Signal-Transduction Pathways that Regulate Smooth Muscle Function I. Signal transduction in phasic (esophageal) and tonic (gastroesophageal sphincter) smooth muscles

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Abstract
Contraction of esophageal (Eso) and lower esophageal sphincter (LES) circular muscle depends on distinct signal-transduction pathways. ACh-induced contraction of Eso muscle is linked to phosphatidylcholine metabolism, production of diacylglycerol and arachidonic acid (AA), and activation of the Ca2+-insensitive PKCε. Although PKCε does not require Ca2+for activation, either influx of extracellular Ca2+or release of Ca2+from stores is needed to activate the phospholipases responsible for hydrolysis of membrane phospholipids and production of second messengers, which activate PKCε. In contrast, the LES uses two distinct intracellular pathways: 1) a PKC-dependent pathway activated by low doses of agonists or during maintenance of spontaneous tone, and 2) a Ca2+-calmodulin-myosin light chain kinase (MLCK)-dependent pathway activated in response to maximally effective doses of agonists during the initial phase of contraction. The Ca2+levels, released by agonist-induced activity of phospholipase C, determine which contractile pathway is activated in the LES. The Ca2+-calmodulin-MLCK-dependent contractile pathway has been well characterized in a variety of smooth muscles. The steps linking activation of PKC to myosin light chain (MLC20) phosphorylation and contraction, however, have not been clearly defined for LES, Eso, or other smooth muscles. In addition, in LES circular muscle, a low-molecular weight pancreatic-like phospholipase A2 (group I PLA2) causes production of AA, which is metabolized to prostaglandins and thromboxanes. These AA metabolites act on receptors linked to heterotrimeric G proteins to induce activation of phospholipases and production of second messengers to maintain contraction of LES circular muscle. We have examined the signal-transduction pathways activated by PGF2αand by thromboxane analogs during the initial contractile phase and found that these pathways are the same as those activated by other agonists. In response to low doses of agonists or during maintenance of tone, presumably due to low levels of calcium release, a PKC-dependent pathway is activated, whereas at high doses of PGF2αand thromboxane analogs, in the initial phase of contraction, calmodulin is activated, PKC activity is reduced, and contraction is mediated, in part, through a Ca2+-calmodulin-MLCK-dependent pathway. The PKC-dependent signaling pathways activated by PGF2αand by thromboxanes during sustained LES contraction, however, remain to be examined, but preliminary data indicate that a distinct PKC-dependent pathway may be activated during maintenance of tonic contraction, which is different from the one activated during the initial contractile response. The initial contractile response to low levels of agonists depends on activation of Gq. Sustained contraction in response to PGF2αmay involve activation of the monomeric G protein RhoA, because the contraction is inhibited by the RhoA-kinase antagonist Y27632. This shift in signal-transduction pathways between initial and sustained contraction has been recently reported in intestinal smooth muscle.