The GqKI mutant α1A does not couple to Gq in myocytes. TOP: The mutant receptor does not mediate activation of phosphoinositide-phospholipase C (PI-PLC) by the α1A agonist A61603 (60 min) in adult mouse ventricular myocytes (AMVMs), measured by A. labeling with 3H-inositol, or B. inositol phosphate mass assay. Sigmoidal dose-response curves were fit in Prism; p values are from unpaired t test of the calculated Emax; N = cultures from different hearts. BOTTOM: C. The mutant receptor does not mediate an inotropic response to A61603 (A6,100 nM, 5–10 min at peak response), shown as percent basal length for individual cells from 4 hearts (left) and for hearts from which the cells were isolated (right). Both a WT and a GqKI mouse were studied on each experimental day; thus for GqKI cells that did not respond to A61603, WT cells on the same day did respond. P values by paired t test for cells, and by unpaired t test for hearts.