Fuels and pathways as designed systems for support of muscle work

Abstract

Muscle in all animals relies upon four potential sources of energy: ATP hydrolysis, phosphagen hydrolysis, fermentations or oxidative metabolism. Although the relative contributions of different fuels varies greatly in different organisms, in none is there a simple reliance on stored ATP. Muscle work therefore requires a balance between rates of utilization and formation of ATP, a provision supplied by one of the three remaining fuels and metabolic pathways. Useful endogenous fuels must be storable at high level, and rapidly mobilizable with minimal perturbation of [ATP] and with minimal end-product effects on pH, charge or osmotic balance. In addition to displaying these properties, good exogenous fuels must be transferable at high rates between depot sites and muscle; actual flux rates of exogenous fuels depend upon respective ATP yields and are lowest for fuels which most amplify the yield of ATP per mol substrate oxidized. Substrate flux rates must be matched with O2 flux rates and with rates of endogenous substrate mobilization in order that the right energy-yielding pathways are activated at the right times. Of various control possibilities, an effective competition for ADP (and possibly Pi) seems at this time to be the dominant strategy for assuring integration of aerobic and anaerobic ATP-yielding pathways.

The Glucose–Succinate Pathway: A Crucial Anaerobic Metabolic Pathway in the Scallop Chlamys farreri Experiencing Heat Stress

“…It has been suggested that the OpDHs pathway serves as a major pathway of anaerobic metabolism for energy demand in the early stages of hypoxia stress in marine invertebrates, confirmed by the level of octopine increased gradually after anaerobic stress [13,17,35,44,56]. Some gastropod mollusks, such as C. toreuma, also relied on the OpDHs pathway for anaerobic metabolism under heat stress.…”

“…Both LDH and OpDH pathways exhibited low efficiency yet high rates of energy production, thereby ensuring a constant flux of glycolysis and a steady supply of ATP by regulating the NADH/NAD+ ratio during hypoxia [23,34]. Increasing proline levels in M. edulis under short-term anaerobic stress could provide substrates for the glucose-opine pathway [13,17,35,36,44]. In addition, there was a study that proposed that the lactate pathway may have originated from opine pathways [13].…”

“…Within the GTP-dependent class, there are cytosolic isoenzyme PEPCK-C and mitochondrial isoenzyme PEPCK-M based on their subcellular localization [31][32][33]. Until now, the primary anaerobic metabolism pathway in mollusks remains incompletely elucidated, with the majority of research on the anaerobic metabolism of marine invertebrates concentrating on enzyme activity and end products of anaerobic metabolism [12,21,23,[34][35][36][37][38]. There exists a significant deficiency in systematic and comprehensive research regarding the anaerobic metabolism mechanisms in marine mollusks and their reaction to heat stress, which could be addressed through the rapidly progressed genome and transcriptome datasets.…”

Exercise and emersion in air and recovery in seawater in the green crab (Carcinus maenas): Effects on nitrogenous wastes and branchial chamber fluid chemistry

“…There appear to no reports on urate‐N excretion rates, and no reports on N‐waste excretion after exercise in this species. Exercise is of particular relevance to nitrogen metabolism because of the potential liberation of N‐wastes from increased rates of amino acid oxidation and adenylate breakdown (Hochachka, 1985). In the blue crab ( Callinectes sapidus ), ammonia‐N excretion increased greatly during enforced swimming activity in seawater (Booth et al, 1984; Milligan et al, 1989).…”