Regulation of the crossbridge cycle in vascular smooth muscle by cAMP signalling.

Urocortin, a novel vasodilatory peptide related to the corticotropin-releasing factor (CRF) increased cAMP levels to 220.8 +/- 27.6% of control in rat tail arteries. The effect was completely abolished by the adenylyl cyclase inhibitor, SQ22536 (100 microM). Urocortin also decreased phosphorylation of the regulatory light chains of myosin (MLC20) in rat tail arteries stimulated with high K+ from 27.5 +/- 0.9% (control) to 13 +/- 2% (n = 5). This suggests that urocortin relaxes blood vessels via cAMP-mediated dephosphorylation of MLC20. Previously we have shown that urocortin-induced vasodilation can be ascribed to a decrease in Ca2+ -sensitivity of tension and activation of smooth muscle myosin phosphatase (SMPP-1M). In this study, we provide evidence that urocortin-induced Ca2+ -desensitization does not affect agonist-induced Ca2+ -sensitization. Urocortin relaxed alpha-toxin permeabilized mouse tail arteries preconstricted with pCa 6.1, but did not prevent the Ca2+ -sensitization induced by 10 microM 5-HT, 100 microM norepinephrine (NE) or 1 microM GTPgammaS. In keeping, the maximally relaxing concentration of urocortin (100 nM) had no effect on the concentration dependence of the phenylephrine-induced Ca2+ -sensitization. By contrast, treatment with the cAMP analogue, cBIMPS (100 microM), or the Rho kinase inhibitor, H-1152 (3 microM) relaxed the mouse vessels to a greater extend and completely inhibited phenylephrine (PE) induced sensitization. The lack of effect of urocortin on agonist-induced sensitization could be due to a alpha-adrenergic receptor mediated inhibition of cAMP generation. Furthermore PE induced Ca2+ -sensitization was reported to occur independent of changes in MLC20 phosphorylation involving caldesmon. Our results are compatible with a model in which urocortin/cAMP signalling only affects the myosin linked regulation of vascular tone while cBIMPS may inactivate in addition the MLC20 phosphorylation independent pathway.