| Abstract: | The antagonism between adenosine and isoprenaline on force of contraction, cyclic AMP (cAMP) and cyclic GMP (cGMP) content, adenylate cyclase activity and transmembrane action potential in isolated electrically driven atrial and ventricular muscle preparations from guinea-pig hearts was investigated. In atrial preparations adenosine added 5 min after isoprenaline decreased force of contraction. Adenosine abolished completely the positive inotropic effect of isoprenaline. Similarly, adenosine prevented the positive inotropic effect of isoprenaline when both substances were added simultaneously. In ventricular preparations adenosine also decreased the isoprenaline-induced increase in force of contraction. The effect was much smaller than it was in the atria. Adenosine reduced the isoprenaline-induced increase in force of contraction only by about 60%. Adenosine did not at all influence the positive inotropic effect of isoprenaline when both substances were added simultaneously. In both preparations the isoprenaline-induced increase in cAMP content of the intact contracting preparations was not diminished by adenosine. cGMP content remained unchanged too. Adenosine inhibited adenylate cyclase activity in broken cell preparations from both tissues. In atrial preparations the decrease in force of contraction of adenosine in the presence of isoprenaline was accompanied by a shortening of the action potential duration. In ventricular preparations adenosine failed to shorten the action potential. In conclusion, the effects of adenosine to inhibit the stimulatory action of isoprenaline on myocardial force of contraction are not due to changes in the cAMP and/or cGMP content. Instead, adenosine may inhibit a step beyond an increased cAMP level, e.g., may exert an inhibition of protein kinases. However, in the atria, but not in the ventricles, an additional direct effect of adenosine on transmembrane ion currents, most likely an increase in potassium conductance, probably is of even greater importance. |
