Characterization of IBE 2254 binding to alpha 1-adrenergic receptors on intact DDT1 smooth muscle cells: comparison with membrane binding and correlation with phosphoinositides breakdown.

The binding of the antagonist IBE 2254 (IBE) to alpha 1-adrenergic receptors was characterized on intact DDT1 smooth muscle cells. IBE binding was rapid, reversible, stable and saturable: Bmax = 113000 +/- 13000 receptors/cell, KD = 110 +/- 13 pM (n = 25). Saturation and competition experiments analysed by non linear curve fitting indicated a single population of binding sites with a pharmacological profile typical for alpha 1-adrenergic receptors. Antagonists competed for IBE binding sites in the following order: prazosin greater than phentolamine = phenoxybenzamine greater than yohimbine. The rank order for agonists was clonidine greater than epinephrine greater than norepinephrine greater than phenylephrine. There was a significant correlation between IBE binding to intact cells, DDT1 membranes and rat cortex membranes. Neither agonists nor antagonists showed noticeable changes in their affinity for IBE binding on either system. There was also a good correlation between IBE binding and breakdown of phosphoinositides (PI) measured in intact cells.