Differences in responsiveness to adipokinetic agents between white epididymal and brown interscapula adipose tissue from rats

Lipolytic activity was studied in brown and white adipose tissue of rats in vitro. 5-Hydroxy-tryptamine (5-HT), phenylephrine, noradrenaline, adrenaline and isoprenaline were used as adipokinetic agents. All stimulated lipolysis in brown adipose tissue, but 5-HT and phenyl-ephrine did not in white adipose tissue. A beta-blocking drug, propranolol, inhibited the stimulatory effect of the agents in both adipose tissues. However, an alpha-blocking drug, phentolamine, further increased the lipolysis induced by noradrenaline or adrenaline in brown adipose tissue and inhibited the effect of isoprenaline. In white adipose tissue, its action was to marginally decrease the effect of adrenaline and noradrenaline. Increase in the pH of the incubation medium stimulated FFA and glycerol release in brown adipose tissue, but not in the epididymal adipose tissue. This effect of pH on lipolysis was further enhanced by phentolamine and decreased by propranolol. Increase of lipolysis with pH was not seen with brown fat tissue from the reserpine-treated rats. These results show that brown adipose tissue of the rat has an alpha-receptor with inhibitory effects on lipolysis that is affected by alpha- or mixed-type adrenergic agonists, noradrenaline and adrenaline.