1. Blowfly flight-muscle mitochondria respiring in the absence of phosphate acceptor (i.e. in state 4) take up greater amounts of K+, Na+, choline, phosphate and Cl-(but less NH4+) than non-respiring control mitochondria. 2. Uptake of cations is accompanied by an increase in the volume of the mitochondrial matrix, determined with the use of [14C]-sucrose and3H2O. The osmolarity of the salt solution taken up was approximately that of the suspending medium. 3. The [14C]sucrose-inaccessible space decreased with increasing osmolarity of potassium chloride in the suspending medium, confirming that the blowfly mitochondrion behaves as an osmometer. 4. Light-scattering studies showed that both respiratory substrate and a permeant anion such as phosphate or acetate are required for rapid and massive entry of K+, which occurs in an electrophoretic process rather than in exchange for H+. The increase in permeability to K+and other cations is probably the result of a large increase in the exposed area of inner membrane surface in these mitochondria, with no intrinsic increase in the permeability per unit area. 5. No increase in permeability to K+and other cations occurs during phosphorylation of ADP in state 3 respiration.
Flight-muscle mitochondria of Lucilia cuprina and Musca domestica. Estimation of the pyridine nucleotide content and of the response of respiration to adenosine diphosphate
