Radioligand studies of α-adrenoceptors in the kidney

Renal alpha-adrenergic receptors and genetic hypertension

The hypothesis has been proposed that an increase in the number of renal alpha-adrenergic receptors may contribute to the pathogenesis of genetic hypertension. Herein we review recent findings regarding expression of renal alpha 1 (alpha 1a, alpha 1b)- and alpha 2 (alpha 2a, alpha 2b)-adrenergic subtypes and we provide an updated revision of the above-stated hypothesis. Enhancement in receptor number or in post-receptor components responsible for alpha 1- and alpha 2-adrenergic-mediated sodium reabsorption in proximal tubule may contribute to sodium retention and an elevation in blood pressure. Perhaps such changes contribute to the increase in blood pressure in genetically determined hypertension in humans, although direct tests of this notion have not yet been performed.

Effects of chemical sympathectomy with 6-hydroxydopamine on alpha- and beta-adrenoceptors and muscarinic cholinoceptors in rat kidney

The autonomic receptors in the rat kidney were characterized using the radioligands [3H]prazosin, [3H]clonidine, [3H]dihydroalprenolol (DHA) and [3H]quinuclidinyl benzilate (QNB). The specific binding of [3H]prazosin, [3H]clonidine, [3H]DHA and [3H]QNB to rat kidney membranes was saturable and of high affinity, and showed a pharmacological specificity as well as stereospecificity which characterized renal alpha 1-, alpha 2- and beta-adrenoceptors and muscarinic cholinoceptors, respectively. There was a relatively greater density of alpha-adrenoceptors than beta-adrenoceptors or muscarinic cholinoceptors in the rat kidney. Chemical sympathectomy of rats with 6-hydroxydopamine X HBr (6-OHDA, 50 X 2 mg/kg i.v., 24 h interval) caused a significant increase (21-56%) in the Bmax values for renal [3H]prazosin, [3H]clonidine and [3H]DHA binding at 1 and 2 weeks following the treatment, without a change in the Kd values. 6-OHDA treatment had no significant effect on the Kd and Bmax values for [3H]QNB binding at 1-3 weeks after the treatment. The norepinephrine (NE) concentration was reduced (68-76%) in the 6-OHDA-treated rat kidney. In conclusion, the present study provides biochemical evidence for the possible localization of postsynaptic alpha 1-, alpha 2- and beta-adrenoceptors and muscarinic cholinoceptors in the rat kidney and also for the regulation of these adrenoceptors by the sympathetic nervous system.

Distribution and function of peripheral alpha-adrenoceptors in the cardiovascular system

alpha-Adrenoceptors may be subdivided based on their anatomical distribution within the synapse. Presynaptic alpha-adrenoceptors are generally of the alpha 2-subtype and modulate neurotransmitter liberation via a negative feedback mechanism. Postsynaptic alpha-adrenoceptors are usually of the alpha 1-subtype and mediate the response of the effector organ. Although this "anatomical" subclassification is generally applicable, many exceptions exist. A more useful classification of alpha-adrenoceptor subtypes is based on a pharmacological characterization in which selective agonists and antagonists are used. Peripheral alpha-adrenoceptors are critical in the regulation of the cardiovascular system. Postsynaptic alpha-adrenoceptors in arteries and veins represent a mixed population of alpha 1/alpha 2-adrenoceptors, with both subtypes mediating vasoconstriction. In the peripheral arterial circulation, postsynaptic vascular alpha 1-adrenoceptors are found in the adrenergic neuroeffector junction, whereas postsynaptic vascular alpha 2-adrenoceptors are located extrajunctionally. In the venous circulation, it appears that alpha 2-adrenoceptors may be predominantly junctional, whereas alpha 1-adrenoceptors may be predominantly extrajunctional. It has been proposed that junctional alpha-adrenoceptors will respond predominantly to norepinephrine liberated from sympathetic neurons, whereas extrajunctional alpha-adrenoceptors likely respond to circulating catecholamines. The functional role of extrajunctional alpha-adrenoceptors may be more important in disease states such as hypertension and congestive heart failure where circulating levels of catecholamines may be high and contribute to the maintenance of elevated vascular resistance. alpha 2-Adrenoceptors are also associated with the intima and may play a role in the release of an endogenous relaxing factor from the endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic localization of prazosin binding sites in rat kidney

[3H]Prazosin is a selective ligand for alpha 1-adrenoceptors. Using an in vitro autoradiographic technique specific binding of [3H]prazosin has been demonstrated, mainly to the renal cortex of the rat. These results provide further evidence that, in the rat, there is a high density of alpha 1-adrenoceptors within the renal cortex.