Agonist regulation of the human platelet α-adrenergic receptor

Acute reduction in human platelet alpha 2-adrenoreceptor affinity for agonist by endogenous and exogenous catecholamines.

“…However, unlike our studies, these studies incubated platelets with high concentrations of l-epinephrine for long periods of time; yet, no data concerning platelet count or size distribution was presented. Furthermore, no significant change in receptor number or function occurred after incubation with concentrations of l-epinephrine below 1o-5 M or for less than 4 h (16,17). In contrast, we found a maximal effect on platelet Ca2-adrenoreceptor agonist affinity after in vitro incubation with 1lo M l-epinephrine for 2 h. Our antagonist binding studies further demonstrate that no change in number of receptors per platelet nor in receptor affinity for antagonist occurs after exposure to more physiological concentrations of catecholamines.…”

“…Prior studies found reductions in platelet a2-adrenoreceptor number and function after in vitro exposure to l-epinephrine concentrations of 10-M or greater (16,17). These results were interpreted as showing "down-regulation" of receptor number or agonist retention by the platelet a2-adrenoreceptor.…”

“…Exposure of platelets to high concentrations of catecholamines in vitro has been reported to decrease the number of assayable platelet a2-adrenoreceptors, either through a reduction' in receptor number or by retention of agonist in the preparation (16,17). Ex-posure to catecholamines at concentrations below lo-5 M has not been found to alter the number of platelet a2-adrenoreceptors (18,19).…”

A possible role of synaptic‐membrane protein phosphorylation in the regulation of muscarinic acetylcholine receptors

“…While the mechanism underlying this receptor loss is not fully understood, recent work has suggested a role for receptor internalisation [8]. Similar receptor loss or densensitisation has been reported for a-adrenergic [9], nicotinic cholinergic [lo] and muscarinic cholinergic receptors [ 11 ,121 as well as several polypeptide hormone receptors [l] following prolonged exposure to agonists. A reduction in muscarinic receptor number has been demonstrated to occur following depolarisation of a synaptosomal preparation [ 131.…”

Changes in .ALPHA.-adrenergic receptors in rat brain in vitro by preincubation with .ALPHA.-adrenergic ligands.

“…A few quantitative changes in a-adrenergic receptors under certain experimental conditions have also been reported (23)(24)(25). Previously, we observed rapid and functional increases in a-adrenergic receptors in rat vas deferens; short term exposure of vas deferens to a-agonists resulted in enhancement of the contractile response with increase in the number of a-receptors, measured quantitatively by (3H)WB4101 binding (26).…”