Serotonin produces a configurational change of cultured smooth muscle cells that is associated with elevation of intracellular cAMP

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Abstract
Early passaged bovine pulmonary artery smooth muscle cells (SMC) respond to serotonin (5‐HT) by developing a reversible change in configuration. (Lee et al. J. Cell. Physiol. 138:145, 1989). This configurational change does not occur in pulmonary artery endothelial cells (EC) subjected to 5‐HT and is adenosine triphosphate (ATP) dependent, lost with passage of SMC, and inhibited by various agents that block high‐affinity 5‐HT uptake. We now report a second configurational change (also dendritic formation) of SMC produced by 5‐HT only in the presence of isobutylmethylxanthine (IBMX), an inhibitor of phosphodiesterase. This configurational change was also ATP dependent, but unlike the first response, (Lee et al., 1989), it occurred in both first and later passaged SMC and was not inhibited by blockade of 5‐HT uptake. Also, unlike the response with 5‐HT alone that failed to elevate cAMP, this one was associated with a large elevation of cAMP (eight fold above control values), similar to the response to the β‐agonist isoproterenol, plus IBMX. The second response was not blocked by a variety of 5‐HT receptor antagonists but was reproduced by (±)‐8‐hydroxy‐DPAT HBr (8‐OH‐DPAT), a reputed 5‐HT1A agonist. The response was not dependent upon Ca2+ and was blocked by 1–2 mM n‐phenylanthranilic acid or anthracene‐9‐carboxylic acid, electrically conductive Cl− channel inhibitors. Hence, 5‐HT in the presence of IBMX causes a marked elevation of cAMP of SMC and this elevation in cAMP likely results in a cellular configurational change through a Cl− channel‐dependent mechanism similar to that we previously described for EC in the presence of β‐adrenergic agonist stimulation (Ueda et al. Circ. Res. 66:951, 1990). EC do not show a similar response to 5‐HT possibly because cAMP is not adequately elevated, even in the presence of IBMX, to enhance Cl− channel activity. We propose that our observations indicate the presence of two sites of action of 5‐HT on the smooth muscle cell, one intracellularly and another at a cell surface receptor.