Relaxation of diaphragm muscle.
Coirault, Catherine, Denis Chemla and Yves Lecarpentier.
1INSERM U 451-LOA-ENSTA-Ecole Polytechnique, Batterie de l'Yvette,
91761 Palaiseau Cedex and 2 Service d'Explorations Fonctionnelles,
Centre Hospitalier et Universitaire de Bic[circumflex]etre,
Assistance Publique-H[circumflex]opitaux de Paris, 94275 Le Kremlin
-Bic[circumflex]etre, France.
APStracts 6:0259A, 1999.
Relaxation is the process by which the muscle actively returns, after
contraction to its initial conditions of length and load. In
rhythmically active muscles such as diaphragm, relaxation is of
physiological importance because diaphragm must return to a
relatively constant resting position at the end of each contraction
-relaxation cycle. Rapid and complete relaxation of the diaphragm is
likely to play an important role in adaptation to changes in
respiratory load and breathing frequency. Regulation of diaphragm
relaxation at the molecular and cellular levels involves Ca2+ removal
from the myofilaments, active Ca2+ pumping by the sarcoplasmic
reticulum (SR) and decrease in the number of working crossbridges.
The relative contribution of these mechanisms mainly depends on
sarcomere length, muscle tension, and the intrinsic contractile
function. Increased capacity of SR to take up Ca2+ can arise from
increased density of active SR pumping sites or in slow-twitch fibers
from phosphorylation of phospholamban, whereas impaired coupling
between ATP hydrolysis and Ca2+ transport into the SR or
intracellular acidosis reduce SR Ca2+ pump activity. In experimental
conditions of decreased contractile performance, slowed, enhanced or
unchanged relaxation rates have been reported in vitro. In vivo, a
slowing in the rate of decline of the respiratory pressure is
generally considered an early, reliable index of respiratory muscle
fatigue. Impaired relaxation rate may in turn favor mismatch between
blood flow and metabolic demand, especially at high breathing
frequencies.
Received 7 April 1999; accepted in final form 1 June 1999.
APS Manuscript Number A277-9.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 14 June 1999