METABOLIC AND ENERGY CORRELATES OF INTRACELLULAR PH IN PROGRESSIVE FATIGUE OF SQUID (L-BREVIS) MANTLE MUSCLE

Squid (Lolliguncula brevis) were exercised at increasing swimming spee ds to allow us to analyze the correlated changes in intracellular meta bolic, acid-base, and energy status of the mantle musculature. Beyond a critical swimming velocity of 1.5 mantle lengths/s, an intracellular acidosis developed that was caused by an initial base loss from the c ells, the onset of respiratory acidification, and, predominantly, octo pine formation. The acidosis was correlated with decreasing levels of phospho-L arginine and, thus, supported ATP buffering at the expense o f the phosphagen. Monohydrogenphosphate, the actual substrate of glyco gen phosphorylase, accumulated, enabling glycogen degradation, despite progressive acidosis. In addition to octopine, succinate, and alpha-g lycerophosphate accumulation, the onset of acidosis characterizes the critical velocity and indicates the transition to a non-steady-state t ime-limited situation. Accordingly, swimming above the critical veloci ty caused cellular energy levels (in vivo Gibbs free energy change of ATP hydrolysis) to fall. A minimal value was reached at about -45 kJ/m ol. Model calculations demonstrate that changes in free Mg2+ levels on ly minimally affect ATP free energy, but mimumum levels are relevant i n maintaining functional concentrations of Mg2+-complexed adenylates. Model calculations also reveal that phosphagen breakdown enabled L. br evis to reach swimming speeds about three times higher than the critic al velocity. Comparison of two offshore squid species (Loligo pealei a nd Ilex illecebrosus) with the estuarine squid L. brevis indicates tha t the latter uses a strategy to delay the exploitation of high-energy phosphates and protect energy levels at higher than the minimum levels (-42 kJ/mol) characterizing fatigue in the other species. A more econ omical use of anaerobic resources and an early reduction in performanc e may enable L. brevis to tolerate more extreme environmental conditio ns in shallow estuarine waters and even hypoxic environments and to pr event a fatal depletion of energy stores.