DUAL EFFECT OF SEROTONIN ON GROWTH OF BOVINE PULMONARY-ARTERY SMOOTH-MUSCLE CELLS IN CULTURE

We have previously reported that serotonin (5-hydroxytryptamine [5HT]) alters cultured bovine pulmonary artery smooth muscle cell (SMC) configuration through two different regulatory mechanisms. We now report that 5HT also regulates SMC growth through these same two mechanisms - a stimulatory event initiated intracellularly and inhibition of growth resulting from a cell surface action. 5HT (1-mu-M) plus 0.1 mM iproniazid (a 5HT metabolic inhibitor) produced a severalfold stimulation of DNA synthesis (as measured by [H-3]thymidine incorporation) of SMCs after a 17 - 24-hour incubation with only a slight elevation of cellular cAMP. This stimulatory effect responded synergistically with other growth factors including platelet-derived growth factor, fibroblast growth factor, and epidermal growth factor and was effectively reversed by 5HT uptake inhibition. It was not produced by 5-hydroxyindoleacetic acid, a metabolite of 5HT. In the presence of 1-mu-M 5HT plus 0.1 mM isobutylmethylxanthine (IBMX), cAMP was elevated eightfold, dendritic formation occurred, and [H-3]thymidine labeling of SMCs was inhibited. Inhibition of labeling by [H-3]thymidine was mimicked by other agents that elevated cellular cAMP (10-mu-M histamine, 1-mu-M isoproterenol plus 0.1 mM IBMX, and 10-mu-M forskolin) and by 1 mM dibutyryl cAMP. This inhibitory effect was not blocked by either inhibition of 5HT uptake or 5HT-receptor antagonists ketanserin (5HT2); methiothepin, spiperone, and mianserin (5HT1/5HT2); and 3-tropanyl-indole-3-carboxylate and 3-tropanyl-3,5-dichlorobenzoate (5HT3). However, similar to 5HT, the 5HT1A agonist, (+/-)-8-hydroxy-(+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalenehydrobromide, in association with IBMX, produced an elevation in cAMP and inhibition of labeling by [H-3]thymidine.5HT, in the presence of either iproniazid or IBMX, did not alter [Ca2+]i, indicating that [Ca2+]i was not a signal for either of these actions. The studies show that 5HT affects SMC growth at two foci of action, one intracellularly by a currently unknown mechanism and another at the cell surface, perhaps via a novel receptor and dependent on transduction of the signal through elevation of cAMP.