Higher environmental temperature-induced change in synaptosomal (Na(+)-K+)-ATPase activity of mammalian brain regions.

Exposure of adult male Albino rats to higher environmental temperature (HET) at 35 degrees for 2-12 h or at 45 degrees for 1-2 h decreases hypothalamic synaptosomal (NA(+)-K+)ATPase activity. Synaptosomal (Na(+)-K+)-ATPase activity in (a) CX of rats exposed to 35 degrees for 12 h, (b) CX and PM of rats exposed to 45 degrees for 1-2 h and also (c) in CS of rats exposed to 45 degrees for 2 h are inhibited. (Na(+)-K+)-ATPase activity of synaptosomes prepared from normal rat brain regions incubated at 39 degrees-43 degrees for 0.5-1 h decreased significantly in H and such inhibition was also observed in other brain regions (CX, CS and PM) after incubation of the tissue slices at 41 degrees-43 degrees for 0.5-1 h. Lineweaver-Burk plots indicate that (a) in vivo and in vitro HET-induced inhibition of brain regional synaptosomal (Na(+)-K+)-ATPase is coupled with a decrease in Vmax without any change in Km, (b) very high temperature (under in vitro condition) causes a decrease in Vmax with an increase of Km irrespective of the brain regions. Arrhenius plots show that there is a decrease in transition temperature (TT) in H of rats exposed to either 35 degrees or 45 degrees, whereas such decrease in TT of PM and CX regions are only observed after exposure at 45 degrees. These results suggest that heat exposure increases the lipid fluidity of synaptosomal membrane which may cause the conformational change of the membrane protein and hence inhibit the activity of synaptosomal (Na(+)-K+)-ATPase of brain regions.