title: Potentiation of fractional sarcoplasmic reticulum calcium release by total and free intra-sarcoplasmic reticulum calcium concentration.
abstract: Our aim was to measure the influence of sarcoplasmic reticulum (SR) calcium content ([Ca](SRT)) and free SR [Ca] ([Ca](SR)) on the fraction of SR calcium released during voltage clamp steps in isolated rabbit ventricular myocytes. [Ca](SRT), as measured by caffeine application, was progressively increased by conditioning pulses. Sodium was absent in both the intracellular and in the extracellular solutions to block sodium/calcium exchange. Total cytosolic calcium flux during the transient was inferred from I(Ca), [Ca](SRT), [Ca](i), and cellular buffering characteristics. Fluxes via the calcium current (I(Ca)), the SR calcium pump, and passive leak from the SR were evaluated to determine SR calcium release flux (J(rel)). Excitation-contraction (EC) coupling was characterized with respect to both gain (integral J(rel)/integral I(Ca)) and fractional SR calcium release. Both parameters were virtually zero for a small, but measurable [Ca](SRT). Gain and fractional SR calcium release increased steeply and nonlinearly with both [Ca](SRT) and [Ca](SR). We conclude that potentiation of EC coupling can be correlated with both [Ca](SRT) and [Ca](SR). While fractional SR calcium release was not linearly dependent upon [Ca](SR), intra-SR calcium may play a crucial role in regulating the SR calcium release process.
authors.affiliation: Department of Physiology, Loyola University Chicago, Maywood, Illinois 60153 USA.
authors.forename: T R
authors.initials: TR
authors.lastname: Shannon
authors.forename: K S
authors.initials: KS
authors.lastname: Ginsburg
authors.forename: D M
authors.initials: DM
authors.lastname: Bers
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006352:Heart Ventricles; D007700:Kinetics; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000502:physiology*
chemical_list: D002110:Caffeine; D002118:Calcium