Metabolism at the Max: How Vertebrate Organisms Respond to Physical Activity

Abstract

Activity metabolism is supported by phosphorylated reserves (adenosine triphosphate, creatine phosphate), glycolytic, and aerobic metabolism. Because there is no apparent variation between vertebrate groups in phosphorylated reserves or glycolytic potential of skeletal muscle, variation in maximal metabolic rate between major vertebrate groups represents selection operating on aerobic mechanisms. Maximal rates of oxygen consumption in vertebrates are supported by increased conductive and diffusive fluxes of oxygen from the environment to the mitochondria. Maximal CO2 efflux from the mitochondria to the environment must be matched to oxygen flux, or imbalances in pH will occur. Among vertebrates, there are a variety of modes of locomotion and vastly different rates of metabolism supported by a variety of cardiorespiratory architectures. However, interclass comparisons strongly implicate systemic oxygen transport as the rate-limiting step to maximal oxygen consumption for all vertebrate groups. The key evolutionary step that accounts for the approximately 10-fold increase in maximal oxygen flux in endotherms versus ectotherms appears to be maximal heart rate. Other variables such as ventilation, pulmonary/gill, and tissue diffusing capacity, have excess capacity and thus are not limiting to maximal oxygen consumption. During maximal activity, the ratio of ventilation to respiratory system blood flow is remarkably similar among vertebrates, and CO2 extraction efficiency increases while oxygen extraction efficiency decreases, suggesting that the respiratory system provides the largest resistance to maximal CO2 flux. Despite the large variation in modes of activity and rates of metabolism, maximal rates of oxygen and CO2 flux appear to be limited by the cardiovascular and respiratory systems, respectively.

What drove the evolution of endothermy?

“…The implication from the Bennett and Ruben paper is that ectothermic vertebrates derive a greater fraction of their total energy from anaerobic contributions compared with endotherms. Recent analyses confirm this implication and indicate that anaerobic metabolism starts to be used in conjunction with aerobic metabolism at between 50 and 70% of V˙O 2,max in all vertebrate classes (see Hedrick et al, 2015). Anaerobic capacity does not seem to vary between vertebrate classes .…”

“…This paper personally shaped for us an ongoing interest in understanding what limits aerobic metabolism of vertebrates from a physiological perspective, starting in the early 1980s at Portland State University when Hillman was an Assistant Professor and Hedrick was a Master's student (e.g. Hillman et al, 1985;Withers and Hillman, 1988;Hedrick et al, 2015). This subsequently led us to a broader level of thinking about how aerobic capacity relates to endurance and dispersal using aerobic capacity and metabolic cost of transport to define physiological vagility, from ecological and evolutionary perspectives (e.g.…”

“…Recent evidence suggests that the delivery of oxygen by the cardiovascular system represents the fundamental rate limitation to maximal oxygen consumption and aerobic metabolism in all vertebrates (Hillman et al, 2013;Hedrick et al, 2015). The remaining convective (ventilation) and diffusive (respiratory and mitochondrial) steps in the oxygen cascade appear to have excess capacity and are therefore not limiting.…”

Fulminante Laktatazidose bei kokaininduziertem agitiertem Delir

“…Infolge des intrazellulären O 2 -Defizits steigt der Anteil der energetisch ineffizienten anaeroben Glykolyse mit Akkumulation von Laktat (Typ-A-Laktatazidose). Bei hochintensiver Muskelarbeit ("all-out"-exercise) kann die Laktatproduktion kurzfristig um einen Faktor > 100 steigen, während gleichzeitig die hepatische Laktat-Clearance sinkt [17,22]. Patienten im agitierten Delir kämpfen mitunter "bis zum Umfallen" gegen die Fixierung.…”

Weighing the evidence for using vascular conductance, not resistance, in comparative cardiovascular physiology

“…Cardiac output (systemic blood flow) is regulated to meet oxygen demands, and can vary severalfold depending on the metabolic state of a given animal (Hedrick et al, 2015;Hillman and Hedrick, 2015). By contrast, arterial blood pressure changes relatively little between rest and exercise (Magder, 2018;Munch et al, 2014;Sandblom et al, 2005).…”