Protein kinase C mediation of Ca2+-independent contractions of vascular smooth muscle

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Abstract
Tumour-promoting phorbol esters induce slow, sustained contractions of vascular smooth muscle, suggesting that protein kinase C (PKC) may play a role in the regulation of smooth muscle contractility. In some cases, e.g., ferret aortic smooth muscle, phorbol ester induced contractions occur without a change in [Ca2+]ior myosin phosphorylation. Direct evidence for the involvement of PKC came from the use of single saponin-permeabilized ferret aortic cells. A constitutively active catalytic fragment of PKC induced a slow, sustained contraction similar to that triggered by phenylephrine. Both responses were abolished by a peptide inhibitor of PKC. Contractions of similar magnitude occurred even when the [Ca2+] was reduced to close to zero, implicating a Ca2+-independent isoenzyme of PKC. Of the two Ca2+-independent PKC isoenzymes, ε and ζ, identified in ferret aorta, PKCε is more likely to mediate the contractile response because (i) PKCε, but not PKCζ, is responsive to phorbol esters; (ii) upon stimulation with phenylephrine, PKCε translocates from the sarcoplasm to the sarcolemma, whereas PKCζ translocates from a perinuclear localization to the interior of the nucleus; and (iii) when added to permeabilized single cells of the ferret aorta at pCa 9, PKCε, but not PKCζ, induced a contractile response similar to that induced by phenylephrine. A possible substrate of PKCε is the smooth muscle specific, thin filament associated protein, calponin. Calponin is phosphorylated in intact smooth muscle strips in response to carbachol, endothelin-1, phorbol esters, or okadaic acid. Phosphorylation of calponin in vitro by PKC (a mixture of α, β, and γ isoenzymes) dramatically reduces its affinity for F-actin and alleviates its inhibition of the cross-bridge cycling rate. Calponin is phosphorylated in vitro by PKCε but is a very poor substrate of PKCζ. A signal transduction pathway is proposed to explain Ca2+-independent contraction of ferret aorta whereby extracellular signals trigger diacylglycerol production without a Ca2+transient. The consequent activation of PKCε would result in calponin phosphorylation, its release from the thin filaments, and alleviation of inhibition of cross-bridge cycling. Slow, sustained contraction then results from a slow rate of cross-bridge cycling because of the basal level of myosin light chain phosphorylation (≈0.1 mol Pi/mol light chain). We also suggest that signal transduction through PKCε is a component of contractile responses triggered by agonists that activate phosphoinositide turnover; this may explain why smooth muscles often develop more force in response, e.g., to α1-adrenergic agonists than to K+.Key words: smooth muscle, protein kinase C, calponin.