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Depletion of endogenous malate by preincubation of mitochondria at 30 degrees C in substrate-free media sharply decreases the rate of citrate oxidation and inhibits mitochondrial respiration in the presence of pyruvate and alpha-ketoglutarate. Addition of catalytic amounts of endogenous malate and its production via succinate oxidation promote rapid oxidation of citrate and pyruvate in the mitochondria and abolishes the lag period with alpha-ketoglutarate Malate increases the rate of membrane potential generation after addition of citrate, pyruvate or alpha-ketoglutarate to mitochondrial suspensions. Studies with controlled malate concentrations revealed that the changes in malate concentrations observed in the mitochondria in the presence of gluconeogenesis-inducing hormones may be due to the influence of these hormones on mitochondrial oxidation.

The calcium-transport inhibitor, caltrin, isolated from bovine seminal fluid inhibits calcium accumulation by bovine epididymal spermatozoa, spermatozoal mitochondria, rat liver mitochondria and beef heart mitochondria. Respiration studies demonstrate a marked stimulation of oxygen consumption by caltrin in filipin-treated spermatozoa and rat liver mitochondria. A biphasic effect of caltrin on rat liver mitochondrial respiration was noted, with stimulation at low caltrin concentrations and inhibition as the concentration of caltrin is increased. The ability of caltrin to uncouple and/or inhibit respiration in filipin-treated spermatozoa and isolated liver mitochondria indicates that inhibition of mitochondrial calcium accumulation by caltrin results from one of these mechanisms. Only a marginal effect of caltrin on respiration of intact spermatozoa was observed; indicating that the plasma membrane is impermeable to this protein. The differential effect of caltrin on respiration of intact and permeabilized spermatozoa indicates that caltrin inhibition of Ca2+ uptake into spermatozoa in vivo occurs at the level of the plasma membrane.

We have examined the ion transport properties and the inhibition of rat liver mitochondrial substrate oxidation by the antibiotic W341C. W341C was able to transport 22Na+ and 42K+ across a bulk carbon tetrachloride layer. A preference was shown for K+ transport. With equal molar antibiotic concentrations, W341C transported 42K+ at a greater rate than the K+-selective ionophore nigericin, but transported 22Na+ at a lesser rate than the Na+-selective ionophore monensin. Like nigericin, W341C was able to deplete mitochondrial K+, but not Mg2+ nor Ca2+. The inhibition of mitochondrial substrate oxidation by W341C paralleled the patterns obtained with nigericin. These data indicate that W341C is a K+-selective ionophore that inhibits mitochondrial substrate oxidation by a mechanism analogous to that of nigericin.

Shortly after the injection of glucagon, epinephrine, norepinephrine, vasopressin, or angiotensin II into fasted rats, mitochondria isolated from their livers contained elevated concentrations of malate and oxidized citrate, alpha-ketoglutarate, and, in some cases, succinate more rapidly than mitochondria from fasted, control rats. The administration of tryptophan, lactate, or ethanol and refeeding of rats fasted 24 h result in similar elevations of mitochondrial malate concentration and oxidation of added substrates. Treatments that resulted in elevated mitochondrial malate resulted also in increased uptake of added citrate, alpha-ketoglutarate, pyruvate, and, in some cases, succinate. It is postulated that the well-documented effect of gluconeogenic hormones on mitochondrial oxidation of carboxylic substrates may be mediated by malate which not only yields oxalacetate to support the tricarboxylic acid cycle but also facilitates the transport of added substrates, and which is regenerated in the tricarboxylic acid cycle.