Hyposensitivity to drugs induced by morphine and some related substances in the cat

Effect of morphine on the cat middle cerebral artery

Morphine (up to 3 X 10(-4) M) elicited concentration-dependent contractions in cat middle cerebral arteries, higher concentrations induced vasodilation. These responses were annulled in the presence of 1-methyl-3-isobutylxanthine, but unaffected by diphenhydramine, cimetidine, phentolamine or naloxone. Ca2+ suppression blocked the morphine-evoked contractions and Ca2+ addition antagonized its vasodilatory effects. Nifedipine induced a marked relaxation in arteries previously contracted with morphine. Preincubation with nifedipine induced a decrease in the contraction elicited by morphine while it increased the vasodilatory phase. Morphine did not produce a significant effect in femoral arteries. In cerebral arteries previously submitted to an active tone, the opioid-induced vasodilation was unchanged by naloxone, cimetidine, diphenhydramine or ouabain, whereas Ca2+ addition antagonized this effect. These results indicate: that the opioid effects are mediated neither by opiate receptors nor by noradrenaline or histamine release; and that an antagonism exists between Ca2+ and the vasodilation caused by morphine.

Non-competitive-non-equilibrium alpha-adrenoceptor blocking properties of N-benzyl iodoacetamide, betsamide

N-Benzyl iodoacetamide, betsamide, at 10 mg kg-1 i.v. blocked the hypertensive and contractile responses of the nictitating membrane of the cat to adrenaline. The blockade had a lag period before full development. Pretreatment of cats with betsamide for 7 or 18 h showed a non-equilibrium type of alpha-adrenoceptor blockade. The responses of the nictitating membrane to adrenaline were markedly depressed and did not recover after high doses of adrenaline. In the same cats, adrenaline caused a profound hypotension. The effect of betsamide lasted for at least 72 h. In the rat isolated vas deferens, 3 X 10(-5) M betsamide non-competitively blocked the contractile responses to noradrenaline; the adrenoceptor blockade was less effective when betsamide was applied with noradrenaline. The blockade lasted for more than 24 h, and was not reversible after extensive washing. Betsamide antagonized the contractile effects of carbachol and 5-hydroxytryptamine on the rat vas deferens, but not the beta-responses of the guinea-pig trachea to adrenaline and isoproenaline. Results are discussed in relation to a probable mechanism of action.

Cardiovascular effects of morphine, pethidine, diamorphine and nalorphine on the cat and rabbit

1. The predominant effect of morphine, diamorphine, pethidine or nalorphine on the blood pressure of the anaesthetized cat or rabbit is hypotension although, occasionally, a pressor action may predominate or intervene.2. Possible mechanisms of the depressor phases of action have been studied on cardiac and vascular preparations both in situ and in vitro.3. While in the whole animal, catecholamine release from the adrenal medulla and histamine liberation may be implicated in the responses, the vasodilator and vascular relaxant actions of morphine and, probably, pethidine, nalorphine and diamorphine on the isolated preparations are not mediated by the liberation of known peripheral transmitters or autacoids or by interaction with their specific receptors.