On electrophysiological responses to phorbol esters which stimulate protein kinase C in rabbit sino-atrial node cells

Summary

Effects of phorbol esters on spontaneously beating rabbit sino-atrial (SA) node cells were investigated by means of voltage clamp technique. In a small SA node specimen, 12-O-tetradecanoylphorbol-13-acetate (TPA) 10⁻⁷ mol/l lengthened the cycle length (CL) and at over 3 × 10⁻⁷ mol/l prolonged the action potential duration (APD). Action potential amplitude (APA), maximum diastolic potential (MDP) and maximum rate of rise (V _max) were unaffected. Amiloride 10⁻³ mol/1, an inhibitor of Na⁺-H⁺ exchange, did not reverse the phorbol ester-induced effects. In voltage-clamp experiments, TPA 1-10 × 10⁻⁷ mol/l slightly increased the slow inward current (I _si) and the time-dependent inward current (I _h) which activates during hyperpolarization. The outward current and the tail current were reduced, although the activation curve was not shifted along the voltage axis. In the presence of 10⁻⁷ mol/l isoprenaline, TPA produced dysrhythmia and a transient inward current in voltage-clamp experiments. In the presence of 5 × 10⁻⁵ mol/l phenylephrine or 2 × 10⁻⁶ mol/l acetylcholine, TPA also elicited dysrhythmia. 4-betaphorbol-12,13-dibutyrate (PBD) induced similar electrophysiological effects as TPA, but 4-alpha-phorbol-12,13-didecanoate (PDD) never did so even in the presence of isoprenaline. These results suggest that TPA and PDB might mobilize intracellular Ca²⁺ via protein kinase C activation in the presence of isoprenaline, phenylephrine or acetylcholine, resulting in dysrhythmia due to delayed afterdepolarization.