Are B-type Ca2+ channels of cardiac myocytes akin to the passive ion channel in the plasma membrane Ca2+ pump?

The present study demonstrates that B-type Ca2+ channels observed in rat ve ntricular myocytes markedly reacted to agents known to affect the ion-motiv e plasma membrane Ca2+-ATPase (PMCA) pump. Chlorpromazine (CPZ)-activated B -type Ca2+ channels were completely blocked by internal application of PMCA pump inhibitors, namely La3+ (100 muM), eosin (10 muM) and AIF(3) (100 muM ). Calmodulin (50 U/ml), the main endogenous positive regulator of PMCA, wa s unable to activate but significantly reduced CPZ-activated B-type channel activity. In the same manner, ATP (1 and 4 mM), the main energizing substr ate of PMCA, was able to reversibly and significantly reduce this activity in a dose-dependent manner. Interestingly, anti-PMCA antibody 5F10, but not anti-Na/K ATPase antibody (used as a negative control) induced a marked Ba 2+-conducting channel activity that shared the same characteristics with th at of CPZ-activated B-type channels. 5F10-Activated channels were mostly se lective towards Ba2+, mainly had three observed conductance levels (23, 47 and 85 pS), were observed with a frequency of about 1 out of 5 membrane pat ches and were completely blocked by 10 muM eosin. These results suggest tha t B-type Ca2+ channels are some form of the PMCA pump.