Potentiation of inhibitory and excitatory effects of catechol amines by bretylium

Mechanism of the initial adrenergic effects of bretylium and guanethidine

Treatment with phenoxybenzamine and dichloroisoprenaline prevented the rise of blood pressure, contraction of the nictitating membrane and increase in cardiac contractile force produced by intravenous injections of bretylium and guanethidine in anaesthetized or spinal cats. Treatment with cocaine, imipramine or reserpine reduced the sensitivity to bretylium and guanethidine of the spinal cat. In the spinal cat treated with reserpine, sensitivity to the drugs could be restored by an infusion of noradrenaline. Chlorpromazine also blocked the pressor and nictitating membrane responses to bretylium and guanethidine. The drug effects were unaltered 1 hr after bilateral adrenalectomy. During the intravenous infusion of noradrenaline into spinal cats the pressor responses to bretylium and guanethidine were increased, whilst those to adrenaline and to noradrenaline were decreased. Guanethidine (3 to 5 mg/kg) injected intravenously into the cat caused a sudden relaxation of the rat isolated stomach-strip bathed in blood. In seven similar experiments bretylium (3 to 5 mg/kg) relaxed the strip only once; in the other six experiments there was either no effect (four experiments) or an increase in the tone of the strip (two experiments). It is concluded that the initial adrenergic effects of bretylium and guanethidine are mediated, at least in part, through a release of catechol amines from stores in the effector organ.

Bretylium potentiation of the contractor responses of isolated rabbit aortic strips to potassium and tyramine

1. Pretreatment of rabbit aortic strips with bretylium potentiated the contractor response to potassium and tyramine but not to noradrenaline. On the other hand, such pretreatment inhibited the response to nicotine.2. Even in reserpinized or cold stored aortic strips, pretreatment with bretylium enhanced the contractor response to potassium and tyramine.3. Pretreatment of fresh, reserpinized, or cold stored aortic strips with pheniprazine potentiated the contractor response to potassium and tyramine.4. Pretreatment of aortic strips with bretylium or pheniprazine did not potentiate the response to 5-hydroxytryptamine (5-HT).5. The results indicate that both bretylium and pheniprazine potentiate the action of tyramine and potassium, not by presynaptic mechanisms, but by postsynaptic action, causing an increase in the sensitivity of the effector cells to the stimulants.