No abstract

Stretch and release experiments carried out on skinned single fibers of frog skeletal muscle under rigor conditions indicate that the elastic properties of the fiber depend on strain. For modulation frequencies below 1000 Hz, the results show an increase in Young's modulus of 20% upon a stretch of 1 nm/half-sarcomere. Remarkably, the strain dependence of Young's modulus decreases at higher frequencies to about 10% upon a 1-nm/half-sarcomere stretch at a modulation frequency of 10 kHz. This suggests that the cause of the effect is less straightforward than originally believed: a simple slackening of the filaments would result in an equally large strain dependence at all frequencies, whereas strain-dependent properties of the actin filaments should show up most clearly at higher frequencies. We believe that the reduction of the strain dependence points to transitions of the cross-bridges between distinct force-producing states. This is consistent with the earlier observation that Young's modulus in rigor increases toward higher frequencies.

Monte Carlo simulations were employed to model FRET in systems that included interactions between multiple donors and acceptors and photobleaching. Simulations revealed that pixel-to-pixel variations in microscopy experiments can be due both to variations in the probe distribution and to photon noise, depending on the intensity and probes per detection pixel. The Monte Carlo simulations were used to describe fluorescence experiments on (single) actin filaments using a conventional fluorescence microscope. The filaments were labeled with TRITC at phalloidin and Cy5-maleimide at cys374 and the donor−acceptor distances were taken from literature. The images show a large pixel-to-pixel variation in the fluorescence intensity and suffer from photobleaching. In experiments, the main source of noise was the probe distribution. The acceptor suffered from rapid photobleaching, which resulted in an increase of the donor intensity in time and hampers the straightforward determination of the energy transfer efficiency. The photobleaching of both the donor and the acceptor have been included in the simulations, enabling accurate modeling of the time course of the fluorescence intensities of both donor and acceptor and the energy transfer efficiency.

1 Dexrazoxane (DXR, ICRF-187) has been shown both in animal studies and clinical trials to provide a substantial cardioprotection when co-administered with anthracycline drugs like Doxorubicin (DOX). In a previous study, we showed that chronic DOX treatment in rats is associated with a clear impairment of the crossbridge kinetics and shift in myosin iso-enzymes. 2 The present study was adopted to investigate whether the cardioprotective action of DXR involves preservation of the normal actin-myosin interaction. Rats were treated for 4 weeks with either DOX at a weekly dose of 2 mg kg 71 (i.v.), or were pre-injected with DXR (40 mg kg 71, i.v.) at a 20 : 1 dose ratio 30 min prior to the DOX infusion. Rats receiving saline or DXR alone were included in the experiments. Cardiac trabeculae were isolated 4 weeks after the last infusion and were skinned with detergent. 3 Crossbridge turnover kinetics were studied after application of rapid length perturbations of varying amplitudes in Ca 2+ -activated preparations. DXR treatment oered a signi®cant protection against the DOX-induced impairment of the crossbridge kinetics in isolated cardiac trabeculae. Time constants describing transitions between dierent crossbridge states were restored to normal in both the quick release protocol and the slack-test. DXR prevented the shift from the`high ATPase' amyosin heavy chain (MHC) isoform towards the`low-ATPase' b-MHC isoform in the ventricles. 4 We conclude that pre-administration of DXR in rats greatly reduces the deleterious eects of chronic DOX treatment on the trabecular actin ± myosin crossbridge cycle. Preventing direct deleterious eects on the actin ± myosin crossbridge system may provide a new target for preventing or reducing DOX-related cardiotoxicity and may enable patients to continue the treatment beyond currently imposed limits.