title: Calcium buffering properties of sarcoplasmic reticulum and calcium-induced Ca(2+) release during the quasi-steady level of release in twitch fibers from frog skeletal muscle.
abstract: Experiments were performed to characterize the properties of the intrinsic Ca(2+) buffers in the sarcoplasmic reticulum (SR) of cut fibers from frog twitch muscle. The concentrations of total and free calcium ions within the SR ([Ca(T)](SR) and [Ca(2+)](SR)) were measured, respectively, with the EGTA/phenol red method and tetramethylmurexide (a low affinity Ca(2+) indicator). Results indicate SR Ca(2+) buffering was consistent with a single cooperative-binding component or a combination of a cooperative-binding component and a linear binding component accounting for 20% or less of the bound Ca(2+). Under the assumption of a single cooperative-binding component, the most likely resting values of [Ca(2+)](SR) and [Ca(T)](SR) are 0.67 and 17.1 mM, respectively, and the dissociation constant, Hill coefficient, and concentration of the Ca-binding sites are 0.78 mM, 3.0, and 44 mM, respectively. This information can be used to calculate a variable proportional to the Ca(2+) permeability of the SR, namely d[Ca(T)](SR)/dt ÷ [Ca(2+)](SR) (denoted release permeability), in experiments in which only [Ca(T)](SR) or [Ca(2+)](SR) is measured. In response to a voltage-clamp step to -20 mV at 15°C, the release permeability reaches an early peak followed by a rapid decline to a quasi-steady level that lasts ~50 ms, followed by a slower decline during which the release permeability decreases by at least threefold. During the quasi-steady level of release, the release amplitude is 3.3-fold greater than expected from voltage activation alone, a result consistent with the recruitment by Ca-induced Ca(2+) release of 2.3 SR Ca(2+) release channels neighboring each channel activated by its associated voltage sensor. Release permeability at -60 mV increases as [Ca(T)](SR) decreases from its resting physiological level to ~0.1 of this level. This result argues against a release termination mechanism proposed in mammalian muscle fibers in which a luminal sensor of [Ca(2+)](SR) inhibits release when [Ca(T)](SR) declines to a low level.
authors.affiliation: Département de physiologie et biophysique, Université de Sherbrooke Faculté de Médecine et des Sciences de la Santé, Sherbrooke, Québec J1H5N4, Canada.
authors.forename: Karine
authors.initials: K
authors.lastname: Fénelon
authors.forename: Cédric R H
authors.initials: CR
authors.lastname: Lamboley
authors.forename: Nicole
authors.initials: N
authors.lastname: Carrier
authors.forename: Paul C
authors.initials: PC
authors.lastname: Pape
mesh_terms: D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002155:Calsequestrin / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism* / Q000502:physiology; D011896:Rana temporaria; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: D002021:Buffers; D002155:Calsequestrin; D002118:Calcium