Intracellular Calcium and Membrane Potential Oscillations in the Guinea Pig and Rat Pulmonary Vein Myocardium

Myocardial intracellular Ca2+ and membrane potential oscillations were studied in the isolated guinea-pig and rat pulmonary veins with immunohistochemical, confocal microscopic and electrophysiological analyses. The myocardial layer was present between the smooth muscle layer and the adventitia, and was more developed in the guinea-pig than in the rat. Intracellular Ca2+ oscillations were observed in both species, which were inhibited by ryanodine. Spontaneous Ca2+ waves were observed to propagate along the longitudinal axis of the cell or as a spiral rotating around a subcellular core; the propagation velocity of these Ca2+ waves was similar to that reported in atrial and ventricular cardiomyocytes. Spontaneous action potentials were present in about 35% and 4% of the preparations from the guinea-pig and rat, respectively. In quiescent preparations from the guinea-pig, noradrenaline induced a slow depolarization of the resting membrane potential followed by constant repetitive generation of action potentials, which were inhibited by ryanodine. In quiescent preparations from the rat, noradrenaline induced an initial hyperpolarization and a subsequent depolarization of the resting membrane potential. This was followed by generation of automatic action potentials which occurred in repetitive bursts. Ryanodine either abolished or reduced the duration of action-potential bursts. These results indicate that the pulmonary vein myocardium generates automatic electrical activity under adrenergic influence, which is probably triggered by intracellular Ca2+ oscillations. The difference in firing pattern between the guineapig and rat may be due to the difference in hyperpolarizing mechanisms.