Immunocytochemical localization, binding, and effects of atrial natriuretic peptide in rat adipocytes

The metabolic effects of atrial natriuretic peptide (ANP) have not been widely investigated. Since adipocyte cells represent a model system extensively used to examine the metabolic actions of many peptide hormones, we sought to establish whether ANP could bind to adipocyte membranes, alter cyclic nucleotide metabolism, and affect spontaneous or hormone-stimulated lipolysis. Using in vitro autoradiographic techniques, radiolabelled ANP was found to bind specifically to mammary gland fat cells. Additionally, endogenous ANP-like immunoreactivity could be localized in the plasma membrane compartment and cytoplasmic matrix of fat cells, but not in fat vacuoles. [125I]ANP bound to single high affinity sites (Kd = 0.72 nM) in fat cell membranes. The binding was rapid (equilibrium within 1 min at 25 degrees C) and specific. The atrial peptide was capable of stimulating a time- and concentration-dependent increase in cGMP accumulation in isolated adipocytes, but had no effect on spontaneous or stimulated [-)-isoproterenol, ACTH, forskolin) cAMP formation. ANP did not alter the increase in glycerol production stimulated by l-epinephrine in isolated fat cells. While i.v. infusion of ANP stimulated a marked increase in circulating levels of cGMP, the atrial peptide did not alter plasma triglyceride levels. These data demonstrate the presence of specific ANP binding sites on adipocyte membranes and internalization of ANP-associated immunoreactivity. These receptors are biochemically functional given the ability of ANP to augment cGMP formation. The peptide, however, does not exert an action on adipocyte lipolysis. Adipocytes, therefore, represent an ANP target tissue in which the physiological action of the peptide is yet to be defined.