Abstract: | We studied in -escin-permeabilized canine tracheal smoothmuscle (CTSM) the effect of the protein kinase C (PKC) agonist phorbol12,13-dibutyrate (PDBu) on isometric force at a constant submaximalCa2+ concentration (i.e., theeffect on Ca2+ sensitivity) andregulatory myosin light-chain (rMLC) phosphorylation. PDBuincreased Ca2+sensitivity, an increase associated with a concentration-dependent, sustained increase in rMLC phosphorylation. PDBu altered therelationship between rMLC phosphorylation and isometric force such thatthe increase in isometric force was less than that expected for the increase in rMLC phosphorylation observed. The effect of four PKCinhibitors calphostin C, chelerythrine chloride, apseudosubstrate inhibitor for PKC, PKC peptide-(19 31) (PSSI), andstaurosporine] on PDBu-inducedCa2+ sensitization as well as theeffect of calphostin C and PSSI on rMLC phosphorylation weredetermined. Whereas none of these compounds prevented or reversed thePDBu-induced increase in Ca2+sensitivity, the PDBu-induced increase in rMLC phosphorylation wasinhibited. We conclude that PDBu increases rMLC phosphorylation byactivation of PKC but that the associated PDBu-induced increases inCa2+ sensitivity are mediated bymechanisms other than activation of PKC in permeabilized airway smoothmuscle. |