The relationship between volume and oxidative activity in flight muscle mitochondria

Abstract

Light-scattering and gravimetric studies revealed that insect flight muscle mitochondria exhibited spontaneous as well as induced changes in volume. Of particular significance was the observation that H +, in contrast to other swelling agents employed, caused a rapid and possibly ‘all-or-none’ type of swelling in which the whole population of mitochondria appeared to swell simultaneously. This type of induced swelling appeared to differ from other types of swelling in that it was immediately followed by a spontaneous and partial contraction. The spontaneous contraction, however, was completely abolished if sucrose was present in the medium. Complete contraction of H + swollen mitochondria could be achieved: (a) if the combination of agents, ATP + Mg 2+ + BSA were added to the suspension of swollen mitochondria; (b) if the H +-induced electrochemical gradient was destroyed by the addition of Tris buffer, pH 7·4. Studies correlating changes in mitochondrial volume with changes in mitochondrial oxidative activity revealed that no simple relationship exists between mitochondrial volume and in vitro mitochondrial function. For example, spontaneous swelling resulted in an almost complete loss of pyruvate oxidation but had little or no effect on α-glycero-P oxidation. This observation suggests that the basis for the often-cited discrepancies between rates of pyruvate and α-glycero-P oxidation exhibited by flight muscle mitochondria may be prolonged mitochondrial swelling, which appears to exert a selectively deleterious effect on pyruvate but to a much lesser extent on α-glycero-P oxidation. These studies also revealed that coincident with the initial stages of induced mitochondrial swelling a ‘jump’ in mitochondrial O 2 consumption occurred. Frequently, the jump in O 2 consumption was followed by a re-establishment of pre-swelling rates of O 2 consumption. These transitions in metabolic state of the mitochondria, however, were variable and critically influenced by such factors as the composition of the assay medium, the substrate, and swelling agent employed. Some evidence has been obtained from these studies to suggest that reversible changes in mitochondrial volume, as a result of alterations in [H +], may play an important rôle in modulating the oxidative activity of insect flight muscle mitochondria during periods of rest and activity. A working hypothesis in which [H +] and changes in mitochondrial volume are critical determinants in regulating the metabolism of insect flight muscle mitochondria is presented.