Sunhinary. A survey has been ii)ade of the p)rol)erties of cori wlitoc(hon(rlia in .swelling and contraction. The mitochoni(lria swell spontalneouisly in KCl but not in sucrose.Aged mitochondria will swell rapidly in sucrose if treate(l Nvith citrate or ED'FA. Swelling does not impair oxidative phosphorylation if bovine ser-niiii albumin is p)resellt. ( 10) studie(l the relationshil) between swelling and( su ccinate oxidationi of lupine mitoclhonidria uisinig both l)acked volunm.e anid light-scatterinig measuremlienits.\Vhile packed volume measuremlienits indicated that succiniate somiietimes prevente(l osmlotically induced swelling and induced the mitochondria to contract, parallel optical studies indicated that succinate in-(luced the mlitochondria to sewell. Osmliotic swelling brought abouit either anl activation or inhibition of succiniate oxidation depending onl the concentration of succinlate used. These studies l)ointed out the need for a more complete characterization of swelling and contraction phenomenia in p)lanit miitochondria. Lyons and Pratt (18)

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Our laboratory has been investigating membranelinked functions of plant mitochondria which might help explain active ion transport. Corn mitochondria will actively accumulate calcium + phosphate in a process very like that of animal mitochondria (9); i.e., a high energy intermediate derived from substrate oxidation or ATP furnishes the energy.'With the substrate-powered system, there appears to be a competition between uptake and ATP formation, for the addition of ADP + hexokinase trap diminished calcium uptake. However, unlike animal mitochondria, the corn mitochondria proved to require phosphate for calcium uptake. Furthermore, the accumulation of magniesium and phosphate could not be demonstrated unless some calcium was present.These experiments were done with low concentrations of calcium (0.1-0.2 mm) in order to avoid uncoupling the mitochondria. We had observed that concentrations of calcium of 1 mM and higher reduced P/O ratios, and we assumed that calcium was acting as an uncoupling agent, a view commonly expressed on the basis of experiments with mammalian mitochondria. Calcium has been long known to stimulate acceptorless respiration of animal mitochondria (16,17) and to induce adenosine triphosphatase activity (13), thus mimicking the action of the uncoupling agent, 2,4-dinitrophenol (DNP). Calcium also accelerates the swelling of animal mitochondria, which is thought to reflect a calcium-stimulated release of fatty acids, or U-factor (10).More recent work suggests that the action of calcium is more complex than initially visualized. Chance (2)

Mg '~ § at an optimal concentration of 2 mM (pH 6.5) induces large increases (up to 30%) in the optical density of bovine heart mitochondria incubated under conditions of low ionic strength (< approx. 0.01). The increases are associated with aggregation (sticking together) of the inner membranes and are little affected by changes in the energy status of the mitochondria. Virtually all of a number of other polyvalent cations tested and Ag § induce increases in mitochondrial optical density similar to those induced by Mg e+, their approximate order of concentration effectiveness in respect to Mg "+ being: La 3+ > Pb "+ = Cu "+ > Cd 2+ > Zn "+ > Ag + > Mn 2+ > Ca .'+ > Mg "+. With the exception of Mg 2+, all these cations appear to induce swelling of the mitochondria concomitant with inner membrane aggregation. The inhibitors of the adenine nucleotide transport reaction carboxyatractyloside and bongkrekic acid are capable of preventing and reversing Mg"+-induced aggregation at the same low concentration required for complete inhibition of phosphorylating respiration, suggesting that they inhibit the aggregation by binding to the adenine nucleotide carrier. The findings are interpreted to indicate (a) that the inner mitochondrial membrane is normally prevented from aggregating by virtue of its net negative outer surface charge, (b) that the cations induce the membrane to aggregate by binding at its outer surface, decreasing the net negative charge, and (c) that carboxyatractyloside and bongkrekic acid inhibit the aggregation by binding to the outer surface of the membrane, increasing the net negative charge.KEY WORDS heart mitochondria inner membrane aggregation 9 polyvalent cations 9 carboxyatractyloside 9 bongkrekic acid 9 surface chargeIn the course of studies on the effects of polyvalent metal cations on adenine nucleotide-induced inner membrane contraction in heart mitochondria by the optical density (OD) method (28), we encountered considerable interference from rapid and extensive increases in mitochondrial OD induced by the cations. Increases of up to 40% were observed and appeared to be largely reversed upon induction of inner membrane contraction.