Procion Blue HB (Reactive Blue 2) is a purinergic antagonist.

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 Ki 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].

[1]. Okuda A, et al. Reactive blue 2 induces calcium oscillations in HeLa cells. Jpn J Physiol. 2001 Jun;51(3):389-93. [2]. Lee JH, et al. Reactive blue 2, an antagonist of rat P2Y4, increases K+ secretion in rat cochlea strial marginal cells. Hear Res. 2006 Sep;219(1-2):66-73. [3]. Coughlan SJ, et al. Reactive blue 2 is a potent inhibitor of a thylakoid protein kinase. Eur J Biochem. 1991 Apr 23;197(2):467-71. [4]. Burnstock G, et al. P2-purinoceptors of two subtypes in the rabbit mesenteric artery: reactive blue 2 selectively inhibits responses mediated via the P2y-but not the P2x-purinoceptor. Br J Pharmacol. 1987 Feb;90(2):383-91. [5]. Uneyama H, et al. Suramin and reactive blue 2 are antagonists for a newly identified purinoceptor on rat megakaryocyte. Br J Pharmacol. 1994 Jan;111(1):245-9.