Bryan K. Slinker

Nonlinear Myofilament Regulatory Processes Affect Frequency-Dependent Muscle Fiber Stiffness

To investigate the role of nonlinear myofilament regulatory processes in sarcomeric mechanodynamics, a model of myofilament kinetic processes, including thin filament on-off kinetics and crossbridge cycling kinetics with interactions within and between kinetic processes, was built to predict sarcomeric stiffness dynamics. Linear decomposition of this highly nonlinear model resulted in the identification of distinct contributions by kinetics of recruitment and by kinetics of distortion to the complex stiffness of the sarcomere. Further, it was established that nonlinear kinetic processes, such as those associated with cooperative neighbor interactions or length-dependent crossbridge attachment, contributed unique features to the stiffness spectrum through their effect on recruitment. Myofilament model-derived sarcomeric stiffness reproduces experimentally measured sarcomeric stiffness with remarkable fidelity. Consequently, characteristic features of the experimentally determined stiffness spectrum become interpretable in terms of the underlying contractile mechanisms that are responsible for specific dynamic behaviors.

The AT4 receptor agonist [Nle1]-angiotensin IV reduces mechanically induced immediate-early gene expression in the isolated rabbit heart

Angiotensin II (ANG II), acting principally at the AT1 receptor, modulates mechanically-induced cardiac growth. The ANG II metabolite Angiotensin IV (ANG IV) has been shown to inhibit ANG II-induced mRNA and protein synthesis in chick cardiomyocytes. This effect did not involve the AT1 receptor, but was likely an action at the AT4 receptor. To determine if ANG IV also modulates a mechanically-induced cardiac growth response, we studied the effects of two AT4 receptor ligands, [Nle1]-ANG IV and [divalinal]-ANG IV, on mechanically-induced immediate-early gene expression (c-fos, egr-1, and c-jun) in the buffer perfused (30 degrees C), ejecting, isolated rabbit heart. Mechanical load alone (high systolic pressure and high end-diastolic volume) induced approximately 23-, 49- and 5-fold increases in c-fos, egr-1 and c-jun mRNA (in comparison to control hearts). Perfusion with [Nle1]-ANG IV (10[-10] mol/l) reduced the mechanically-induced expression of c-/fos and egr-1 by 42% and 48%, respectively (P < 0.05). Mechanically-induced c-jun expression was not significantly reduced. Perfusion with [divalinal]-ANG IV (10[-8] mol/l) had no effect on mechanically-induced immediate-early gene expression. We conclude that AT4 receptor agonism influences mechanical immediate-early gene expression, and propose the hypothesis that AT1 and AT4 receptors initiate opposing effects on mechanically-induced immediate-early gene expression in the isolated rabbit left ventricle.

Myofilament kinetics in isometric twitch dynamics.

AbstractTo better understand the relationship between kinetic processes of contraction and the dynamic features of an isometric twitch, studies were conducted using a mathematical model that included: (1) kinetics of cross bridge (XB) cycling; (2) kinetics of thin filament regulatory processes; (3) serial and feedback interactions between these two kinetic processes; and (4) time course of calcium activation. Isometric twitch wave forms were predicted, morphometric features of the predicted twitch wave form were evaluated, and sensitivities of wave form morphometric features to model kinetic parameters were assessed. Initially, the impulse response of the XB cycle alone was analyzed with the findings that dynamic constants of the twitch transient were much faster than turnover number of steady-state XB cycling, and, although speed and duration of the twitch wave form were sensitive to XB cycle kinetic constants, parameters of wave shape were not. When thin filament regulatory unit (RU) kinetics were added to XB cycle kinetics, the system impulse response was slowed with only little effect on wave shape. When cooperative neighbor interactions between RU and XB were added, twitch wave shape (as well as amplitude, speed and duration) proved to be sensitive to variation in cooperativity. Importantly, persistence and shape of the falling phase could be strongly modified. When kinetic coefficients of XB attachment were made to depend on sarcomere length, changes in wave shape occurred that did not occur when only sliding filament mechanisms were operative. Indeed, the force–length relationship proved to be highly sensitive to length-dependent XB attachment in combination with cooperative interactions. These model findings are the basis of hypotheses for the role of specific kinetic events of contraction in generating twitch wave form features.

Angiotensin IV has mixed effects on left ventricle systolic function and speeds relaxation.

OBJECTIVE A novel angiotensin receptor has been described and named AT4. Ligands for this receptor include the angiotensin II (Ang II) metabolite Ang II (3-8), known as angiotensin IV (Ang IV). There is 10-fold more AT4 receptor than AT1 receptor in rabbit myocardium. The AT4 receptor has a high affinity for Ang IV (Ki in rabbit myocardium < 2 x 10(-9)) and similar ligands, but very low affinity for Ang II (Ki in rabbit myocardium > 10(-6)). Although several functions have been attributed to the novel Ang IV peptide/AT4 receptor system, the effect of this system on left ventricular (LV) function has not been studied. We hypothesized (1) that Ang IV would affect LV function and (2) that any effects would be opposite to those of Ang II. METHODS Using the buffer-perfused (30 degrees C) isolated rabbit heart, we studied the effect of the AT4 agonist Nle1-Ang IV on LV systolic function, quantified using both Frank-Starling and end-systolic pressure-volume relationships, and relaxation. We also studied the effect of the AT1/AT2 agonist, Sar1-Ang II on LV function. Finally, because the profile of effect of Nle1-Ang IV was similar to the reported effect of nitric oxide (NO), we also studied the effect of Nle1-Ang IV in the presence of the NO synthase inhibitor NG-monomethyl-L-arginine. RESULTS Nle1-Ang IV reduced LV pressure-generating capability at any volume but increased the sensitivity of pressure development to volume change. Nle1-Ang IV reduced LV ejection capability. Sar1-Ang II had the opposite effect-increasing both pressure generation and ejection capability. Finally, both Sar1-Ang II and Nle1-Ang IV speeded LV relaxation. Inhibition of NO synthase did not alter the effect of Nle1-Ang IV on LV systolic function or relaxation. CONCLUSIONS AT4 receptor agonism has mixed effects on LV systolic function, depressing pressure-generation and ejection capabilities, but enhancing the sensitivity of pressure development to volume change. It also speeds relaxation. The effect of Ang IV on systolic function is generally opposite to the effect of Ang II, whereas the Ang IV influence on relaxation is similar to the effect of Ang II.

Overall cardiac functional effect of positive inotropic drugs with differing effects on relaxation.

Recent interest in so-called calcium-sensitizing positive inotropic drugs has highlighted the potential problem of a positive effect on force development being offset, at least partially, by the negative effect that many of these drugs have on relaxation. The purpose of this study was to examine the interplay of contraction and relaxation in determining the overall cardiac effect of different positive inotropic drugs. Using a buffer-perfused isolated rabbit heart preparation, we studied four drugs (calcium, dobutamine, EMD 57033, and CGP 48506) that were given at doses sufficient to increase similarly left ventricular pressure-generating capability by approximately 20%. We show that, even though they produce equivalent changes in pressure-generating capability, these four agents produce dissimilar changes in relaxation capability, with dobutamine speeding relaxation, EMD 57033 slowing relaxation, and calcium and CGP 48506 having little effect of relaxation. Similar relative effects were observed for drug-induced changes in the timing of pressure-generation events. These effects combine to produce different drug-induced changes in overall cardiac pump function judged by the relation between cardiac output and heart rate. Dobutamine shifted the maximal cardiac output to a higher heart rate. In contrast, both calcium sensitizers shifted the maximum in cardiac output to a lower heart rate, whereas calcium had no effect. Thus even though positive inotropic drugs may have similar effects on left ventricular pressure generation, the overall benefit of such drugs on ventricular pump function will depend on how the drug also affects ventricular relaxation and ejection capabilities.

Relaxation effect of CGP-48506, EMD-57033, and dobutamine in ejecting and isovolumically beating rabbit hearts

Because it is not known whether ejection influences the negative effect of the Ca(2+)-sensitizing drugs on ventricular relaxation, we extended our previous analysis of stress-dependent relaxation in isovolumic beats to encompass ejecting beats and evaluated the relationships between both the time of onset of relaxation and the rate of relaxation and wall stress in a broader analysis framework. Furthermore, because the sites of action of the Ca(2+)-sensitizing drugs CGP-48506 and EMD-57033 may be different, and thus CGP-48506 may have fewer adverse effects on resting muscle length or force, we compared these two drugs to test the hypothesis that CGP-48506 would have less effect than EMD-57033 on relaxation in the isolated buffer-perfused rabbit heart. This analysis of stress-dependent relaxation in both ejecting and isovolumic beats readily differentiates between the negative lusitropic effect of 2 x 10(-6) M EMD-57033, the negligible lusitropic effect of 6 x 10(-6) M CGP-48506, and the positive lusitropic effect of 1.25 x 10(-6) M dobutamine. Furthermore, comparison of the effect of the two Ca(2+)-sensitizing drugs in ejecting versus isovolumic contractions shows that CGP-48506 affects relaxation differently in ejecting contractions than it does in isovolumic contractions, whereas EMD-57033 affects relaxation similarly in both ejecting and isovolumic contractions.Because it is not known whether ejection influences the negative effect of the Ca2+-sensitizing drugs on ventricular relaxation, we extended our previous analysis of stress-dependent relaxation in isovolumic beats to encompass ejecting beats and evaluated the relationships between both the time of onset of relaxation and the rate of relaxation and wall stress in a broader analysis framework. Furthermore, because the sites of action of the Ca2+-sensitizing drugs CGP-48506 and EMD-57033 may be different, and thus CGP-48506 may have fewer adverse effects on resting muscle length or force, we compared these two drugs to test the hypothesis that CGP-48506 would have less effect than EMD-57033 on relaxation in the isolated buffer-perfused rabbit heart. This analysis of stress-dependent relaxation in both ejecting and isovolumic beats readily differentiates between the negative lusitropic effect of 2 × 10-6 M EMD-57033, the negligible lusitropic effect of 6 × 10-6 M CGP-48506, and the positive lusitropic effect of 1.25 × 10-6 M dobutamine. Furthermore, comparison of the effect of the two Ca2+-sensitizing drugs in ejecting versus isovolumic contractions shows that CGP-48506 affects relaxation differently in ejecting contractions than it does in isovolumic contractions, whereas EMD-57033 affects relaxation similarly in both ejecting and isovolumic contractions.

Left ventricular pressure response to small-amplitude, sinusoidal volume changes in isolated rabbit heart

The objective was to determine the dynamics of contractile processes from pressure responses to small-amplitude, sinusoidal volume changes in the left ventricle of the beating heart. Hearts were isolated from 14 anesthetized rabbits and paced at 1 beats/s. Volume was perturbed sinusoidally at four frequencies (f) (25, 50, 76.9, and 100 Hz) and five amplitudes (0.50, 0.75, 1.00, 1.25, and 1.50% of baseline volume). A prominent component of the pressure response occurred at the f of perturbation [infrequency response, delta Pf(t)]. A model, based on cross-bridge mechanisms and containing both pre- and postpower stroke states, was constructed to interpret delta Pf(t). Model predictions were that delta Pf(t) consisted of two parts: a part with an amplitude rising and falling in proportion to the pressure around that which delta Pf(t) occurred [Pr(t)], and a part with an amplitude rising and falling in proportion to the derivative of Pr(t) with time. Statistical analysis revealed that both parts were significant. Additional model predictions concerning response amplitude and phase were also confirmed statistically. The model was further validated by fitting simultaneously to all delta Pf(t) over the full range of f and delta V in a given heart. Residual errors from fitting were small (R2 = 0.978) and were not systematically distributed. Elaborations of the model to include noncontractile series elastance and distortion-dependent cross-bridge detachment did not improve the ability to represent the data. We concluded that the model could be used to identify cross-bridge rate constants in the whole heart from responses to 25- to 100-Hz sinusoidal volume perturbations.The objective was to determine the dynamics of contractile processes from pressure responses to small-amplitude, sinusoidal volume changes in the left ventricle of the beating heart. Hearts were isolated from 14 anesthetized rabbits and paced at 1 beats/s. Volume was perturbed sinusoidally at four frequencies ( f ) (25, 50, 76.9, and 100 Hz) and five amplitudes (0.50, 0.75, 1.00, 1.25, and 1.50% of baseline volume). A prominent component of the pressure response occurred at the f of perturbation [in-frequency response,[Formula: see text] ( t)]. A model, based on cross-bridge mechanisms and containing both pre- and postpower stroke states, was constructed to interpret[Formula: see text] ( t). Model predictions were that[Formula: see text] ( t) consisted of two parts: a part with an amplitude rising and falling in proportion to the pressure around that which[Formula: see text] ( t) occurred [Pr( t)], and a part with an amplitude rising and falling in proportion to the derivative of Pr( t) with time. Statistical analysis revealed that both parts were significant. Additional model predictions concerning response amplitude and phase were also confirmed statistically. The model was further validated by fitting simultaneously to all[Formula: see text] ( t) over the full range of f and ΔV in a given heart. Residual errors from fitting were small ( R 2 = 0.978) and were not systematically distributed. Elaborations of the model to include noncontractile series elastance and distortion-dependent cross-bridge detachment did not improve the ability to represent the data. We concluded that the model could be used to identify cross-bridge rate constants in the whole heart from responses to 25- to 100-Hz sinusoidal volume perturbations.

Myocardial contractile depression from high-frequency vibration is not due to increased cross-bridge breakage

Experiments were conducted in 10 isolated rabbit hearts at 25°C to test the hypothesis that vibration-induced depression of myocardial contractile function was the result of increased cross-bridge breakage. Small-amplitude sinusoidal changes in left ventricular volume were administered at frequencies of 25, 50, and 76.9 Hz. The resulting pressure response consisted of a depressive response [ΔPd( t), a sustained decrease in pressure that was not at the perturbation frequency] and an in-frequency response [ΔP f ( t), that part at the perturbation frequency]. ΔPd( t) represented the effects of contractile depression. A cross-bridge model was applied to ΔP f ( t) to estimate cross-bridge cycling parameters. Responses were obtained during Ca2+ activation and during Sr2+ activation when the time course of pressure development was slowed by a factor of 3. ΔPd( t) was strongly affected by whether the responses were activated by Ca2+ or by Sr2+. In the Sr2+-activated state, ΔPd( t) declined while pressure was rising and relaxation rate decreased. During Ca2+ and Sr2+ activation, velocity of myofilament sliding was insignificant as a predictor of ΔPd( t) or, when it was significant, participated by reducing ΔPd( t) rather than contributing to its magnitude. Furthermore, there was no difference in cross-bridge cycling rate constants when the Ca2+-activated state was compared with the Sr2+-activated state. An increase in cross-bridge detachment rate constant with volume-induced change in cross-bridge distortion could not be detected. Finally, processes responsible for ΔPd( t) occurred at slower frequencies than those of cross-bridge detachment. Collectively, these results argue against a cross-bridge detachment basis for vibration-induced myocardial depression.Experiments were conducted in 10 isolated rabbit hearts at 25 degrees C to test the hypothesis that vibration-induced depression of myocardial contractile function was the result of increased cross-bridge breakage. Small-amplitude sinusoidal changes in left ventricular volume were administered at frequencies of 25, 50, and 76.9 Hz. The resulting pressure response consisted of a depressive response [delta Pd(t), a sustained decrease in pressure that was not at the perturbation frequency] and an infrequency response [delta Pf(t), that part at the perturbation frequency]. delta Pd(t) represented the effects of contractile depression. A cross-bridge model was applied to delta Pf(t) to estimate cross-bridge cycling parameters. Responses were obtained during Ca2+ activation and during Sr2+ activation when the time course of pressure development was slowed by a factor of 3. delta Pd(t) was strongly affected by whether the responses were activated by Ca2+ or by Sr2+. In the Sr(2+)-activated state, delta Pd(t) declined while pressure was rising and relaxation rate decreased. During Ca2+ and Sr2+ activation, velocity of myofilament sliding was insignificant as a predictor of delta Pd(t) or, when it was significant, participated by reducing delta Pd(t) rather than contributing to its magnitude. Furthermore, there was no difference in cross-bridge cycling rate constants when the Ca(2+)-activated state was compared with the Sr(2+)-activated state. An increase in cross-bridge detachment rate constant with volume-induced change in cross-bridge distortion could not be detected. Finally, processes responsible for delta Pd(t) occurred at slower frequencies than those of cross-bridge detachment. Collectively, these results argue against a cross-bridge detachment basis for vibration-induced myocardial depression.