Calcium transport in rat liver mitochondria

In the present study, it was found that various components of the rat cytosol affect Ca²⁺ transport into and out of mitochondria when examined in vitro. The ³H-palmitate-binding protein when saturated with palmitate caused Ca²⁺ release. The Ca-binding protein and albumin helped prolong Ca²⁺ retention in the mitochondria. Palmitoyl CoA and palmitate caused release of Ca²⁺ from preloaded mitochondria. On the other hand palmitoylcarnitine caused Ca²⁺ retention. The mitochondria were able to accumulate 4 - 9 % of the externally added Mg²⁺. The presence of varying concentration of external Mg²⁺ did not inhibit or increase Ca²⁺ uptake by the mitochondria, however, the extra Mg²⁺ accumulated prolonged Ca²⁺ retention in the mitochondria. During Ca²⁺ release from the mitochondria, there was a corresponding release of mitochondrial Mg²⁺. During Ca²⁺ retention, the level of mitochondrial Mg²⁺ remained steady. ATP or respiratory substrate was required for Ca²⁺ uptake. The presence of both respiratory substrate and ATP favoured Ca²⁺ retention. During Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ release and a relatively reduced state favours Ca²⁺ retention) was confirmed and further extended in this study. The adenine nucleotides were not significantly different from the control during Ca²⁺ 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 Ca²⁺ retention in the mitochondria, caused the Ca²⁺ released from the mitochondria by oxaloacetate to be taken up again. Externally added NAD⁺ and possibly NADH prolonged Ca²⁺ retention in the mitochondria. On the other hand, NADPH caused an earlier Ca²⁺ release. NADP⁺ did not affect Ca²⁺ uptake or release, The effect of cAMP on Ca²⁺ release was reinvestigated in the present study based on the findings of Lehninger et al (1978) and Christiansen (1977). Cyclic AMP caused Ca²⁺ 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⁺ ratio and released their Ca²⁺ earlier than the mitochondria from fed rats. It was suggested that cAMP stimulates Ca²⁺ release from the mitochondria presumably by altering the redox state of the mitochondrial pyridine nucleotides