A calcium conducting channel akin to a calcium pump

Calcium conducting channels were studied in blebs of sarcoplasmic reticulum described by Stein & Palade (1988). The calcium channels had at least three conductance states (70 pS, 50 pS and 37 pS) and were weakly selective for calcium ions, with a permeability ratio Ca2+ to K+ of about 3.4. The open probability of the channel was strongly voltage dependent, decreasing at positive membrane voltages. 10 microM ryanodine and 5 microM ruthenium red had no effect on this channel; neither did millimolar concentrations of ATP, Mg2+, caffeine, and Ca2+, implying that the calcium conducting channels are not ryanodine receptors. Several calcium pump inhibitors--namely, vanadate, AlF4-, reactive red 120, and cyclopiazonic acid--had obvious effects on the calcium conducting channels, suggesting that the calcium conducting channel of SR membrane blebs is some form of the SR calcium pump.