Glycogen depletion of the intrafusal fibers in a mouse muscle spindle during prolonged swimming

Abstract

This study investigated the recruitment of different types of intrafusal fibers during prolonged swimming at 60-75% of VO2max. We used 56 male adult mice and examined depletion of glycogen in soleus (Sol) and extensor digitorum longus (EDL) muscle spindles by visual inspection and a newly developed optical scanning method. More than 80% of all spindles from six mice consisted of four fibers: one type I nuclear bag (bag1) fiber, one type II nuclear bag (bag2) fiber, and two nuclear chain fibers. Glycogen content was estimated in muscle fibers from groups of six mice that had rested or swum for either 0.5, 1, 2, 4, or 8 h. The optical scanning intensity of periodic acid Schiff (PAS)-stained sections was correlated with their biochemically determined glycogen content (r = 0.93). Both methods showed fundamentally the same result: each type of intrafusal fiber has its own typical recruitment pattern during exercise. In the initial phase (0-0.5 h), glycogen depletion was largest in nuclear bag1 fibers and insignificant in the bag2 and chain fibers. With the bag1 fibers having become fatigued, nuclear bag2 fibers mainly took over during the middle phase (2-4 h). During the last phase (4-8 h), only the glycogen content of chain fibers decreased significantly (4-8 h). There were significant correlations between the recruitment pattern of bag1 and extrafusal type I fibers in both Sol and EDL, between nuclear bag2 and type IIa fibers in Sol, and between nuclear chain and type IIb fibers in EDL. This suggests that, during moderately intense exercise, glycogen depletion occurs first in the slow, then the intermediate, and, finally, the fast intrafusal fibers.

Excitability of the soleus reflex arc during intensive stretch–shortening cycle exercise in two power-trained athlete groups

“…*P < 0.05 refers to the statistical significance between the values compared to the before-condition lead to presynaptic inhibition of the Ia afferent terminal (Duchateau and Hainaut 1993). If muscle damage occurs, biochemical substances such as bradykin and prostaglandins are known to be released, and these stimulate the spontaneous discharge of the small muscle afferents (Rotto and Kaufman 1988;Yoshimura et al 1996). As mentioned earlier, the present exercise did not cause any stressful metabolic loading.…”

Exercise assessment of transgenic models of human cardiovascular disease

“…Mice have significantly higher weight-corrected values of resting V O 2 than larger mammals (9,11,65). Similar to the variation seen in murine heart rate and blood pressure measurements, routine activities such as grooming or feeding and stresses such as handling can increase V O 2 by up to 50% (9).…”

“…V O 2 max during swimming exercise has also been measured but has not been as well validated. Thus there are discrepancies between values of V O 2 max reported during treadmill exercise and those reported during swimming, with the swimming values usually lower (15,65). This may reflect the difficulty of performing graded exercise in the swimming model and calls into question whether true V O 2 max can be measured with this method.…”

Reduced Bone Mass Accrual in Swim-Trained Prepubertal Mice

“…Swim mice underwent a training program starting at 3.5 wk of age (prepuberty) (6,30). Previous studies report that swimming is a strenuous activity with oxygen consumption (V O 2 ) ranging between 70% and 85% of V O 2max or three to four times of resting values in mice (18,41) and rats (2,23). Swim duration was increased gradually from 5 to 55 min during a period of 2.5 wk (Fig.…”