Zusammenfassung
Die Homöothermie steht am Ende eines Evolutionsprozesses, in dem jeder Anstieg des Sauerstoffangebots mit einer Zunahme des Energieumsatzes beantwortet und damit auch neue Abhängigkeiten geschaffen wurden. In Anpassung an die winterliche Nahrungsknappheit hat sich in gemäßigten Zonen der Winterschlaf entwickelt, der mit einer kontrollierten Absenkung der Körpertemperatur bis nahe 0°C einhergeht. Er bildet somit das eindrucksvollste Beispiel dafür, dass bei Homöothermen nicht nur die Körperschale poikilotherm ist, sondern auch die Körperkerntemperatur den Umgebungsbedingungen variabler angepasst werden kann, als vielfach angenommen. Allerdings stellen die natürlichen Torpiditätszustände anders als die klinische Hypothermie keine kältebedingte Stoffwechseldrosselung, sondern eine endogene Umsatzreduktion mit konsekutiver Temperatursenkung dar. Als stoffwechselsuppressiver Faktor wurde u. a. der pH-Wert diskutiert, der bei winterschlafenden Säugetieren – im Gegensatz zu den passiven Fluktuationen in der poikilothermen Körperschale (α-stat) – durch relative Hypoventilation bei 7,4 konstant gehalten wird (pH-stat). Bei der klinischen Hypothermie bestimmt die Temperatur insofern über die Umsatzrate, als es je nach Erhalt oder Ausschaltung der Thermoregulation entweder zu einer kältegegenregulatorischen Steigerung (akzidentelle Hypothermie) oder zu einer kältebedingten Drosselung der Stoffwechselrate (induzierte Hypothermie) kommt. Dagegen wird die absolute Hypothermietoleranz – wie ebenfalls am Beispiel des Winterschlafes erkennbar wird – eher von einem einheitlichen Minimalumsatz als von der artspezifischen Körpertemperatur bestimmt, bei der dieser je nach Körpergröße und Grundumsatz erreicht wird. Dementsprechend sollte die Hypothermie auch weniger nach dem Grad der Abkühlung als vielmehr nach den metabolischen Auswirkungen beurteilt werden, die sie in Abhängigkeit von den jeweiligen Rahmenbedingungen hervorruft.
Abstract
Homeothermy is the result of an evolutionary process during which every increase in oxygen supply led to a consecutive increase in metabolic rate and, thus, to a new dependence on favorable ambient conditions. In response to the food scarcity of winter months, some inhabitants of temperate zones developed an ability to hibernate which is characterized by a fully thermocontrolled reduction in body temperature down to near zero values. Hibernation thus illustrates that in homeotherms, not only the body shell is poikilothermic, but also the core temperature is more variable than often assumed. However, in contrast to clinical hypothermia, natural torpidity does not consist of a cold-induced reduction in metabolic rate, but of an endogenous metabolic reduction with subsequent lowering of body temperature. As a factor of metabolic suppression, the pH has been suspected which, in hibernators, is kept constant at 7.4 by relative hypoventilation (pH-stat) which differs from its passive shift in the poikilothermic body shell (α-stat). In clinical hypothermia, temperature governs the metabolic rate in that, depending on the state of thermoregulation, either a cold defense reaction with an increased metabolic rate (accidental hypothermia) or a cold-induced reduction in metabolic rate (induced hypothermia) occurs. However, as can be learned from hibernators, the lower limit of hypothermia tolerance seems to be due to a uniform minimal metabolic rate rather than to the species-specific body temperature at which this metabolic limit is reached, depending on body size and basal metabolic rate. Accordingly, in judging the sequelae of hypothermia, the degree of cooling should be given less emphasis than the resulting effects on metabolic rate.
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Singer, D. Warum 37°C?. Anaesthesist 56, 899–906 (2007). https://doi.org/10.1007/s00101-007-1220-y
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DOI: https://doi.org/10.1007/s00101-007-1220-y