Potentiation of P2Y Receptors by Physiological Elevations of Extracellular K+via a Mechanism Independent of Ca2+Influx

Abstract

Many physiological and pathophysiological situations generate a significant increase in extracellular K+ concentration. This is known to influence a number of membrane conductances and exchangers, whereas direct effects of K+ on the activation of G protein-coupled receptors have not been reported. We now show that Ca2+ release evoked by P2Y1 receptors expressed in 1321-N1 astrocytoma cells is markedly potentiated by small increases in external K+ concentration. This effect was blocked by the phospholipase-C inhibitor U-73122 (1-[6-[[17 beta]-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), but not by its analog U-73343 (1-[6-[[17 beta]-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-2,5-pyrrolidinedione), and not by nifedipine, Ni2+, Cd2+, or Gd3+. Thus, K+ enhances d-myo-inositol 1,4,5-trisphosphate-dependent Ca2+ release without a requirement for Ca2+ influx. The cation dependence of this effect displayed the order K+ > Rb+ > N-methyl-D-glucamine+, and Cs+ and choline+ were ineffective. The potentiation by K+ is half-maximal at an increase of 2.6 mM (total K+ of 7.6 mM). K+ caused a reduction in EC50 (2.7-fold for a 29 mM increase) without a change of slope; thus, the greatest effect was observed at near-threshold agonist levels. The response to K+ can be explained in part by depolarization-dependent potentiation of P2Y1 receptors [J Physiol (Lond) 555:61-70, 2004]. However, electrophysiological recordings of 1321-N1 cells and megakaryocytes demonstrated that K+ also amplifies ADP-evoked Ca2+ responses independently of changes in membrane potential. Elevated K+ also amplified endogenous UTP-dependent Ca2+ responses in human embryonic kidney 293 cells, suggesting that other P2Y receptors are K(+)-dependent. P2Y receptors display a widespread tissue distribution; therefore, their modulation by small changes in extracellular K+ may represent a novel means of autocrine and paracrine regulation of cellular activity.