Estimates of beat to beat handling of activator calcium using measurements of [Ca2+]i in aequorin loaded ferret cardiac muscle.

Autor:	Urthaler F; Department of Medicine, University of Alabama at Birmingham 35294., Walker AA, Reeves RC, Hefner LL
Jazyk:	angličtina
Zdroj:	Cardiovascular research [Cardiovasc Res] 1994 Jan; Vol. 28 (1), pp. 40-6.
DOI:	10.1093/cvr/28.1.40
Abstrakt:	Objective: The aims were (1) to measure simultaneously and on a beat to beat basis intracellular calcium concentration ([Ca2+]i) transients and force transients in isolated ferret cardiac trabeculae; (2) to obtain and compare independent estimates of the recirculating fraction of Ca2+ using the [Ca2+]i data and the force data (recirculating fraction is the fraction of activator Ca2+ taken up by the sarcoplasmic reticulum in each beat and, in the steady state twitch, the fraction of activator Ca2+ released by the sarcoplasmic reticulum); and (3) to estimate the amount of Ca2+ that returns to the sarcoplasmic reticulum and the amount that, during steady state contractions, enters the cytosol, presumably from the extracellular compartment, with each beat. Methods: Eight trabeculae were mounted in the myograph. The servo-controlled muscle length was 98% of the length at which developed force was maximal. A modified technique was used for chemical loading of aequorin, and a new method for computer controlled low level photon counting, storage, calibration, and analysis. [Ca2+]i transients and force transients were simultaneously recorded during potentiated beats, together with their respective decays toward control steady state [Ca2+]i transients and force transients. A modified test of postextrasystolic potentiation achieved with a brief train of rapid pacing followed by a pause was used to evoke the potentiated beats. Results: At 2.0 mM extracellular Ca2+ ([Ca2+]o), resting [Ca2+]i was 283(SD 77) nM. The resting tension was 1.6(0.3) g.mm-2. The steady state [Ca2+]i transient and the peak potentiated [Ca2+]i transient averaged 992(165) and 1290(154) nM respectively. The corresponding tensions were 4.0(1.9) and 8.7(3.1) g.mm-2 respectively. The recirculating fraction of Ca2+ calculated from the dissipation of the potentiated [Ca2+]i transient averaged 45(4)%. This recirculating fraction was indistinguishable from the one calculated with another method from the decay of the force potentiation. Conclusions: This is the first study to estimate the recirculating fraction of activator Ca2+ using measurements of [Ca2+]i. The results indicate that over a wide range of [Ca2+]i and tensions the Ca(2+)-force relationship is well approximated by a straight line. At 2.0 mM [Ca2+]o it appears that some 450 nM of Ca2+ recirculates and that a similar amount per steady state beat enters the cytosol, probably from the extracellular compartment.
Databáze:	MEDLINE
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