Cultured rat retinal pigment epithelium cells are shown to contain serotonergic, 5-HT2, receptors associated with phosphoinositide turnover and mobilization of intracellular calcium. Serotonin at a concentration of 10 microM induced a 2.5-fold increase in [3H]-inositol phosphates (more than 75% is in the form of [3H]-inositol-1-phosphate) accumulation within 30 min in cells preincubated in [3H]-myo-inositol and exposed to 5 mM lithium chloride. The EC50 value of serotonin was approx. 0.9 microM and the saturation concentration was 100 microM. Serotonin analogues like tryptamine, 5-methoxytryptamine, alpha-methyl-serotonin and the 5-HT2 agonists quipazine and DOI (1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane) all stimulated InsPs accumulation to some degree. Carbachol, noradrenaline, isoproterenol, dopamine, tryptophan, 5-hydroxytryptophan, 8-hydroxy-2(di-n-propyl-amino) tetralin, 2-methyl-serotonin and NECA (5'-[N-ethyl]-carboxamidoadenosine) were inactive. The serotonin-induced response was blocked most effectively by ketanserin and methysergide but not by 5-HT3 or 5-HT1 antagonists. The serotonin response was attenuated by the active phorbol ester, 4 beta-phorbol 12-myristate 13-acetate and this was attenuated by the non-selective protein kinase C inhibitor, staurosporine. Pertussis toxin failed to influence the serotonin-mediated phosphoinositide turnover. Addition of serotonin to cultures loaded with Fura-2 showed a transient increase in calcium concentrations in most of the cells. This change in calcium was independent of external calcium and the serotonin response was attenuated by ketanserin but not by the 5-HT3 antagonist granisetron.(ABSTRACT TRUNCATED AT 250 WORDS)