The effects of temperature and thermal acclimation upon the osmotic properties and nonelectrolyte permeability of liver and gill mitochondria from rainbow trout (Salmo gairdneri)

Thermal acclimation of rainbow trout (Salmo gairdneri) taken from 20°C to 7°C resulted in adaptation of mitochondrial function, as evidenced by increases in the specific activities of NADH‐ and succinate‐cytochrome c reductase of 1.93‐ and 2.7‐fold respectively. Mitochondria from both gill and liver obeyed the Boyle‐van't Hoff relationship in the range from 400 to 60 mosM. Thermal acclimation had no effect on the osmotic properties of liver mitochondria, whereas gill mitochondria from cold‐acclimated trout were more sensitive to osmotic swelling than mitochondria from warm‐acclimated individuals. The non‐electrolyte permeability of liver mitochondria was assessed by optically monitoring mitochondrial swelling rates in isosmotic solutions of urea, glycerol, mannitol and glucose. Two parameters of mitochondrial swelling were determined: (a) initial swelling rates, d(1/A)dt, and (b) swelling constants, k_s, derived from the time required to swell a fixed volume. Regardless of the assay temperature or the permeant employed, liver mitochondria from cold‐acclimated trout exhibited greater initial swelling rates than mitochondria from warm‐acclimated trout, indicating properties of temperature‐compensated permeability. The apparent ranking of nonelectrolyte permeabilities was urea > glycerol > mannitol > glucose. k_s values for urea and glycerol from cold‐acclimated trout were greater than values typical of warm‐acclimated populations; however k_s values for glucose and mannitol were not influenced by thermal acclimation. Regardless of the permeant considered, activation energies for k_s values were 3‐ to 5‐fold greater than those for initial swelling rates. The time course of mitochondrial swelling consists of two components, an initial rapid swelling phase characterized by a half life of 3–12 seconds, and a slower swelling phase characterized by a half life of 1–6 minutes. Initial swelling rates, which approximate the rapid swelling component, are considered to be the least ambiguous index of permeability, whereas k_s values are more complex and strongly influenced by the slower swelling component.