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., 19891, 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 wa5 associated with a large elevation of cAMP (eight fold above control values), similar to the response to the P-agonist isoproterenol, plus IBMX. The second response was not blocked by a variety of 5-HT receptor antagonists but was reproduced by (*)-&hydroxy-DPAT HBr (8-OH-DPAT), a reputed 5-HT,, agonist. The response was not dependent upon Ca''and was blocked by 1-2 mM n-phenylanthranilic acid or anthracene-9-carboxylic acid, electrically conductive CI-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 CI-channel-dependent mechanism similar to that we previously described for EC in the presence of P-adrenergic agonist stimulation (Ueda et al. Circ. Kes. h6: 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 CI-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.

Among other possible actions, serotonin (5-HT) is widely recognized to function both as a neurotransmitter and a regulator of smooth muscle tension (Van Nueten et al., 1985). Although serotonergic receptors and related cellular biologic function have been studied extensively in neural cells (Wang and Peroutka, 19881, little information is available about the mechanism of action of 5-HT on smooth muscle cells (SMC). Relaxation of smooth muscle cells by 5-HT in canine coronary arteries is mediated through release of endothelial derived relaxing factor (Cohen et al., 1983; Gutterman et al., 1986). Contraction, on the other hand, is thought to occur by direct action of 5-HT on smooth muscle by phosphoinositide hydrolysis (Roth et al., 1984; Nakaki et al., 1985; Conn and Sanders-Bush, 1986) and subsequent release of intracellular stores of Caz+(Doyle et al., 1986; Somlyo et al., 1985). We have previously reported that 5-HT is accumulated by bovine pulmonary artery smooth muscle cells in culture and that