Phospholipase C modulates automaticity of canine cardiac Purkinje fibers

Alpha-1 adrenergic agonists increase cardiac Purkinje fiber automaticity and elevate D-myo-inositol-trisphosphate (IP₃) levels. To learn about the relationship between phosphoinositide metabolism and the modulation of cardiac rhythm, we used phospholipase C to activate phosphoinositide hydrolysis in an alpha-1 receptor-independent fashion and determined whether this intervention modulated automaticity. We used standard microelectrode techniques to study automaticity in adult Purkinje fiber bundles, fluorescence microscopy to study fura-2 fluorescence in isolated Purkinje and ventricular myocytes and standard biochemical techniques to measure inositol phosphate production in ventricular myocytes. Phospholipase C increased Purkinje fiber automaticity, a process that was enhanced by 10 mM lithium (which had no effect alone) and suppressed by verapamil or ryanodine (both 10 μM). Superfusion with 12-O-tetradecanoyl-phorbol-13-acetate phorbol ester, phospholipase D and A₂, as well as L-α-phosphatidic acid, trypsin and D-myo-inositol-1-phosphate, D-myo-inositol-1,4-bisphosphate, IP₃ and D-myo-inositol-1,4,5,6-tetrakisphosphate did not affect automatic rate or transmembrane potentials. Biochemical studies of ventricular myocytes demonstrated a phospholipase C-induced increase in intracellular and extracellular IP₃, D-myo-inositol-1,4-bisphosphate and D-myo-inositol-1-phosphate at 3 min, with the extracellular increase persisting thereafter. Fluorescence microscopy with fura-2 revealed that phospholipase C increased systolic-free calcium. Given that phospholipase C superfusion elevates intra- and extracellular phosphoinositide metabolites, that the external application of these metabolites has no effect on automaticity, and that phospholipase C-dependent increases in automaticity are associated with an elevated intracellular calcium and are blocked by verapamil or ryanodine and potentiated by lithium, we postulate that the intracellular formation of biologically active inositol phosphate metabolites can modulate cardiac rhythm by increasing automaticity.