EVIDENCE FOR THE INVOLVEMENT OF SEVERAL INTRACELLULAR DOMAINS IN THE COUPLING OF OXYTOCIN RECEPTOR TO G-ALPHA(Q 11)/

In order to probe the nature of oxytocin receptor (OTR)/G alpha(q/11) protein coupling, we examined the effect of co-expression of OTR intra cellular domains on oxytocin stimulated phosphoinositide turnover in C OSM6 cells overexpressing OTR and G alpha(q). Co-expression of G alpha (q) enhanced the oxytocin response maximally al a pOTR/pC alpha(q) pla smid transfection ratio of 1:0.16. In cells co-expressing OTR and G al pha(q/11), oxytocin stimulated phosphoinositide turnover with an EC50 of 48 nM. Co-transfection with plasmids expressing OTR intracellular d omains inhibited oxytocin stimulated phosphoinositide turnover by 23 /- 6% (ii), 37 +/- 4% (2i), 55 +/- 6% (3i), and 40 +/- 6% (4i), respec tively (P < 0.01). Expression of the 3i loop of the alpha(1B)-adrenerg ic receptor, which also couples to G alpha(q/11), inhibited phosphoino sitide tumover by 35 +/- 2% (P < 0.01), while expression of the 3i loo p of the dopamine mine 1A receptor, which couples to G alpha(s), had n o effect. While these data indicate a functional role for the OTR 3i L oop, they also suggest that interactions with more than one intracellu lar domain probably mediate the coupling of OTR to the G alpha(q/11) c lass of GTP-binding proteins. (C) 1998 Elsevier Science Inc.