Abstract
Inward rectifier K(+) channels are important for maintaining normal electrical function in many cell types. The proper function of these channels requires the presence of membrane phosphoinositide 4,5-bisphosphate (PIP(2)). Stimulation of the Ca(2+)-sensing receptor CaR, a pleiotropic G protein-coupled receptor, activates both G(q/11), which decreases PIP(2), and phosphatidylinositol 4-kinase (PI-4-K), which, conversely, increases PIP(2). How membrane PIP(2) levels are regulated by CaR activation and whether these changes modulate inward rectifier K(+) are unknown. In this study, we found that activation of CaR by the allosteric agonist, NPSR568, increased inward rectifier K(+) current (I (K1)) in guinea pig ventricular myocytes and currents mediated by Kir2.1 channels exogenously expressed in HEK293T cells with a similar sensitivity. Moreover, using the fluorescent PIP(2) reporter tubby-R332H-cYFP to monitor PIP(2) levels, we found that CaR activation in HEK293T cells increased membrane PIP(2) concentrations. Pharmacological studies showed that both phospholipase C (PLC) and PI-4-K are activated by CaR stimulation with the latter played a dominant role in regulating membrane PIP(2) and, thus, Kir currents. These results provide the first direct evidence that CaR activation upregulates currents through inward rectifier K(+) channels by accelerating PIP(2) synthesis. The regulation of I (K1) plays a critical role in the stability of the electrical properties of many excitable cells, including cardiac myocytes and neurons. Further, synthetic allosteric modulators that increase CaR activity have been used to treat hyperparathyroidism, and negative CaR modulators are of potential importance in the treatment of osteoporosis. Thus, our results provide further insight into the roles played by CaR in the cardiovascular system and are potentially valuable for heart disease treatment and drug safety.