Journal of Applied Physiology, Vol 69, Issue 3 1093-1097, Copyright © 1990 by American Physiological Society
Depression of posttetanic twitch potentiation by low calcium and calcium channel antagonists
J. H. Williams
Muscular Function Laboratory, Virginia Polytechnic Institute and State University, Blacksburg 24061.
The purpose of this investigation was to determine if antagonizing
extracellular calcium influx altered posttetanic twitch potentiation (PTP).
Whole muscles and muscle fiber bundles (less than or equal to 25 fibers)
dissected from frog sartorius and semitendinosus muscles were mounted at
optimal length in a normal Ringer solution (NR). To determine PTP,
isometric twitches were evoked every 10 s (0.1 Hz) before and after a 2.5-s
tetanic contraction (80 Hz). To antagonize calcium influx, low-calcium
Ringer [LCR, calcium replaced by 3 mM magnesium and 1 mM ethylene
glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid], NR plus
diltiazem (Dilt, 30 microM), NR plus nifedipine (Nif, 10 microM), and NR
plus D 600 (30 microM) were also used (n = 8 for each condition). These
conditions altered pretetanic twitch tension by only -1.2 +/- 2.4, 4.2 +/-
2.3, 4.7 +/- 3.7, and 1.6 +/- 3.7% (SE), (LCR, Dilt, Nif, and D 600, P
greater than 0.05) but caused a noticeable decrease in tension at the end
of the tetanus. Under NR conditions, twitches evoked immediately after the
tetanus were potentiated by 49.5 +/- 0.4% with the peak rate of tension
development (dP/dt) increased by 44.9 +/- 0.5% (P less than 0.05).
Antagonizing calcium influx depressed the PTP response by 59.8 +/- 6.2,
55.9 +/- 10.1, 73.2 +/- 6.8, and 29.8 +/- 3.6% (P less than 0.05) and
increased dP/dt by 65.8 +/- 11.1, 45.7 +/- 8.6, 55.6 +/- 4.4% and 49.0 +/-
10.5% (P less than 0.05). Addition of drugs immediately after the tetanus
only slightly reduced PTP but accelerated recovery of the twitch.(ABSTRACT
TRUNCATED AT 250 WORDS)
