ENDOTHELIN-1 AND VASOPRESSIN ACTIVATE CA2-PERMEABLE NONSELECTIVE 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 intracel lular calcium [Ca2+](i) and the patch-damp techniques. Vasopressin and endothelin-1 (100 nM) evoked an initial peak followed by a smaller su stained rise of [Ca2+](i) in the presence of extracellular calcium [Ca 2+](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 du e to calcium release from the storage sites, whereas the later sustain ed rise of [Ca2+](i) was due to the calcium entry from outside. The su stained rise of [Ca2+](i) was unaffected by nifedipine (10 mu M) signi ficantly, but was completely abolished by La3+ (1 mM). Under current c lamp 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, bot h vasopressin and endothelin-1 first activated the outward current, th en 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 Ca2+-sensitive K+ current (i(K.Ca)). The inward current was still elicited with the patch pipette containing Cs+-internal sol ution, and reversed at about 0 mV. The reversal potential was not sign ificantly altered by the replacement of [Cl-](i) or [Cl-](o), proposin g that the inward current is a cation selective channel (I-N.S.). The inward current was also observed even when extracellular cations are C a2+. La3+ (1 mM), Cd2+ (1 mM) completely abolished the vasopressin-ind uced I-N.S., however, nifedipine (10 mu M) failed to inhibit it signif icantly. Single channel activities were recorded in the cell-attached configurations when vasopressin or endothelin-l was applied to the bat hing solution. The unitary conductance of the channels was approximate ly 20 pS with 140 mM Na+, Cs+, or K+ in the pipette, but was 15 pS wit h 110 mM Ca2+ in the pipette. Permeabilities sequence calculated from the reversal potentials was Na+ Cs+ K+ Ca+. These results provide evid ence that calcium entry and membrane depolarization elicited by vasopr essin or endothelin-1 are mediated by a receptormediated Ca2+-permeabl e non-selectire cation channel in aortic smooth muscle cells. (C) 1990 Academie Press Limited