The relationships between membranes and intramembrane compartments of isolated heart mitochondria are inadequately defined to express the induced morphological changes associated with the structural organization. The inner membrane and matrix are the major structural entities which undergo transformation upon alteration of metabolism or incubation conditions. To better express these morphological changes within a mitochondrion, two inner membranes plus enclosed matrix are defined as an inmerix (plural inmerices). Three general morphological forms of mitochondria can be distinguished by the size and shape of inmerices. These are distended, condensed, and coalesced inmerixal configurations. Hypotonic conditions and Pi in isotonic sucrose generate distended configurations. This Pi distention is apparently dependent on utilization of energy. It does not occur under anaerobic conditions. Oxidizable substrates generate condensed configurations. ADP and dADP generate coalesced configurations and stop formation of condensed and distended inmerixal configurations in the absence of inhibitors. ADP coalescence is apparently not dependent on an energy input. It occurs under aerobic and anaerobic conditions, and in isotonic and hypotonic media. Atractyloside completely inhibits the effects of ADP on inmerixal membranes whereas oligomycin does not. Distention by Pi is unaffected by the two inhibitors. Distended inmerices, without added Pi (12 mM and 62 mM sucrose), are coalesced by ADP. These studies indicate that coalescence of inmerixal membranes probably reflects the consequences of specific stoichiometric binding or translocation of adenine nucleotides.

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