Effect of Phorbol Esters on Cytosolic Ca²⁺ Level, Myosin Phosphorylation and Muscle Tension in High K⁺ -Stimulated Bovine Tracheal Smooth Muscle

抄録

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 μM)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 μM 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 μM)increased MLC phosphorylation and force without changing the MLC phosphorylation/force relationship. DPB (1 μM) 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.