Physiological Regulation of Cardiac Alpha Adrenergic Receptors

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In the present work, we first characterized alpha-1 adrenergic receptors in rat cardiac membrane preparation, utilizing alpha-1 selective antagonist [^3H]prazosin as a radioligand. Then, the in vivo regulation of these receptors in a variety of physiologic and pathophysiological situations including aging,thyroid hormone and pheochromocytoma was studied Comparing young (2 month-old) and older (12 month-old)rats, we found that there was an age-related decreae in the number of myocardial alpha-1 receptors. With thyroid hormone treatment, the density of ventricular alpha-1 receptors was found to be further reduced even in the older rats, suggesting that thyroid hormone and aging have different influences on the cardiac alpha-1 receptors and these two physiological variables are independently interrelated Next, we have examined the consequences of prolonged stimulation of the cardiac alpha receptors in a rat model harboring pheochromocytoma. A marked down-regulation of alpha-1 receptors was found in the hearts of pheochromocytoma rats.without any significant change in the affinities of these receptors for the circulating catecholamine. In summary,these studies cleary demonstrate that the cardiac alpha adrenergic receptors can be dynamically regulated in vivo like cardiac beta receptors.