β2-adrenoceptor agonist-induced down-regulation after short-term exposure

We examined the effect of duration of beta(2)-adrenergic receptor (beta(2)AR) occupancy by isoproterenol on specific binding of I-125-lodocyanopindolol (I-125-ICYP) in membranes from rat L6 myoblasts. Ten minute exposure caused a time- and concentration-dependent maximal decrease in I-125-ICYP binding 24 hours after exposure equal to that following continuous exposure (p < 0.05). Low temperature, concanavalin A, H89 and ICI 118,551 blocked the decline in I-125-ICYP binding during the first hour following exposure probably representing receptor sequestration to a compartment or change to a form incapable of ligand binding. Compared to controls, receptor binding 4 and 24 hours following exposure was reduced 56 +/- 8.7% and 72 +/- 8.8%, respectively (p < 0.05), and was blocked by ICI 118,551 but not CGP12177. Isoproterenol-induced, but not forskolin-stimulated, cAMP accumulation was reduced 35% 24 hours following exposure (p < 0.05). I-125-ICYP binding in intact L6 cells 4 and 24 hours after exposure were respectively 56 +/- 8.9 and 61 +/- 13% of controls (p < 0.05). Following agonist exposure, CHO cell membranes expressing human beta(2)ARs exhibited I-125-ICYP binding 85 +/- 2.0% and 6 +/- 2.8% of control values 4 and 24 hours, respectively (p < 0.05). A model predicting that full occupation of the beta(2)AR activates receptor degradation explains our results that agonist-induced downregulation of beta(2)AR does not require continuous presence of the agonist.