REGULATION OF 5-HYDROXYTRYPTAMINE(2) (5-HT(2)) RECEPTOR EXPRESSION INCULTURED RAT AORTIC SMOOTH-MUSCLE CELLS BY SR-46349B, A SELECTIVE 5-HT(2) RECEPTOR ANTAGONIST

Regulation of 5-hydroxytryptamine (5-HT2) receptor expression by SR 46 349B, a potent and selective 5-HT2 receptor antagonist, was investigat ed in cultured rat aortic smooth muscle cells. Binding of [H-3]SR 4634 9B to rat vascular smooth muscle cells was time-dependent, reversible, and saturable. [H-3]SR 46349B bound to one class of specific binding sites with high affinity (K(D) = 1.3 +/- 0.3 nm; B(max) = 176 +/- 42 f mol/10(5) cells). Exposure of cells to a 1 muM concentration of the 5- HT2 agonist +/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((+/-)- DOI) or the antagonist ketanserin led to a significant decrease in 5-H T2 receptor density as measured by [H-3]SR 46349B binding. In contrast , exposure of cells to 1 muM SR 46349B caused a marked increase in the maximal binding capacity of [H-3]SR 46349B, with a maximal effect at 24 h (73% increase). The affinity constant was not affected by prior e xposure to (+/-)-DOI, ketanserin, or SR 46349B. Furthermore, exposure of cells to 1 muM (+/-)-DOI or ketanserin produced, 48 h later, a decr ease in the ability of (+/-)-DOI to stimulate phosphoinositide turnove r in the cells, whereas treatment with SR 46349B induced a significant stimulation of the 5-HT2 receptor-linked signal transduction. This ef fect occurred with no changes in the amount of 5-HT2 receptor mRNAs as measured by quantitative polymerase chain reaction. These results ind icate that SR 46349B increases 5-HT2 receptor binding and functions wi thout altering steady-state 5-HT2 mRNA levels in cultured rat aortic s mooth muscle cells.