Modulation by extracellular ATP of L-type calcium channels in guinea-pig single sinoatrial nodal cell.
1. The effects of extracellular adenosine 5'-triphosphate ([ATP]zero) on the L-type Ca2+ channel currents in guinea-pig single sinoatrial nodal (SAN) cells, isolated by enzymatic dissociation, were investigated by use of whole-cell patch-clamp techniques. 2. The application of [ATP]zero (2 microM-1 mM) produced an inhibitory effect on the L-type Ca2+ channel current peak amplitude (10 mM Ba2+ as charge carrier) in a concentration-dependent and reversible manner with an IC50 of 100 microM and a Hill coefficient of 1.83. 3. The presence of the adenosine receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 microM) and 8-phenyltheophylline (10 microM) did not affect the [ATP]zero-induced inhibition of the Ca2+ channel currents. Adenosine (100 microM) had little effect on the basal Ca2+ channel currents. Adenosine 500 microM, caused 23% inhibition of the Ca2+ channel current, which was abolished by 0.1 microM DPCPX. 4. The presence of the P2-purinoceptor antagonists, 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) failed to affect the inhibitory action of [ATP]zero on Ca2+ channel currents. 5. The relative rank order of potency of different nucleotides and nucleosides, at a concentration of 100 microM, on the inhibition of the Ca2+ channel currents is as follows: adenosine 5'-triphosphate (ATP) = alpha,beta-methylene-ATP (alpha,beta MeATP) > > 2-methylthioATP (2-MeSATP) > or = adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) > > uridine 5'-triphosphate (UTP) = adenosine 5'-diphosphate (ADP) > adenosine 5'-monophosphate (AMP) > or = adenosine. 6. These results suggest that [ATP]zero may play an important role in the heart beat by inhibiting the L-type Ca2+ channel currents in single SAN cells. This inhibitory effect is not due to the formation of adenosine resulting from the enzymatic degradation of [ATP]zero. Based on the relative order of inhibitory potency of different nucleotides and nucleosides on the L-type Ca2+ channel currents and the ineffectiveness of the purinoceptor antagonists tested, a novel type of purinoceptor may be involved.
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Department of Pharmacology, Faculty of Medicine, Chinese University of Hong Kong, Shatin, New Territories, Hong Kong.
Kwan
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YW
D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D004594:Electrophysiology; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D009705:Nucleosides / Q000494:pharmacology; D009711:Nucleotides / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D058914:Purinergic Antagonists; D058915:Purinergic P1 Receptor Antagonists; D011732:Pyridoxal Phosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D012849:Sinoatrial Node / Q000166:cytology / Q000378:metabolism*; D013806:Theophylline / Q000031:analogs & derivatives / Q000494:pharmacology; D014970:Xanthines / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D009705:Nucleosides; D009711:Nucleotides; D010975:Platelet Aggregation Inhibitors; D058914:Purinergic Antagonists; D058915:Purinergic P1 Receptor Antagonists; D014970:Xanthines; C077792:pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid; D011732:Pyridoxal Phosphate; D000255:Adenosine Triphosphate; C051360:1,3-dipropyl-8-cyclopentylxanthine; D013806:Theophylline; C028322:8-phenyltheophylline