Publisher Summary
Alpha-1 (α1) receptors by and large appear to occur in a single-affinity state with respect to both agonists and antagonists. By comparison, α2 receptors may exist in multiple-affinity states reflecting the ability of the α2 binding-site protein to form a complex with additional membrane proteins that themselves are receptors for the physiological substrates guanosine 5'-triphosphate (GTP), Na+, Mg2+, and possibly Ca2+-calmodulin. Binding studies have also strongly indicated that α2 receptors in most, if not all, tissues are probably coupled in an inhibitory manner to adenylate cyclase, as has been demonstrated in platelets, adipocytes, and NG 108-15 cells. Nothing is known about the mechanisms of regulation of α1 receptors, although both up- and down-regulations of α1 receptor have been demonstrated. In this regard, the ability to label and study α1 receptors on cells in culture would be particularly useful. With regard to α2 receptors, the number of affinity states that exist and their role in terms of the kinetics of α2-receptor coupling to adenylate cyclase are still not clear.

Characterization of alpha 1- and alpha 2-adrenergic receptors.

Just as multiple forms or \"isoenzymes\" of particular enzymes exist, so there may be multiple receptors for an individual neurotransmitter. Some of these have been well known for many years; for instance, the distinction of muscarinic and nicotinic cholinergic receptors, aand P-adrenergic receptors, and , more recently, distinct 6,and P,adrenergic receptors. In different parts of the body histamine acts at distinct H, and H, receptor sites. All of these discriminations of receptor populations were based initially on pharmacological variations. During the past few years it has been possible to label neurotransmitter receptors by binding techniques (Snyder, 1978), an approach which affords more subtle molecular discriminations of receptor properties. In pharmacological experiments drugs may vary in potency because of differential bioavailability and metabolism, even in isolated organs, rather than distinct populations of receptors. Measuring receptor binding in isolated membranes overcomes potentially artifactual sources of variations in drug potency and has permitted identification of several new subclasses of neurotransmitter receptors.

Multiple neurotransmitter receptors.

Existence and functional role of alpha1-adrenoceptors in the mammalian heart

Demonstration of two classes of opiate binding sites in the nervous tissue of the marine mollusc Mytilus edulis. Positive homotropic cooperativity of lower affinity binding sites.

Prazosin is know to block postsynaptic α-adrenoceptors. In this study 3H-prazosin has been used to label biochemically central α-adrenoceptors. In rat brain membranes 3H-prazosin bound specifically in a rapid, reversible and saturable manner to a single class of high affinity sites. The relative order of potencies for inhibition of 3H-prazosin binding was WB4101 > ARC 239 > phentolamine ≫ piperoxane > yohimbine which is a characteristic of the α1 type of adrenoceptors. In contrast, the relative order of potencies for inhibition of 3H-clonidine binding was yohimbine > piperoxane > WB4101 > ARC239 > prazosin which is a characteristic of the peripheral ‘α1’-and ‘α1’-adrenoceptors. These results indicate that 3H-prazosin binds to central ‘α1’ and 3H-clonidine to ‘α’2-receptor and confirm the presence of two classes of α-adrenoceptors in rat brain membranes.

3H-Prazosin binds specifically to 'alpha 1'-adrenoceptors in rat brain.

In crude rat cardiac membrane preparations, [3H]dihydroergocryptine (3H-DHE) appears to bind to two classes of sites with limited capacity, differing in their specificities and their affinities. The first class of binding sites interacts preferentially with the postsynaptic alpha-adrenoceptor blocker ARC239, as can be expected for postsynaptic alpha-adrenoceptors. The binding of 3H-DHE to these receptors follows the law of mass action, with a high affinity for 3H-DHE (Kd 25 degrees C = 1.67 +/- 0.37 nM). Postsynaptic saturating levels of 3H-DHE are necessary to occupy the second class of binding sites. These sites exhibit a preferential affinity for presynaptic ligands such as clonidine and yohimbine, as would be expected for presynaptic alpha-adrenergic receptors. This presynaptic binding shows a markedly positive homotropic cooperativity (Hill n = 2.88) with initial and final apparent Kds of 23 and 0.83 nM, respectively. Free energy of interaction between sites is of the order of 2 kcal (8.36 kJ)/mol of sites. These characteristics provide a rational molecular basis for the functional role of presynaptic alpha-adrenoceptors that mediate the inhibition of norepinephrine release from nerve endings.

Biochemical evidence for presynaptic and postsynaptic alpha-adrenoceptors in rat heart membranes: positive homotropic cooperativity of presynaptic binding

In rat cardiac membranes, [3H] dihydroergocryptine (3H-DHE) appears to bind to two classes of α-adrenoceptors. The 3H-DHE binding to the postsynaptic α-adrenoceptors follows the law of mass action while the presynaptic binding shows a markedly positive homotropic cooperativity.

Claire Levy-Marchal

Papers

Biochemical evidence for presynaptic and postsynaptic alpha-adrenoceptors in rat heart membranes: positive homotropic cooperativity of presynaptic binding

Positive homotropic cooperativity of presynaptic [3h]-dihydro-ergocryptine binding in rat heart membranes