Abstract
Myosin light-chain phosphorylation is the primary mechanism for activating smooth-muscle contraction and occurs principally at Ser-19 of the 20 kDa light chains of myosin (LC(20)). In some circumstances, Thr-18 phosphorylation may also occur. Protein kinase C (PKC) can regulate LC(20) phosphorylation indirectly via signalling pathways leading to inhibition of myosin light-chain phosphatase. The goal of this study was to determine the relative importance of myosin light-chain kinase (MLCK) and PKC in basal and stimulated LC(20) phosphorylation in rat tail arterial smooth-muscle strips (RTA). Two MLCK inhibitors (ML-9 and wortmannin) and two PKC inhibitors (chelerythrine and calphostin C) that have different mechanisms of action were used. Results showed the following: (i) basal LC(20) phosphorylation in intact RTA is mediated by MLCK; (ii) alpha(1)-adrenoceptor stimulation increases LC(20) phosphorylation via MLCK and PKC; (iii) Ca(2+)-induced LC(20) phosphorylation in Triton X-100-demembranated RTA is catalysed exclusively by MLCK, consistent with the quantitative loss of PKCs alpha and beta following detergent treatment; (iv) very little LC(20) diphosphorylation (i.e. Thr-18 phosphorylation) occurs in intact or demembranated RTA at rest or in response to contractile stimuli; and (v) the level of LC(20) phosphorylation correlates with contraction in intact and demembranated RTA, although the steady-state tension-LC(20) phosphorylation relationship is markedly different between the two preparations such that the basal level of LC(20) phosphorylation in intact muscles is sufficient to generate maximal force in demembranated preparations. This may be due, in part, to differences in the phosphatase/kinase activity ratio, resulting from disruption of a signalling pathway leading to myosin light-chain phosphatase inhibition following detergent treatment.