Hepatic calcium efflux during cytochrome P-450-dependent drug oxidations at the endoplasmic reticulum in intact liver.

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Abstract
During metabolism of (type I) drugs by cytochrome P-450-dependent monooxygenase of the endoplasmic reticulum, the NADPH/NADP+ ratio in rat liver selectively decreases to .apprx. 1/2 of the control values, but the NADH/NAD+ ratio remains practically unaffected. Showed stimulated Ca2+ efflux upon nicotinamide nucleotide oxidation. The selective oxidation of NADPH in cytosol and mitochondria during drug oxidations was a useful experimental tool to determine whether the oxidation of NADPH or of NADH is responsible for Ca2+ efflux. With perfused livers from phenobarbital-treated rats, Ca2+ efflux was demonstrated, amounting to 8 nmol/min per of liver (wet wt), with aminopyrine, ethylmorphine or hexobarbital as drug substrates. Drug-associated Ca2+ release was diminished when the inhibitor metyrapone was present, or when drug oxidation was suppressed during N2 anoxia or in the presence of antimycin A in livers from fasted rats. Ca2+ efflux was elicited by infusion of the thiol oxidant diamide, and by tert-butyl hydroperoxide. Ca2+ efflux elicited by these compounds was restricted upon addition of the thiol dithioerythritol, but there was little sensitivity of the drug-associated Ca2+ efflux to the thiol. Further mitochondrial oxidation of NADPH by addition of ammonium chloride had no effect on drug-associated Ca2+ efflux. Prior addition of the .alpha.-agonist phenylephrine suppressed the Ca2+ release by drug addition. While the molecular mechanism involved in Ca2+ efflux from liver mitochondria and from hepatocytes and the regulatory significance are not known, apparently in the case of nicotinamide nucleotide-linked Ca2+ efflux the oxidation of NADPH may suffice, with oxidation of NADH not being a requirement.

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