Calcium and acidosis in renal hypoxia

The effects of lowering extracellular calcium concentration on hypoxic injury in the thick ascending limb of Henle (TAL) were studied in the isolated perfused rat kidney. At standard conditions of pH 7.4 and total perfusate calcium 1.9 mM, widespread TAL necrosis results from the combined effects of low medullary O2 delivery and the demands of solute transport activity. Reducing calcium to 0.5 or 0.1 mM, effectively prevented TAL membrane fragmentation. This cytoprotection was not accompanied by improved O2 delivery or by any consistent effects on renal physiology (glomerular filtration rate, sodium reabsorption, free water clearance or O2 consumption) that might have suggested that the mechanism was reduced O2 demand. In addition, the medullary ATP depletion which characteristically precedes TAL necrosis was not reversed. Finally, reduced perfusate calcium also markedly decreased TAL damage in an alternative model of hypoxia-like injury caused by a respiratory uncoupler. In aggregate, these findings indicate that reducing extracellular calcium does not prevent hypoxia itself, but rather disrupts the mechanism of its effects on cell integrity. The relationship of H+ and Ca2+ in the pathogenesis of hypoxic TAL injury was also studied. Lowering media pH to 7.0 reduced TAL damage but this cytoprotection was overcome by increasing media calcium concentration. Furthermore, with more severe acidosis (media pH 6.5 or 6.0), progressively greater perfusate calcium concentrations were required to reproduce severe TAL damage. These results indicate that extracellular calcium promotes the development of hypoxic TAL necrosis and that the cytoprotective effect of acidosis in hypoxia may be to counteract the calcium-dependent mechanism of injury.