Anti adrenergic effect of adenosine on Na+-Ca2+ exchange current recorded from guinea-pig ventricular myocytes

The Na+-Ca2+ exchanger is a protein present in the cell membrane of many ce ll types. In heart it plays important roles in Ca homeostasis and ionic cur rent generation. Recently, it has been reported that the beta -adrenergic a gonist isoprenaline (ISO) can increase directly Na+-Ca2+ exchanger activity in guinea-pig Ventricular myocytes. Adenosine (ADO) exerts anti-adrenergic properties that make it effective against some arrhythmias and the aim of the present study was to determine whether or not ADO can antagonize the di rect modulatory effect of ISO on the exchanger. Whole-cell patch clamp measurements of Na+-Ca2+ exchanger current (I-NaCa) were made from guinea-pig ventricular myocytes, with major interfering curr ents inhibited. I-NaCa was measured at 37 degreesC as current sensitive to external nickel (Ni2+, 10 mM) during an applied descending voltage ramp, IS O (I muM) significantly increased both inward and outward I-NaCa This effec t was abolished in the presence of ADO (200 muM). ADO alone did not signifi cantly alter the amplitude of I-NaCa The effect of ADO on the response of I -NaCa to ISO was mimicked by the A(1) ADO receptor agonist N-6-cyclopentyla denosine (CPA, 10 muM), whereas the effect of ADO on the response of I-NaCa to ISO was inhibited by the A(1) ADO receptor antagonist 8-cyclopentyl-1,3 -dipropylxanthine (DPCPX, 2 muM). These data suggest that the A(1) ADO rece ptor mediated the response, The anti-adrenergic effects on I-NaCa Of ADO we re not affected by the protein kinase C (PKC) inhibitor, chelerythrine (CLT , 1 muM), nor by the nitric oxide (NO) synthase inhibitor, N (G)-nitro-L-ar ginine methyl ester (L-NAME, 0.5 mM). Moreover, in the presence of PKC acti vator phorbol le-myristate 13-acetate (PMA, 1 muM) or exogenous NO donor so dium nitroprusside (SNP, 100 muM), ISO preserved its stimulatory effect on I-NaCa. However, prior incubation of myocytes with pertussis toxin (PTX, 5 mug ml(-1)) did prevent the effect of ADO. The anti-adrenergic effect of AD O on I-NaCa was mimicked by externally applied carbachol (CCh, 10 muM), a m uscarinic receptor agonist. We conclude that ADO antagonized the effect of beta -adrenergic stimulation of I-NaCa by directly activating inhibitory G- protein (G(i))-linked A(1) receptors in guinea-pig ventricular myocytes. Th ese findings may suggest a novel mechanism by which adenosine exerts some o f its antiarrhythmic effects. (C) 2001 Harcourt Publishers Ltd.