Effect of length and cross-bridge attachment on Ca2+ binding to cardiac troponin C

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Ca2+ binding to cardiac troponin C in the myofilament lattice and its relation to the myofibrillar ATPase activity

The Ca(2+)-binding properties of troponin C in the intact myofilament lattice and their relation to the activation of ATPase were investigated with isolated porcine cardiac myofibrils. Ca2+ binding, which is composed of two classes of binding sites with different affinities (classes 1 and 2), was clearly detected by a novel method for subtracting the large background activity of myofibrillar Ca2+ binding. The classes 1 and 2 were equivalent stoichiometrically to the two high-affinity sites (sites III and IV) and a single low-affinity site (site II) of troponin C. In the presence of ATP, positive cooperativity was observed in the Ca2+ binding of class-2 sites and the Hill equation parameters were in excellent agreement with those for the Ca(2+)-activated myofibrillar ATPase activity, which indicated that the activation of ATPase is a linear function of the Ca2+ occupancy of site II. In the absence of ATP, a marked increase in the affinity of only class-2 sites was observed while the cooperativity was lost. These results provide direct evidence that some feedback mechanism exists between myosin crossbridge attachment and the Ca2+ binding to site II of troponin C, which may thus confer positive cooperativity on the Ca2+ activation of myofibrillar ATPase activity.

Improved mineral balance and skeletal metabolism in postmenopausal women treated with potassium bicarbonate

BACKGROUND

In normal subjects, a low level of metabolic acidosis and positive acid balance (the production of more acid than is excreted) are typically present and correlate in degree with the amount of endogenous acid produced by the metabolism of foods in ordinary diets abundant in protein. Over a lifetime, the counteraction of retained endogenous acid by base mobilized from the skeleton may contribute to the decrease in bone mass that occurs normally with aging.

METHODS

To test that possibility, we administered potassium bicarbonate to 18 postmenopausal women who were given a constant diet (652 mg [16 mmol] of calcium and 96 g of protein per 60 kg of body weight). The potassium bicarbonate was given orally for 18 days in doses (60 to 120 mmol per day) that nearly completely neutralized the endogenous acid.

RESULTS

During the administration of potassium bicarbonate, the calcium and phosphorus balance became less negative or more positive--that is, less was excreted in comparison with the amount ingested (mean [+/- SD] change in calcium balance, +56 +/- 76 mg [1.4 +/- 1.9 mmol] per day per 60 kg; P = 0.009; change in phosphorus balance, +47 +/- 64 mg [1.5 +/- 2.1 mmol] per day per 60 kg; P = 0.007) because of reductions in urinary calcium and phosphorus excretion. The changes in calcium and phosphorus balance were positively correlated (P < 0.001). Serum osteocalcin concentrations increased from 5.5 +/- 2.8 to 6.1 +/- 2.8 ng per milliliter (P < 0.001), and urinary hydroxyproline excretion decreased from 28.9 +/- 12.3 to 26.7 +/- 10.8 mg per day (220 +/- 94 to 204 +/- 82 mumol per day; P = 0.05). Net renal acid excretion decreased from 70.9 +/- 10.1 to 12.8 +/- 21.8 mmol per day, indicating nearly complete neutralization of endogenous acid.

CONCLUSIONS

In postmenopausal women, the oral administration of potassium bicarbonate at a dose sufficient to neutralize endogenous acid improves calcium and phosphorus balance, reduces bone resorption, and increases the rate of bone formation.

Differential effects of length on maximum force production and myofibrillar ATPase activity in rat skinned cardiac muscle

1. The fall of maximum Ca(2+)-activated force of cardiac myofibrils at short muscle lengths could be due to a reduction of cross-bridge cycling or to development of an opposing (restoring) force. To try to distinguish between these possibilities, we measured simultaneously myofibrillar force development and MgATPase activity (a measure of cross-bridge cycling) in rat skinned trabeculae at different muscle lengths. ATPase activity was measured photometrically from the utilization of NADH in a coupled enzyme assay. Muscle length was varied to give estimated 0.2 micron changes in sarcomere length (SL) over the range 1.4-2.4 microns. 2. Both Ca(2+)-activated force development and ATPase activity were optimal at a muscle length (Lo) where the resting SL was 2.2 microns. At Lo the maximum ATPase activity at 21 degrees C was 0.56 +/- 0.05 mM s-1 (mean +/- S.E.M., n = 6), which was equivalent to an ATP turnover per myosin S1 head of 3.3 s-1. 3. The relationship between ATPase activity and SL was curved, with rather little change in ATPase activity over the SL range 2.0-2.4 microns, but significant falls at 1.8 microns and below. At 65% of Lo (corresponding to a mean active SL of approximately 1.4 microns), the ATPase activity was only 50% of its value at 2.2 microns SL. 4. Force development decreased linearly as SL was reduced below 2.2 microns. Force fell by more than ATPase activity, particularly at SL 1.6 and 1.8 microns. 5. The fall of ATPase activity indicates that some of the decline of force production at short SL results from a fall in the net rate of cross-bridge cycling. This is probably the result of double overlap of thin filaments. However, the differential effect on force and ATPase reveals that, in the intermediate range of SL, decreased cross-bridge cycling can account for only part of the fall of force; the remainder is probably due to an increase in a restoring force, which may arise from deformation of the connective tissue in the muscle preparations used.

Effect of fluoride on the rate of dentin apposition and caries progression in young and old Wistar rats

The effect of fluoride in drinking water on the progression of dentinal caries and dentin apposition was studied separately in young and old Wistar rats. The animals were inoculated with Streptococcus sobrinus and fed ad libitum on a 43% sucrose diet and distilled water supplemented with 0, 1, or 19 ppm fluoride. After a 7-wk (young) or 13-wk (adult) cariogenic challenge, the areas of dentinal caries and dentin apposition were quantified after tetracycline staining. Fluoride in the drinking water reduced the progression of dentinal caries and the speed of dentin formation in the young animals but only the progression of dental caries in the adult ones. Both the progression of carious lesions in the dentin and the rate of dentin apposition were 10 times faster during primary dentinogenesis.

Filament overlap affects TnC extraction from skinned muscle fibres

Recent studies on calcium regulation of muscle contraction selectively extract troponin C (TnC) from skinned skeletal muscle fibres with a low ionic strength rigor solution containing a Ca2+/Mg2+ chelator. As previous results from this laboratory and others demonstrate a crossbridge effect, especially rigor, on many of the properties of TnC, the effects of filament overlap on TnC extraction from skinned rabbit psoas muscle fibres were investigated. Tension-pCa relationships at a sarcomere length of 2.7 microns were determined before and after a 5 min TnC extraction at sarcomere lengths of 2.3, 2.5, 2.7, 3.1, 3.3 or 3.5 microns with 20 mM Tris, pH 7.8, 5 mM EDTA. The decrease in the post-extraction maximum Ca2+ activated tension, an indicator of the amount of TnC extracted, was linearly related to the overlap of the thick and thin filaments with decreases in tension being associated with a decrease in filament overlap. The smaller fibre diameter at the longer sarcomere length could facilitate diffusion of TnC from fibre segments. However, the wide range of measured diameters, 40-120 microns, accounted for only 14% of the observed tension decrement and shrinking the fibre with polyvinylpyrrolidone did not increase the tension decrement. Increasing the sarcomere length before extraction was also found to decrease the TnC content of fibre segments along with the post-extraction maximum tension. Thus, TnC appears to be preferentially extracted from non-overlap than overlap regions of the sarcomere. These results further indicate that rigor crossbridges affect TnC other than through increased Ca2+ binding and that under the conditions used here, they retard its extraction.

Alterations in intracellular calcium and tension of activated ferret papillary muscle in response to step length changes

1. To study the effects of mechanical constraints on the calcium (Ca2+) affinity of cardiac troponin C, we analysed the tension and aequorin light (AL, intracellular Ca2+) transients in response to a step length change in aequorin-injected ferret right ventricular papillary muscles. The muscle preparations were continuously activated with ouabain (10(-4) M) (ouabain contracture) or with high frequency stimuli in the presence of ryanodine (5 microM) (tetanic contraction). 2. The tension transient in response to either the release or stretch was oscillatory: tension decreased rapidly during the release and then increased, after which it lapsed into a new steady level in a series of damped oscillations. The opposite was true for the stretch. The oscillatory responses were conspicuous and less damped in ouabain-activated preparations (oscillation frequency of 2.2-2.3 Hz at 22 degrees and 4.5-4.6 Hz at 30 degrees C) and much more damped in ryanodine-treated preparations. 3. The transient AL response was also oscillatory, the time course of which corresponded to that of the transient tension response. Regardless of the difference in the time course of the transients in two different preparations and at two different temperatures, the increase in AL corresponded to the decrease in tension, likewise the decrease in AL to the increase in tension. 4. The mean level of AL after release was lower than the control level present just prior to the release in ouabain-activated preparations, but the AL after release finally returned to the nearly control level in ryanodine-treated preparations. 5. When the ryanodine-treated muscle was further treated with 2,3-butanedione monoxime (BDM) (20 mM), the tetanic tension decreased remarkably without affecting the AL signal. The tension transient of this preparation was quite similar to that of the resting muscle, which changed in a nearly stepwise fashion; AL was hardly affected by step length changes, as in the resting muscle, in spite of the higher AL level. 6. These results suggest that the Ca2+ affinity of cardiac troponin C is increased with an increase in tension (i.e. the cross-bridge attachment) and decreased with a decrease in tension i.e. the cross-bridge detachment), and that the mean [Ca2+]i is lowered by release, at least in a Ca(2+)-overloaded condition, mainly through the sarcoplasmic reticulum.

Subsarcomeric distribution of calcium in demembranated fibers of rabbit psoas muscle

Direct measurements were made of the Ca distribution within sarcomeres of glycerinated rabbit psoas muscle fibers in rigor using electron probe x-ray microanalysis. Both analogue raster analysis and digital x-ray imaging were used to quantitate the Ca distribution along thick and thin filaments as a function of the concentration of free Ca2+. Even when corrected for the estimated contribution of Ca bound to thick filaments, the Ca measured in the region of overlap between thick and thin filaments significantly exceeded the Ca in the I-band at subsaturating concentrations of free Ca2+. At saturating levels of free Ca2+, the excess Ca in the overlap region was diminished but still statistically significant. The data thus suggest that the formation of rigor linkages exerts multiple effects on the binding of Ca2+ to thin filaments in the overlap region by increasing the affinity of troponin C for Ca2+ and possibly by unmasking additional Ca2+ binding sites. The data also show that the cooperativity invested in the thin filaments is insufficient to permit the effects of rigor cross-bridge formation on Ca2+ binding to propagate far along the thin filaments into the I-band.

Calcium kinetic and mechanical regulation of the cardiac muscle

A comprehensive dynamic model of the excitation contraction coupling, developed for a single cardiac muscle, is extended to a multi-cell system (duplex). The model defines the mechanical activation level based on calcium kinetics and crossbridge cycling and emphasizes the central role of the troponin regulatory proteins in regulating muscle activity. The intracellular control mechanism includes two feedback loops that affect the affinity of troponin for calcium and the crossbridge cycling. The model is used to simulate the basic mechanical characteristics of the cardiac muscle, i. e. the force-length and the force-velocity relationships, and describes their dependence on the mechanical activation level. The two-cell duplex unit is used to study the influence of inter-cellular interactions and the effect of inhomogeneity on muscle performance, due to non-uniformity in the electrical stimulation or inhomogeneity in calcium kinetics. Better understanding of the performance of the inhomogeneous muscle is obtained due to our ability to describe the control of the activation level in each cell.

Pyrene-labeled cardiac troponin C. Effect of Ca2+ on monomer and excimer fluorescence in solution and in myofibrils

The two cysteine residues (Cys-35 and Cys-84) of bovine cardiac troponin C (cTnC) were labeled with the pyrene-containing SH-reactive compounds, N-(1-pyrene) maleimide, and N-(1-pyrene)iodoacetamide in order to study conformational changes in the regulatory domain of cTnC associated with cation binding and cross-bridge attachment. The labeled cTnC exhibits the characteristic fluorescence spectrum of pyrene with two sharp monomer fluorescence peaks and one broad excimer fluorescence peak. The excimer fluorescence results from dimerization of adjacent pyrene groups. With metal binding (Mg2+ or Ca2+) to the high affinity sites of cTnC (sites III and IV), there is a small decrease in monomer fluorescence but no effect on excimer fluorescence. In contrast, Ca2+ binding to the low affinity regulatory (site II) site elicits an increase in monomer fluorescence and a reduction in excimer fluorescence. These results can be accounted for by assuming that the pyrene attached to Cys-84 is drawn into a hydrophobic pocket formed by the binding of Ca2+ to site II. When the labeled cTnC is incorporated into the troponin complex or substituted into cardiac myofibrils the monomer fluorescence is enhanced while the excimer fluorescence is reduced. This suggests that the association with other regulatory components in the thin filament might influence the proximity (or mobility) of the two pyrene groups in a way similar to that of Ca2+ binding. With the binding of Ca2+ to site II the excimer fluorescence is further reduced while the monomer fluorescence is not changed significantly. In myofibrils, cross-bridge detachment (5 mM MgATP, pCa 8.0) causes a reduction in monomer fluorescence but has no effect on excimer fluorescence. However, saturation of the cTnC with Ca2+ reduces excimer fluorescence but causes no further change in monomer fluorescence. Thus, the pyrene fluorescence spectra define the different conformations of cTnC associated with weak-binding, cycling, and rigor cross-bridges.

Calcium concentration in the myoplasm of skinned ferret ventricular muscle following changes in muscle length

1. Ferret ventricular muscles were skinned by prolonged application of Triton X-100. Aequorin was allowed to diffuse into the myoplasmic space and the resulting light emission was used to monitor the myoplasmic [Ca2+]. The muscle was then activated with a lightly buffered Ca2+ solution and the changes in myoplasmic [Ca2+] and tension in response to length changes were investigated. 2. A sudden reduction in muscle length led to a rapid increase in myoplasmic [Ca2+] to a new level which was maintained as long as muscle length was reduced and which was reversed when the muscle was stretched back to the control length. The rate of increase of [Ca2+] when the muscle length was reduced was greater than the rate of decrease in [Ca2+] when the muscle was stretched. 3. Increasing the concentration of EGTA in the activating solution, so as to increase its Ca2+-buffering capacity, eliminated the changes in myoplasmic [Ca2+] in response to a length change but had little effect on developed tension. 4. On stretching the muscle there was a slow component of recovery of tension with a time course broadly similar to the rate of decrease of myoplasmic [Ca2+]. The time course of tension redevelopment and of the accompanying reduction in myoplasmic [Ca2+] both decreased to a similar extent when the [Ca2+] used to activate the muscle was increased. 5. Step reductions of length of increasing amplitude caused increases in myoplasmic [Ca2+] which were larger in proportion to the size of the step. 6. Step reductions of length of equal size but from different starting lengths caused changes in myoplasmic [Ca2+] the amplitude of which correlated with the change in tension rather than the change in length. 7. The increase in myoplasmic [Ca2+] when muscle length is reduced suggests that Ca2+ is released from a site in the muscle, probably troponin C. The time course and magnitude of the changes in myoplasmic [Ca2+] correlate more closely with the changes in developed tension than muscle length.