Activation of three types of voltage-independent Ca2+ channel in A7r5 cells by endothelin-1 as revealed by a novel Ca2+ channel blocker LOE 908

摘要

class="enumerated" style="list-style-type:decimal">We have shown that in addition to voltage-operated Ca²⁺ channel (VOC), endothelin-1 (ET-1) activates two types of Ca²⁺-permeable nonselective cation channel (NSCC) in A7r5 cells: its lower concentrations (⩽1 nM; lower [ET-1]) activate only an SK&F 96365-resistant channel (NSCC-1), whereas its higher concentrations (⩾10 nM; higher [ET-1]) activate an SK&F 96365-sensitive channel (NSCC-2) as well.We now characterized the effects of a blocker ofCa²⁺ entry channel LOE 908 on NSCCs and store-operatedCa²⁺ channel (SOCC) in A7r5 cells, and using twodrugs, clarified the involvement of these channels in the ET-1-inducedincrease in the intracellular free Ca²⁺ concentrations([Ca²⁺]i). Whole-cell recordingsand [Ca²⁺]i monitoring withfluo-3 were used.LOE 908 up to 10 μM had no effect on increases in [Ca²⁺]i induced by thapsigargin or ionomycin, but SK&F 96365 abolished them.In the cells clamped at −60 mV, both lower and higher [ET-1] induced inward currents with linear iv relationships and the reversal potentials of −15.0 mV. Thapsigargin induced no currents.In the presence of nifedipine, lower [ET-1] induced a sustained increase in [Ca²⁺]i, whereas higher [ET-1] induced a transient peak and a sustained increase. The sustained increases by lower and higher [ET-1] were abolished by removal of extracellular Ca²⁺, and they were suppressed by LOE 908 to 0 and 35%, respectively, with the LOE 908-resistant part being abolished by SK&F 96365.These results show that LOE 908 is a blocker of NSCCs without effect on SOCC, and that the increase in [Ca²⁺]i at lower [ET-1] results from Ca²⁺ entry through NSCC-1 in addition to VOC, whereas the increase at higher [ET-1] involves NSCC-1, NSCC-2 and SOCC in addition to VOC.