Effect of hormones and 3', 5'-cyclic AMP on very low density lipoprotein receptors

Human 125I-labelled VLDL interacts with rat adipocytes in vitro, with properties typical of a ligand-receptor interaction. This VLDL-receptor interaction is modulated by hormones which are known to change cyclic AMP levels. Norepinephrine and isoproterenol, both of which elevate cycle AMP, increase the binding of VLDL to adipocytes. Dibutyryl-cyclic AMP, a derivative of cyclic AMP, also increases the VLDL binding to adipocytes. Insulin reverses the catecholamine-induced increase in VLDL binding. This parallels insulin's effect on the catecholamine-induced changes in cyclic AMP. Direct addition of cyclic AMP itself increases VLDL binding to adipocyte membranes, a system in which no lipolysis of new protein synthesis occurs. Based on the competition between unlabelled VLDL and 125I-labelled VLDL, we conclude that catecholamines act on adipocytes, and cyclic AMP on membrane fractions, by increasing their capacity rather than their affinity to bind VLDL.