Journal of Applied Physiology, Vol 65, Issue 3 1410-1415, Copyright © 1988 by American Physiological Society
Effect of theophylline on the velocity of diaphragmatic muscle shortening
G. S. Supinski, J. S. Arnold and S. G. Kelsen
Department of Medicine, Case Western Reserve University, Cleveland, Ohio 44106.
The present study examined the effect of theophylline on the shortening
velocity of submaximally activated diaphragmatic muscle (i.e., muscles were
activated by the use of a level of stimulation, 50 Hz, within the range of
phrenic neural firing frequencies achieved during breathing, whereas
maximum activation is achieved at 300 Hz). Experiments were performed in
vitro on strips of diaphragmatic muscle obtained from 21 Syrian hamsters.
Muscle shortening velocity was assessed during isotonic contractions
against a range of afterloads, and Hill's characteristic equation was used
to calculate velocity at zero load. In addition, unloaded shortening
velocity was also measured by the slack test, i.e., from the time required
for muscles to take up slack after a sudden reduction in muscle length.
Theophylline (160 mg/l) increased the velocity of muscle shortening against
a wide range of external loads (0-14 N/cm2) and increased the extrapolated
unloaded velocity of shortening from 6.4 +/- 0.9 to 7.9 +/- 1.1 (SE)
lengths/s (P less than 0.01). Theophylline reduced the time required to
take up slack for any given step change in muscle length, increasing the
unloaded velocity of shortening assessed by the slack test from 7.6 +/- 0.9
to 9.3 +/- 1.1 lengths/s (P less than 0.002). The effect of theophylline on
diaphragmatic shortening velocity was evident at concentrations as low as
40 mg/l and increased progressively as theophylline concentrations were
increased to 320 mg/l. Theophylline increased the shortening velocity of
fatigued as well as fresh muscles.(ABSTRACT TRUNCATED AT 250 WORDS)
