Mg 2+ ‐sensitive non‐capacitative basolateral Ca 2+ 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 T ) that is inhibited by extracellular Mg 2+ . Elevation of free cytosolic [Ca 2+ ] ([Ca 2+ ] i ) is known to increase C T . Therefore, we examined [Ca 2+ ] i dynamics and their sensitivity to extracellular Mg 2+ 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 H 2 O) −1 . Reduction of apical osmolality alone did not affect [Ca 2+ ] i . In the absence of extracellular Mg 2+ , the hyposmotic shock induced a biphasic rise in [Ca 2+ ] i . The first phase peaked within 40 s and [Ca 2+ ] i increased from 245 ± 12 to 606 ± 24 n m . This phase was unaffected by removal of extracellular Ca 2+ , 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 [Ca 2+ ] i rise followed swelling‐induced ATP release. The PLC inhibitor, the ATP treatment or suramin did not affect a second rise of [Ca 2+ ] i to a maximum of 628 ± 31 n m . The second phase depended on Ca 2+ in the basolateral perfusate and was largely suppressed by 2 m m basolateral Mg 2+ . Acute exposure of the basolateral membrane to Mg 2+ during the upstroke of the second phase caused a rapid decline in [Ca 2+ ] i . Basolateral Mg 2+ inhibited Ca 2+ entry in a dose‐dependent manner with an inhibition constant ( K i ) of 0.60 m m . These results show that polarized A6 epithelia respond to hyposmotic shock by Ca 2+ release from inositol trisphosphate‐sensitive stores, followed by basolateral Ca 2+ influx through a Mg 2+ ‐sensitive pathway. The second phase of the [Ca 2+ ] i response is independent of the initial intracellular Ca 2+ release and therefore constitutes non‐capacitative Ca 2+ entry.