Possible mechanisms for the increased sensitivity to glucagon and catecholamines of chicken adipose tissue during hatching

Abstract

The lipolytic sensitivity of chicken adipocytes to glucagon increases about 10-fold over the hatching period. Experiments were conducted to determine whether this change in sensitivity was related to changes in cyclic AMP metabolism within adipocytes. The capacity of glucagon to increase adipocyte cyclic AMP content was increased in 2-day-old chicks compared to 19-day embryos. This effect was seen at all glucagon concentrations, without any marked shift in the glucagon concentration for half-maximal effect. Cyclic AMP responses to catecholamines and theophylline were also greater in chick adipocytes than in cells from embryos. Lipolysis was maximally activated when cyclic AMP was increased to only a small fraction of maximum. There was some indication that a given increase in cyclic AMP content was associated with higher rates of lipolysis in chick than in embryo adipocytes. Phosphodiesterase activity was at a minimum on the day of hatching, but levels in 2-day-old chicks were similar to those in 19-day embryos. It is concluded that the increase in lipolytic sensitivity to hormones over the hatching period is associated with an increased capacity for cyclic AMP accumulation, probably due to increased adenylate cyclase activity. There may also be an increase in the sensitivity of the lipase activation system to cyclic AMP.