Masaomi Tajimi (#), Masatoshi Hori, Hiroshi Ozaki (*) and Hideaki Karaki
Department of Veterinary Pharmacology, Graduate School of Agriculture
and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
(#) Present address: Bayer Yakuhin, Ltd., Institute of Allergy, Research Center Kyoto, Soraku-gun, Kyoto 619-02, Japan
(*) To whom correspondence should be addressed.
Abstract: To determine the role of protein kinase C (PKC) in bovine tracheal smooth muscle contractility, we examined the effects of phorbol esters on cytosolic Ca2+ level ([Ca2+]i), myosin light chain (MLC) phosphorylation and contractile force in intact muscle and contraction in a permeabilized preparation. In intact muscle, 12-deoxyphorbol 13-isobutyrate (DPB, 1 microM) increased the force without changing [Ca2+]i. High K+ (72.7 mM) induced sustained contraction with sustained increase in [Ca2+]i. In the muscle stimulated by high K+, 50 nM DPB increased the contractile force without changing [Ca2+]i, and 1 microM DPB increased the contractile force with decreasing [Ca2+]i. Thus DPB shifted the [Ca2+]i/force relationship for high K+ to the lower [Ca2+]i in a concentration-dependent manner. In permeabilized muscle, DPB did not induce contraction in the absence of Ca2+ (<<0 nM), but shifted the Ca2+/force relationship to the lower Ca2+ levels. In the muscle stimulated with high K+, DPB (50 nM and 1 microM) increased MLC phosphorylation and force without changing the MLC phosphorylation/force relationship. DPB (1 microM) increased PKC activity estimated by the translocation from the cytoplasm to the membrane. These results suggest that DPB increases the Ca2+ sensitivity of MLC phosphorylation via the activation of PKC. Furthermore, DPB at higher concentration has an inhibitory effect on stimulated [Ca2+]i.
Keywords: Protein kinase C, Phorbol ester, Myosin phosphorylation, Cytosolic Ca2+ level, Tracheal smooth muscle