Received for publication December 1, 2004.
Revised February 7, 2005.
Accepted for publication February 14, 2005.

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 U73122, but not by its analogue U73343, nor by nifedipine, Ni²⁺, Cd²⁺ or Gd³⁺. Thus, K⁺ enhances IP₃-dependent Ca²⁺ release without a requirement for Ca²⁺ influx. The cation-dependence of this effect displayed the order K⁺>Rb⁺>NMDG⁺, with Cs⁺ and Choline⁺ being ineffective. The potentiation by K⁺ increase 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 (Martinez-Pinna et al. 2004 J.Physiol. 555, 61-70). 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 HEK 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.

Key words: Purinergic, Gq/11 family, Phospholipase C's, Ca imaging, Platelets

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