Competitive vs. noncompetitive blockade of epinephrine-sensitive GTPase activity in human platelets

The manner of inhibition of the epinephrine-stimulated GTPase activity by phentolamine and yohimbine was studied in human platelet membranes. (-)-Epinephrine(1-microM)-stimulated GTPase activity was inhibited concentration-dependently by phentolamine and yohimbine with IC50 values of 150 nM and 25 nM, respectively. Addition of increasing concentrations of phentolamine shifted the concentration-response curve to (-)-epinephrine to the right in a parallel manner. Schild plot analysis gave a KB value of 13 nM and a slope factor of 0.99. Yohimbine inhibited the activity, on the contrary, in a noncompetitive fashion. This inhibition by yohimbine was reversible after thorough washing of the membranes.