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

Abstract

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. British Journal of Pharmacology (1999) 126, 1107–1114; doi:10.1038/sj.bjp.0702416