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J Appl Physiol 87: 1243-1252, 1999;
8750-7587/99 $5.00
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Vol. 87, Issue 4, 1243-1252, October 1999

INVITED REVIEW
Relaxation of diaphragm muscle

Catherine Coirault1, Denis Chemla2, and Yves Lecarpentier2

1 Laboratoire d'Optique Appliquée, Ecole Nationale Supérieure des Techniques Avancées, Ecole Polytechnique, Institut National de la Santé et de la Recherche Médicale U 451, Batterie de l'Yvette, 91761 Palaiseau Cedex; and 2 Service d'Explorations Fonctionnelles, Centre Hospitalier et Universitaire de Bicêtre, Assistance Publique-Hôpitaux de Paris, 94275 Le Kremlin-Bicêtre, France

Relaxation is the process by which, after contraction, the muscle actively returns 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 cross bridges. 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 reduces 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.

in vivo; diaphram muscle; in vitro


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