Seasonal variation in the response of adipose tissue to a serum fat-mobilizing factor may be explained in terms of reduced availability of fat cell receptors.

The lipolytic activity of serum obtained from fasted rats on rat isolated rat cells, exhibits seasonal trends. If it is assumed that the serum lipolytic agonist combines with receptors on fat cell membranes, and that the measured physiological response (glycerol release) is proportional to the number of such combinations, then the observed seasonal variation could be due to changes in serum concentration of lipolytic agonist, or the sensitivity of cells towards the same concentration of agonist (or both). On the basis of the observations described herein, it is proposed that the seasonal fluctuation in fat cell sensitivity is the result of some measure of altered availacbility of receptors for the lipolytic agonist. Possible changes in the concentration of circulating agonist, which were not investigated, may also play a role.