Biomedical subjects

Kenneth S Campbell

Publications and source records attributed to Kenneth S Campbell.

8 recordsLinked to original sources

PubMed2006

The rate of tension recovery in cardiac muscle correlates with the relative residual tension prevailing after restretch.

Isolated cardiac muscles generate tension more quickly at higher levels of Ca(2+) activation. We investigated the molecular mechanisms underlying this effect in permeabilized rat myocardial preparations by measuring the rate of tension recovery following brief shortening/restretch perturbations. Separate series of experiments used Ca(2+)-activating solutions with different pH values (pH 6.75, 7.00, and 7.25) and different phosphate (P(i)) concentrations (0, 2.5, and 5.0 mM added P(i)) to modulate the recovery kinetics. Subsequent analysis showed that the rate of tension recovery correlated (P < 0.001) with the relative residual tension, that is, the minimum tension measured immediately after restretch normalized to the steady-state isometric tension for the experimental condition. This new finding suggests that the rate at which cardiac muscles develop force increases with the proportion of cross bridges bound to the thin filament and is strong evidence of cooperative contractile activation.

Actin Cytoskeleton↗

PubMed2006

Filament compliance effects can explain tension overshoots during force development.

Spatially explicit stochastic simulations of myosin S1 heads attaching to a single actin filament were used to investigate the process of force development in contracting muscle. Filament compliance effects were incorporated by adjusting the spacing between adjacent actin binding sites and adjacent myosin heads in response to cross-bridge attachment/detachment events. Appropriate model parameters were determined by multi-dimensional optimization and used to simulate force development records corresponding to different levels of Ca(2+) activation. Simulations in which the spacing between both adjacent actin binding sites and adjacent myosin S1 heads changed by approximately 0.06 nm after cross-bridge attachment/detachment events 1), exhibited tension overshoots with a Ca(2+) dependence similar to that measured experimentally and 2), mimicked the observed k(tr)-relative tension relationship without invoking a Ca(2+)-dependent increase in the rate of cross-bridge state transitions. Tension did not overshoot its steady-state value in control simulations modeling rigid thick and thin filaments with otherwise identical parameters. These results underline the importance of filament geometry and actin binding site availability in quantitative theories of muscle contraction.

Actins↗

PubMed2005

Tension recovery in permeabilized rat soleus muscle fibers after rapid shortening and restretch.

Permeabilized rat soleus muscle fibers were subjected to rapid shortening/restretch protocols (20% muscle length, 20 ms duration) in solutions with pCa values ranging from 6.5 to 4.5. Force redeveloped after each restretch but temporarily exceeded the steady-state isometric tension reaching a maximum value approximately 2.5 s after relengthening. The relative size of the overshoot was <5% in pCa 6.5 and pCa 4.5 solutions but equaled 17% +/- 4% at pCa 6.0 (approximately half-maximal Ca2+ activation). Muscle stiffness was estimated during pCa 6.0 activations by imposing length steps at different time intervals after repeated shortening/restretch perturbations. Relative stiffness and relative tension were correlated (p < 0.001) during recovery, suggesting that tension overshoots reflect a temporary increase in the number of attached cross-bridges. Rates of tension recovery (k(tr)) correlated (p < 0.001) with the relative residual force prevailing immediately after restretch. Force also recovered to the isometric value more quickly at 5.7 < or = pCa < or = 5.9 than at pCa 4.5 (ANOVA, p < 0.05). These results show that k(tr) measurements underestimate the rate of isometric force development during submaximal Ca2+ activations and suggest that the rate of tension recovery is limited primarily by the availability of actin binding sites.

Animals↗

PubMed2005

Developmental changes in rat cardiac titin/connectin: transitions in normal animals and in mutants with a delayed pattern of isoform transition.

Rat cardiac titin undergoes developmental changes in isoform expression during the period from late embryonic through the first 20-25 days of life. At least five size classes of titin isoforms have been identified using SDS agarose gel electrophoresis. The longest normal isoform is expressed in the embryonic stages, and it is progressively replaced with increasingly smaller versions. The isoform switching is consistent with changes in resting tension from lower values in one-day neonates to higher levels in adult myocytes. Considerable micro-heterogeneity in alternative splicing patterns also was found, particularly in the N2BA PEVK region of human, rat, and dog ventricle. A rat mutation has been identified in which the embryonic-neonatal titin isoform transitions are markedly delayed. These mutant animals may prove useful for examining the role of titin in stretch-activated signal transduction and in the Frank-Starling relationship.

Alternative Splicing↗

PubMed2004

Titin isoform changes in rat myocardium during development.

Developmental changes in the alternative splicing patterns of titin were observed in rat cardiac muscle. Titin from 16-day fetal hearts consisted of a single 3710 kDa band on SDS agarose gels, and it disappeared by 10 days after birth. The major adult N2B isoform (2990 kDa) first appeared in 18-day fetal hearts and its proportion in the ventricle increased to approximately 85% from 20 days of age and older. Changes in three other intermediate-sized N2BA isoform bands also occurred during this same time period. The cDNA sequences of fetal cardiac, adult ventricle, and adult soleus were different in the PEVK and alternatively spliced middle Ig domain. Extensive heterogeneity in splice patterns was found in the N2BA PEVK region. The extra length of the fetal titin isoforms appeared to be due to both a greater number of middle Ig domains expressed plus the inclusion of more PEVK exons. Passive tension measurements on myocyte-sized fragments indicated a significantly lower tension in neonate versus adult ventricles at sarcomere lengths greater than 2.1 microm, consistent with the protein and cDNA sequence results. The time course of the titin isoform switching was similar to that occurring with myosin and troponin I during development.

Alternative Splicing↗

PubMed2003

SLControl: PC-based data acquisition and analysis for muscle mechanics.

SLControl is a computerized data acquisition and analysis system that was developed in our laboratory to help perform mechanical experiments using striated muscle preparations. It consists of a computer program (Windows 2000 or later) and a commercially available data acquisition board (16-bit resolution, DAP5216a, Microstar Laboratories, Bellevue, WA). Signals from the user's existing equipment representing force, fiber length (FL), and (if desired) sarcomere length (SL) are connected to the system through standard Bayonet Neill Concelman cables and saved to data files for later analysis. Output signals from the board control FL and trigger additional equipment, e.g., flash lamps. Windows dialogs drive several different experimental protocols, including slack tests and rate of tension recovery measurements. Precise measurements of muscle stiffness and force velocity/power characteristics can also be accomplished using SL and tension control, respectively. In these situations, the FL command signal is updated in real time (at rates > or =2.5 kHz) in response to changes in the measured SL or force signals. Data files can be exported as raw text or analyzed within SLControl with the use of built-in tools for cursor analysis, digital filtering, curve fitting, etc. The software is available for free download at http://www.slcontrol.com.

Algorithms↗

PubMed2003

Cycling cross-bridges increase myocardial stiffness at submaximal levels of Ca2+ activation.

Permeabilized multicellular preparations of canine myocardium were subjected to controlled length changes to investigate the extent to which cross-bridges augment passive stiffness components in myocardium at low levels of Ca(2+) activation. When the preparations were immersed in pCa 9.0 solution (negligible free [Ca(2+)]) they behaved as simple elastic systems (i.e., tension increased proportionately with length). In contrast, when the muscles were stretched in Ca(2+) activating solutions, tension rose much more rapidly during the initial phase of the movement than thereafter. Several lines of evidence suggest that the nonlinear response represents the displacement of populations of cycling cross-bridges that are perturbed by interfilamentary movement and take some time to recover. 1), The stiffness of the initial phase increased proportionately with the level of Ca(2+) activation. 2), The magnitude of the short-range response increased with stretch velocity. 3), The initial response was reversibly reduced by 5-mM 2,3-butanedione monoxime, a known cross-bridge inhibitor. The initial stiffness of the passive elastic (pCa 9.0) response was equivalent to the Ca(2+) dependent component at 2% (pCa approximately 6.2) of the maximal (pCa 4.5) level. These results suggest that cross-bridges may significantly affect diastolic chamber stiffness.

Adaptation, Physiological↗

PubMed2002

History-dependent mechanical properties of permeabilized rat soleus muscle fibers.

Permeabilized rat soleus muscle fibers were subjected to repeated triangular length changes (paired ramp stretches/releases, 0.03 l(0), +/- 0.1 l(0) s(-1) imposed under sarcomere length control) to investigate whether the rate of stiffness recovery after movement increased with the level of Ca(2+) activation. Actively contracting fibers exhibited a characteristic tension response to stretch: tension rose sharply during the initial phase of the movement before dropping slightly to a plateau, which was maintained during the remainder of the stretch. When the fibers were stretched twice, the initial phase of the response was reduced by an amount that depended on both the level of Ca(2+) activation and the elapsed time since the first movement. Detailed analysis revealed three new and important findings. 1) The rates of stiffness and tension recovery and 2) the relative height of the tension plateau each increased with the level of Ca(2+) activation. 3) The tension plateau developed more quickly during the second stretch at high free Ca(2+) concentrations than at low. These findings are consistent with a cross-bridge mechanism but suggest that the rate of the force-generating power-stroke increases with the intracellular Ca(2+) concentration and cross-bridge strain.

Animals↗