Early post-hatching starvation delays p70 S6 kinase activation in the muscle of neonatal chicks

Chicken muscle ribosomal protein S6 kinase (S6KI) has been recently characterised and its enzymic activity is regulated by the nutritional and hormonal (insulin) status in vivo. The regulation of S6KI is still unknown in neonatal chicks. The present study aimed to compare the activation of S6KI in early-feeding (EF) and 48h-delayed-feeding (DF) chicks from hatching to 4d of age. During post-hatching starvation, S6KI activity remained at the basal level measured in the control-hatched chicks. The maximum S6KI activity was recorded on the first day of feeding with an increase of about 2.5-fold in the EF and DF chicks (P<0ʹ01). S6KI activity was correlated with plasma insulin level, suggesting a probable insulin-dependent S6KI activation. The feeding-induced increase in S6KI activity was related to its Thr389 residue phosphorylation. A similar pattern for protein kinase B phosphorylation was observed, upstream from S6KI. The S6KI pathway was stimulated to the same extent in the EF and DF chicks, which indicates that post-hatching starvation did not increase S6KI activation. It is concluded that muscle S6KI is activated as soon as food is available without improvement in the response of the S6KI pathway after post-hatching starvation.