Urocortin increases the intracellular cAMP concentration and thus decreases the degree of phosphorylation of MYPT1 and increases the myosin phosphatase activity

Abstract

Urocortin is a peptide hormone related to corticotrophin-releasing factor. It is assumed that urocortin is involved in blood pressure regulation by dilating the peripheral blood vessels. In rat tail arteries, urocortin-induced vasodilation is caused by a decrease in the myofilament Ca2+ sensitivity, the mechanism of which is still unclear. In this study, the hypothesis was tested that the decrease in the Ca2+ sensitivity in mouse tail arteries results from the activation of myosin light chain phosphatase. The relaxation of KCl (42 mM) precontracted intact mouse tail arteries by 1 and 10 nM urocortin was significantly inhibited by 1 μM antisauvagine-30, a CRF-2 receptor antagonist (p < 0.05, n = 3). The addition of 1 μM KT 5720, a protein kinase A inhibitor, to intact rat tail arteries did not affect the KCl-induced force but significantly attenuated the urocortin-induced relaxation (n = 5). In α-toxin-permeabilized mouse tail arteries, urocortin relaxed activated preparations at constant pCa 6.1 by 37.6 ± 8.2% (n = 5) as compared with reference vessels (n = 5, p < 0.001). The relaxation of vessels with impaired membranes was inhibited by pretreatment with 30 μM Rp-8-COT-cAMPS, an inactive analog of cAMP. In permeabilized mouse arteries, treatment with 100 nM urocortin was related to dephosphorylation of MLC20Ser 19 and MYPT1Thr696/Thr850. The effect of urocortin on MYPT1 dephosphorylation was completely abolished by 30 μM Rp-8-CPT-cAMPS and mimicked by Sp-5,6-DCl-cBiMPS, an active cAMP analog. On the basis of these findings, it was assumed that the urocortin-induced relaxation is a consequence of a decrease in the calcium sensitivity mediated by a cAMP-dependent increase in the activity of myosin light chain phosphatase.