REACTION-DIFFUSION ANALYSIS OF THE EFFECTS OF TEMPERATURE ON HIGH-ENERGY PHOSPHATE DYNAMICS IN GOLDFISH SKELETAL-MUSCLE

Thermal acclimation results in dramatic changes in the fractional volu me of mitochondria within skeletal muscle of teleost fish. We investig ated the hypothesis that changes in mitochondrial volume represent a c ompensatory response to temperature-induced changes in intracellular d iffusion coefficients (D) of the high-energy phosphate compounds ATP a nd creatine phosphate (PCr). Using P-31 nuclear magnetic resonance spe ctroscopy, we determined D-PCr and D-ATP in goldfish (Carassius auratu s) skeletal muscle at 25 degrees C and 5 degrees C: D-PCr was 3.28+/-0 .18x10(-6) cm(2) s(-1) at 25 degrees C and 2.00+/-0.09x10(-6) cm(2)s(- 1) at 5 degrees C; D-ATP was 2.13+/-0.16x10(-6) cm(2)s(-1) at 25 degre es C and was estimated to be 1.30x10(-6)cm(2)s(-1) at 5 degrees C. The re was no evidence for an effect of acclimation temperature or fiber t ype on D-ATP or D-PCr. A mathematical reaction-diffusion model was use d to calculate profiles of [ATP], [PCr] and the free energy of ATP hyd rolysis (Delta G(ATP)) in activated goldfish muscle fibers at 5 degree s C and 25 degrees C. The results showed spatial and temporal constanc y of [ATP], [PCr] and Delta G(ATP) in red fibers at both temperatures, regardless of changes in acclimation temperature or mitochondrial den sity. The model also showed spatial and temporal constancy of [ATP] in white fibers at 5 degrees C and 25 degrees C, but gradients in [PCr] and Delta G(ATP) developed in white fibers under all conditions of tem perature and acclimation temperature. These gradients were attenuated in cold-acclimated animals by cold-induced increases in mitochondrial density. However, the model shows that the proximal stimulus for tempe rature-induced changes in mitochondrial volume density in muscle is no t a disruption in intracellular diffusion of high-energy phosphates.