Abstract
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.
Highlights
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
We have previously reported that the same maximal rate of glycerol production could be induced by either epinephrine or ACTH [11].Addition of both together at maximally effective concentrations or in combinations with glucagon did not further stimulate lipolysis
I n the experiment shown in Fig. 1, two preparations of fat cells, one from rats weighing about 175 g and the other from 500-g rats, were incubated with concentrations of epinephrine, ACTH, or glucagon that had been previously shown to produce maximal effects on lipolysis
Summary
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 75-80y0 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. I n 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 consistentwith 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. As rats grow, the fat cells exhibit a selective change in their responsiveness to glucagon, i.e., the maximal stimulation of lipolysis inducible with glucagon relative to that produced by epinephrine or ACTH declines with increasing age and size of the donor rat. A parallel change in the effect of glucagon on fat cell adenyl cyclase activity has been demonstrated
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