Regulation of myocardial contraction as revealed by intracellular Ca2+ measurements using aequorin

Figures

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  Fig. 1. Structure and arrangement of cardiac thin filament proteins in the absence and presence of Ca2+. Upon Ca2+ binding to TnC, the C-terminus region of TnI dissociates from actin, allowing for Tm movement and, consequently, myosin binding to actin (indicated by arrows). Tm, tropomyosin; TnT, troponin T; TnI, troponin I; TnC, troponin C. C, COOH terminus; N, NH2 terminus. The equilibrium between the “off” state and the “on” state is a function of [Ca2+]i. H+ competes with Ca2+ for the regulatory Ca2+-binding site of TnC and thereby apparently inhibits the actomyosin interaction. Modified from Fig. 3 of [38] in an open access article published by Springer Nature with permission.

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  Fig. 2. Schematic illustration showing the intracellular structure of a cardiac myocyte. The influx of Ca2+ from the interstitial fluid upon excitation causes the release of Ca2+ from the sarcoplasmic reticulum (SR). The released Ca2+ binds to troponin on thin filaments and triggers sarcomeric contraction (as in Fig. 1). Relaxation occurs as a result of uptake of Ca2+ by the SR Ca2+ pump, by extrusion of intracellular Ca2+ by Na+-Ca2+ exchangers, and partially by the sarcolemmal Ca2+ pump. It still remains elusive whether mitochondria play a significant role in Ca2+ handling under physiological conditions. The T-tubules and Z-lines run in parallel, causing Ca2+ sparks at/near the Z-lines. Thick and thin filaments, and titin are shown in the sarcomere (for simplicity, two titin molecules per half thick filament are shown). Troponin and tropomyosin exist on thin filaments, regulating actomyosin interaction in a [Ca2+]i-dependent manner (as in Fig. 1). LTCC, L-type Ca2+ channel; RyR, ryanodine receptor; PMCA, plasma membrane Ca2+ ATPase; NCX, Na+-Ca2+ exchanger; NHE, sodium-hydrogen exchanger. Modified from Fig. 2 of [38] in an open access article published by Springer Nature with permission.

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  Fig. 3. Schematic illustration showing the Ca2+-dependent light generation by aequorin. Apo-aequorin is converted to the active form of aequorin when reconstituted by a luminophore coelenterazine in the presence of O2. Coelenterazine is oxidized by binding with three Ca2+ molecules to the respective EF-hands in aequorin; the subsequent conformational change of the protein in association with the release of CO2 produces the singlet-excited coelenteramide that emits blue light (hv ~ 470 nm). The portion of coelenterazine where intramolecular changes occur is indicated in green. Modified from Fig. 1 of [70] with permission.

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  Fig. 4. Effects of changes in muscle length on CaT and tension. A Experimental records showing the changes in CaT and tension following length changes. CaT and tension were measured at Lmax and when a length change was applied to 82% Lmax. Top trace, CaT; middle trace, muscle length; bottom trace, tension. In this experiment, muscle length was changed by a micrometer, and therefore, the CaT recording was discontinued for ~ 20 s during the length change. Cat ventricular papillary muscle was used. B Averaged records of CaT (top) and tension (middle) from 32 twitch contractions over the periods shown in A, i.e., (i), (ii), (iii), (iv) and (v). The “notch” on each bottom trace indicates the stimulation point. Modified from Fig. 2 of [4] with permission.

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  Fig. 5. Effects of quick release on CaT and tension during twitch. CaT and tension were measured at Lmax and when a sudden length change was applied to 92% Lmax at various times before (A) and after (B–D) stimulation (indicated by arrows). Ferret ventricular papillary muscle was used. In each panel, two records obtained at Lmax and 92% Lmax are superimposed. Top trace, muscle length; second trace, CaT; third trace, tension; bottom trace, difference of CaT at Lmax and 92% Lmax (i.e., Extra-Ca2+ as clearly seen in C and D). In A, muscle length was shortened 50 ms before the stimulus. In B–D, muscle length was shortened 22, 75 and 138 ms after the stimulus, respectively. Note that Extra-Ca2+ becomes greater with greater magnitude of a fall of tension (i.e., Extra-Ca2+ greater in D than in C). ΔT, fall of tension upon shortening. Modified from Fig. 1 of [45] with permission.

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  Fig. 6. Effects of isoprenaline on CaT and tension during twitch. Left panel (A–D) shows dose-dependent changes in CaT (top) and tension (bottom). Ferret ventricular papillary muscles were used. Iso, isoprenaline. A, control (without Iso); B, 0.2 µM Iso; C, 0.5 µM Iso; D 1.0 µM Iso. Arrows indicate the stimulation point. Right panel indicates the normalized and superimposed CaT (top) and tension (bottom) with and without 0.2 µM Iso. Left and right panels were obtained from the same preparation. Modified from Fig. 6 of [46] with permission.