title: The sarcoplasmic reticulum and sarcolemma together form a passive Ca2+ trap in colonic smooth muscle.
abstract: In smooth muscle, active Ca(2+) uptake into regions of sarcoplasmic reticulum (SR) which are closely apposed to the sarcolemma has been proposed to substantially limit the increase in the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) following Ca(2+) influx, i.e. the 'superficial buffer barrier hypothesis'. The present study has re-examined this proposal. The results suggest that the SR close to the sarcolemma acts as a passive barrier to Ca(2+) influx limiting [Ca(2+)](c) changes; for this, SR Ca(2+) pump activity is not required. In single voltage-clamped colonic myocytes, sustained opening of the ryanodine receptor (RyR) (and depletion of the SR) using ryanodine increased the amplitude of depolarisation-evoked Ca(2+) transients and accelerated the rate of [Ca(2+)](c) decline following depolarisation. These results could be explained by a reduction in the Ca(2+) buffer power of the cytosol taking place when RyR are opened (i.e. the SR is 'leaky'). Indeed, determination of the Ca(2+) buffer power confirmed it was reduced by approximately 40%. Inhibition of the SR Ca(2+) pump (with thapsigargin) also depleted the SR of Ca(2+) but did not reduce the Ca(2+) buffer power or increase depolarisation-evoked Ca(2+) transients and slowed (rather than accelerated) Ca(2+) removal. However, thapsigargin prevented the ryanodine-induced increase in [Ca(2+)](c) decline following depolarisation. Together, these results suggest that when the SR was rendered 'leaky' (a) more of the Ca(2+) entering the cell reached the bulk cytoplasm and (b) Ca(2+) was removed more quickly at the end of cell activation. Under physiological circumstances in the absence of blocking drugs, it is proposed that the SR limits the [Ca(2+)](c) increase following influx without the need for active Ca(2+) uptake. The SR and sarcolemma may form a passive physical barrier to Ca(2+) influx, a Ca(2+) trap, which limits the [Ca(2+)](c) rise occurring during depolarisation by about 50% and from which the ion only slowly escapes into the main part of the cytoplasm.
authors.affiliation: Neuroscience and Biomedical Systems, Institute of Biomedical and Life Sciences, West Medical Building, University of Glasgow, Glasgow, G12 8QQ, UK.
authors.forename: Karen N
authors.initials: KN
authors.lastname: Bradley
authors.forename: John W
authors.initials: JW
authors.lastname: Craig
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Muir
authors.forename: John G
authors.initials: JG
authors.lastname: McCarron
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002469:Cell Separation; D003106:Colon / Q000378:metabolism*; D006168:Guinea Pigs; D008297:Male; D008954:Models, Biological; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D012433:Ryanodine / Q000494:pharmacology; D012508:Sarcolemma / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
chemical_list: D012433:Ryanodine; D019284:Thapsigargin; D002118:Calcium