ION TRANSPORT BY HEART MITOCHONDRIA .15. MORPHOLOGICAL CHANGES ASSOCIATED WITH PENETRATION OF SOLUTES INTO ISOLATED HEART MITOCHONDRIA

1. The morphology of isolated beef heart mitochondria has been compared with the water content, cation content, and mannitol-permeable water volume. Mitochondria suspended in isotonic potassium acetate take up K+ and acetate spontaneously by an energy-linked reaction. The ion uptake is accompanied by extensive swelling of the mannitol (or sucrose)-impermeable water volume and by a dilation of the matrix space in electron micrographs. A similar ion uptake, increase in mannitol-impermeable water, and dilation of the matrix is seen in isotonic potassium acetate in the absence of energy when the membrane is made permeable to K+ by the addition of gramicidin. 2. Expansion of the matrix and increases in the mannitol-impermeable water volume similar to those noted above for osmotic swelling in potassium acetate also occur when mitochondria are suspended in hypotonic solutions of sucrose or KCl. 3. In isotonic KCl the impermeant Cl- prevents the accumulation of K+ either by the energy-linked reaction or by the passive, gramicidin-induced reaction. The uptake of K+, increases in mannitol-impermeable water, and dilation of the matrix do not occur in this medium in the presence of a source of energy or in the absence of energy when gramicidin is added. Dilation of the matrix can be seen in isotonic KCl only when the membrane is made permeable to Cl- by either the addition of p-hydroxymercuribenzoate or by the combination of gramicidin and metabolic energy. 4. It can be concluded that the matrix space in electron micrographs correlates well with the mannitol-impermeable water volume which is observed in isotope experiments. It can also be concluded that the morphological changes reported here are closely related to osmotic factors. © 1969.