On the in vitro lipolytic activity of peptide hormones in human adipose tissue

Peptide hormones are potent stimulators of lipolysis in incubated adipocytes or adipose tissue of laboratory animals. However, none out of 40 synthetic or highly purified peptide hormones or sequences from the pituitary, the gastrointestinal tract and of different origin was able to stimulate glycerol release from 'incubated' human adipocytes. Yet, this model of the 'incubated adipocyte' offers several disadvantages: rapid decrease of pH in the incubation medium, release and increment of lipolysis inhibiting substances such as FFA and adenosine and rapid unspecific degradation of the peptides. In an improved in vitro model--pH-stat titration--which avoids or diminishes the disadvantages of 'incubated adipocytes', a group of pituitary peptides was tested. One peptide hormone, beta-lipotropin, stimulated lipolysis significantly in human adipose tissue. Two other peptides which were highly active in adipose tissue of laboratory animals were ineffective in this system too. The lipolytic response to beta-lipotropin was comparable to the effect of noradrenaline at equimolar concentrations. The data indicate that results on the lipolytic activity of peptide hormones in adipose tissue of laboratory animals cannot be transferred to human adipose tissue. In advanced in vitro test systems the ability to stimulate lipolysis could also be shown for a peptide hormone suggesting a role of such substances in the endocrine regulation of adipose tissue mass in men.