Procion Blue HB (Reactive Blue 2) is a purinergic antagonist. In Vitro: Reactive Blue 2 is used as an ATP receptor antagonist and induces Ca²⁺ oscillations in HeLa cells. Reactive Blue 2 enhances a Ca²⁺ response to histamine that is linked to the PLC cascade. Reactive Blue 2 may activate the PLC cascade in an extracellular Ca²⁺-dependent manner and induce Ca²⁺ oscillations^[1]. The application of Reactive Blue 2 increases K⁺ secretion in a dose-dependent manner, and this increase is characterized as a peak followed by a partial relaxation to a steady-state. Reactive Blue 2 has antagonistic activities at P2Y4, and the antagonist potency at P2Y4 paralleled the potency of K+ secretion^[2].The anthraquinone dye reactive blue 2 is found to be a potent inhibitor of a protein kinase isolated and purified from thylakoids. The mode of inhibition is noncompetitive, with a K_i of 8 μM for the membrane-bound kinase, and 6 microM for the purified kinase. The inhibitor does not modify the substrate preference of the endogenous kinase and could be removed from the membrane by washing^[3]. Reactive blue 2 selectively inhibits responses mediated via the P2ypurinoceptor, at least within a limited concentration range. In preparations where the tone has been raised with noradrenaline, ATP and 2-methylthio ATP, but not α,β-methylene ATP, produce relaxations of the vessel. These relaxations are inhibited in the presence of reactive blue 2^[4]. Reactive blue 2, at concentrations of 0.3-10 μM blocks the ATP-induced oscillation in a concentration-dependent manner^[5].