INHIBITION BY EXTRACELLULAR ATP OF L-TYPE CALCIUM-CHANNEL CURRENTS INGUINEA-PIG SINGLE SINOATRIAL NODAL CELLS - INVOLVEMENT OF PROTEIN-KINASE-C

It is generally accepted that ATP is costored and coreleased with nora drenaline from the sympathetic nerve terminals. The pacemaker region o f the mammalian heart is highly innervated with the sympathetic nervou s system. It is possible, therefore, that ATP released from nerve term inals can act on pacemaker cells and modulate heart beat activity. How ever, the physiological role(s) of extracellular ATP in mammalian hear t beat has been little evaluated, even though the effect of extracellu lar ATP observed in in vivo studies has been attributed to the formati on of adenosine, the catabolic product of ATP, which is a cardiodepres sant. The present study investigated the effect of extracellular ATP o n L-type calcium channel currents of guinea-pig single sinoatrial noda l cells, by using the whole cell patch clamp technique. Application of extracellular ATP caused a concentration-dependent and reversible inh ibition of calcium channel currents with a 50% inhibitory concentratio n of 100 mu M The presence of the P-2-purinoceptor antagonist suramin (1, 10 and 100 mu M), reactive blue 2 (1 and 10 mu M) and pyridoxal ph osphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 50 and 100 mu M) or the adenosine receptor anatgonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 mu M) and 8-phenyltheophylline (10 mu M) failed to affect the inhibitory action of extracellular ATP on cal cium channel curren ts. The relative rank order of potency of different nucleotides and nu cleosides, at a concentration of 100 mu M, on the inhibition of calciu m channel currents was as follows: ATP = alpha,beta-methylene-ATP much greater than 2-methylthioATP greater than or equal to adenosine 5'-O- (3-thiotriphosphate) much greater than UTP = ADP > AMP greater than or equal to adenosine. Dialysis, by way of the patch pipette, of the cel l interior with specific protein kinase C inhibitor staurosporine (70 nM) or calphostin C (500 nM) abolished extracellular ATP-induced inhib ition of calcium channel currents. Therefore, extracellular ATP-mediat ed inhibition oi calcium channel currents in guineapig single sinoatri al nodal cells involves activation of protein kinase C.