Activation of two types of Ca2+‐permeable nonselective cation channel by endothelin‐1 in A7r5 cells

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Abstract
1 In A7r5 cells loaded with the Ca2+ indicator fura‐2, we examined the effect of a Ca2+ channel blocker SK&F 96365 on increases in intracellular free Ca2+ concentrations ([Ca2+]i) and Mn2+ quenching of fura‐2 fluorescence by endothelin‐1 (ET‐1). Whole‐cell patch‐clamp was also performed. 2 Higher concentrations (10 nm) of ET‐1 (higher [ET‐1]) evoked a transient peak and a subsequent sustained elevation in [Ca2+]i: removal of extracellular Ca2+ abolished only the latter. A blocker of l‐type voltage‐operated Ca2+ channel (VOC) nifedipine at 1 μm reduced the sustained phase to about 50%, which was partially sensitive to SK&F 96365 (30 μm). 3 Lower [ET‐1] (1 nm) evoked only a sustained elevation in [Ca2+]i which depends on extracellular Ca2+. The elevation was partly sensitive to nifedipine but not SK&F 96365. 4 In the presence of 1 μm nifedipine, higher [ET‐1] increased the rate of Mn2+ quenching but lower [ET‐1] had little effect. 5 In whole‐cell recordings, both lower and higher [ET‐1] induced inward currents at a holding potential of −60 mV with linear I‐V relationships and reversal potentials close to 0 mV. The current at lower [ET‐1] was resistant to SK&F 96365 but was abolished by replacement of Ca2+ in the bath solution with Mn2+. The current at higher [ET‐1] was abolished by the replacement plus SK&F 96365. 6 In a bath solution containing only Ca2+ as a movable cation, ET‐1 evoked currents: the current at lower [ET‐1] was sensitive to Mn2+, whereas that at higher [ET‐1] was partly sensitive to SK&F 96365. 7 These results indicate that in addition to VOC, ET‐1 activates two types of Ca2+‐permeable nonselective cation channel depending on its concentrations which differ in terms of sensitivity to SK&F 96365 and permeability to Mn2+. British Journal of Pharmacology (1998) 124, 1541–1549; doi:10.1038/sj.bjp.0701984