P. Schiereck

Effect of sarcomere length and filament lattice spacing on force development in skinned cardiac and skeletal muscle preparations from the rabbit

SummarySkinned cardiac and skeletal muscle freeze-dried preparations were activated in solutions strongly buffered for Ca2+. The response of single skeletal muscle fibres or thin strips of papillary muscle was investigated in relation to changes in Ca content of the perfusate. Sarcomere length was set and controlled during the experiments. The relation between the negative logarithm of the Ca concentration, the pCa, and the normalized developed force proved to be sigmoidal. The exact position of these curves proved to be dependent upon both sarcomere length and the distance between the filaments. The latter was shown by means of osmotic compression of the fibres using dextran. As a consequence of these observations. it was concluded that the length-tension relation is dependent upon the actual Ca concentration. The results are discussed in terms of cross-bridge interaction.

Doxorubicin interacts directly with skinned single skeletal muscle fibres

The effect of doxorubicin, a highly effective anticancer agent, on the contractile apparatus of skinned single muscle fibres was tested in a concentration of 1 microM. Sarcomere length was set and held at 2 microns. Doxorubicin induced an increase in tension dependent on the Ca2+ concentration and time of incubation. The rise was up to 25% at [Ca2+] 40 microM. A parallel, small but significant shift of the calcium sensitivity curve, the relation between normalized tension and the negative logarithm of [Ca2+], the pCa, was observed. The results of this study suggest a direct interaction of doxorubicin with the actin myosin structure, possibly by an effect on myosin-ATP activity.

Influence of the velocity of changes in end-diastolic volume on the starling mechanism of isolated left ventricles.

Abstract1.In this study the relationships between active developed systolic pressure, end-diastolic pressure and different diastolic volumes are studied in Tyrode perfused isolated rabbit left ventricles. Contractions were isovolumic.2.Rapid diastolic volume changes were imposed on top of different preset basic diastolic volumes. These volume changes are shown to produce systolic and diastolic pressure values that cannot be explained by assuming a single pressure-volume relation during systole and diastole. The changes in pressure are in the same direction but higher than is expected on the basis of the increase or decrease of the ventricular end-diastolic volume alone.3.The variation of the diastolic pressure-volume relation cannot be explained by assuming variations of the hearts passive elasticity or viscous effects within its wall. During diastole the effect is completely reversible without concomitant systolic effects. No velocity dependent effect of the quick volume change could be observed if the time duration was varied between 10 and 65 ms. The results are in keeping with the hypothesis that active force generating mechanisms may be present during the diastolic pause.4.The effects observed during systole suggest the possibility of length dependent activation of the myocardial cells. This results in different inotropic conditions of the heart at identical volumes, depending on how these volumes were installed. These volumes may be considered to affect intrinsic properties of the muscle cells on a beat to beat basis.

Correlated neurocardiologic and fitness changes in athletes interrupting training.

PURPOSE We studied nine male Dutch top marathon skaters during a 1-month interruption of their training schedules after their last contest in the winter to investigate a possible decline in baroreflex sensitivity. METHODS Before and after this period, a maximal exercise test was done, and at days 0, 4, 7, 14, and 28 neurocardiologic measurement sessions--heart rate and noninvasive baroreflex sensitivity, recumbent and tilt--were performed. RESULTS Interruption of training resulted in a significant and relevant decrease in the maximal oxygen uptake (from 65.7 +/- 5.8 to 61.6 +/- 4.7 mL O2 x kg(-1) x min(-1); P = 0.03), most likely associated with decreased competitive possibilities. Resting heart rate modestly increased (from 54.6 +/- 7.2 to 58.8 +/- 7.5 bpm), however, not significantly. Heart rate during 60 degrees tilt increased considerably (from 70.1 +/- 6.1 to 80.1 +/- 9.1 bpm; P = 0.01), possibly due to a decrease in blood volume and an increase in cardiopulmonary baroreflex gain. Arterial baroreflex sensitivity decreased significantly in the recumbent (from 13.3 +/- 5.4 to 9.8 +/- 3.8 ms x mm Hg(-1), P = 0.04), but not in the 60 degrees tilt position (from 6.7 +/- 2.0 to 6.0 +/- 2.5 ms x mm Hg(-1)). The relative decrease in baroreflex sensitivity and maximal oxygen uptake correlated significantly (r = 0.71, P = 0.02). CONCLUSIONS In summary, our data show that correlated detrimental changes in fitness and baroreflex sensitivity are measurable in these athletes after a month of interruption of training.

Ca2+ channel antagonists enhance tension in skinned skeletal and heart muscle fibres.

Striated muscle fibres, both skeletal and cardiac of different species including human, skinned by freeze-drying, were activated in solutions strongly buffered for Ca2+. The single fibres were immersed in solutions with different [Ca2+]. Sarcomere length was set and controlled by laser diffraction. Fibre type was determined by Sr2+ activation. The relation between the negative logarithm of the Ca2+ concentration and the normalized tension, the Ca2+ sensitivity curve, was investigated. The effect on the contractile machinery of three different Ca2+ channel antagonists (verapamil, diltiazem and nifedipine) in a therapeutic concentration (10(-6) M) was investigated. The possible effects on the Ca2+ sensitivity curve were quantified by: (1) the change in maximal tension developed at pCa2+ = 4.4; (2) the change in pCa2+ value at which 50% of the tension induced at pCa2+ = 4.4; (3) the steepness of the Ca2+ sensitivity curve in this point. The three drugs tested, at a therapeutic concentration of 1 microM, all enhanced maximal induced tension by respectively 25, 20 and 7%. The sarcomere length dependency of the effect proved to be dependent upon the drug, but also slightly on fibre type (skeletal or cardiac), or on species. It is concluded that the drug influences the cooperativity of the two different types of binding sites on troponin-C (low- and high-affinity sites). Tension enhancement was due to increased stiffness of the actin-myosin interaction site.

Hypertensive Stress Increases Dispersion of Repolarization

Several electrocardiographic indices for repolarization heterogeneity have been proposed previously. The behavior of these indices under two different stressors at the same heart rate (i.e., normotensive gravitational stress, and hypertensive isometric stress) was studied. ECG and blood pressure were recorded in 56 healthy men during rest (sitting with horizontal legs), hypertensive stress (performing handgrip), and normotensive stress (sitting with lowered legs). During both stressors, heart rates differed <10% in 41 subjects, who constituted the final study group. Heart rate increased from 63 ± 9 beats/min at rest to 71 ± 11 beats/min during normotensive, and to 71 ± 10 beats/min during hypertensive stress (P < 0.001). Systolic blood pressure was 122 ± 15 mmHg at rest and 121 ± 15 mmHg during normotensive stress, and increased to 151 ± 17 mmHg during hypertensive stress (P < 0.001). The QT interval was larger during hypertensive (405 ± 27) than during normotensive stress (389 ± 26, P < 0.001). QT dispersion did not differ significantly between the two stressors. The mean interval between the apex and the end of the T wave (Tapex‐Tend) of the mid‐precordial leads was larger during hypertensive (121 ± 17 ms) than during normotensive stress (116 ± 15 ms, P < 0.001). The singular value decomposition T wave index was larger during hypertensive (0.144 ± 0.071) than during normotensive stress (0.089 ± 0.053, P < 0.001). Most indices of repolarization heterogeneity were larger during hypertensive stress than during normotensive stress. Hypertensive stressors are associated with arrhythmogeneity in vulnerable hearts. This may in part be explained by the induction of repolarization heterogeneity by hypertensive stress.

Left Ventricular Force-Velocity Relations Measured from Quick Volume Changes

Left ventricles of rabbit hearts were subjected to series of quick volume releases (QVR) — taking placw within 6 ms — applied at fixed times in the cardiac cycle. The hearts were paced artificially and allowed to contract isovolumically. The QVR was used as a tool for realizing predetermined pressure values at any time during the ascending limb of the intra-ventricular pressure curve. Any desired pressure could be attained by suitable choice of the QVR amplitude. By relatingdP/dt values occuring immediately after the QVR to the pressure attained by the QVR for different QVR amplitudes, instantaneousdP/dt relations were obtained. Time effects on these relations were studied by repeating the QVR series with increasing amplitudes at different but constant times. Influences of volume and contractile state were examined by varying end diastolic pressure (EDP) and the perfusate [Ca2+]. The data were fitted with adP/dt-P relation derived from the Hill equation using a simple geometric model of the ventricle and a two element model of the myocardium. The experimental relations were described adequately by the model. The parameters in the Hill equation estimated for heart muscle were compared to those previously reported on heart muscle experiments. Parameter values obtained were:a/F0: 0.001–1.3;F0 (mean maximal force forVCE=0): 12.2-3.5 N;b: 1.7–13.2 cm/s.F0 rises at the beginning of systole and shows a plateau from ca. 60–100% time of peak pressure. This time course was influenced by changes in EDP and [Ca2+]. Parameterb exhibits a time course comparable to that of ventricular pressure. It was not influenced by EDP changes and only slightly increased by an increase in [Ca2+].

A fluorescence depolarization study of the orientational distribution of crossbridges in muscle fibres.

A fluorescence depolarization study of the orientational distribution of crossbridges in dye-labelled muscle fibres is presented. The characterization of this distribution is important since the rotation of crossbridges is a key element in the theory of muscle contraction. In this study we exploited the advantages of angle-resolved experiments to characterize the principal features of the orientational distribution of the crossbridges in the muscle fibre. The directions of the transition dipole moments in the frame of the dye and the orientation and motion of the dye relative to the crossbridge determined previously were explicitly incorporated into the analysis of the experimental data. This afforded the unequivocal determination of all the second and fourth rank order parameters. Moreover, this additional information provided discrimination between different models for the orientational behaviour of the crossbridges. Our results indicate that no change of orientation takes place upon a transition from rigor to relaxation. The experiments, however, do no rule out a conformational change of the myosin S 1 during the transition.

The effect of applied mechanical vibration on two different phases of rat papillary muscle relaxation

Abstract Applying external mechanical vibration during the relaxation phase of rat papillary muscle decreases the duration of the first part of the relaxation phase. To elucidate the basic mechanism responsible for this shortening of the relaxation period, we applied a controlled vibration to isolated twitching rat papillary muscles during various phases in the relaxation of a twitch. The first part of the relaxation phase was accelerated when length perturbations were applied in the first part of the relaxation of a twitch, dependent on both amplitude and frequency of the perturbation. When vibrations were applied in the first half of the relaxation, the second phase of relaxation was slightly slower (about 20%), but when no vibrations were applied in the first phase, relaxation could be accelerated by applying vibration in the latter half of the relaxation phase. Thus, in the latter half of relaxation, the acceleration of relaxation depended upon perturbation events earlier during that twitch. This study indicates that vibration-induced acceleration of relaxation is due (at least in part) to an apparent increase in detachment rate of attached cross-bridges from the thin filament without substantial reattachment.

Tetragonal deformation of the hexagonal myofilament matrix in single skinned skeletal muscle fibres owing to change in sarcomere length

SummarySingle skinned skeletal muscle fibres were immersed in solutions containing two different levels of activator calicium (pCa: 4.4; 6.0). Sarcomere length was varied from 1.6 to 3.5 Μm and recorded by laser diffraction. Slack length was 2.0 Μm. Small-angle equatorial X-ray diffraction patterns of relaxed and activated fibres at different sarcomere lengths were recorded using synchrotron radiation. The position and amplitude of the diffraction peaks were calculated from the spectra based on the hexagonal arrangement of the myofilament matrix, relating the position of the (1.0)- and (1.1)-diffraction peaks in this model by √3. The diffraction peaks were fitted by five Gaussian functions (1.0, 1.1, 2.0, 2.1 and Z-line) and residual background was corrected by means of a hyperbola. The coupling of the position of the (1.0)- and (1.1)-peak was expressed as a factor: FAC=[d(1.0)/d(1.1)]/√3. In the relaxed state this coupling factor decreased at increasing sarcomere length (0.9880±0.002 at 2.0 Μm; 0.900±0.01 at 3.5 Μm). The coupling factor tends toward the one that will be obtained from the squared structure of actin filaments near the Z-discs. At shorter sarcomere lengths a decrease of the coupling factor has also been seen (0.9600±0.005 at 1.6 Μm), giving rise to an increased uniform deformation of the hexagonal matrix, when sarcomere length is changed from slack length. From these experiments we conclude that a change in sarcomere length (from slack length) increases the deformation of the actin-myosin matrix to a tetragonal lattice.