The relationship between contractile force and intracellular [Ca2+] in intact rat cardiac trabeculae.
The control of force by [Ca2+] was investigated in rat cardiac trabeculae loaded with fura-2 salt. At sarcomere lengths of 2.1-2.3 microns, the steady state force-[Ca2+]i relationship during tetanization in the presence of ryanodine was half maximally activated at a [Ca2+]i of 0.65 +/- 0.19 microM with a Hill coefficient of 5.2 +/- 1.2 (mean +/- SD, n = 9), and the maximal stress produced at saturating [Ca2+]i equalled 121 +/- 35 mN/mm2 (n = 9). The dependence of steady state force on [Ca2+]i was identical in muscles tetanized in the presence of the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA). The force-[Ca2+]i relationship during the relaxation of twitches in the presence of CPA coincided exactly to that measured at steady state during tetani, suggesting that CPA slows the decay rate of [Ca2+]i sufficiently to allow the force to come into a steady state with the [Ca2+]i. In contrast, the relationship of force to [Ca2+]i during the relaxation phase of control twitches was shifted leftward relative to the steady state relationship, establishing that relaxation is limited by the contractile system itself, not by Ca2+ removal from the cytosol. Under control conditions the force-[Ca2+]i relationship, quantified at the time of peak twitch force (i.e., dF/dt = 0), coincided fairly well with steady state measurements in some trabeculae (i.e., three of seven). However, the force-[Ca2+]i relationship at peak force did not correspond to the steady state measurements after the application of 5 mM 2,3-butanedione monoxime (BDM) (to accelerate cross-bridge kinetics) or 100 microM CPA (to slow the relaxation of the [Ca2+]i transient). Therefore, we conclude that the relationship of force to [Ca2+]i during physiological twitch contractions cannot be used to predict the steady state relationship.
Backx
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PH
Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Gao
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Azan-Backx
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Marban
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D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D005260:Female; D016257:Fura-2; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000502:physiology / Q000648:ultrastructure; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism / Q000648:ultrastructure; D010210:Papillary Muscles / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D012433:Ryanodine / Q000494:pharmacology; D012518:Sarcomeres / Q000502:physiology / Q000648:ultrastructure; D013050:Spectrometry, Fluorescence
D007211:Indoles; D012433:Ryanodine; D000252:Calcium-Transporting ATPases; D002118:Calcium; D016257:Fura-2; C000543:cyclopiazonic acid