Cardiac hyperactivity and its consequent metabolically induced coronary vasodilation (MCD) were studied in isolated, perfused, electrically paced rat hearts. The alpha-adrenoceptor agonists, phenylephrine and methoxamine, produced a concentration-dependent inhibition of the inotropic responses to noradrenaline, dobutamine, isoprenaline, tyramine, and glucagon, while relatively potentiating their MCD reactions. This inhibition was unrelated to the alpha-agonists' known inotropic action and was not affected by catecholamine depletion of the heart. Withdrawal of the alpha-agonists or administration of the alpha-adrenoceptor antagonists phentolamine, phenoxybenzamine, or prazosin returned the inotropic and MCD reactions to normal. Neither the MCD response to electrically induced tachycardia nor the inotropic reactions produced by calcium chloride were affected by alpha-adrenoceptor agonists or antagonists. Alone, alpha-adrenoceptor antagonists were shown to potentiate the inotropic responses to noradrenaline and isoprenaline while the MCD was relatively diminished. The responses to glucagon were unaltered by alpha-antagonists. We postulate that myocardial reactivity to sympathetic stimulation can be modulated through alpha-adrenoceptors by the inhibition of processes that mediate cardiostimulation at post-beta-adrenoceptor sites, together with facilitation of those leading up to MCD. Accordingly, this modulation would act to prevent ischaemic damage to the heart by acting to limit the inotropic responses to increasing sympathetic stimulation while maximizing the blood supply to the myocardium.
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