The Energy-linked Reaction of Calcium with Mitochondria

Abstract

The reaction of divalent cations with mitochondria has been a subject of investigation from various standpoints for over 10 years (1). While a large number of contributions particularly from 1961 onward (see below) emphasized the accumulation of large amounts of calcium, this contribution and our previous work emphasize the reaction of low concentrations of divalent cations with phosphorylating mitochondria. These reactions lead to a short term activation of respiration that is followed by a restitution of respiratory control. Under these conditions, we found in 1955 the reaction of calcium to be similar to that with adenosine diphosphate (2) and unlike t.hat with uncoupling agents (but cf. References 3-5). Calcium activation of respiration in mitochondria was observed in 1953 by Siekevitz and Potter (3) and by Potter, Siekevitz, and Simonson (4)) who reported approximately 3-fold stimulation of respiration on addition of 500 PM calcium to rat liver mitochondria. This observation was confirmed by Lindberg and Ernster (6). However, there was no indication t,hat the respiratory activation was a reversible process, or that there was a calcium to oxygen stoichiometry. Also in 1953, Slater and Cleland (7) observed the tight binding of calcium to heart sarcosomes; the binding was so tight that the “isolated sarcosomes contained all the calcium in the heart muscle.” Our 1955 report (2) showed that the respiratory stimulation observed by Siekevitz and Potter (3) and by Potter et al. (4) was not the uncoupling event pictured by Lehninger (5, 8) in which Ca* acted like dinitrophenol and was not expended in the reaction. Instead it was found that the reaction involved calcium expenditure in the same sense as the reaction of mitochondria with ADP, providing tightly coupled mitochondria were supplemented with low concentrations of calcium (approximately 100 mpmoles per mg of protein). Respiration was stimulated to a level in excess of that obtained by the addition of ADP and phosphate in agreement with Potter et al. (4). But we found that the response resembled that of added ADP plus Pi in many respects, particularly in the proportionality between the duration of the interval of stimulated respiration and the amount of added calcium. Accompanying this interval of stimulated respiration were cycles of oxidation-reduction changes of pyridine nucleotide and other respiratory components that also closely resembled those caused by ADP plus Pi. Thus, the respiratory activation resembles the transition from the resting State 4 to active State 3 and the return to the resting state that is observed upon addition of small concentrations of ADP to tightly coupled mitochondria (State 4 to 3 to 4 transition) (9). All of the phenomena associated with the reaction of low concentrations of calcium with mitochondria