Molecular mechanism of action by atrial natriuretic peptide in rat vascular smooth muscle cells

The mechanism by which atrial natriuretic peptide (ANP) acts on cells remains obscure. Using cultured vascular smooth muscle cells (VSMC) from rat aorta, we studied the structure-activity relationship of alpha-human(h) ANP and attempted to clarify its cellular mechanism of action. Binding studies using a variety of synthetic alpha-hANP analogs with deletion and substitution of amino-acid residue(s) within the ring structure and deamino-dicarba analogs with replacement of the disulfide bond with an ethylene linkage, suggested that the original cyclic structure is a minimum requirement for biological activity and the disulfide bond is not essential for receptor binding. The present study using VSMC loaded with a fluorescent Ca2+ indicator quin-2 revealed that alpha-rat(r) ANP has no effect on increases in cytosolic free Ca2+ concentration stimulated by angiotensin (A) II or arginine-vasopressin (AVP). While both vasoconstrictive hormones rapidly stimulated phosphatidylinositol (PI) response in VSMC, pretreatment with rANP did not affect AII- or AVP-induced PI response. However, AII-stimulated phosphorylation level of 20 K-dalton (Da) myosin light chain (MLC), a regulatory contractile protein of VSMC, was attenuated by pretreatment with rANP. These data suggest that ANP may not act at site of either agonist-induced membrane PI hydrolysis or intracellular Ca2+-signal system, but may partly be involved in phosphorylation/dephosphorylation of 20 K-Da MLC in cultured rat VSMC.