Atrial natriuretic peptide inhibits the agonist-induced increase in extent of myosin light chain phosphorylation in aortic smooth muscle

The effect of atrial natriuretic peptide (ANP) on angiotensin II- and histamine-induced contraction and muscle light chain phosphorylation was examined in strips of rabbit aorta smooth muscle. Preincubation of strips with 10^-7 M ANP prior to addition of either agonist inhibits both the increase in extent of myosin light chain phosphorylation and the contractile response to either 5 x 10^-8 M angiotensin II or 10^-5 M histamine without inhibiting the agonist-induced increase in the intracellular free Ca²⁺ concentration. Furthermore, in muscle strips precontracted with either angiotensin II or histamine, addition of ANP leads to a prompt relaxation and a prompt decrease in the extent of myosin light chain phosphorylation. These data argue that ANP uncouples the initial agonist-induced Ca²⁺ transient from the increase in extent of myosin light chain phosphorylation either by inhibiting the Ca²⁺-dependent activation of myosin light chain kinase or stimulating the activity of a phosphoprotein phosphatase capable of bringing about the rapid dephosphorylation of phosphorylated myosin light chains.