The uptake and release of calcium by heart mitochondria

Abstract

The kinetics of uptake of Ca²⁺ by rat heart mitochondria were studied by a spectrophotometric method with Arsenazo III indicator. The exponential rate coefficients measured with or without added phosphate increase with the amount of Ca²⁺ added up to about 24μm. Evidence is given that the effect is attributable to a combination of formation of chelates at low concentrations to act as Ca²⁺ buffers, with co-transport of substrate to provide more respiratory fuel. The inhibitory effect of Mg²⁺ depends on the Ca²⁺ concentration, so with a constant [Mg²⁺] the low concentrations of Ca²⁺ are most inhibited, and the rate coefficients are still more Ca²⁺-dependent. Ca²⁺ uptake is slowed by local anaesthetics such as butacaine and dibucaine, and also by propranolol and palmitoyl-CoA. After an uptake, the release of Ca²⁺ was investigated. The spontaneous release involves an initially slow and small appearance of free Ca²⁺ and is followed by an auto-accelerated phase. The release is accompanied by a gradual decrease in internal ATP; it is initiated by palmitoyl-CoA (reversed by carnitine), by lysophosphatidylcholine, by Na⁺ salts (reversed by oligomycin) and by K⁺ salts added to a K⁺-free medium containing valinomycin. The process is probably a response to an increased energy load imposed on the mitochondria by the various conditions, which include the spontaneous action of phospholipase activated by traces of Ca²⁺. The problem of how much mitochondrial activity is participating in normal heart Ca²⁺ turnover is discussed, and experiments showing only 7–14% exchange of the mitochondrial Ca²⁺ occurring in vivo in 10 or 20min are reported.