MORPHOLOGICAL AND ELECTROPHYSIOLOGICAL CHANGES INDUCED BY CALCIUM IONOPHORES (A23187 AND X-537A) IN SPONTANEOUSLY BEATING RABBIT SINOATRIAL NODE CELLS

1. Effects of calcium ionophores (A23187 and X-537A) on the spontaneously beating sino-atrial (SA) node cells of rabbit heart were examined using electron microscopic and an electrophysiological techniques. 2. During exposure to A23187 or X-537A (2 x 10(-5) M), the cycle length was significantly prolonged by 11% (n = 12) or 118% (n = 11), respectively. But neither ionophore affected other action potential parameters. 3. X-537A (2 x 10(-5) M) induced irregular rhythm (dysrhythmia), probably due to cellular calcium overload. Similarly, ouabain (3 x 10(-7) M) also elicited dysrhythmia. In the presence of isoproterenol (ISP, 10(-7) M), X-537A potentiated dysrhythmia, and A23187 newly induced it. 4. In ultrastructural analyses, X-537A caused swelling of the cisternae of Golgi apparatus within 10 min, whereas A23187 and ouabain did not produce any changes even after 30 min-application. 5. Addition of high Ca2+ (10 mM) and/or ISP (10(-7) M) to X-537A produced a further dilation and vacuolization. In A23187 or ouabain, however, the addition of Ca2+ and ISP did not cause any changes, even during dysrhythmia. 6. These results indicate that X-537A elicited a more potent calcium overload than A23187, and that a discrepancy between ultrastructural damages and electrical changes exists.