Seasonal And Diurnal Variation Of Metabolism In Phrynocephalus Erythrurus

The red tail toad-headed lizard Phrynocephalus erythrurus, which inhabits the Qiangtang Plateau in northern Tibet, is considered to be the highest living lizard in the world(mostly 4500–5000 m above sea level). Reptiles adjust their physiology by regulating metabolism to adapt the seasonal and daily changes of light, temperature, and nutrient availability. To test the adjustment strategy of metabolism in P. erythrurus during hibernation, we examined the organ(heart, liver, lungs, abdominal fat bodies, testes, brain, stomach and intestine) fresh weight and water content, tissue(heart, liver, muscle) and plasma metabolite contents, the structure of the heart, the key metabolic enzymes activity(pyruvate kinase, glutamate dehydrogenase and lipase) in liver and skeletal muscle. Compared with Phrynocephalus przewalskii, which inhabits in low altitude(1400 m above sea level), we investigated the response mechanism of P. erythrurus to low temperature. Compared with activity period, the liver, fat bodies and testes weight in P. erythrurus significantly increased but intestines and stomach weight markedly decreased before hibernation. P. erythrurus significantly reduced the fat body, brain and testes weight after hibernation. During hibernation, the tissues of P. erythrurus and P. przewalskii had dehydration with tissue-specific and specie-specific. The arrangement of spongy myocardial layer was found to change dramatically and the area density of lacunary spaces(La) was remarkable decreased by cardiomyocyte hypertrophy during hibernation. All individuals could accumulate glucose, triglyceride and glycogen as energy source during winter dormancy. There were remarkable changes in the activity of pyruvate kinase(PK), glutamate dehydrogenase(GLDH) and lipase(LPS) with tissue-specific and specie-specific during hibernation in P. erythrurus and P. przewalskii. Compared to P. przewalskii, P. erythrurus maintained higher glucose levels of blood and tissues at activity stage and accumulated more glycogen before hibernation, which may be related to adaption of P. erythrurus at the plateau low temperature environment.In the second section we studied the effect of temperature on the metabolic circadian rhythms in P. erythrurus. We investigated the diurnal variation of body temperature, heart rate, standard metabolic rate(SMR) and plasma glucose concentration under two temperature regimes(18℃ and 28℃). Circadian rhythms were present in the SMR of P. erythrurus with higher values during the day than that during the night in constant conditions and the magnitude of the daily change was reduced at 18℃. Additionally, the variation of heart rate was consistent with SMR except the nadir which came earlier at 18℃, implying a metabolic suppression in the low temperature. However, the body temperature did not show obvious circadian rhythm at constant conditions. The plasma glucose concentration at 28℃ was significantly higher than that at 18℃ with different diurnal fluctuation: the value at 16:00 was significantly higher than that at 20:00 under 28℃, 0:00 and 12:00. However, the values at 20:00 and 08:00 were significantly lower than that at 00:00 and 12:00 under 18℃, and the values at 04:00 and 16:00 were moderate. The present study confirms the fact that prevalence of circadian rhythms in the SMR, heart rate and plasma glucose concentration of P. erythrurus. The control of some endogenous factors plays a decisive role in the metabolic circadian rhythms, while temperature has notable effects in entraining these rhythms, which may be associated with the secretion of some hormones.