ADRENERGIC RECEPTOR-MEDIATED REGULATION OF CULTURED RABBIT AIRWAY SMOOTH-MUSCLE CELL-PROLIFERATION

To evaluate the potential role of adrenergic receptors in regulating a irway smooth muscle (ASM) cell proliferation, the mitogenic effects an d mechanisms of action of selective alpha-and beta-adrenoceptor activa tion were investigated in cultured rabbit ASM cells. The alpha(1)-adre noceptor agonists, phenylephrine and methoxamine, elicited significant dose-dependent (10(-10)-10(-4) M) stimulation of ASM cell mitogenesis , with mean +/- SE peak increases in ASM cell count amounting to 17.0 +/- 3.5 and 44.0 +/- 6.8% above unstimulated (control) levels, respect ively. Similarly, the alpha(2)-adrenoceptor agonist clonidine (10(-8)- 10(-4)) also induced ASM cell proliferation, with a 41.1 +/- 5.5% peak increase in cell count above control. The promitogenic responses to a lpha-adrenoceptor activation were blocked by pertussis toxin (PT; 100 ng/ml), which ADP-ribosylates G protein negatively coupled to adenylat e cyclase activation. In additional studies, we found that 1) treatmen t with agents inducing intracellular adenosine 3',5'-cyclic monophosph ate (cAMP) accumulation including the beta-adrenergic agonist, isoprot erenol, the cAMP analogue, 8-(4-chlorophenylthio)-cAMP and forskolin, all produced significant dose-dependent inhibition of serum-stimulated ASM cell growth; 2) alpha-adrenoceptor activation inhibited isoproter enol-induced cAMP accumulation; and 3) the anti-proliferative effects of isoproterenol and PT were additive. Collectively, the above finding s provide new evidence that adrenergic receptors exert an opposing dua lity of action in regulating ASM cell proliferation, wherein receptors which are negatively coupled (i.e., alpha-adrenergic) and those which are positively coupled (i.e., beta-adrenergic) to activation of the a denylate cyclase/cAMP signal-transduction pathway are promitogenic and growth inhibitory to ASM cells, respectively.