A collection of analogues of atnal natnuretic peptide (ANP) were screened for their ability to inhibit ANP-induced cGMP stimula-tion. The antagonists revealed through this screen are structurally related; almost all are substituted at either aspartate-1 3 or phenylalanine-26. This tendency is consistent throughout several families of small ANP analogues, suggesting that these two amino acid residues are involved in the process of ANP/cGMP signal transduction. One compound, A741 86, was studied in some detail. A741 86 is a potent inhibitor of the activation of guanylate cyclase by ANP; it acts with a pA2 of 7.1 2 in rat vascular smooth muscle cells and shifts the ANP/cGMP dose-response curve by 3 orders of magnitude at a 10 pM concentration. It also inhibits cGMP release in vivo, and at an infusion rate of 10 g/kg-min it completely abolishes ANP-induced natriuresis and diuresis. A741 86 does not, however, antagonize the hypotensive or vasorelaxant effects of ANP; in fact it acts as an agonist in these assays. It thus appears that cGMP, although it may mediate the renal responses to ANP, is not responsible for the vascular and hemodynamic effects that result from the action of the hormone.

The ANPs are a family of peptide hormones released from atrial cardiocytes in response to increases in central venous pressure(1-3). They are intimately involved in the regulation of blood pressure and fluid volume status and, for this reason, have attracted recent attention as possible therapeutic agents (4, 5). ANPs are known to have a variety of microscopic and macroscopic actions; they stimulate sodium and water excretion by the kidneys, cause a shift of fluid from the circulation into extravascular space, enhance the production of cGMP by the membrane-bound form of guanylate cyclase while lowering intracellular cAMP levels, inhibit aldosterone biosynthesis, and oppose the actions of the renin-angiotensin system. This complex cascade of responses is mediated through a number of different receptors(6-11). In particular, two major classes of peripheral ANP receptors have been closely studied. ANP-R1, the high molecular weight or B-type receptor, has been shown to be functionally coupled to the production of cGMP. There is strong physical evidence for this linkage; the receptor has recently been sequenced and found to consist of an extracellular binding domain and an intracellular domain that bears close homology to soluble guanylate cyclase (12). ANP-R2, the C-receptor, is also membrane bound but contains only a vestigial intracellular domain. This receptor, which predominates in most cell lines, has been suggested to serve a