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

Abstract

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. 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. The sarcoplasmic/endoplasmic reticulum Ca 2+ ATP-ase (SERCA) inhibitor thapsigargin (10 -1 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. 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 μg 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. © 2005 Nature Publishing Group All rights reserved.