Abstract: | The presence of Ca2+ in a hypo-osmotic reaction medium reducessuccinate: cytochrome c reductase activity and the release ofouter membrane-specific antimycin A-insensitive NADH: cytochromec reductase. The action of Ca2+ is non-competitive and approximately30 mmol m3 Ca2+ affords half-maximal (I50) protection.The effect of a range of inorganic and organic multivalent cationson succinate: cytochrome c reductase activity suggests thatthe action of Ca2+ is non-specific and probably involves Ca2+binding to outer membrane component(s) which may be proteins. Valinomycin- or gramicidin-induced passive swelling of isolatedcorn mitochondria in isotonic K.C1 is also non-competitivelyinhibited by up to 50% with Ca2+. Half-maximal inhibition (I50)occurs at 0-35 mol m3 Ca2+ for valinomycin and 1-0 molm3 Ca2+ for gramicidin. Other divalent cations, Mg2+,Sr2+ and Ba2+, seem to inhibit similarly while the trivalentcations La3+ and Ho3+ show a maximum inhibition of up to 85%,with an I50 of 0.1 mol m3 for valinomycin. It is suggestedthat non-specific cation binding may reduce membrane fluiditythereby slowing down the rate of ionophore penetration throughthe inner membrane. Key words: Calcium, Mitochondria, Membranes |