Rhythmic electrical and mechanical activity in stomach of toad and
frog.
Prosser, C. Ladd.
Department of Physiology and Biophysics, University of Illinois,
Urbana, IL 61801
APStracts 2:0060G, 1995.
Rhythmic slow spikes and contractions at 2-5 per minute occur in
frog/toad stomach when longitudinal muscle is present, not in the
circular layer after removal of the longitudinal layer. Interstitial
cells of Cajal (ICs) occur in frog stomach in the longitudinal layer,
in toad stomach mainly in longitudinal layer, some between layers.
They were identified by staining with methylene blue, with
fluorescent dye rhodamine 123, toluidine blue and by electron
microscopy. Interstitial cells resemble in ultrastructure those
observed in mammalian intestine by small cell diameter, long
processes forming a network, large nuclei, mitochondria, caveolae,
endoplasmic reticulum and absence of fibrillae. Rhythmic activity in
stomachs stained with methylene blue is abolished by bright
illumination. Rhythmic slow spikes and contractions are reduced and
frequency are lowered when calcium concentration in the medium is
significantly decreased; in high calcium these functions are
increased. Threshold concentration of calcium for maintaining slow
spikes was between 10-7 and 10-6 M Ca2+. Barium at 2.5 mM increases
slow spike frequency and amplitude; barium increases contractions
proportionately more than spikes. Slow spikes are reduced or blocked
by verapamil, nifedipine, and Cd2+ and are enhanced by Bay K 8644.
Quaternary ammonium compounds TEA and TPA prolong slow spikes and
enhance contractions. Replacement of NaCl by LiCl, methyl glutamine,
or treatment with amiloride reduces slow spike amplitude and to a
lesser extent frequency. This indicates efflux of Ca2+ by Ca/Na
exchange. Contractions, but not spikes are reduced by the calmodulin
blocker trifluoperazine. Contractions are enhanced but slow spikes
are not altered by ouabain. This suggests retention of Ca2+ when a
tonic Na/K pump is blocked. A model for rhythmicity includes: influx
of Ca2+ via L-channels, repolarization by IKCa+ efflux of Ca2+ by
Na/Ca exchange and efflux of Na+ by a Na/K pump. Frequency is
determined by rate constants for both influx and efflux of calcium.
Received 19 October 1994; accepted in final form 29 March 1995.
APS Manuscript Number G421-4.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 19 April 1995.