Modulation of ionic currents in smooth muscle balls of the rabbit intestine by intracellularly perfused ATP and cyclic AMP

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Abstract
The effects of intracellularly perfused ATP and cyclic-AMP (c-AMP) on ionic currents recorded from fragmented smooth muscle cells (smooth muscle ball; SMB) were investigated, using the single electrode whole cell voltage clamp method. The Ca2+ current was distinguished from K+ currents, using pipette solution containing Cs+, TEA+ and 4 mM EGTA. ATP enhanced the Ca2+ current, dose-dependently between 0.3 and 10. mM, and slightly slowed the slow component of the decay of the Ca2+ current, while the steady-state inactivation curve remained unaffected. Intracellular application of 5′-adenylyl-imidodiphosphate (AMP-PNP; 1mM) inhibited the Ca2+ current by competitionwith ATP, but c-AMP (up to 300 μM) had no effect. With a high-K+ solution containing 0.3 mM EGTA and ATP in the pipette and physiological salt solution in the bath, a net inward current with transient (Ca2+ dependent) and delayed (Ca2+ independent) K+ outwart currents were evoked. Increased concentrations of ATP (above 1 mM) but not c-AMP (up to 100 μM) in the pipette enhanced the transient K+ outward current Neither agent had any effect on the delayed outward current. When repetitive stimulations of intervals shorter than 5 s were applied, the amplitude of the transient outward current was markedly reduced, and 100 μM c-AMP partially prevented this attenuation. ATP may act on the Ca2+ channel either by phosphorylating the channel protein or by other ATP requiring mechanisms, independently from those induced by the action of c-AMP. Thus, the different responses of cardiac and visceral smooth muscles induced by β-adrenoceptor stimulation may be explained in part by the different natures of the Ca2+ channel in response to c-AMP.