pH-dependent effect of mitochondria on calcium influx into Jurkat cells; a novel mechanism of cell protection against calcium entry during energy stress.

Loss of the mitochondrial membrane potential results in a significant inhibition of calcium influx through calcium release-activated channels (CRAC) in Jurkat cells suspended in the medium of pH lower than 7.4. This effect disappears when the medium pH increases. Alkalinisation of the cytosol achieved by the addition of NH(4)Cl to the cells pretreated with thapsigargin, CCCP and CaCl(2), suspended in the medium of pH 7.2, does not affect CRAC activity, while alkalisation of the extracellular milieu by NaOH results in a strong stimulation of calcium entry. Thus, the mitochondrial effect on CRAC is exclusively related to the extracellular pH. Coupled mitochondria are able to take up Ca(2+) accumulated in the close proximity of CRAC. This protects these channels against feedback inhibition exerted by high [Ca(2+)](c). We conclude that CRAC may exist in two conformations: inhibitable and not inhibitable by cytosolic Ca(2+). Lower extracellular pH promotes the former one. This explains a much higher inhibitory effect of mitochondrial uncouplers on the calcium influx into the cells exposed to pH 7.2 than that observed in the cells suspended in the medium of pH 7.8. This phenomenon may provide an additional mechanism protecting cells against calcium overloading in transient episodes of energy stress.