Actions of bretylium and guanethidine on the uptake and release of [3H]-noradrenaline

The effect of bretylium and guanethidine has been studied on the uptake and the spontaneous and reserpine-induced release of [(3)H]-noradrenaline in the rat heart and in the splenic nerve endings of the cat. Bretylium and guanethidine inhibited the uptake by the heart of circulating [(3)H]-noradrenaline. Bretylium blocked spontaneous and reserpine-induced release of [(3)H]-noradrenaline, while guanethidine caused release and partially antagonized the reserpine-induced release. Both compounds produced a transient liberation of [(3)H]-noradrenaline from splenic nerves, but blocked the release of the [(3)H]-catechol amine following stimulation of the splenic nerve.

Effect of bretylium tosylate (Darenthin) on pulmonary circulation

In a dose sufficient to produce a mild systemic hypotension, bretylinm increases pulmonary arterial pressure and venous admixture in the experimental animal. The clinical significance of these side effects appears to deserve some consideration.

Action of bretylium and guanethidine at the neuromuscular junction

Bretylium and guanethidine produced a block of neuromuscular transmission in the rat phrenic nerve diaphragm and cat sciatic gastrocnemius preparations, but had a potentiating effect on acetylcholine-induced contracture of the frog rectus. On the sciatic gastrocnemius preparation of the cat the compounds had a twofold action, consisting of an initial transient, and a delayed but more sustained, block of neuromuscular transmission. Bretylium and guanethidine had no immediate effect on the response of the muscle to direct stimulation, though a weak delayed depressant effect was observed. Intravenous injection of these compounds produced a flaccid paralysis in pigeons. Possible mechanisms of the neuromuscular blocking action of bretylium and guanethidine are discussed. Inhibition of acetylcholine release from motor nerve endings resulting from local anaesthesia by these compounds is suggested as a possible mechanism of their neuromuscular blocking action.

Mechanism of action of guanethidine

The early hypotensive action of intravenous guanethidine in rabbits, rats and cats anaesthetized with urethane is reversed after pretreatment with iproniazid. The fall in blood pressure following injection of guanethidine in rabbits is reduced after previous administration of reserpine. Reserpine, like adrenalectomy and splenectomy, suppresses the early pressor effect of guanethidine in cats anaesthetized with chloralose. Guanethidine inhibits the action of tyramine and nicotine, but potentiates the effect of noradrenaline on isolated rabbit atria. Guanethidine is also a weak inhibitor of monoamine oxidase activity. The results are discussed and compared with those shown by reserpine. It is concluded that the early effects of guanethidine are mainly due to the release of endogenous catechol amines.

Potentiation of inhibitory and excitatory effects of catechol amines by bretylium

Enhancement of both excitatory and inhibitory responses to adrenaline, noradrenaline and isoprenaline was demonstrated with bretylium both in vivo and in vitro. Excitatory responses studied included rise of blood pressure and contraction of the nictitating membrane in cats and the contraction of aortic strip in rabbits; the inhibitory responses studied were fall of cat blood pressure and relaxation of rat uterus and rabbit tracheal chain. Possible mechanisms of potentiation of the effects of catechol amines by bretylium are discussed. A sensitization of the peripheral effector cells is suggested as a likely mechanism of potentiation.

Supersensitivity of salivary glands following treatment with bretylium or guanethidine

The sensitivity of the submaxillary glands to noradrenaline was estimated once a week in cats anaesthetized with hexobarbitone. Daily subcutaneous injections of bretylium or guanethidine were found to produce a supersensitivity of the glands, similar to that caused by extirpation of the superior cervical ganglion.

Action of chlorothiazide on the distribution excretion and hypotensive effect of pempidine in man

When chlorothiazide is given to hypertensive patients who are receiving pempidine a rise in plasma pempidine concentration occurs and this is proportionately greater than the additional fall in blood pressure. After pempidine has been added to human whole blood in vitro or in vivo the ratio of the pempidine concentration in the red cells to that in the plasma falls in the course of 1 hr from an initial value greater than 2 to about 1.2. If chlorothiazide is present also, however, the ratio remains constant at 0.7. Changes in the plasma pempidine concentration in vivo probably result from the binding of pempidine to plasma protein in the presence of chlorothiazide. This has been observed in vitro by a dialysis technique.