title: Direct measurement of Ca2+ uptake and release by the sarcoplasmic reticulum of saponin permeabilized isolated smooth muscle cells.
abstract: To make direct measurements of Ca2+ uptake and release by the sarcoplasmic reticulum (SR) of isolated smooth muscle cells, a fluorometric method for monitoring Ca2+ uptake by striated muscle SR vesicles (Kargacin, M.E., C.R. Scheid, and T.W. Honeyman. 1988. American Journal of Physiology. 245:C694-C698) was modified. With the method, it was possible to make continuous measurements of SR function in saponin-skinned smooth muscle cells in suspension. Calcium uptake by the SR was inhibited by thapsigargin and sequestered Ca2+ could be released by Br-A23187 and thapsigargin. From the rate of Ca2+ uptake by the skinned cells and the density of cells in suspension, it was possible to calculate the Ca2+ uptake rate for the SR of a single cell. Our results indicate that the SR Ca2+ pump in smooth muscle cells can remove Ca2+ at a rate that is 45-75% of the rate at which Ca2+ is removed from the cytoplasm of intact cells during transient Ca2+ signals. From estimates of SR volume reported by others and our measurements of the amount of Ca2+ taken up by the skinned cells, we conclude that the SR of a single cell can store greater than 10 times the amount of Ca2+ needed to elicit a single transient contractile response.
authors.affiliation: Department of Medical Physiology, University of Calgary, Alberta, Canada.
authors.forename: M E
authors.initials: ME
authors.lastname: Kargacin
authors.forename: G J
authors.initials: GJ
authors.lastname: Kargacin
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D009130:Muscle, Smooth / Q000502:physiology*; D010539:Permeability; D011893:Rana esculenta; D012503:Saponins / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000502:physiology*; D013270:Stomach / Q000502:physiology*; D013997:Time Factors
chemical_list: D012503:Saponins; D002118:Calcium