STATUS OF CA2-CHANNELS IN HEARTS PERFUSED WITH CA2+-FREE MEDIUM AS WELL AS UPON REPERFUSION (CA2+-PARADOX)()

Although a wide variety of biochemical changes have been observed duri ng the occurrence of Ca2+-paradox, very little is known about membrane alterations during Ca2+-free perfusion which may predispose the heart to the development of intracellular Ca2+-overload in the Ca2+-paradox phenomenon. In view of the marked influx of Ca2+ into the myocardial cell during Ca2+-paradox and the involvement of Ca2+-channels in the e ntry of Ca2+, we determined the status of Ca2+-channels by measuring t he binding of a Ca2+-antagonist, [H-3] PN200-110, with membranes obtai ned from rat hearts perfused with Ca2+-free medium. The density of Ca2 +-channels in the membranes was increased upon perfusing the heart wit h Ca2+-free medium for >2 min or when the perfusion medium contained l ess than 25 mu M concentration of Ca2+. The increase in Ca2+-channel d ensity was attenuated when the hearts were perfused with Ca2+-free med ium in the presence of a low concentration (35 mM) of Na+ or at low te mperature (21 degrees C); two conditions which are known to prevent th e occurrence of Ca2+-paradox. These results indicate that increased de nsity of Ca2+-channels due to Ca2+-free perfusion may contribute rewar ds the massive Ca2+-influx into the myocardial cell for the induction of intracellular Ca2+-overload associated with Ca2+-paradox during rep erfusion with Ca2+-containing medium.