SEASONAL CHANGES IN PLASMA NITROGENOUS COMPOUNDS OF THE EUROPEAN BROWN BEAR (URSUS ARCTOS ARCTOS)

and p ≈ 0.003, respectively) lower, while ornithine significantly (p < 0.001) and citrulline somewhat higher in winter as compared with the other seasons. These changes indicate active urea recycling, and the synthesis of non-essential amino acids with ammonium and glycerol as precursors. Of essential amino acids, the levels of phenylalanine, lysine and methionine and of non-essential ones, alanine and histidine were significantly elevated during the denning period. No consistent changes occurred in other amino acids. One indication of the degradation of muscle protein as a source of amino acids in winter was the significant increase of 3-methylhistidine concentration in plasma. However, the degradation of type I collagen also increased, since the concentration of the cross-linked carboxyterminal telopeptide of type I collagen (ICTP) was about 3.5-fold higher in winter than in summer. These changes indicate that de novo synthesis of essential amino acids is not necessary in winter sleeping bears.