP prolonged the rate of decay of the Ca signals. No changes in either the rate of rise or in the latent period between stimulation and onset of the Ca signal were observed. Release of cAMP reduced the amplitude of Ca signals, whereas release of ATP had the opposite effect. Our results show that cAMP and ATP, released above their endogenous levels, modulate intracellular Ca2+ release. The cAMP modulation is more significant and may be of physiological importance.