Calcium transport in rat liver mitochondria

In the present study, it was found that various components of the rat cytosol affect Ca¬¨‚⧂ÄövÖ‚à´ transport into and out of mitochondria when examined in vitro. The ¬¨‚â•H-palmitate-binding protein when saturated with palmitate caused Ca¬¨‚⧂ÄövÖ‚à´ release. The Ca-binding protein and albumin helped prolong Ca¬¨‚⧂ÄövÖ‚à´ retention in the mitochondria. Palmitoyl CoA and palmitate caused release of Ca¬¨‚⧂ÄövÖ‚à´ from preloaded mitochondria. On the other hand palmitoylcarnitine caused Ca¬¨‚⧂ÄövÖ‚à´ retention. The mitochondria were able to accumulate 4 - 9 % of the externally added Mg¬¨‚⧂ÄövÖ‚à´. The presence of varying concentration of external Mg¬¨‚⧂ÄövÖ‚à´ did not inhibit or increase Ca¬¨‚⧂ÄövÖ‚à´ uptake by the mitochondria, however, the extra Mg¬¨‚⧂ÄövÖ‚à´ accumulated prolonged Ca¬¨‚⧂ÄövÖ‚à´ retention in the mitochondria. During Ca¬¨‚⧂ÄövÖ‚à´ release from the mitochondria, there was a corresponding release of mitochondrial Mg¬¨‚⧂ÄövÖ‚à´. During Ca¬¨‚⧂ÄövÖ‚à´ retention, the level of mitochondrial Mg¬¨‚⧂ÄövÖ‚à´ remained steady. ATP or respiratory substrate was required for Ca¬¨‚⧂ÄövÖ‚à´ uptake. The presence of both respiratory substrate and ATP favoured Ca¬¨‚⧂ÄövÖ‚à´ retention. During Ca¬¨‚⧂ÄövÖ‚à´ release induced by palmitoyl CoA, PEP or glucose plus hexokinase, the total ATP concentration (i.e. that in the medium plus mitochondria) decreased, while the total AMP increased. During prolonged Ca¬¨‚⧂ÄövÖ‚à´ retention in the mitochondria, a high concentration of total ATP and a low concentration of total AMP was observed. The findings of Lehninger et al (1978) (that a more oxidised steady state of the mitochondrial pyridine nucleotide favours Ca¬¨‚⧂ÄövÖ‚à´ release and a relatively reduced state favours Ca¬¨‚⧂ÄövÖ‚à´ retention) was confirmed and further extended in this study. The adenine nucleotides were not significantly different from the control during Ca¬¨‚⧂ÄövÖ‚à´ release in the presence of the oxidants of mitochondrial NADH such as acetoacetate or oxaloacetate. Bovine serum albumin or ethane-1-hydroxy-diphosphonic acid (EHDP), substances known to help Ca¬¨‚⧂ÄövÖ‚à´ retention in the mitochondria, caused the Ca¬¨‚⧂ÄövÖ‚à´ released from the mitochondria by oxaloacetate to be taken up again. Externally added NAD‚ÄövÖ‚à´ and possibly NADH prolonged Ca¬¨‚⧂ÄövÖ‚à´ retention in the mitochondria. On the other hand, NADPH caused an earlier Ca¬¨‚⧂ÄövÖ‚à´ release. NADP‚ÄövÖ‚à´ did not affect Ca¬¨‚⧂ÄövÖ‚à´ uptake or release, The effect of cAMP on Ca¬¨‚⧂ÄövÖ‚à´ release was reinvestigated in the present study based on the findings of Lehninger et al (1978) and Christiansen (1977). Cyclic AMP caused Ca¬¨‚⧂ÄövÖ‚à´ release provided that palmitoyl CoA or palmitoylcarnitine were substrates and that the mitochondria prepared from fed rats were used within 1 hr after isolation. It was also noted in this study that the mitochondria isolated from starved rats had a lower NADH/NAD‚ÄövÖ‚à´ ratio and released their Ca¬¨‚⧂ÄövÖ‚à´ earlier than the mitochondria from fed rats. It was suggested that cAMP stimulates Ca¬¨‚⧂ÄövÖ‚à´ release from the mitochondria presumably by altering the redox state of the mitochondrial pyridine nucleotides.