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Abstract
1. The modulation of noradrenaline-induced membrane currents by papaverine was studied with the nystatin whole-cell technique in freshly isolated rabbit ear artery and portal vein cells. 2. At a holding potential of -50 mV (in potassium-free conditions), papaverine inhibited the Ca(2+)-activated inward chloride current ICl(Ca) evoked by ionophoretic application of noradrenaline in rabbit portal vein cells in a concentration-dependent (10(-5)-10(-4) M) manner. 3. With potassium-containing solutions (at 0 mV) 10(-4) M-papaverine reversibly abolished the Ca(2+)-activated outward potassium current IK(Ca) induced by noradrenaline in both protal vein and ear artery cells. Occasionally, 10(-4) M-papaverine itself activated IK(Ca) before it abolished the response to noradrenaline. 4. Higher concentrations of papaverine (up to 10(-3) M) always evoked both ICl(Ca) and IK(Ca) at -50 and 0 mV respectively, before it abolished these responses to noradrenaline in both ear artery and portal vein cells. In the presence of 10(-3) M-papaverine higher doses of noradrenaline evoked the non-selective cation current in some portal vein cells. 5. In the presence of 10(-2) M-caffeine, which elicited both IK(Ca) and ICl(Ca), papaverine (10(-3) M) failed to activate either current and 10(-3) M-papaverine reduced the caffeine-induced IK(Ca) by 94%. In contrast, IK(Ca) induced by the calcium ionophore ionomycin was unaffected by papaverine. In Ca(2+)-free, 1 mM-EGTA external solutions, 10(-3) M-papaverine still induced IK(Ca) and blocked the response to noradrenaline. 6. The alpha 1-adrenoceptor antagonist prazosin (5 x 10(-8) M) completely blocked the noradrenaline-induced IK(Ca) but currents evoked by papaverine (10(-3) M) were unaffected by prazosin. 7. It was concluded that papaverine modulated noradrenaline-induced membrane currents in vascular smooth muscle cells by releasing and depleting caffeine-sensitive intracellular Ca2+ stores.
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