title: Inhibition by extracellular ATP of L-type calcium channel currents in guinea-pig single sinoatrial nodal cells: involvement of protein kinase C.
abstract: It is generally accepted that ATP is costored and coreleased with noradrenaline from the sympathetic nerve terminals. The pacemaker region of the mammalian heart is highly innervated with the sympathetic nervous system. It is possible, therefore, that ATP released from nerve terminals can act on pacemaker cells and modulate heart beat activity. However, the physiological role(s) of extracellular ATP in mammalian heart beat has been little evaluated, even though the effect of extracellular ATP observed in in vivo studies has been attributed to the formation of adenosine, the catabolic product of ATP, which is a cardiodepressant. The present study investigated the effect of extracellular ATP on L-type calcium channel currents of guinea-pig single sinoatrial nodal cells, by using the whole cell patch clamp technique. Application of extracellular ATP caused a concentration-dependent and reversible inhibition of calcium channel currents with a 50% inhibitory concentration of 100 microM. The presence of the P2-purinoceptor antagonist suramin (1, 10 and 100 microM), reactive blue 2 (1 and 10 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 50 and 100 microM) or the adenosine receptor antagonists 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX, 0.1 microM) and 8-phenyltheophylline (10 microM) failed to affect the inhibitory action of extracellular ATP on calcium channel currents. The relative rank order of potency of different nucleotides and nucleosides, at a concentration of 100 microM, on the inhibition of calcium channel currents was as follows: ATP = alpha, beta-methylene-ATP >> 2-methylthioATP > or = adenosine 5'-O-(3-thiotriphosphate) >> UTP = ADP > AMP > or = adenosine. Dialysis, by way of the patch pipette, of the cell interior with specific protein kinase C inhibitor staurosporine (70 nM) or calphostin C (500 nM) abolished extracellular ATP-induced inhibition of calcium channel currents. Therefore, extracellular ATP-mediated inhibition of calcium channel currents in guinea-pig single sinoatrial nodal cells involves activation of protein kinase C.
authors.affiliation: Department of Pharmacology, Faculty of Medicine, Chinese University of Hong Kong, Shatin, New Territories, Hong Kong. b136701@mailserv.cuhk.edu.hk
authors.forename: Y W
authors.initials: YW
authors.lastname: Kwan
authors.forename: A D
authors.initials: AD
authors.lastname: Qi
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects*; D002675:Child, Preschool; D005110:Extracellular Space; D006321:Heart / Q000502:physiology; D006801:Humans; D011493:Protein Kinase C / Q000494:pharmacology*; D012849:Sinoatrial Node / Q000166:cytology* / Q000502:physiology
chemical_list: D015220:Calcium Channels; D000255:Adenosine Triphosphate; D011493:Protein Kinase C