Tonic and phasic smooth muscle contraction is not regulated by the PKCα - CPI-17 pathway in swine stomach antrum and fundus.

Regulation of myosin light chain phosphatase (MLCP) via protein kinase C and the 17 kDa inhibitor of myosin light chain phosphatase (CPI-17) has been reported as a Ca(2+) sensitization signaling pathway in smooth muscle (SM), and thus may be involved in tonic vs. phasic contractions. This study examined the protein expression and spatial-temporal distribution of and CPI-17 in intact SM tissues. KCl or carbachol (CCh) stimulation of tonic stomach fundus SM generates a sustained contraction while the phasic stomach antrum generates a transient contraction. In addition, the tonic fundus generates greater relative force than phasic antrum with 1 µM phorbol 12, 13-dibutyrate (PDBu) stimulation which is reported to activate the duanyu1531α - CPI-17 pathway. Western blot analyses demonstrated that this contractile difference was not caused by a difference in the protein expression of duanyu1531α or CPI-17 between these two tissues. Immunohistochemical results show that the distribution of duanyu1531α in the longitudinal and circular layers of the fundus and antrum do not differ, being predominantly localized near the SM cell plasma membrane. Stimulation of either tissue with 1 µM PDBu or 1 µM CCh does not alter this peripheral duanyu1531α distribution. There are no differences between these two tissues for the CPI-17 distribution, but unlike the duanyu1531α distribution, CPI-17 appears to be diffusely distributed throughout the cytoplasm under relaxed tissue conditions but shifts to a primarily peripheral distribution at the plasma membrane with stimulation of the tissues with 1 µM PDBu or 1 µM CCh. Results from double labeling show that neither duanyu1531α nor CPI-17 co-localize at the adherens junction (vinculin/talin) at the membrane but they do co-localize with each other and with caveoli (caveolin) at the membrane. This lack of difference suggests that the duanyu1531α - CPI-17 pathway is not responsible for the tonic vs. phasic contractions observed in stomach fundus and antrum.

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