Phorbol 12,13-dibutyrate-mediated contraction of the stimulated bronchial smooth muscles of mice

Increasing evidence suggests that protein kinase C (PKC) is involved in the Ca2+ sensitization of various smooth muscle contractions. However, the exact role of PKC on bronchial smooth muscle (BSM) contraction is still unclear. In the present study, to determine the role of PKC activation in the BSM contraction, the effects of phorbol 12,13-dibutyrate (PDBu) on BSM tone were examined in the absence and presence of contractile stimulation. Although PDBu had no effect on the basal tone, the contraction induced by acetylcholine, high K+ depolarization or Ca2+ ionophore A23187 was significantly augmented by PDBu. Western blot analyses also revealed that the increase in the level of phosphorylated myosin light chain (MLC) induced by high K+ depolarization was significantly augmented by PDBu treatment. Interestingly, neither high K+ depolarization alone nor PDBu alone caused CPI-17 phosphorylation, but a significant phosphorylation of CPI-17 was observed when BSMs were co-stimulated by high K+ and PDBu. Thus, a certain level of intracellular Ca2+ might be needed both for an activation of CPI-17 and an induction of contraction induced by PDBu in mouse BSMs.