De. Rassier et Br. Macintosh, Length dependence of staircase potentiation: interactions with caffeine and dantrolene sodium, CAN J PHYSL, 78(4), 2000, pp. 350-357
In skeletal muscle, there is a length dependence of staircase potentiation
for which the mechanism is unclear. In this study we tested the hypothesis
that abolition of this length dependence by caffeine is effected by a mecha
nism independent of enhanced Ca2+ release. To test this hypothesis we have
used caffeine, which abolishes length dependence of potentiation, and dantr
olene sodium, which inhibits Ca2+ release. In situ isometric twitch contrac
tions of rat gastrocnemius muscle before and after 20 s of repetitive stimu
lation at 5 Hz were analyzed at optimal length (L-o), L-o - 10%, and L-o 10%. Potentiation was observed to be length dependent, with an increase in
developed tension (DT) of 78 +/- 12, 51 +/- 5, and 34 +/- 9% (mean +/- SEM)
, at L-o - 10%, L-o, and L-o + 10%, respectively. Caffeine diminished the l
ength dependence of activation and suppressed the length dependence of stai
rcase potentiation, giving increases in DT of 65 +/- 13, 53 +/- 11, and 45
+/- 12% for L-o - 10%, L-o, and L-o + 10%, respectively. Dantrolene adminis
tered after caffeine did not reverse this effect. Dantrolene alone depresse
d the potentiation response, but did not affect the length dependence of st
aircase potentiation, with increases in DT of 58 +/- 17, 26 +/- 8, and 18 /- 7%, respectively. This study confirms that there is a length dependence
of staircase potentiation in mammalian skeletal muscle which is suppressed
by caffeine. Since dantrolene did not alter this suppression of the length
dependence of potentiation by caffeine, it is apparently not directly modul
ated by Ca2+ availability in the myoplasm.