The infiltration of neutrophils following passive‐stretch‐conditioning is required for protection from contraction‐induced injury

Abstract

Exposing skeletal muscle to repeated stretches without activation (passive stretches) results in no evidence of overt damage, but reduces injury following subsequent lengthening contractions. Despite no overt damage, passive stretches cause neutrophil infiltration. We hypothesized that the neutrophil infiltration serves to initiate protective adaptations that result from passive-stretch-conditioning. To deplete the level of neutrophils, mice were administered RB6-8C5 antibody prior to exposure of the extensor digitorum longus (EDL) muscles to either 75 lengthening contractions (LCs) or 75 passive stretches. Fourteen days later, when neutrophil levels had recovered to control levels, passive-stretch-conditioned muscles were administered 75 LCs. Muscles of untreated mice also underwent 75 LCs. Maximum isometric force, damaged fibers, and infiltrating neutrophils were measured 3 days after LCs. For unconditioned muscles of RB6-8C5-treated compared with untreated mice, the deficits in isometric force and numbers of injured fibers were less by half, confirming previous observations that a reduction in neutrophil infiltration following lengthening contractions is sufficient to reduce injury. The deficits in isometric force and numbers of injured fibers for passive-stretch-conditioned muscles of RB6-8C5-treated mice were not different from unconditioned muscles of untreated mice. We conclude that neutrophil infiltration following passive stretches is required for the protection from injury provided by passive-stretch-conditioning. Supported by AG-20591.