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Anästhesiol Intensivmed Notfallmed Schmerzther 2002; 37(8): 441-460
DOI: 10.1055/s-2002-33168
Aktuelle Medizin und Forschung
© Georg Thieme Verlag Stuttgart · New York

Phylogenese des Stoffwechsels der Säugetiere

Phylogeny of Mammalian MetabolismD.  Singer
  • 1Universitäts-Kinderklinik Würzburg
Diese Arbeit ist meinem langjährigen Mentor in Fragen der vergleichenden Säugetierkunde, Herrn Prof. Dr.med. Hans-Jürg Kuhn, Anatomisches Institut der Universität und Deutsches Primatenzentrum Göttingen, in Dankbarkeit gewidmet.
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Publication History

Publication Date:
07 August 2002 (online)

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Zusammenfassung

Die Säugetiere stehen am Ende eines metabolischen Evolutionsprozesses, innerhalb dessen mit dem Schritt vom anaeroben zum aeroben Zellstoffwechsel und dem Übergang der Wirbeltiere vom Wasser- zum Landleben die Grundlagen für eine Steigerung des Energieumsatzes (vom Brady- zum Tachymetabolismus) geschaffen wurden. Die gesteigerte Stoffwechselrate und die damit verbundene endogene Wärmeproduktion bilden die Voraussetzungen für eine erhöhte aerobe Dauerleistungsfähigkeit und für die Homöothermie, die den Säugetieren und Vögeln die Besiedelung gemäßigter Klimazonen ermöglicht hat. Die zugrundeliegende Zunahme der Membranpermeabilität bedingt aber auch einen gesteigerten Energiebedarf (für die Membranpumpenaktivität), der für die verminderte Hypoxietoleranz von Säugetieren verantwortlich ist und eine permanente Substratzufuhr voraussetzt. In Anpassung an eine saisonale Diskrepanz zwischen erhöhtem thermoregulatorischem Energiebedarf und vermindertem Nahrungsangebot hat sich daher bei einigen Kleinsäugern - offenbar aus der „Eintrittspforte” des bei den Warmblütern neu aufgetretenen Non-REM-Schlafes heraus - der Winterschlaf entwickelt. Der Winterschlaf der Säugetiere ist durch eine ausgeprägte Umsatzreduktion gekennzeichnet, die durch eine Azidose moduliert und durch die erhaltene Thermoregulation auf ein verträgliches Maß begrenzt wird. Die Untergrenze der Abkühlung ist dabei offenbar durch einen kritischen Minimalumsatz vorgegeben, der allen Säugetieren gemeinsam ist und bei um so tieferen Körpertemperaturen erreicht wird, je höher der normotherme Energieumsatz ist. Da der spezifische (d. h. auf das Körpergewicht bezogene) Grundumsatz mit abnehmender Körpergröße zunimmt, weisen kleinere Säuger eine höhere Hypothermietoleranz auf als größere. Andererseits bedeutet der Abfall auf ein einheitliches Minimalniveau eine Inaktivierung der üblichen Körpergrößenbeziehung des Energieumsatzes und bildet so das Gegenstück zu dem nach der Geburt erfolgenden Anstieg der Stoffwechselrate von einem niedrigen fetomaternalen auf das der Körpergröße entsprechende höhere Niveau. Dieser postnatale Energieumsatzanstieg, der die einsetzende Thermoregulation erleichtert, verläuft dem zunehmenden Sauerstoffpartialdruck am Übergang vom fetalen zum adulten Kreislauf parallel und liegt damit möglicherweise auch der bei neugeborenen Säugetieren zu beobachtenden Fähigkeit zur Absenkung des Energiebedarfes bei Sauerstoffmangel zugrunde. Es mehren sich Hinweise darauf, dass, wie schon beim Schritt von der Anaerobiose zu Aerobiose, die Steigerung des Stoffwechselrate mit jeder Zunahme des Sauerstoffangebotes ein generelles Prinzip der Evolution darstellt, um - abgesehen von den daraus resultierenden Selektionsvorteilen - die Gewebe vor Sauerstoffüberangebot und oxidativem Stress zu schützen.

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.








Schlüsselwörter

Evolution - Energieumsatz - Homöothermie - Hypoxie - Hypothermie - Säugetiere - Neugeborene

Key words

Evolution - Metabolic Rate - Homeothermy - Hypoxia - Hypothermia - Mammals - Neonates

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PD Dr. med. D. Singer

Universitäts-Kinderklinik

Josef-Schneider-Straße 2

97080 Würzburg

Email: d.singer@mail.uni-wuerzburg.de