alpha 1-Adrenergic receptors of a smooth muscle cell line: guanine nucleotides do not regulate agonist affinities.

Using [3H]-dihydroergocryptine, we have identified in membranes prepared from the DDT1 MF-2 smooth muscle cell line a binding site with characteristics of the alpha 1-adrenergic receptor. Specific binding (90-95% of total binding) was saturable with a binding site concentration of 197 +/- 44 fmol/mg protein and was of high affinity with a dissociation constant of 1.7 +/- 0.4 nM. The order of agonist competition for the binding site was epinephrine (Ki = 2.3 +/- 0.5 microM) greater than or equal to norepinephrine (Ki = 4.4 +/- 1.3 microM) much greater than isoproterenol (Ki = 195.5 +/- 27.6 microM), consistent with an alpha-adrenergic interaction. Computer modelling of competition curves obtained with prazosin (alpha 1-selective) and yohimbine (alpha 2-selective) indicated that the DDT1 cell alpha-adrenergic receptor was predominantly (greater than 95%) of the alpha 1-subtype. Guanine nucleotides, either GTP or 5'-guanylylimidodiphosphate, did not reduce the affinity of either epinephrine of phenylephrine for the [3H]-dihydroergocryptine binding site.