Abstract
The manner of inhibition of the epinephrine-stimulated GTPase activity by phentolamine and yohimbine was studied in human platelet membranes. (−)-Epinephrine (1 μM)-stimulated GTPase activity was inhibited concentration-dependently by phentolamine and yohimbine with IC 50 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 K B 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.
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