Smooth muscle length-dependent PI(4,5)P₂ synthesis and paxillin tyrosine phosphorylation

We studied effects of increasing the length of porcine trachealis muscle on 5.5 μM carbachol (CCh)-evoked phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] synthesis and other parameters of phosphatidylinositol (PI) turnover. PI(4,5)P₂ resynthesis rates in muscle held at 1.0 optimal length (L₀), measured over the first 6 min of CCh stimulation, were 140 ± 12 and 227 ± 14% of values found in muscle held at 0.5 L₀ and in free-floating muscle, respectively. Time-dependent changes in cellular masses of PI(4,5)P₂, PI, and phosphatidic acid, and PI resynthesis rates, were also altered by the muscle length at which contraction occurred. In free-floating muscle, CCh did not evoke increases in tyrosine-phosphorylated paxillin (PTyr-paxillin), an index of β₁-integrin signaling; however, there were progressive increases in PTyr-paxillin in muscle held at 0.5 and 1.0 L₀ during contraction, which correlated with increases in PI(4,5)P₂ synthesis rates. These data indicate that PI(4,5)P₂ synthesis rates and other parameters of CCh-stimulated inositol phospholipid turnover are muscle length-dependent and provide evidence that supports the hypothesis that length-dependent β₁-integrin signals may exert control on CCh-activated PI(4,5)P₂ synthesis.