Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.2.853

Regulation of Contraction in Striated Muscle

Calcium channels in excitable membranes are of great importance for many cellular functions. Modulation of these channels by neurotransmitters and drugs regulates calcium influx into the cell and thereby alters the functional state of the cell. Recently it has become possible to measure properties of single calcium channels directly and to obtain evidence on mechanisms of their modulation.

Calcium channel modulation by neurotransmitters, enzymes and drugs

Equations have been developed to describe cardiac action potentials and pacemaker activity. The model takes account of extensive developments in experimental work since the formulation of the M.N.T. (R. E. McAllister, D. Noble and R. W. Tsien, J. Physiol., Lond. 251, 1-59 (1975)) and B.R. (G. W. Beeler and H. Reuter, J. Physiol., Lond. 268, 177-210 (1977)) equations. The current mechanism i $\_{K2}$ has been replaced by the hyperpolarizing-activated current, i $\_f$ . Depletion and accumulation of potassium ions in the extracellular space are represented either by partial differential equations for diffusion in cylindrical or spherical preparations or, when such accuracy is not essential, by a three-compartment model in which the extracellular concentration in the intercellular space is uniform. The description of the delayed K current, i $\_K$ , remains based on the work of D. Noble and R. W. Tsien (J. Physiol., Lond. 200, 205-231 (1969a)). The instantaneous inward-rectifier, i $\_{K1}$ , is based on S. Hagiwara and K. Takahashi's equation (J. Membrane Biol. 18, 61-80 (1974)) and on the patch clamp studies of B. Sakmann and G. Trube (J. Physiol., Lond. 347, 641-658 (1984)) and of Y. Momose, G. Szabo and W. R. Giles (Biophys. J. 41, 311a (1983)). The equations successfully account for all the properties formerly attributed to i $\_{K2}$ , as well as giving more complete descriptions of i $\_{K1}$ and i $\_K$ . The sodium current equations are based on experimental data of T. J. Colatsky (J. Physiol., Lond. 305, 215-234 (1980)) and A. M. Brown, K. S. Lee and T. Powell (J. Physiol., Lond. 318, 479-500 (1981)). The equations correctly reproduce the range and magnitude of the sodium \`window' current. The second inward current is based in part on the data of H. Reuter and H. Scholz (J. Physiol., Lond. 264, 17-47 (1977)) and K. S. Lee and R. W. Tsien (Nature, Lond. 297, 498-501 (1982)) so far as the ion selectivity is concerned. However, the activation and inactivation gating kinetics have been greatly speeded up to reproduce the very much faster currents recorded in recent work. A major consequence of this change is that Ca current inactivation mostly occurs very early in the action potential plateau. The sodium-potassium exchange pump equations are based on data reported by D. C. Gadsby (Proc. natn. Acad. Sci. U.S.A. 77, 4035-4039 (1980)) and by D. A. Eisner and W. J. Lederer (J. Physiol., Lond. 303, 441-474 (1980)). The sodium-calcium exchange current is based on L. J. Mullins' equations (J. gen. Physiol. 70, 681-695 (1977)). Intracellular calcium sequestration is represented by simple equations for uptake into a reticulum store which then reprimes a release store. The repriming equations use the data of W. R. Gibbons & H. A. Fozzard (J. gen. Physiol. 65, 367-384 (1975b)). Following Fabiato & Fabiato's work (J. Physiol., Lond. 249, 469-495 (1975)), Ca release is assumed to be triggered by intracellular free calcium. The equations reproduce the essential features of intracellular free calcium transients as measured with aequorin. The explanatory range of the model entirely includes and greatly extends that of the M.N.T. equations. Despite the major changes made, the overall time-course of the conductance changes to potassium ions strongly resembles that of the M.N.T. model. There are however important differences in the time courses of Na and Ca conductance changes. The Na conductance now includes a component due to the hyperpolarizing-activated current, i $\_f$ , which slowly increases during the pacemaker depolarization. The Ca conductance changes are very much faster than in the M.N.T. model so that in action potentials longer than about 50 ms the primary contribution of the fast gated calcium channel to the plateau is due to a steady-state \`window' current or non-inactivated component. Slower calcium or Ca-activated currents, such as the Na-Ca exchange current, or Ca-gated currents, or a much slower Ca channel must then play the dynamic role previously attributed to the kinetics of a single type of calcium channel. This feature of the model in turn means that the repolarization process should be related to the inotropic state, as indicated by experimental work. The model successfully reproduces intracellular sodium concentration changes produced by variations in [Na] $\_o$ , or Na-K pump block. The sodium dependence of the overshoot potential is well reproduced despite the fact that steady state intracellular Na is proportional to extracellular Na, as in the experimental results of D. Ellis J. Physiol., Lond. 274, 211-240 (1977)). The model reproduces the responses to current pulses applied during the plateau and pacemaker phases. In particular, a substantial net decrease in conductance is predicted during the pacemaker depolarization despite the fact that the controlling process is an increase in conductance for the hyperpolarizing-activated current. The immediate effects of changing extracellular [K] are reproduced, including: (i) the shortening of action potential duration and suppression of pacemaker activity at high [K]; (ii) the increased automaticity at moderately low [K]; and (iii) the depolarization to the plateau range with premature depolarizations and low voltage oscillations at very low [K]. The ionic currents attributed to changes in Na-K pump activity are well reproduced. It is shown that the apparent K $\_m$ for K activation of the pump depends strongly on the size of the restricted extracellular space. With a 30% space (as in canine Purkinje fibres) the apparent K $\_m$ is close to the assumed real value of 1 mM. When the extracellular space is reduced to below 5%, the apparent K $\_m$ increases by up to an order of magnitude. A substantial part of the pump is then not available for inhibition by low [K] $\_b$ . These results can explain the apparent discrepancies in the literature concerning the K $\_m$ for pump activation.

A Model of Cardiac Electrical Activity Incorporating Ionic Pumps and Concentration Changes

Magnesium: Nature's physiologic calcium blocker

Assessment of left ventricular systolic and diastolic pump properties is fundamental to advancing the understanding of cardiovascular pathophysiology and therapeutics, especially for heart failure. The use of end-systolic and end-diastolic pressure-volume relationships derived from measurements of instantaneous left ventricular pressure-volume loops emerged in the 1970s as a comprehensive approach for this purpose. As invasive and noninvasive techniques for measuring ventricular volume improved over the past decades, these relations have become commonly used by basic, translational, and clinical researchers. This review summarizes 1) the basic concepts underlying pressure-volume analysis of ventricular and myocardial systolic and diastolic properties, 2) deviations from ideal conditions typically encountered in real-life applications, 3) how these relationships are appropriately analyzed, including statistical analyses, and 4) the most common problems encountered by investigators and the appropriate remedies. The goal is to provide practical information and simple guidelines for accurate application and interpretation of pressure-volume data as they pertain to characterization of ventricular and myocardial properties in health and disease.

Assessment of systolic and diastolic ventricular properties via pressure-volume analysis: a guide for clinical, translational, and basic researchers.

1. The calcium-sensitive photoprotein aequorin was micro-injected into cells of rat and cat ventricular muscles. The resulting light emission is a function of intracellular free calcium concentration ([Ca2+]i). The transient increases in [Ca2+]i that accompany contraction were monitored. 2. After an increase in muscle length, the developed tension increased immediately and then showed a slow increase over a period of minutes. The peak [Ca2+]i in each contraction was initially unchanged after an increase in muscle length but then showed a slow increase with a time course similar to that of the slow tension change. 3. As a consequence of these slow changes, the shape of the tension-length relation depends on the procedure used to determine it and this change in shape can be attributed to changes in activation. 4. Immediately after an increase in muscle length the calcium transient was abbreviated. 5. When a quick release was performed during a contraction, a short-lived increase in the [Ca2+]i was observed following the release. 6. The two previous observations can both be explained if the binding constant of troponin for calcium is a function of developed tension.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1982.sp014221

The effects of muscle length on intracellular calcium transients in mammalian cardiac muscle.

The calcium-sensitive photoprotein aequorin was microinjected into cells of rat and cat ventricular muscle. During subsequent stimulation of the muscle light emission could be detected and this signal is a function of the intracellular [Ca++]. The time course and amplitude of the intracellular [Ca++] transient occurring during contraction is described. The effects on tension and light emission of changing external [Ca++] and stimulus frequency and of adding adrenaline and caffeine to the bathing solution are described. These results show that changes in external calcium and stimulus frequency alter tension by means of changes in the intracellular [Ca++] whereas adrenaline in addition alters the sensitivity of the contractile system to intracellular [Ca++]. The results also suggest that although a fall in intracellular [Ca++] always precedes relaxation, the time course of the fall of [Ca++] is not generally a rate limiting step in the lime course of relaxation.

S. Kurihara

Papers

The effects of muscle length on intracellular calcium transients in mammalian cardiac muscle.

Calcium transients in mammalian ventricular muscle.