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Department of Pharmacology, John Curtin School of Medical Research, Australian National University, Canberra.

1. The ionic basis of the depolarizing responses resulting from ionophoresis of noradrenaline onto the smooth muscle of mesenteric veins has been investigated using electrically short segments of vessel. 2. Isolated cut segments of vein were effectively isopotential as assessed by the voltage response to a step change in current. The mean input resistance and time constant of the smooth muscle were 24 M omega and 131 ms respectively. 3. Data on the noradrenaline-induced slow depolarization indicated that it resulted from a decrease in conductance to potassium ions consistent with the finding of Suzuki (1981). 4. The fast noradrenaline-induced depolarization was found to have a reversal potential of about -22 mV. 5. Exposure to low-chloride solution caused greater than 90% suppression of this fast response with a 50% reduction occurring in less than 2 min. This suppression was not due to a negative shift in reversal potential. 6. The fast response underwent a large positive shift in reversal potential directly after changeover to low-chloride solution at times when any inactivation of the response was minimal. By contrast the fast response showed no evidence implicating either sodium or calcium as charge-carrying ions. 7. It is concluded the fast depolarization is carried by chloride ions.

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