Calcium-independent activation of the contractile apparatus in smooth muscle of mouse aorta by protein phosphatase inhibition

Smooth muscle contraction is primarily regulated by reversible phosphorylation of the 20-kDa light chains of myosin (MLC(20)) involving Ca(2+)-calmodulin-dependent myosin light chain kinase (MLCK) and serine/threonine protein phosphatases (PP).The aim of this study was to investigate the effects of the protein phosphatases (PP) type 1 (PP1) and type 2A (PP2A) inhibitor cantharidin (Cant), its structural analogue endothall (ETA) and microcystin LR (MC) on force of contraction and MLC(20)-phosphorylation in arterial smooth muscle of mouse aorta. Cant increased force of contraction and MLC(20)-phosphorylation in intact arterial rings of mouse aorta in the presence of Ca(2+) whereas ETA and MC were ineffective under the same experimental conditions. In contrast, all compounds induced contraction and led to enhanced MLC(20)-phosphorylation in nominally Ca(2+)-free solution in fibers of mouse aorta permeabilised (skinned) with Triton X-100.In addition, Western blot analysis revealed that skinning of mouse aorta did not result in a loss of PP1 and PP2A compared to intact rings. Thus, both PP must be tightly bound to structural proteins, e.g. myosin. The findings indicate a Ca(2+)-independent mechanism of smooth muscle contraction involving inhibition of PP1- and/or PP2A-activities leading to enhanced force and MLC(20)-phosphorylation of arterial smooth muscle.