Low Ca²⁺ impedes cross-bridge detachment in chemically skinned Taenia coli

Abstract

Muscle force is generated by cycling cross-bridges between actin and myosin filaments. In smooth muscle, cyclic attachment and detachment of cross-bridges is thought to be induced by a Ca²⁺- and calmodulin-dependent myosin light chain kinase which phosphorylates myosin. The relaxation that occurs after Ca²⁺ removal is usually ascribed to dephosphorylation of myosin by a phosphatase as non-phosphorylated myosin is unable to form force-generating criss-bridges¹. Recently, Dillon et al.² claimed, however, that dephosphorylation of attached cross-bridges may impede cross-bridge detachment, thus forming so-called ‘latch bridges’. Here we present evidence that after a Ca²⁺- and calmodulin-induced contraction of chemically skinned guinea pig Taenia coli³, the rapid removal of Ca²⁺ impedes the detachment of the myosin cross-bridges from the actin filament; force can then be maintained without energy consumption. The extremely slowly detaching cross-bridges which maintain the force after Ca²⁺ removal may indeed correspond to the ‘latch bridges’ mentioned above.