Cold sensitivity of heart rate and succinate oxidation by myocardial homogenates from hibernating ground squirrels and diet-treated rats

Mitochondrial membrane transitions in heart and other organs of a hibernator

Critical temperatures (T) for transitions in both lipid structure and enzyme function of mitochondrial membranes from liver, kidney, brown fat, and heart tissues were determined for the hibernator Spermophilus lateralis at two weekly intervals from early summer to late autumn and during hibernation. For all tissues T fell into one of three groups: those below 4 degrees C (the minimal level of accurate determination), those centered about a mean of 11.9 +/- 1.4 degrees C, and those centered about a mean of 20.9 +/- 1.8 degrees C. The T for tissues from torpid animals and from heart, at all sampling periods, was below 4 degrees C. For liver, kidney, and brown fat the mean T was approximately 21 degrees C in early summer but was lowered later in the season in a two-step process, falling to below 4 degrees C before the animals were exposed to cold and entering torpor. It is concluded that for mitochondria the thermal response of the membrane lipids is altered such that the transition in structure and function is always below the minimum body temperature likely to be experienced by this animal. Heart tissue is exceptional in that the transition is at a temperature consistent with a body temperature of torpor even in summer-active animals.

Dietary lipid modulation of rat liver mitochondrial succinate: cytochrome c reductase

Diets supplemented with high levels of either saturated fatty acids or unsaturated fatty acids were fed to adult rats for a period of 9 weeks and changes in the liver mitochondrial membrane phospholipid fatty acid composition and thermal behaviour of succinate: cytochrome c reductase were determined. The dietary treatment induced a change in the omega 6 to omega 3 unsaturated fatty acid ratio in the membrane lipids, with the ratio being highest with the unsaturated fatty acid and lowest with the saturated fatty acid diet. Arrhenius plots of succinate: cytochrome c reductase activity exhibited differences in both critical temperature (Tf) and Arrhenius activation energy (Ea) depending on the type of dietary treatment. The Tf was elevated from 23 degrees C in control to 32 degrees C in the saturated fatty acid-supplemented group. No significant effect on the Tf was observed in the unsaturated fatty acid-supplemented group however higher Ea values were observed due to the unsaturated fatty acid diet. The changes in succinate: cytochrome c reductase are probably due to changes in the lipid-protein interactions in the membrane, induced by the dietary lipid supplementation.

Concentrations of lactate and pyruvate and temperature effects on lactate dehydrogenase activity in the tissues of the big brown bat (Eptesicus fuscus) during arousal from hibernation

1. The rectal temperatures, steady state concentrations of lactate and pyruvate, and the LDH isoenzyme composition in the heart, liver, and pectoral muscle of hibernating and arousing Eptesicus fuscus were measured. 2. Bat rectal temperature increased from 8.86 to 33.1 degrees C during arousal. 3. During arousal, steady state concentrations of pyruvate and lactate increased significantly in the tissues, however they remained generally below the level necessary to saturate LDH at the respective temperature. 4. The activities of the two LDH isoenzymes, M4, the predominant form in bat liver, and H4, the main form in bat heart and pectoral muscle, show substrate-dependent temperature effects described by the equation, mu = (E beta S + E alpha K t)/(K t + S). 5. Temperature effects (mu) on bat LDH activity increased during arousal but remained significantly lower than mu determined at saturating concentrations of substrate (E beta). 6. The parameters E beta-E alpha, E alpha and K tau are particularly important in describing the temperature dependence of LDH activity in tissues of the arousing bat.