The biochemical mechanism responsible for the osmotic sensitivity of glutamate oxidation in oyster (Crassostrea virginica Gmelin) gill mitochondria was examined. The relative roles of osmotically induced changes in matrix divalent cation concentration and ionic strength were determined. The calcium-magnesium ionophore A23187 inhibited glutamate oxidation in both low- (40% of control rate) and high- (25% of control rate) osmolarity media. Addition of MgCl2 reversed A23187 inhibition, but at each MgCl2 concentration (1–5 mmol l−1) the rate of glutamate oxidation in the high-osmolarity medium was about 45% that in the low-osmolarity medium. EDTA (4 mmol l−1) stimulated glutamate oxidation at both osmolarities (more pronounced at high osmolarity), removing the dependency on assay medium osmolarity. The stimulation in response to EDTA was correlated with mitochondrial swelling, which required the presence of monovalent cations for maximal effect. These data suggest that osmotically induced changes in matrix [Mg2+] or [Ca2+] are not responsible for the osmotic sensitivity of glutamate oxidation by oyster gill mitochondria in vitro. Changes in extramitochondrial [Mg2+] may be involved in regulating the rate of glutamate oxidation in vivo, through effects on mitochondrial volume. The maximal rate of glutamate dehydrogenase was not very sensitive to the ionic strength of the assay medium. The rate of electron transport was highly dependent on ionic strength, with the maximal rate occurring in 50 mmol l−1 salt. The osmotic sensitivity of glutamate oxidation in oyster gill mitochondria is apparently due to the effects of changes in matrix ionic strength on the rate of electron transport.

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