Intracellular convection, homeostasis and metabolic regulation

Abstract

In its historic definition, the term homeostasis refers to the constancy of the internal milieu in the face of external perturbations; the latter in principle may be caused by extracellular factors, or in the case we shall consider, by change in intracellular biological function. Of all the tissues in the vertebrate body, skeletal muscle displays the special quality of being able to routinely sustain very large changes in work and metabolic rates. Compared to the 1.5-to 2-fold differences in metabolic rates between resting and activated states, which is common to many tissues (liver and brain, to mention two), skeletal muscles in most animals must be able to sustain up to, or even more than, 100-fold changes in ATP turnover rates. Amongst vertebrate endotherms, the highest muscle metabolic rate (in the range of 600·µmol·ATP·g -1 ·min -1 ) appears to be that of hummingbird breast muscle during hovering flight, which is a rate over 500 times muscle RMR (Suarez, 1992;Suarez et al., 1990Suarez et al., , 1991. During muscle ischemia, hypoxemia or hypoxia, the metabolism of muscle, like that of many other tissues under conditions of oxygen lack, may need to sustain a suppression of metabolism even below resting rates (Hochachka and Guppy, 1987), thus extending even further the enormous range between the lowest and highest sustainable ATP turnover rates of this remarkable tissue.Current popular interpretations of such large scale differences in steady state energy turnover are regulated assume cybernetic feedback control circuitry. The standard

The proteomic response of cheliped myofibril tissue in the eurythermal porcelain crabPetrolisthes cinctipesto heat shock following acclimation to daily temperature fluctuations

“…Given that the temperature fluctuations applied in our experiment probably affected reaction rates and therefore caused high and fluctuating ATP consumption rates, we assume that arginine kinase's main function under these circumstances is the transfer of phosphoryl groups from arginine phosphate to MgADP -to maintain ATP levels and thereby energy homeostasis (Ellington, 2001;Hochachka, 2003). However, arginine kinase also affects glycolytic rates through the release of inorganic phosphate, which serves as (1) a substrate for glycogen phosphorylase, leading to the breakdown of glycogen to glucose-1-phosphate, and (2) a H + buffer during glycogenolysis and glycolysis (Ellington, 2001;Griffiths, 1981).…”

“…The proteomic fingerprint we identified suggests that such a network exists in claw muscle and that it is modified during temperature fluctuations and acute heat shock conditions that temporarily demand high ATP turnover. This further suggests that the energetics of temperature acclimation are dependent on the restructuring of the cellular architecture, specifically the microstructure of those networks that are able to maintain ATP concentrations despite greatly fluctuating rates of ATP consumption (Hochachka, 2003).…”

Cell Thermoregulation: Problems, Advances and Perspectives

“…It is a rule to think that diffusion and possibly convection are the primary means to passively remove the heat generated inside the cell (Hochachka, 2003).…”

The micro-architecture of the cerebral cortex: Functional neuroimaging models and metabolism

“…Therefore, the demands of homeostasis prevail versus metabolic regulation. The [s] 4 stability paradox-why most metabolite concentrations are homeostatic over large changes in pathway fluxes-constitutes a major problem in cellular metabolism and two models have been proposed for metabolic regulation so far (Hochachka, 2003): a) the classical [the cells behave like a watery bag of enzymes] and b) the alternative [3-dimensional order and structure of cells constrain metabolite movement and conversion]. Even though the classical model is consistent with the metabolite homeostasis, it fails while providing a global explanation for the [s] stability paradox.…”

“…dyneins, kinesins) tracks and the convection system acts as an over-riding aid mechanism which facilitates the enzymesubstrate encounter. Hochachka (2003) called attention to the fact that classical and alternative models of metabolic regulation have operated as 'two solitudes',, each considering the other incompatible with its own experimental modus operandi. Cellular metabolism is robustly organized among species, resulting in a network with heterogeneous scale-free design, where a few hubs play protagonist roles (Jeong et al, 2000; Table 1, supplementary material I).…”