ADENOSINE AND THE CONTROL OF ADRENERGIC REGULATION OF ADIPOSE-TISSUE LIPOLYSIS DURING LACTATION

Adenosine is a locally active factor that is produced intracellularly and extracellularly in adipose tissue. Adenosine binds to receptors in the plasma membrane of adipocytes; this activates a guanine triphosphate binding protein that inhibits adenylate cyclase activity and, hence, lipolysis. Lactation results in an enhanced responsiveness of adipocytes to beta-agonists, which stimulate lipolysis, and, paradoxically, to adenosine, which inhibits lipolysis. These adaptations are partly due to increases in ligand binding and to changes in postreceptor components of the signal transduction systems. Somatotropin is implicated in the chronic adaptations of the beta-adrenergic system, whereas insulin, somatotropin, glucocorticoids, and at least one unidentified factor have a role in the chronic control of the adenosine system of adipocytes.