Platelets null for PRKAR2B are hyporesponsive. (A) PRKAR2B immunoblot. Lysates of mouse wild-type (wt) brain, wt platelets, and platelets from Prkar2b^−/− mice (KO) were subjected to sodium dodecyl sulfate–polyacrylamide gel electrophoresis and immunoblotted with polyclonal antisera raised against recombinant mouse protein as described.²⁹  Approximately equal loading is shown by immunoblotting for β-actin. (B) Platelet surface expression of the activation marker, P-selectin, was quantified by flow cytometry on platelets from wild-type (WT) mice and mice null for Prkar2b (KO).²⁹  Platelets were stimulated with a fixed, subthreshold concentration of PAR4AP (0.8mM) and the indicated increasing concentrations of epinephrine.