Cell Morphology and Early-phase Ca²⁺ Transients of Guinea-Pig Pulmonary Vein Cardiomyocytes Compared with Atrial and Ventricular Cardiomyocytes

Abstract

The cellular morphology and spatio-temporal pattern of the rise in Ca²⁺ during the early-phase Ca²⁺ transients of isolated guinea pig pulmonary vein cardiomyocytes were studied using confocal microscopy and compared with those of ventricular and atrial cardiomyocytes. The pulmonary vein cardiomyocytes had cross striations and were rich in sarcoplasmic reticulum but lacked T-tubules; this was similar to atrial cardiomyocytes but different from ventricular cardiomyocytes, in which T-tubules were present throughout the cell. The sarcoplasmic reticulum was present throughout the cytoplasm in all cell types. On the induction of Ca²⁺ transients by electrical field stimulation of the pulmonary vein cardiomyocytes, the rise in Ca²⁺ concentration first occurred at the subsarcolemmal region and then spread to the cell interior; this phenomenon was similar to that in atrial cardiomyocytes but different from that in ventricular cardiomyocytes, in which the Ca²⁺ concentration rose simultaneously throughout the cytoplasm. The properties of the Ca²⁺ sparks and Ca²⁺ waves in the pulmonary vein cardiomyocytes were the same as those in atrial and ventricular cardiomyocytes. In pulmonary vein cardiomyocytes showing spontaneous activity, the Ca²⁺ transients were preceded by increased firing of Ca²⁺ sparks. These results indicate that, in guinea pig pulmonary vein cardiomyocytes that lack T-tubules, Ca²⁺ transients involve the propagation of Ca²⁺ waves, which is similar to that of atrial but not ventricular cardiomyocytes. Ca²⁺ sparks appear to be involved in the generation of spontaneous Ca²⁺ transients of pulmonary vein cardiomyocytes.