Expression of CPI-17 and myosin phosphatase correlates with Ca²⁺ sensitivity of protein kinase C-induced contraction in rabbit smooth muscle

1. Various smooth muscles have unique contractile characteristics, such as the degree of Ca²⁺ sensitivity induced by physiological and pharmacological agents. Here we evaluated six different rabbit smooth muscle tissues for protein kinase C (PKC)-induced Ca²⁺ sensitization. We also examined the expression levels of myosin light chain phosphatase (MLCP), the MLCP inhibitor phosphoprotein CPI-17, and the thin filament regulator h-calponin. 2. Immunohistochemical and Western blot analyses indicated that CPI-17 was found primarily in smooth muscle, although expression varied among different tissues. Vascular muscles contained more CPI-17 than visceral muscles, with further distinction existing between tonic and phasic subtypes. For example, the tonic femoral artery possessed approximately 8 times the cellular CPI-17 concentration of the phasic vas deferens. 3. In contrast to CPI-17 expression patterns, phasic muscles contained more MLCP myosin-targeting subunit than tonic tissues. Calponin expression was not statistically different. 4. Addition of phorbol ester to α-toxin-permeabilized smooth muscle caused an increase in contraction and phosphorylation of both CPI-17 and myosin light chain (MLC) at submaximal [Ca²⁺]_i. These responses were several-fold greater in femoral artery as compared to vas deferens. 5. We conclude that the expression ratio of CPI-17 to MLCP correlates with the Ca²⁺ sensitivities of contraction induced by a PKC activator. PKC stimulation of arterial smooth muscle with a high CPI-17 and low MLCP expression generated greater force and MLC phosphorylation than stimulation of visceral muscle with a relatively low CPI-17 and high MLCP content. This implicates CPI-17 inhibition of MLCP as an important component in modulating vascular muscle tone.