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1-adrenergic receptor binding in rat vascular smooth muscle
Glucocorticoids are known to have marked effects on blood pressure regulation, predominantly through altering cardiovascular sensitivity to noradrenaline. However, the molecular mechanisms underlying this action remain unclear. As part of our studies into these we have measured
1-adrenergic receptor binding using the ligand [3H]prazosin in plasma membrane fractions of aortas prepared from control, adrenalectomized and dexamethasone-treated adrenalectomized rats. In controls there were 50±8 (S.E.M.; n=6) fmol 1-adrenergic receptors/mg membrane protein (Bmax) with a dissociation constant (Kd) of 0·52±0·10 nM (n=6). Adrenalectomy 8 days before tissue preparation caused a 40% decrease in Bmax and a 60% decrease in Kd. Dexamethasone replacement after adrenalectomy returned these values close to those of controls. Noradrenaline competed for the [3H]prazosin-binding sites. Computer analysis by a non-linear curve-fitting program (LIGAND) showed that noradrenaline binding was to a heterogeneous population of high- and low-affinity receptors with Kd values of 1·87±0·73 µM and 0·48±0·12 mM (n=5) respectively. Guanosine thiotriphosphate (GTP[S]) caused the conversion of high-affinity to low-affinity binding, consistent with the model of the high-affinity sites being coupled to a G protein. After adrenalectomy, noradrenaline binding was to a homogeneous population of low-affinity receptors; hence, the effect of GTP[S] was no longer apparent, suggesting that under these conditions the 1-adrenergic receptors were unable to couple to a G protein. The two-site model of binding and GTP[S] effect was returned by dexamethasone treatment. These data provide evidence that glucocorticoids not only modulate the number of 1-adrenergic receptors on vascular smooth muscle, but also cause disruptions in receptor—G protein coupling. This may be an important mechanism by which glucocorticoids exert their effect on cardiovascular sensitivity.
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