Long‐lasting activation of cation current by low concentration of endothelin‐1 in mouse fibroblasts and smooth muscle cells of rabbit aorta

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Abstract
Recombinant human ETA receptors were expressed in a mouse fibroblast cell line (Ltk−cell) and functional coupling of the receptors with Ca2+ permeable channels at low concentrations of endothelin‐l (ET‐1) was investigated using whole‐cell recordings and monitoring the changes in intracellular free Ca2+ concentrations ([Ca2+]i) with a Ca2+ indicator, fluo‐3. A similar type of coupling was investigated in freshly dispersed vascular smooth muscle cells (VSMCs) of rabbit thoracic aorta by use of whole‐cell recordings. In Ltk−cells expressing recombinant human ETA receptors, concentrations of ET‐1 (10−8 M, 10−9 M) evoked an initial transient peak and a subsequent sustained elevation in [Ca2+]i whereas a lower concentration of ET‐1 (10−10 M) evoked only a sustained elevation of [Ca2+]i. After removal of extracellular Ca2+, ET‐1 evoked only an initial peak without a sustained elevation of [Ca2+]i. The sustained elevation induced by 10−10 M ET‐1 was blocked by 300 μM mefenamic acid (a cation channel blocker) but not by 10 μM nifedipine (a blocker of voltage‐operated Ca2+ channel). In whole‐cell recordings with Ltk− cells, a brief (3–5 min) application of ET‐1 (10−10 M) induced a sustained inward current at a holding potential of −60 mV. The current‐voltage relationship revealed that the reversal potential of the ET‐1‐induced current was close to 0 mV (1.9 mV) and was not altered by reducing the concentration of C1− in the bath solution, indicating that the current is carried by cations. The current was reversibly blocked by 300 μM mefenamic acid, and it persisted after all cations in the bath solution had been replaced by Ca2+ (5 or 30 mM) and nonpermeant cation N‐methyl‐D‐glucamine, indicating that the ET‐1‐activated channel is permeable to Ca2+. Activation of the current was independent of membrane potential and the current was induced even after addition of a high concentration (10 mM) of a Ca2+ chelator, EGTA, to the pipette solution. In whole‐cell recordings from rabbit aortic VSMCs, ET‐1 (10−10 M) induced a sustained inward current at a holding potential of −60 mV. The reversal potential was −12 mV and was not altered when the concentration of Cl− in the pipette solution was decreased, indicating that the current is carried by cations. Again activation of the current was independent of membrane potential and was observed even after addition of a high concentration (10 mM) of a Ca2+ chelator, EGTA to the pipette solution. The current was reversibly blocked by 300 μM mefenamic acid and was permeable to Ca2+, showing marked similarities to ET‐1‐induced cationic current in Ltk− cells. These results indicate that in Ltk− cells transfected with cDNA for recombinant ETA receptors and VSMCs, ETA receptors can functionally couple with a nonselective cation channel permeable to Ca2+. Thus the present data suggest that the cation channel plays an essential role in the sustained elevation of [Ca2+]i at low concentrations of ET‐1 by causing Ca2+ entry through the channel.