66
impulse stimulation (34). This relationship is maintained
for rested as well as fatigued muscles. It would appear
that the reduced AT of the fatigued muscle was due to
reduced activation.
A reduced activation of skeletal muscle could,
theoretically be the result of: a) reduced neuromuscular
transmission, b) reduced Ca2 + release, or c) reduced
responsiveness of the contractile elements to Ca2+.
Neuromuscular transmission appears to be intact (34).
This agrees with others (3, 35, 37) who have found only
minimal changes in muscle action potentials or electro
myographic response following comparable stimulation
O i
periods. There is a possibility that Ca release has
been attenuated. This could cause a reduced AT and
dP/dt without altering Ct or Rt 1/2 (15). Brust (10),
who observed similar fatigue patterns with the mouse
soleus muscle, suggests that the fatigue he observed was
a result of reduced Ca2+ release (see also (63) for the
converse). The factor(s) responsible for the reduced
Ca2+ release is/are not known. Bonner et al. (5) have
reported that muscle mitochondria accumulate Ca2+ during
exercise. This would reduce the pool of Ca2+ available
for recycling in the sarcoplasmic reticulum (67) and
consequently limit the amount available for release.
Dantrolene sodium apparently reduces the amount of
Ca2+ released per impulse (24). This drug was used in
these experiments to determine the effects of reduced