| Summary: | In isolated fat cells, the same maximal rate of glycerol production can be induced by epinephrine or ACTH, alone or in combination with each other or with glucagon. With fat cells from rats weighing 150–175 g, the maximal rate of lipolysis attained with glucagon was 75–80% of that produced by epinephrine or ACTH, and with increasing size of the donor rat, the magnitude of the effect of glucagon relative to that of the other hormones declined markedly. In particulate preparations from fat cells of rats weighing 100–125 g, the maximal effect of glucagon on adenyl cyclase activity was about 60% of that of epinephrine, and was significantly less (30%) in preparations from 350–400 g rats. These data are consistent with the hypothesis that with growth of the rat there is a selective decline in the number of glucagon receptors relative to those for epinephrine or ACTH in the fat cell membrane.
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