Receptor-controlled calcium discharge in frog heart cells

A brief account is given of previous work concerned with the effects which ATP (P2-'purinergic') and alpha-adrenaline receptors have on the activity of the sarcoplasmic reticulum (SR) in frog heart atrial cells. More recent experiments made with single atrial trabeculae are also described which suggest, among other things, that ATP and alpha-adrenaline receptors both facilitate the process by which an action potential induces SR calcium discharge in atrial heart cells. The facilitatory effect involved is considerable; at high (greater than or equal to 2 microM) ATP doses, SR calcium discharge seems to be increased about 14-fold and, quite possibly, more than this. Results obtained with different levels of [Ca2+]o during ATP action suggest that the process of calcium-induced calcium release plays little part in the facilitatory process. Its putative mechanism is briefly discussed.