KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively "simple" mechanism involving activation of voltage-operated Ca2+ channels that leads to increases in cytosolic free Ca2+ ([Ca2+]i), Ca2+-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca2+ sensitization, the relationship between [Ca2+]i and force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca2+]i and force does not exist: for a given increase in [Ca2+]i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca2+ desensitization. Such changes in Ca2+ sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca2+ sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca2+ sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca2+ sensitivity of smooth muscle contraction. KCl as a "simple" stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca2+ sensitivity.
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