Effect of clonidine on responsiveness of venous smooth muscle to sympathetic neurotransmitter

The presence of both longitudinal and circular smooth muscle makes the canine accessory cephalic vein a sensitive preparation for the evaluation of venomotor activity. Infusion of clonidine (10 μg/kg) into this intact, peripheral vein results in transient, localized venoconstriction and prolonged arterial hypotension. Bradycardia, paralleling the hypotension in onset and duration, is a consistent concomitant. Exposure of the test vein smooth muscle to clonidine diminishes responsiveness of this vein to subsequent close injections of norepinephrine. This inhibition, though pronounced, is short‐lived and usually abates within an hour.