Regulation of Ca2+ channel currents by intracellular ATP in smooth muscle cells of rat mesenteric artery.

Regulation of L-type Ca2+ channels of vascular smooth muscle (VSM) cells by adenosine 3',5'-cyclic monophosphate (cAMP)-dependent and guanosine 3',5'-cyclic monophosphate (cGMP)-dependent phosphorylation, which requires Mg2+ATP as a phosphate donor, has been reported (T. Ishikawa, J. R. Hume, and K. D. Keef. Circ. Res. 73: 1128-1137, 1993; Z. Xiong, N. Sperelakis, and C. Fenoglio-Preiser. J. Vasc. Res. 31: 271-279, 1994), and regulation by ATP has been demonstrated (Y. Ohya and N. Sperelakis. Circ. Res. 64: 145-154, 1989). However, it has not been elucidated whether the regulation by ATP is mediated by a mechanism that is distinct from phosphorylation. In the present study, we examined the effects of intracellularly perfused ATP on Ca2+ channel currents of VSM cells isolated from rat mesenteric arteries using a whole cell voltage clamp combined with an intracellular perfusion technique. Ba2+ currents (I(Ba)) through Ca2+ channels were evoked by depolarizing pulses from a holding potential of -80 mV with 130 mM Cs+ in the pipette and 100 mM Ba2+ in the bath. The decrease in the ATP concentration (from 5 to 0.1 mM) in the pipette caused a 45 +/- 5% (n = 8) reduction of maximal I(Ba) obtained at +40 mV within 10 min. The dose-response relation between I(Ba) and ATP showed a dissociation constant of 0.53 mM ATP. This concentration is much higher than that usually required for phosphorylation (e.g., few micromolar). Increase in the ATP (from 0.1 to 5 mM) caused an enhancement of maximal I(Ba) by 57 +/- 10% (n = 6), and this enhancement was not prevented in the presence of 30 microM H-7, a nonspecific inhibitor of protein kinases, or 1 microM protein kinase inhibitor, an inhibitor protein of cAMP-dependent protein kinase. These results indicate that slow Ca2+ channels in VSM cells are regulated by intracellular ATP, independently of phosphorylation, implying a direct regulatory action, such as a requirement for ATP binding to the inner surface of the channel, to exhibit activity.