Abstract-Urocortin, a vasodilatory peptide related to corticotropin-releasing factor, may be an endogenous regulator of blood pressure. In vitro, rat tail arteries are relaxed by urocortin by a cAMP-mediated decrease in myofilament Ca 2ϩ sensitivity through a still unclear mechanism. Here we show that contraction of intact mouse tail arteries induced with 42 mmol/L KCl or 0.5 mol/L noradrenaline was associated with a Ϸ2-fold increase in the phosphorylation of the regulatory subunit of myosin phosphatase (SMPP-1M), MYPT1, at Thr696, which was reversed in arteries relaxed with urocortin. Submaximally (pCa 6.1) contracted mouse tail arteries permeabilized with ␣-toxin were relaxed with urocortin by 39Ϯ3% at constant [Ca 2ϩ ], which was associated with a decrease in myosin light chain (MLC 20 Ser19 ), MYPT1 Thr696 , and MYPT1 Thr850 phosphorylation by 60%, 28%, and 52%, respectively. The Rho-associated kinase (ROK) inhibitor Y-27632 decreased MYPT1 phosphorylation by a similar extent. Inhibition of PP-2A with 3 nmol/L okadaic acid had no effect on MYPT1 phosphorylation, whereas inhibition of PP-1 with 3 mol/L okadaic acid prevented dephosphorylation. Urocortin increased the rate of dephosphorylation of MLC 20 Ser19 Ϸ2.2-fold but had no effect on the rate of contraction under conditions of, respectively, inhibited kinase and phosphatase activities. The effect of urocortin on MLC 20Ser19 and MYPT1 phosphorylation was blocked by Rp-8-CPT-cAMPS and mimicked by Sp-5,6-DCl-cBIMPS. In summary, these results provide evidence that Ca 2ϩ -independent relaxation by urocortin can be attributed to a cAMP-mediated increased activity of SMPP-1M which at least in part is attributable to a decrease in the inhibitory phosphorylation of MYPT1. ) is a 40 amino acid polypeptide that belongs to the corticotropin-releasing factor (CRF) family. 1 Urocortin-like immunoreactivity 2 and expression of the peripheral subtype of CRF receptors, CRF-2R, 3 has been detected in the circulatory system. Urocortin, which has a higher affinity for CRF-2R than CRF itself, relaxes blood vessels both in vitro 4 and in vivo, 5 causing a CRF-2R-mediated decrease in the mean arterial blood pressure. 6 Interestingly, in CRF-2R-deficient mice, the resting blood pressure was elevated. 6 These findings suggest that urocortin is an endogenous regulator of blood pressure and blood flow. In addition, urocortin, the plasma levels of which are increased in human heart failure, 7 has beneficial effects in experimental heart failure. 8 The mechanism by which urocortin relaxes blood vessels appears to be complex, eg, vasodilation has been reported to be both endothelium dependent and independent. 3,4,9,10 The endothelium-independent vasodilation was suggested to be mediated by activation of the cAMP/protein kinase A (PKA) signaling cascade, 2,11 which is in line with the observation that activation of CRF-2R increases cAMP levels in Ltk cells. 12 In some vessels, activation of PKA was associated with activation of potassium channels and membrane potential hype...

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