title: Force relaxes before the fall of cytosolic calcium in the photomechanical response of rat sphincter pupillae.
abstract: In the rat sphincter pupillae, as in other smooth muscles, the primary signal transduction cascade for agonist activation is receptor --> G protein --> phospholipase C --> inositol trisphosphate --> intracellular Ca(2+) concentration ([Ca(2+)](i)) --> calmodulin --> myosin light chain kinase --> phosphorylated myosin --> force development. Light stimulation of isolated sphincters pupillae can be very precisely controlled, and precise reproducible photomechanical responses (PMRs) result. This precision makes the PMR ideal for testing models of regulation of smooth muscle myosin phosphorylation. We measured force and [Ca(2+)](i) concurrently in sphincter pupillae following stimulation by light flashes of varying duration and intensity. We sampled at unusually short (0.01-0.02 s) intervals to adequately test a PMR model based on the myosin phosphorylation cascade. We found, surprisingly, contrary to the behavior of intestinal muscle and predictions of the phosphorylation model, that during PMRs force begins to decay while [Ca(2+)](i) is still rising. We conclude that control of contraction in the sphincter pupillae probably involves an inhibitory process as well as activation by [Ca(2+)](i).
authors.affiliation: Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA. krivoshik.andrew@mayo.edu
authors.forename: A P
authors.initials: AP
authors.lastname: Krivoshik
authors.forename: L
authors.initials: L
authors.lastname: Barr
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism*; D008027:Light; D009119:Muscle Contraction / Q000502:physiology*; D009130:Muscle, Smooth / Q000502:physiology*; D009994:Osmolar Concentration; D010775:Photic Stimulation / Q000379:methods; D011680:Pupil / Q000502:physiology* / Q000528:radiation effects*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D002118:Calcium