The Autonomic Nervous System and Energy Metabolism

Summary

The mechanism by which fat stores are mobilized is of central importance to an understanding of energy metabolism in general. Apart from the fact that the dynamic state of adipose tissue can be regulated by the influx of glucose into the fat cell, a number of hormones are able to stimulate lipolysis as well as glycogenolysis by a direct interaction with a “Metabolic Receptor”.

In this context it is difficult to overestimate the importance of the work of Sutherland and coworkers who developed the concept that cyclic 3’, 5’-AMP is a “Second Messenger” in metabolic processes. This nucleotide, formed by adenyl cyclase in all nucleated cells, is the trigger through which a physiologic event — the release of a hormone — is translated to a metabolic event — lipolysis or glycogenolysis. Thus, it appears that the metabolic receptor is identical with the enzyme adenylcyclase which converts ATP to cyclic 3’, 5’-AMP. In this reaction catecholamines exert a catalytic function by interaction with the enzyme and substrate as well.

Our experiments with various stimulants and inhibitors of lipolysis indicate that the adenyl cyclase — ATP — complex in adipose tissue behaves like a sympathetic β-receptor. Although both, α- and β-adrenolytics, are able to block the action of noradrenaline and adrenaline, only β-adrenolytics produce this effect already in very low concentrations, stero-specifically and in a competitive manner. In addition, β-adrenolytics have a second point of attack in the lipolytic systems, which is probably located beyond the adenyl cyclase step and possibly identical with that of α-adrenolytics. This suggestion, however, remains speculative as long as nothing is known about the interconversion of inactive and active triglyceride lipase.