Abstract

Human 5-hydroxytryptamine₇ (5-HT₇) receptors display characteristics shared with receptors believed to form a tight physical coupling with G protein in the absence of ligand. Some receptors apparently preassociated with G_i/o and G_q/11 are reported to inhibit the signaling of other similarly coupled G protein-coupled receptors by limiting their access to activate a common G protein pool. Therefore, we determined whether 5-HT₇ receptor expression was sufficient to limit signaling of endogenously expressed G_s-coupled receptors in human embryonic kidney (HEK) 293 cells. Using the ecdysone-inducible expression system, which allows for the titration of increasing receptor density in the same clonal cell line, we compared the effects of 5-HT_4(b) and 5-HT_7(a,b,d) receptor expression on adenylyl cyclase (AC) stimulation by the endogenous G_s-coupled β-adrenergic (βAR) and prostanoid EP (EPR) receptors. βAR- and EPR-stimulated AC activity was attenuated by 5-HT₇ receptor expression in both membrane preparations and intact HEK293 cells. βAR- and EPR-stimulated AC activity was unaffected by expression of the G_s-coupled 5-HT₄ receptor. The mechanism of this heterologous desensitization seems independent of protein kinase A activation, nor does it occur at the level of G protein activation because 1) βAR- and EPR-stimulated AC activity was not restored to control values when Gα_s was overexpressed; and 2) β₁AR and β₂AR activation of Gα_s was unaffected by the expression of 5-HT₇ receptors. In addition, overexpression of AC isoforms was unable to rescue βAR- and EPR-stimulated AC activity. Therefore, 5-HT₇ receptors probably limit access and/or impede activation of AC by βAR and EP receptors. Although the 5-HT₇ receptor may preassociate with G protein and/or AC, the mechanism of this heterologous desensitization remains elusive.