Two physiological agents that appear to facilitate calcium discharge from the sarcoplasmic reticulum in frog heart cells: adrenalin and ATP

The $\alpha $-adrenergic effect of adrenalin and the action of ATP and other nucleotides have been examined in single trabeculae of frog heart by means of procedures developed in the preceding paper (Niedergerke & Page 1981). The results suggest that both adrenalin and ATP are able, in conjunction with the action potential, to facilitate the discharge of calcium from the sarcoplasmic reticulum in at least some of the cardiac cells. As a result, the strength of the twitch is enhanced. As shown previously for the action of caffeine, this calcium discharge was not maintained, declining rapidly, together with the twitch tension, as the sarcoplasmic reticulum calcium store became depleted. Trabeculae from atrium and ventricle differed in their propensity to respond to these two substances. Thus, $\alpha $-adrenergic responses were obtained in some 30% of the atrial, but in none of the ventricular, trabeculae examined. On the other hand, both kinds of trabecula gave ATP responses, but these tended to be weaker and required higher concentrations in ventricle than atrium. The possibility that the two responses are of physiological importance is suggested by the low concentrations ($\leq $ 5 $\times $ 10$^{-7}$ M) needed to produce large tension increases. A tentative hypothesis is advanced according to which $\alpha $-catecholamine and ATP effects participate in circulatory control by initiating a rapid boost of cardiac pump activity, preparatory to the slower but better maintained $\beta $-catecholamine action.