of endothermy is of major interest in the understanding of mammalian and avian radiation. Importantly, Rowland. Reptilia, potentially tegu lizards, no BAT, no thermogenic function of UCP1. Not surprisingly then, the local heating of BAT can be even detected by thermal imaging of skin (e.g., Symonds., ). Dumonteil., 1995 Mammals Monotremata Anecdotal evidence for echidnas No BAT, no evidence for thermogenic UCP1; Speculations about alternative rewarming mechanism from nightmare essay pdf torpid states.
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It has recently been shown that serca1a and SLN are significantly reduced during the hibernation season in skeletal muscles of thirteen-lined ground squirrels. It seems, however, that this up-regulation is based on the relatively slow process of increasing the enzyme's expression level (Clarke., or changing the fatty acid composition or cholesterol content of the surrounding membrane (Cornelius, ). Up to date at least 214 species of heterothermic mammals and birds have been identified (Ruf and Geiser, ) and it is now widely accepted that heterothermy is a plesiomorphic, ancient trait from which homeothermy has evolved (Grigg., ; Lovegrove, ). Interestingly, SLN expression is high in newborn mice and rats - two species that have muscle as well as UCP1-mediated NST - and is usually down-regulated during neonatal development (Babu., ; Pant., however, continuous cold exposure can prevent this down-regulation, leading. Once SLN induces slippage of Ca2 from serca, this means Ca2 ions are captured by serca from the sarcoplasma, which is followed by ATP cleavage and heat generation, and by the release of two Ca2 back to the sarcoplasma. If this is not the case, the SLN-induced slippage of Ca2-ions without involvement of the RyR (Mitidieri and de Meis, ; Mall., ) would be an alternative form of thermogenesis restricted to mammals. The data on piglets discussed above (Berthon., 1994 on the other hand, suggest that muscle NST,.e., heat production that is not proportional to fiber contraction intensity, and shivering thermogenesis are not mutually exclusive. This process is fostered by SLN although it is not yet possible to explain the effect of SLN on slippage in molecular terms (Mall., ).
This permitted us to assess the effects of both genetic and pharmacologic manipulations on the generation of heat and allowed us, for the first time, to measure the heat (uCal/sec/cell) of BAT in the basal and stimulated state. Nevertheless, piglet mortality is still high and often attributed to thermoregulatory problems (Herpin., 2002 which could be due to the lack of BAT. It has also been proposed that BMR was elevated by increased membrane leakiness caused by the incorporation of polyunsaturated fatty acids (pufa) (Hulbert and Else, ; Hulbert, a view that has been challenged by a comparative study on mammals (Valencak and Ruf, ). Another regulator of serca, the protein phospholamban, is not involved in thermogenesis (Sahoo., ; Shaikh., ). Instead it seems that increasing levels of endothermy were achieved by recruiting additional mechanisms of thermogenesis to muscular work during locomotion, including specialized shivering thermogenesis, increases in mitochondrial density and membrane leakage, increases in sodium-potassium pump activity, shifts in serca activity toward NST. Berthon., 1994, 1996 ; de Meis, 2001a, b ; de Meis., 2005b ; Babu., 2007 ; Bal., 2012, 2016 ; Anderson, 2016 ; Anderson., 2016 ; Pant., 2016 Open in a separate window. enhanced muscle NST in transgenic mice, which have increased capacity for muscular fatty acid uptake, was accompanied by an increase in oxidative fibers. The extra heat produced by the heart under these conditions was mainly derived from both an increase of serca2a activity and an enhancement of mitochondrial oxidative phosphorylation.