CELLULAR METABOLIC HOMEOSTASIS DURING LARGE-SCALE CHANGE IN ATP TURNOVER RATES IN MUSCLES

The term homeostasis traditionally refers to the maintenance of a rela tively constant internal milieu in the face of changing environmental conditions or changing physiological function. Tissues such as skeleta l and cardiac muscles must sustain very large-scale changes in ATP tur nover rate during equally large changes in work. In many skeletal musc les, these changes can exceed 100-fold. In unique biological circumsta nces (for example, during periods of oxygen limitation, vasoconstricti on and hypometabolism), tissues such as skeletal muscles may be oblige d to sustain further decreases in ATP turnover rates and operate for v arying periods at seriously suppressed ATP turnover rates. Examination of a number of cellular and whole-organism systems identifies ATP con centration as a key parameter of the interior milieu that is nearly un iversally 'homeostatic'; it is common to observe no change in ATP conc entration even while the change in its turnover rate can increase or d ecrease by two orders of magnitude. A large number of other intermedia tes of cellular metabolism are also regulated within narrow concentrat ion ranges, but none seemingly as precisely as is [ATP]. In fact, the only other metabolite in aerobic energy metabolism that is seemingly a s 'homeostatic' is oxygen - at least in working muscles, regulatory qu estion is how such homeostasis intermediates in pathways of energy sup ply and energy demand is achieved.