Status of Ca(2+)-channels in hearts perfused with Ca(2+)-free medium as well as upon reperfusion (Ca(2+)-paradox)

Although a wide variety of biochemical changes have been observed during the occurrence of Ca(2+)-paradox, very little is known about membrane alterations during Ca(2+)-free perfusion which may predispose the heart to the development of intracellular Ca(2+)-overload in the Ca(2+)-paradox phenomenon. In view of the marked influx of Ca2+ into the myocardial cell during Ca(2+)-paradox and the involvement of Ca(2+)-channels in the entry of Ca2+, we determined the status of Ca(2+)-channels by measuring the binding of a Ca(2+)-antagonist, [3H] PN200-110, with membranes obtained from rat hearts perfused with Ca(2+)-free medium. The density of Ca(2+)-channels in the membranes was increased upon perfusing the heart with Ca(2+)-free medium for > 2 min or when the perfusion medium contained less than 25 microM concentration of Ca2+. The increase in Ca(2+)-channel density was attenuated when the hearts were perfused with Ca(2+)-free medium in the presence of a low concentration (35 mM) of Na+ or at low temperature (21 degrees C); two conditions which are known to prevent the occurrence of Ca(2+)-paradox. These results indicate that increased density of Ca(2+)-channels due to Ca(2+)-free perfusion may contribute towards the massive Ca(2+)-influx into the myocardial cell for the induction of intracellular Ca(2+)-overload associated with Ca(2+)-paradox during reperfusion with Ca(2+)-containing medium.