OSCILLATORY CL- CURRENT-INDUCED BY ANGIOTENSIN-II IN RAT PULMONARY ARTERIAL MYOCYTES - CA2+ DEPENDENCE AND PHYSIOLOGICAL IMPLICATION

We have previously reported that angiotensin II (ANG II) induces oscil lations in the cytoplasmic calcium concentration ([Ca2+](i)) of pulmon ary vascular myocytes. The present work was undertaken to investigate the effect of ANG II in comparison with ATP and caffeine on membrane c urrents and to explore the relation between these membrane currents an d [Ca2+](i). In cells clamped at -60 mV, ANG 11 (10 mu M) or ATP (100 mu M) induced an oscillatory inward current. Caffeine (5 mM) induced o nly one transient inward current. In control conditions, the reversal potential (E-rev) Of these currents was close to the equilibrium poten tial for Cl- ions (E-Cl = -2.1 mV) and was shifted towards more positi ve values in low-Cl- solutions. Niflumic acid (10-50 mu M) and DIDS (0 .25-1 mM) inhibited this inward current. Combined recordings of membra ne current and [Ca2+](i) by Indo-1 microspectrofluorimetry revealed th at ANG II- and ATP-induced currents occurred simultaneously with oscil lations in [Ca2+](i), whereas the caffeine-induced current was accompa nied by only one transient increase in [Ca2+](i). Niflumic acid (25 mu M) had no effect on agonist-induced [Ca2+](i) responses, whereas thap sigargin (1 mu M) abolished both membrane current and the [Ca2+](i) re sponse. Heparin (5 mg/ml in the pipette solution) inhibited both [Ca2](i) responses and membrane currents induced by ANG II and ATP, but no t by caffeine. In pulmonary arterial strips, ANG It-induced contractio n was inhibited by niflumic acid (25 mu M) or nifedipine (1 mu M) to t he same extent and the two substances did not have an additive effect. This study demonstrates that, in pulmonary vascular smooth muscle, AN G II, as well as ATP, activate an oscillatory calcium dependent chlori de current which is triggered by cyclic increases in [Ca2+](i) and tha t both oscillatory phenomena are primarily IP3-mediated. It is suggest ed that ANG Ii-induced oscillatory chloride current could depolarise t he cell membrane leading to activation of voltage-operated Ca2+ channe ls. The resulting Ca2+ influx contributes to the component of ANG Ii-i nduced contraction that is equally sensitive to chloride or calcium ch annel blockade.