Implication of ATP released from atrial, but not papillary, muscle segments of guinea pig by isoproterenol and forskolin.

Effects of isoproterenol and forskolin, which increases intracellular cyclic AMP, on contraction and ATP release in atrial and papillary muscles of guinea pig were evaluated. In the electrically driven-left atrial muscle segments, isoproterenol and forskolin at 0.1 microM produced an ATP release coupled with a positive inotropic effect, the values of net ATP release at 5 min after these drugs being 5.20 +/- 0.59 and 5.37 +/- 0.55 nmoles/g wet weight, respectively. The forskolin evoked-ATP release was unaffected by prazosin plus propranolol or by guanethidine, implying that ATP is released from non-neuronal origin. In contrast, in papillary muscle segments, the test cardiotonics did not elicit any ATP release despite producing contractile response similar to that in atrial preparations. However, there is no difference in ectoATPase activities of both tissues. Adenosine added exogenously inhibited electrically evoked-contraction of the atrium, but not that of the papillary, although inhibitions by verapamil of the contractions were approximately equal in these preparations. These findings suggest that cardiotonics such as isoproterenol produce a liberation of ATP from auricle muscles, but not from ventricle muscles, and that the liberated ATP may mainly be catabolized to adenosine by ectoenzymes and the resultant nucleoside may serve as a functional modulator through stimulation of pre or postsynaptic A1-receptors.