The Effects of Ca2+ and MgADP on Force Development during and after Muscle Length Changes

Fabio C Minozzo,Dilson E Rassier,Miklos S Kellermayer

doi:10.1371/journal.pone.0068866

Fabio C Minozzo, Dilson E Rassier + Show 1 more

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https://doi.org/10.1371/journal.pone.0068866

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Journal: PLoS ONE	Publication Date: Jul 16, 2013
Citations: 16	License type: CC BY 4.0

Affiliation: McGill University, Christie (Canada)

Abstract

The goal of this study was to compare the effects of Ca2+ and MgADP activation on force development in skeletal muscles during and after imposed length changes. Single fibres dissected from the rabbit psoas were (i) activated in pCa2+4.5 and pCa2+6.0, or (ii) activated in pCa2+4.5 before and after administration of 10 mM MgADP. Fibres were activated in sarcomere lengths (SL) of 2.65 µm and 2.95 µm, and subsequently stretched or shortened (5%SL at 1.0 SL.s−1) to reach a final SL of 2.80 µm. The kinetics of force during stretch were not altered by pCa2+ or MgADP, but the fast change in the slope of force development (P1) observed during shortening and the corresponding SL extension required to reach the change (L1) were higher in pCa2+6.0 (P1 = 0.22±0.02 Po; L1 = 5.26±0.24 nm.HS.1) than in pCa2+4.5 (P1 = 0.15±0.01 Po; L1 = 4.48±0.25 nm.HS.1). L1 was also increased by MgADP activation during shortening. Force enhancement after stretch was lower in pCa2+4.5 (14.9±5.4%) than in pCa2+6.0 (38.8±7.5%), while force depression after shortening was similar in both Ca2+ concentrations. The stiffness accompanied the force behavior after length changes in all situations. MgADP did not affect the force behavior after length changes, and stiffness did not accompany the changes in force development after stretch. Altogether, these results suggest that the mechanisms of force generation during and after stretch are different from those obtained during and after shortening.

Highlights

Length changes imposed to activated muscles are commonly used to study the molecular mechanisms of contraction [1,2,3,4,5,6]
It has been suggested that stretch can induce changes in crossbridge kinetics leading to a long-lasting increase in the number of crossbridges attached to actin [16], and that shortening can cause crossbridge deactivation due to newly overlap zone formed between myosin and actin filaments [15,17]
Since we activated fibers on average sarcomere lengths between 2.7 mm and 2.9 mm, the force should be 20–30% lower than that produced at the plateau of the force-length relation, and our values are in close agreement with the literature

Summary

Introduction

Length changes imposed to activated muscles are commonly used to study the molecular mechanisms of contraction [1,2,3,4,5,6]. Force stabilizes at a higher or a lower level than that produced during isometric contractions at corresponding lengths, respectively (phase 4); these phenomena have been referred to as residual force enhancement and force depression [13,14,15]. Attempts to correlate these last-longing changes in force with changes that happen during length changes have proved inconclusive. It has been suggested that stretch can induce changes in crossbridge kinetics leading to a long-lasting increase in the number of crossbridges attached to actin [16], and that shortening can cause crossbridge deactivation due to newly overlap zone formed between myosin and actin filaments [15,17]

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