Intracellular calcium homeostasis changes induced in rat spinal cord neurons by extracellular acidification.

Changes in intracellular Ca2+ induced by extracellular acidification to pH = 6 were studied in isolated rat spinal dorsal horn neurons using indo-1 fluorescent technique. In all neurons such treatment induced a decrease of basal [Ca2+]i level by 20.8%, preceded in some of them by temporary increase. The changes were completely reversible. The depolarization-induced [Ca2+]i transients became strongly and also reversibly depressed. If tested after termination of acidification, they demonstrated substantial prolongation of their decay phase, reaching 310% at 120 sec after the application of depolarization. To analyze the mechanisms of such changes, mitochondrial protonophore CCCP has been applied between the end of acidification and the depolarizing pulse. This completely eliminated the described slowing of the transients' decay. To the contrary, application of caffeine to induce Ca2+ release from the endoplasmic reticulum did not show significant changes in the corresponding [Ca2+]i transients. A conclusion is made that in mammalian neurons extracellular acidification, apart from inhibiting voltage-operated Ca2+ channels, also substantially alters the Ca2+ exchange function of mitochondria responsible for rapid accumulation of ions and their delayed release back into the cytosol.