AJP - Renal Physiology, Vol 260, Issue 1 12-F18, Copyright © 1991 by American Physiological Society

ARTICLES

Phosphate depletion increases cytosolic calcium of brain synaptosomes

S. G. Massry, S. M. Hajjar, P. Koureta, G. Z. Fadda and M. Smogorzewski
Division of Nephrology, University of Southern California School of Medicine, Los Angeles 90033.

Phosphate depletion (PD) is associated with a rise in resting levels of [Ca2+]i in pancreatic islets. It is not known whether this derangement occurs in other cells, and the mechanisms by which PD affects [Ca2+]i have not been delineated. This study examined the effect of PD on [Ca2+]i of brain synaptosomes and evaluated potential mechanisms that may lead to rise in their [Ca2+]i. [Ca2+]i levels in synaptosomes of PD rats (460 +/- 18.3 nM) were higher (P less than 0.01) than those of pair-weighed (PW) rats (358 +/- 12.5 nM). Verapamil treatment of PD rats (PD-V) normalized [Ca2+]i in their synaptosomes (361 +/- 8.5 nM). In verapamil-treated PW rats (PW-V), synaptosomal [Ca2+]i (359 +/- 8.3 nM) was not affected. ATP content and Na(+)-K(+)-ATPase activity of synaptosomes were lower (P less than 0.01) in PD rats than in PW, PD-V, and PW-V rats. The values of these parameters from the latter three groups were not different. Km of synaptosomal Ca2(+)-ATPase was not affected by PD but Vmax of this enzyme (2.5 +/- 0.33 mumol Pi.mg protein-1.h-1) was lower (P less than 0.05) than in PW (5.4 +/- 0.66 mumol Pi.mg protein-1.h-1), PD-V, and PW-V rats. Our data indicate that PD raises [Ca2+]i in brain synaptosomes and suggest that PD increases calcium entry into synaptosomes. This would inhibit mitochondrial ATP production, with a consequent fall in ATP content of synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

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