Temperature dependence of coupled and uncoupled oxidations in brain synaptosomes from hibernators and non-hibernators

In the presence of saturating glucose concentrations, Arrhenius plot for gl ucose oxidation in the ground squirrel synaptosomes showed no discontinuity , while a sharp break was present when rat synaptosomes were used. The addi tion of 2,4-dinitrophenol (DNP), a potent uncoupler of oxidative phosphoryl ation, resulted in an increase of glucose oxidation rates. The stimulatory effect was significantly higher in the case of hibernator's synaptosomes, a nd a break occurred in the Arrhenius plot for both species. Multiple-dose e xperiments with DNP provided bell-shaped dose-response profiles, quantitati vely defined by an optimal stimulatory dose (EDopt) and the corresponding m aximum stimulatory effect (S-max). Cooling from 32 degrees to 5 degreesC re sulted in the decrease of both EDopt and S-max (shifting to the left and fl attening of the dose-response profiles). The effect of temperature was enha nced by factors capable of further reducing the rate of ATP generation (low substrate concentration), and opposed by factors leading to reduced rates of ATP utilization (suppression of active transport by the elimination of s odium ions in the medium). In comparison to rat darn, ground squirrel synap tosomes showed a markedly increased responsiveness to DNP stimulation (much higher S-max), and an increased resistance to cold-induced alterations of the dose-response profiles. In addition, the use of DNP and low incubation temperature (5 degreesC) revealed differences between active and actually h ibernating hibernator, in correlation with data on their respective toleran ces to reduced body temperature.