Mg2+-sensitive non-capacitative basolateral Ca2+ entry secondary to cell swelling in the polarized renal A6 epithelium

Polarized renal A6 epithelia respond to hyposmotic shock with an increase in transepithelial capacitance (C-r) that is inhibited by extracellular Mg2+. Elevation of free cytosolic [Ca2+] ([Ca2+](i)) is known to increase C-T. Therefore, we examined [Ca2+](i) dynamics and their sensitivity to extracellular Mg2+ during hyposmotic conditions. Fura-2-loaded A6 monolayers, cultured on permeable supports were subjected to a sudden reduction in osmolality at both the basolateral and apical membranes from 260 to 140 mosmol (kg H2O)(-1). Reduction of apical osmolality alone did not affect [Ca2+](i). In the absence of extracellular Mg2+, the hyposmotic shock induced a biphasic rise in [Ca2+](i). The first phase peaked within 40 s and [Ca2+](i) increased from 245 +/- 12 to 606 +/- 24 nM. This phase was unaffected by removal of extracellular Ca2+, but was abolished by activating P2Y receptors with basolateral ATP or by exposing the cells to the phospholipase C (PLC) inhibitor U73122 prior to the osmotic shock. Suramin also severely attenuated this first phase, suggesting that the first phase of the [Ca2+](i) rise followed swelling-induced ATP release. The PLC inhibitor, the ATP treatment or suramin did not affect a second rise of [Ca2+](i) to a maximum of 628 +/- 31 nM. The second phase depended on Ca2+ in the basolateral perfusate and was largely suppressed by 2 mM basolateral W. Acute exposure of the basolateral membrane to Mg2+ during the upstroke of the second phase caused a rapid decline in [Ca2+](i). Basolateral Mg2+ inhibited Ca2+ entry in a dose-dependent manner with an inhibition constant (K-i) of 0.60 mm. These results show that polarized A6 epithelia respond to hyposmotic shock by Ca2+ release from inositol trisphosphate-sensitive stores, followed by basolateral Ca2+ influx through a Mg2+-sensitive pathway. The second phase of the [Ca2+](i) response is independent of the initial intracellular Ca2+ release and therefore constitutes non-capacitative Ca2+ entry.