Inducible expression of alpha 1B-adrenoceptors in DDT1 MF-2 cells: comparison of receptor density and response.

Hamster DDT1 MF-2 smooth muscle cells natively express alpha 1B-adrenoceptors which are linked to phosphatidylinositol hydrolysis. We studied the relationship between alpha 1B-adrenoceptor density and response in this cell line by stable transfection with an isopropyl-beta-D-thiogalactoside (IPTG)-inducible vector (pOP alpha 1B) containing the hamster alpha 1B-adrenoceptor cDNA. Transfected cells showed a 2-fold increase in receptor density compared to untransfected cells due to constitutive activity of the uninduced vector. Induction of vector expression caused a time-dependent increase in receptor density, reaching a maximum 8-fold increase after 48 h. Exposure to different concentrations of inducing agent for 16 h caused a graded increase in both receptor density and norepinephrine-stimulated [3H]inositol phosphate (InsP) formation. A linear correlation between receptor density and maximum InsP response was observed. Induction of receptor expression did not alter the potency of norepinephrine in stimulating [3H]InsP formation, suggesting that there was no receptor reserve, even at very high expression levels. This inducible expression system should be useful for relating receptor density and responsiveness, and comparing the coupling efficiency of closely related subtypes in activating different signal transduction mechanisms.