Force relaxes before the fall of cytosolic calcium in the photomechanical response of rat sphincter pupillae |
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Authors: | Krivoshik A P Barr L |
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Institution: | Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. krivoshik.andrew@mayo.edu |
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Abstract: | In the ratsphincter pupillae, as in other smooth muscles, the primary signaltransduction cascade for agonist activation is receptor G protein phospholipase C inositol trisphosphate intracellularCa2+ concentration (Ca2+]i) calmodulin myosin light chain kinase phosphorylated myosin force development. Light stimulation of isolated sphincters pupillaecan be very precisely controlled, and precise reproducible photomechanical responses (PMRs) result. This precision makes the PMRideal for testing models of regulation of smooth muscle myosinphosphorylation. We measured force andCa2+]i concurrently in sphincter pupillaefollowing stimulation by light flashes of varying duration andintensity. We sampled at unusually short (0.01-0.02 s) intervalsto adequately test a PMR model based on the myosin phosphorylationcascade. We found, surprisingly, contrary to the behavior of intestinalmuscle and predictions of the phosphorylation model, that during PMRsforce begins to decay while Ca2+]i is stillrising. We conclude that control of contraction in the sphincterpupillae probably involves an inhibitory process as well as activationby Ca2+]i. |
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