Amino acids measured in this study are shown in Table 1
. The PFAA concentrations increased in the control dogs and the GlcNAc treated dogs whereas those from the GlcN- or the Glc-treated dogs significantly decreased after one hour (Figure 1
). The levels of Glu, Gly, and Ala concentrations were significantly lower than observed fort he GlcNAc-treated dogs (Table 2
After administration of GlcNAc, no remarkable change was observed in either PFAA concentration or each amino acid level compared to the control.
In a healthy human report, postprandial PFAA concentrations were raised compared to those before a meal [14
]. Following feeding, PFAA increased in the control dogs and the GlcNAc-treated dogs. However, PFAA decreased compared to the control group after administration of GlcN or Glc. In an in vitro
study using mesenchymal stem cells, treatment with 100 μM or 1,000 μM GlcN increased expression of aggrecan and type II collagen. Moreover, 100 μM GlcN treatment led to increased sGAG content [15
]. In humans, plasma GlcN concentration reached 150–300 μM after oral administration of 20 mg/kg GlcN [16
]. In dogs, plasma GlcN concentration was reported to reach 50 μM after oral administration of 125 mg/kg GlcN [6
], and reached 100 μM after oral administration of 300 mg/kg GlcN (Figure 2
). Although the maximum plasma GlcN concentration achieved after oral administration of 500 mg/kg GlcN has not been reported, a previous study indicated that it exceeds 100 μM. These findings suggested that high levels of GlcN were provided to the tissue by the circulatory system in dogs. Naito K et al.
described that GlcN has the potential to exert a chondroprotective action on an experimentally induced OA by inhibiting type II collagen degradation and enhancing type II collagen synthesis in the articular cartilage [13
]. Therefore, proteoglycan and type II collagen were likely to be synthesized actively in cartilage. Gly, Ala and Glu are the main components of type II collagen [17
], and the levels of these amino acids became lower than those of the control and GlcNAc-treated dogs after administration of GlcN or Glc. These results suggest that GlcN or Glc stimulated proteoglycan and type II collagen synthesis in the dogs.
No change in total or individual amino acid concentrations was observed after administration of GlcNAc compared to the control group. The maximum concentration of GlcNAc in dogs reached about 20 μM after 300 mg/kg GlcNAc administration (Figure 2
). Therefore, absorption of GlcNAc by the canine gut may be inferior to that of GlcN. However, the mechanisms of GlcNAc absorption are unclear; further investigation into the absorption and metabolism of GlcNAc is necessary.
We did not directly confirm type II collagen and proteoglycan synthesis in dogs after administration of GlcN. However, oral administration of GlcN induced obvious functional recovery in various kinds of canine orthopedic diseases [2
]. To confirm these phenomena occur in dogs is required to understand the mechanism of GlcN in dog joint diseases.