BIOCHEMICAL AND ULTRASTRUCTURAL ASPECTS OF CA2+ TRANSPORT BY MITOCHONDRIA OF THE HEPATOPANCREAS OF THE BLUE CRAB CALLINECTES SAPIDUS

Mitochondria isolated from the hepatopancreas of the blue crab Callinectes sapidus show up to 12-fold stimulation of respiration on addition of Ca²⁺, which is accompanied by Ca²⁺ accumulation (Ca²⁺:site = 1.9) and H⁺ ejection (H⁺:Ca²⁺ = 0.85). Sr²⁺ and Mn²⁺ are also accumulated; Mg²⁺ is not. A strongly hypertonic medium (383 mosM), Mg²⁺, and phosphate are required for maximal Ca²⁺ uptake. Ca²⁺ uptake takes precedence over oxidative phosphorylation of ADP for respiratory energy. Once Ca²⁺ is accumulated by the crab mitochondria, it is stable and only very slowly released, even by uncoupling agents. ATP hydrolysis also supports Ca²⁺ uptake. Respiration-inhibited crab hepatopancreas mitochondria show both high-affinity and low-affinity Ca²⁺-binding sites, which are inactive in the presence of uncoupling agents. Crab hepatopancreas mitochondria have an enormous capacity for accumulation of Ca²⁺, up to 5,500 ng-atoms Ca²⁺ per mg protein, with an equivalent amount of phosphate. Freshly isolated mitochondria contain very large amounts of Ca²⁺, Mg²⁺, phosphate, K⁺, and Na⁺; their high Ca²⁺ content is a reflection of the vary large amount of extra-mitochondrial Ca²⁺ in the whole tissue. Electron microscopy of crab mitochondria loaded with Ca²⁺ and phosphate showed large electron-dense deposits, presumably of precipitated calcium phosphate. They consisted of bundles of needle-like crystals, whereas Ca²⁺-loaded rat liver mitochondria show only amorphous deposits of calcium phosphate under similar conditions. The very pronounced capacity of crab hepatopancreas mitochondria for transport of Ca²⁺ appears to be adapted to a role in the storage and release of Ca²⁺ during the molting cycle of this crustacean.