Ionic basis for the antagonism between adenosine and isoproterenol on isolated mammalian ventricular myocytes

We studied the effects of adenosine and isoproterenol on membrane currents of isolated bovine and guinea pig ventricular myocytes with a two-microelectrode voltage clamp technique. Adenosine (50 microM to 0.2 mM) alone had no effect on any of the membrane currents measured, but it antagonized the effects induced by 10 nM isoproterenol. Peak calcium membrane current was augmented by isoproterenol from a control of 4.8 +/- 0.6 to 8.6 +/- 0.8 nA and adenosine reduced it to 5.7 +/- 0.7 nA (mean +/- SEM of six cells). The inactivation time constant was not altered by isoproterenol alone or isoproterenol plus adenosine, and neither was the voltage dependence of peak calcium membrane current. Thus, the changes caused by isoproterenol could be described as an increase in maximal calcium conductance from 0.86 +/- 0.7 to 1.55 +/- 0.04 mS/cm2 and partially antagonized by adenosine to 0.97 +/- 0.04 mS/cm2. Isoproterenol also increased the non-inactivating component of calcium membrane current from 17 +/- 1 to 24 +/- 4%, and adenosine reduced it to 18 +/- 2% (n = 4). The steady state activation and inactivation variables remained unchanged. Consistent with these effects on calcium membrane current, adenosine completely antagonized the isoproterenol-induced increase of the slow action potentials obtained in sodium-free medium. Isoproterenol increased the steady state outward currents at potentials between -90 and -30 mV (i.e., probable iK1). Adenosine alone had no effect on potassium membrane current, but it antagonized the effects of isoproterenol. Slow action potentials in 25 mM potassium were enhanced by isoproterenol, but were only moderately attenuated by adenosine. Accordingly, in 25 mM potassium the isoproterenol-induced changes in membrane currents were not antagonized by adenosine. This lack of inhibition by adenosine of the isoproterenol effects in 25 mM potassium could not be mimicked by 1-minute-long conditioning prepulses to -45 mV. The results indicate that adenosine by itself (absence of isoproterenol) has no effect on maximal calcium conductance, that the isoproterenol-induced increase in cyclic adenosine 3',5'-monophosphate, which leads to an increase in maximal calcium conductance, is antagonized by adenosine, and that such action can account for the ability of adenosine to attenuate the stimulatory effects of isoproterenol.