RELATIONSHIP BETWEEN PAXILLIN AND MYOSIN PHOSPHORYLATION DURING MUSCARINIC STIMULATION OF SMOOTH-MUSCLE

The tyrosine phosphorylation of paxillin increases in association with force development during tracheal smooth muscle contraction, suggesti ng that paxillin plays a role in the contractile activation of smooth muscle [Z. L. Wang, F. M. Pavalko, and S. J. Gunst. Am. J. Physiol. 27 1 (Cell Physiol. 40): C1594-C1602, 1996]. We compared the Ca2+ sensiti vity of the tyrosine phosphorylation of paxillin and myosin light chai n (MLC) phosphorylation in tracheal muscle and evaluated whether MLC p hosphorylation is necessary to induce paxillin phosphorylation. Ca2+-d epleted muscle strips were stimulated with 10(-7)-10(-4) M acetylcholi ne (ACh) in 0, 0.05, 0.1, or 0.5 mM extracellular Ca2+. In the absence of extracellular Ca2+, 10(-4) M ACh induced a maximal increase in pax illin phosphorylation without increasing MLC phosphorylation or force. Increases in extracellular Ca2+ concentration did not further increas e paxillin phosphorylation. However, during stimulation with 10(-6) MA Ch, paxillin phosphorylation increased with increases in extracellular Ca2+ concentration. We conclude that the tyrosine phosphorylation of paxillin can be stimulated by signaling pathways that do not depend on Ca2+ mobilization and that the activation of contractile proteins is not required to elicit paxillin phosphorylation.