Contractile properties of the rat external abdominal oblique and diaphragm muscles during development

We studied the in vitro contractile and fatigue properties of the rat external abdominal oblique (EAO) and costal diaphragm (DIA) muscles during postnatal development. Isometric twitch contraction (CT) and half-relaxation (RT( 1/2 )) times were longer in both the EAO and DIA muscles during the early postnatal period and decreased with age. In the first postnatal week, the CT and RT( 1/2 ) were longer in the EAO than the DIA muscle. At 14 days of age and thereafter, the CT and RT( 1/2 ) were shorter in the EAO than in the DIA muscle. Force-frequency relationships of the EAO and DIA muscles changed during postnatal development such that the relative force (percent maximum) generated at lower frequencies (<15 pulses/s) decreased with age. Moreover the relative force generated by the EAO muscle at lower frequencies was greater than that of the DIA muscle during the early postnatal period but less than that of the DIA muscle in adults. The specific force of both the EAO and DIA muscles increased progressively with age. There were no differences in specific force between the EAO and DIA muscles at any age. The fatigability of the EAO and DIA muscles was comparable during the early postnatal period and increased in both muscles with postnatal development. In adults the EAO muscle was more fatigable than the DIA muscle. We conclude that the contractile and fatigue properties of the EAO and DIA muscles undergo significantly different postnatal transitions, which may reflect their functional involvement in sustaining ventilation.