Endothelin-1 and vasopressin activate Ca(2+)-permeable non-selective cation channels in aortic smooth muscle cells: mechanism of receptor-mediated Ca2+ influx.
The effects of vasopressin and endothelin-1 on cultured aortic smooth muscle cell lines (A7r5) were investigated by measurements of intracellular calcium [Ca2+]i and the patch-clamp techniques. Vasopressin and endothelin-1 (100 nM) evoked an initial peak followed by a smaller sustained rise of [Ca2+]i in the presence of extracellular calcium [Ca2+]o. In the absence of [Ca2+]o, only the initial peak of [Ca2+]i was observed. Therefore, the initial peak of [Ca2+]i was mainly due to calcium release from the storage sites, whereas the later sustained rise of [Ca2+]i was due to the calcium entry from outside. The sustained rise of [Ca2+]i was unaffected by nifedipine (10 microM) significantly, but was completely abolished by La3+ (1 mM). Under current clamp conditions with K(+)-internal solution, vasopressin and endothelin-1 (100 nM) produced hyperpolarization, then followed by depolarization. Under voltage clamp conditions at a holding potential of -40 mV, both vasopressin and endothelin-1 first activated the outward current, then followed by a long-lasting inward current with a high noise level. The first outward current was abolished by charybdotoxin (100 nM), Cs+ in the patch pipette and high EGTA (10 mM) in the pipette, suggesting that it was a Ca(2+)-sensitive K+ current (IK.Ca). The inward current was still elicited with the patch pipette containing Cs(+)-internal solution, and reversed at about 0 mV. The reversal potential was not significantly altered by the replacement of [Cl-]i or [Cl-]o, proposing that the inward current is a cation selective channel (IN.S.). The inward current was also observed even when extracellular cations are Ca2+. La3+ (1 mM), Cd2+ (1 mM) completely abolished the vasopressin-induced (IN.S.), however, nifedipine (10 microM) failed to inhibit it significantly. Single channel activities were recorded in the cell-attached configurations when vasopressin or endothelin-1 was applied to the bathing solution. The unitary conductance of the channels was approximately 20 pS with 140 mM Na+, Cs+, or K+ in the pipette, but was 15 pS with 110 mM Ca2+ in the pipette. Permeabilities sequence calculated from the reversal potentials was Na+ not equal to Cs+ not equal to K+ > Ca+. These results provide evidence that calcium entry and membrane depolarization elicited by vasopressin or endothelin-1 are mediated by a receptor-mediated Ca(2+)-permeable non-selective cation channel in aortic smooth muscle cells.