Effects of purinergic stimulation on the Ca current in single frog cardiac cells

Ca current (I _Ca) was measured by whole-cell voltage clamp in single cells isolated from frog ventricle, in which the Na current was inhibited by tetrodotoxin (0.3 M) and K currents were blocked by substituting K with 120 mM intracellular and 20 mM extracellular Cs. The influence of stimulation by ATP (0.1–100 M) was assessed in the presence of propranolol (1 M) or pindolol (0.1 M), prazozin (0.1 M) and atropine (10 M). ATP, in the micromolar range, had two types of effect. Like other P₁-purinoagonists, it antagonized the increase in I _Ca elicited by -adrenostimulation. When added alone, 1 M ATP could increase I _Ca up to twofold. An increase in I _Ca was also observed even after it had been maximally enhanced by intracellularly applied cAMP (50 M). Voltage dependence and kinetics of I _Ca were not affected. These effects were considered to be related to P₂-purinoceptor activation. At higher ATP concentrations the increase in I _Ca was less; at 100 M, ATP reduced I _Ca. The ATP-induced increase in I _Ca was prevented by internal perfusion of the cells with GDP -S] or neomycin, respectively, to block signal transduction to phospholipase C or its phosphodiesterase activity on the polyphosphoinositides. We conclude that P₂purinoceptor stimulation increases the Ca current in frog ventricular cells by a pathway that might involve phosphoinositide turnover.