Nitric oxide relaxes circular smooth muscle of rat distal colon through RhoA/Rho‐kinase independent Ca 2+ desensitisation

1 The aim of this study in circular smooth muscle of rat distal colon was to determine whether Ca 2+ desensitisation, in addition to mechanisms lowering cytosolic free Ca 2+ concentration ([Ca 2+ ] cyt ), was involved in the relaxation elicited by nitric oxide (NO). Changes in isometric tension and [Ca 2+ ] cyt were recorded simultaneously in fura‐2‐loaded strips. 2 In methacholine (10 −5   M )‐precontracted preparations, exogenous NO (10 −4   M ), adenosine 5′‐triphosphate (ATP; 10 −3   M ) and electrical field stimulation (EFS; 1 ms, 40 V, 4 Hz, 1 min) induced a decrease in smooth muscle tension, which was accompanied by a fall in [Ca 2+ ] cyt . 3 The sarcoplasmic/endoplasmic reticulum Ca 2+ ATP‐ase (SERCA) inhibitor thapsigargin (10 −6   M ) did not exert an influence on the decrease in tension produced by exogenous NO, but significantly attenuated the fall in [Ca 2+ ] cyt . Both the relaxation and the fall in [Ca 2+ ] cyt to ATP and EFS were unaffected by thapsigargin. 4 Calyculin‐A (10 −6   M ), a myosin light chain phosphatase (MLCP) inhibitor, significantly reduced the decrease in tension elicited by exogenous NO, but did not alter the fall in [Ca 2+ ] cyt to exogenous NO. Inactivating RhoA by exoenzyme C3 (2  μ g ml −1 ) or inhibiting Rho‐kinase with (+)‐( R )‐ trans ‐4‐(1‐aminoethyl)‐ N ‐(4‐pyridyl) cyclohexanecarboxamide dihydrochloride monohydrate (Y‐27632; 10 −5   M ) had no effect on the decrease of both tension and [Ca 2+ ] cyt generated by exogenous NO. 5 This paper demonstrates that a RhoA/Rho‐kinase independent Ca 2+ desensitisation pathway contributes to the relaxation by NO in circular smooth muscle strips of rat distal colon.British Journal of Pharmacology (2005) 144 , 588–594. doi: 10.1038/sj.bjp.0706111