REGULATION OF CA2-MUSCLE CELLS OF RAT MESENTERIC-ARTERY( CHANNEL CURRENTS BY INTRACELLULAR ATP IN SMOOTH)

Regulation of L-type Ca2+ channels of vascular smooth muscle (VSM) cel ls by adenosine 3',5'-cyclic monophosphate (cAMP)-dependent and guanos ine 3',5'-cyclic monophosphate (cGMP)-dependent phosphorylation, which requires Mg2+ ATP as a phosphate donor, has been reported (T. Ishikaw a, J. R. Hume, and K. D. Keef. Circ. Res. 73: 1128-1137, 1993; Z. Xion g, N. Sperelakis, and C. Fenoglio-Preiser. J. Vasc. Res. 31: 271-279, 1994), and regulation by ATP has been demonstrated (Y. Ohya and N. Spe relakis. Circ. Res. 64: 145-154, 1989). However, it has not been eluci dated 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 VS M cells isolated from rat mesenteric arteries using a whole cell volta ge clamp combined with an intracellular perfusion technique. Ba2+ curr ents (I-Ba) through Ca2+ channels were evoked by depolarizing pulses f rom a holding potential of -80 mV with 130 mM Cs+ in the pipette and 1 00 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 maxi mal 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. Th is concentration is much higher than that usually required for phospho rylation (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 thi s enhancement was not prevented in the presence of 30 mu M H-7, a nons pecific inhibitor of protein kinases, or 1 mu M protein kinase inhibit or, an inhibitor protein of cAMP-dependent protein kinase. These resul ts indicate that slow Ca2+ channels in VSM cells are regulated by intr acellular ATP, independently of phosphorylation, implying a direct reg ulatory action, such as a requirement for ATP binding to the inner sur face of the channel, to exhibit activity.