Phylogenese des Stoffwechsels der Säugetiere

Abstract

Mammals are at the end of a gradual metabolic evolution in the course of which the step from anaerobic to aerobic cellular metabolism and the transition from water to land life formed the basis for an increase in metabolic rate (from brady- to tachymetabolism). The increased metabolic rate and the resulting endogenous heat production were the preconditions for enhanced long-term performance as well as for homeothermy which allowed mammals and birds to invade temperate zones. However, the underlying increase in membrane permeability also results in an increased energy demand (for membrane pump activity) which leads to the reduced hypoxia tolerance of mammals and requires a permanent substrate supply. As an adaptation to a seasonal discrepancy between increased thermoregulatory energy demand and decreased food supply, some small mammals apparently extended the newly evolved non-REM-sleep into hibernation. Mammalian hibernation is characterized by a profound metabolic reduction which is influenced by acidosis and limited to a tolerable degree by maintained thermoregulation. The lower limit of cooling seems to be determined by a critical minimal metabolic rate which is common to all mammals. The higher the normothermic metabolic rate, the lower is the temperature at which this minimal metabolic rate is reached. Since specific (i. e., weight-corrected) basal metabolic rate increases with decreasing body mass, small mammals exhibit a higher hypothermia tolerance than larger ones. On the other hand, the metabolic decrease to a uniform minimal level reflects an inactivation of the overall metabolic size relationship and, thus, forms a counterpart to the metabolic increase from a lower fetomaternal to the higher size-related level, occurring after birth. The postnatal metabolic increase which favours the onset of thermoregulation, parallels the increase in oxygen tension at the transition from fetal to adult circulation and, thus, probably enables mammalian neonates to readjust their metabolic needs in response to hypoxia. There is increasing evidence that, similar to the step from anaerobiosis to aerobiosis, the increase in metabolic rate resulting from any increase in oxygen supply is a general principle of evolution that, apart from its further adaptive benefits, protects tissues from oxygen excess and subsequent oxidative stress.

Pediatric Hypothermia: An Ambiguous Issue

“…When the cold defense is overwhelmed or exhausted, a drop in body temperature occurs, which is referred to as accidental hypothermia in contrast to the induced hypothermia in cardiac surgery or neurocritical care. Whereas in the latter, a direct temperature-dependent decrease in metabolic rate (by a factor of 1/2.0–1/2.5 per 10 °C drop in body temperature) is achieved by pharmacological suppression of thermoregulation, the temperature decline in accidental hypothermia is preceded by a maximal (i.e., roughly five-fold) increase in heat production rate [ 5 , 6 ]. However, especially in children, the boundary between the two types of hypothermia may be blurred: For example, when caregivers keep pediatric critical care patients “a little cooler” in an attempt to spare their metabolism, this can actually result in unintended cold stress due to the hidden onset of thermogenesis.…”

“…Although there is no strict correlation between the (endogenous) metabolic reduction and the (concomitant) temperature decline in hibernators (both in hazel mice and in black bears, the magnitude of metabolic reduction exceeds the pure temperature effect), this relationship suggests an increasing hypothermia tolerance with decreasing body mass among mammals. This may also explain why human infants (HI) and small children tend to tolerate a higher degree of cold-induced metabolic reduction than human adults (HA), owing to their larger metabolic “drop height” [ 5 , 6 ].…”

Warum 37°C?

“…Indessen hat Luft -trotz ihrer geringeren Wärmeleitfähigkeit ("Isolationswirkung") -eine erheblich geringere Wärmekapazi tät, sodass landlebende Wirbeltiere deut lich stärkeren Schwankungen der Umge bungstemperatur ausgesetzt sind als was serlebende. So hat der WasserLandÜber gang der Wirbeltiere zwar einerseits einen gewissen "Evolutionsdruck" in Richtung auf, andererseits aber -vor allem durch die gesteigerte O 2 Verfügbarkeit -auch güns tige Ausgangsvoraussetzungen für die Ent wicklung der Homöothermie (s. unten) mit sich gebracht [42,45].…”

“…Der Fetus erreicht noch nicht -wie das Neuge borene kurz nach der Geburt -einen sei ner geringeren Körpergröße entsprechend höheren spezifischen Energieumsatz, son dern verhält sich metabolisch vielmehr "wie ein Organ der Mutter" [34,45,46]. Hierbei scheint es sich um eine Anpas sung an die niedrige Sauerstoffspannung in utero zu handeln, die den unreifen Or ganismus offenbar vor toxischen Neben wirkungen schützen soll, aber nur mit einem entsprechend reduzierten Ener gieumsatz tolerabel ist.…”

Den Mangel überleben: Natürliche Anpassungen bei Neugeborenen

“…Denn nicht nur während der intrauterinen Periode, sondern auch und v. a. während der Geburt und der perinatalen Transition kann es zu Versorgungsengpässen kommen, die gerade für kleine Säugetiere besonders kritisch sind. Ausschlaggebend hierfür sind hauptsächlich 3 Gründe [1,2]:…”