# Strain rate of stretch affects crossbridge detachment during relaxation of intact cardiac trabeculae

## TL;DR

This study explores how the speed of stretching affects the detachment of crossbridges in heart muscle during relaxation.

## Contribution

The paper introduces new insights into how strain rate influences crossbridge detachment in intact cardiac tissue during early diastole.

## Key findings

- Higher strain rates during relaxation lead to enhanced crossbridge detachment.
- The stress response after ramp-stretches shows features similar to step-stretches in activated myocardium.
- Minimum stress and time-to-minimum stress are not affected by strain rate.

## Abstract

Mechanical Control of Relaxation refers to the dependence of myocardial relaxation on the strain rate just prior to relaxation, but the mechanisms of enhanced relaxation are not well characterized. This study aimed to characterize how crossbridge kinetics varied with strain rate and time-to-stretch as the myocardium relaxed in early diastole. Ramp-stretches of varying rates (amplitude = 1% muscle length) were applied to intact rat cardiac trabeculae following a load-clamp at 50% of the maximal developed twitch force, which provides a first-order estimate of ejection and coupling to an afterload. The resultant stress-response was calculated as the difference between the time-dependent stress profile between load-clamped twitches with and without a ramp-stretch. The stress-response exhibited features of the step-stretch response of activated, permeabilized myocardium, such as distortion-dependent peak stress, rapid force decay related to crossbridge detachment, and stress recovery related to crossbridge recruitment. The peak stress was strain rate dependent, but the minimum stress and the time-to-minimum stress values were not. The initial rapid change in the stress-response indicates enhanced crossbridge detachment at higher strain rates during relaxation in intact cardiac trabeculae. Physiologic considerations, such as time-varying calcium, are discussed as potential limitations to fitting these data with traditional distortion-recruitment models of crossbridge activity.