Inhibition of lipolysis by agents acting via adenylate cyclase in fat cells from infants and adults.

The in vitro lipolytic effect of catecholamines is poor during infancy because of enhanced alpha 2-adrenoceptor activity. The mechanisms behind this were investigated in isolated fat cells obtained from 1- to 4-mo-old infants and from adults. The cells were incubated with agents that inhibit lipolysis through distinct receptors coupled to adenylate cyclase via the inhibitory GTP binding coupling protein, Gi. The sensitivity to the alpha 2-adrenoceptor agonist clonidine was 14 times higher in the infant group as compared to the adults, whereas that to an adenosine analogue was 14 times lower. The sensitivities to prostaglandin E2 and nicotinic acid were similar in both age groups. Preincubation of the adipocytes with pertussis toxin abolished the antilipolytic effects of all agents. The density of alpha 2-adrenoceptor binding sites determined with [3H]yohimbine was increased by about 25% in the infants. In conclusion, the antilipolytic sensitivity of adenosine and alpha 2-adrenoceptors develops separately and may play different roles in the regulation of lipolysis in man. Furthermore, the enhanced alpha 2-adrenoceptor sensitivity during infancy seems at least in part to be due to an increase in the number of receptors.