Characteristics of a store-operated calcium-permeable channel - Sarcoendoplasmic reticulum calcium pump function controls channel gating

We examined the single channel properties and regulation of store-operated calcium channels (SOCC). In human submandibular gland cells, carbachol (CCh) induced flickery channel activity while thapsigargin (Tg) induced burst-like activity, with relatively lower open probability (NPo) and longer mean open time. Tg- and CCh-activated channels were permeable to Na+ and Ba2+, but not to NTUDG, in the absence of Ca2+. The channels exhibited similar Ca2+, Na2+, and Ba2+ conductances and were inhibited by 2-aminoethoxydiphenylborate, xestospongin C, Gd3+, and La3+. CCh stimulated flickery activity changed to burst-like activity by (i) addition of Tg, (ii) using Na+ instead of Ca2+, (iii) using Ca2+-free bath solution, or (iv) buffering [Ca2+](i) with BAPTA-AM. Buffering [Ca2+](i) induced a 2-fold increase in NPo of Tg-stimulated SOCC. Reducing free [Ca2+] in the endoplasmic reticulum with the divalent cation chelator, N,N,N ' ,N ' -tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), induced burst-like channel activity similar to that seen with CCh + Tg. Thus, SOCC is activated by stimulation of muscarinic receptors, inhibition of the sarcoendoplasmic Ca2+ pump, and lowering [Ca2+] in the internal store. Importantly, SOCC activity depends on [Ca2+](i) and the free [Ca2+] in the internal store. These novel findings reveal that SERCA plays a major role in the gating of SOCC by (i) refilling the internal Ca2+ store(s) and (ii) decreasing the [Ca2+](i)-dependent inhibition.