MODULATION OF INTRACELLULAR CA2- ROLE OF ENDOPLASMIC-RETICULUM CA2+-ATPASE( BY GLUCOSE IN MDCK CELLS )

Intracellular free calcium ([Ca2+](i)) has multiple functional roles i n renal epithelia, including mediating ligand- and volume-activated K and Cl- channels, modulating the permeability of apical membrane to N a+, and regulating tubuloglomerular feedback. We investigated glucose effects on intracellular pH (pH(i)) and [Ca2+](i) in Madin-Darby canin e kidney (MDCK) cells using fluorescent probes, SNARF-1 and fura 2, re spectively. The addition of glucose decreased both pH(i) and [Ca2+](i) in a dose-dependent fashion. Thapsigargin (TG) and cyclopiazonic acid (CPA), well-known endoplasmic reticulum (ER) Ca2+-adenosinetriphospha tase (Ca2+-ATPase) inhibitors, abolished the glucose-induced [Ca2+](i) decrease. Without glucose, 1 mu M TG induced a sustained elevation in [Ca2+](i), which increased further with glucose addition, whereas 15 mu M CPA induced a transient increase in [Ca2+](i) that was not affect ed by further addition of glucose. The sustained elevation in [Ca2+](i ) induced by TG was dependent on extracellular Ca2+. TG-induced [Ca2+] (i) increase was modulated by glucose, i.e., at higher glucose concent rations, TG induced a larger and more rapid rise in [Ca2+](i). We conc lude that glucose has dual effects on [Ca2+](i) regulation. Glucose al one reduces [Ca2+](i) by activating ER-type Ca2+ ATPase, since this ph enomenon is TG and CPA sensitive. In the presence of TG, glucose incre ases [Ca2+](i) probably by increasing Ca2+ entry. Our data suggest a m odel in which TG activates capacitative Ca2+ entry by depletion of the ER Ca2+ pool. Glucose increases TG-induced [Ca2+](i) elevation by fur ther enhancing capacitative Ca2+ entry.