The paradox of smooth muscle physiology

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (contraction) and the release of force (relaxation). The signaling events that activate contr action include Ca2+-dependent myosin light chain phosphorylation. The signa ling events that mediate relaxation include the removal of a contractile ag onist (passive relaxation) and activation of cyclic nucleotide-dependent si gnaling pathways in the continued presence of a contractile agonist (active relaxation). The major questions that remain in contractile physiology inc lude (1) how is tonic force maintained when intracellular Ca2+ levels and m yosin light chain phosphorylation have returned to basal levels; and (2) wh at is the mechanism of cyclic nucleotide-dependent relaxation? This review focuses on these specific controversies surrounding the molecular mechanism s of contraction and relaxation of vascular smooth muscle. (C) 2001 Elsevie r Science Ireland Ltd. All rights reserved.