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Department of Physiology and Cell Biology, University of Nevada School of Medicine, Reno, Nevada, 89557, USA
Cyclical periods of depolarization (slow waves) underlie peristaltic contractions involved in mixing and emptying of contents in the gastric antrum. Slow waves originate from a myenteric network of interstitial cells of Cajal (ICC-MY). In this study we have visualized the sequence and propagation of Ca2+ transients associated with pacemaker potentials in the ICC network and longitudinal (LM) and circular muscle (CM) layers of the isolated guinea-pig gastric antrum. Gastric antrum was dissected to reveal the ICC-MY network, loaded with Fluo-4 AM and activity was monitored at 37°C. Ca2+ waves propagated throughout the ICC-MY network at an average velocity of 3.24 ± 0.12 mm s–1 at a frequency of 4.87 ± 0.16 cycles min–1 (n= 4). The propagation of the Ca2+ wave often appeared ‘step-like’, with separate regions of the network being activated after variable delays. The direction of propagation was highly variable (
angle of propagation 44.3 ± 10.9 deg per cycle) and was not confined to the axes of the longitudinal or circular muscle. Ca2+ waves appeared to spread out radially from the site of initiation. The initiating Ca2+ wave in ICC-MY was correlated to secondary Ca2+ waves in intramuscular interstial cells of Cajal, ICC-IM, and smooth muscle cells, and the local distortion (contraction) in a field of view. TTX (1 µM) had little effect on slow wave or pacemaker potential activity, but 2-APB (50 µM) blocked all Ca2+ waves, indicating a pivotal role for intracellular Ca2+ stores. Nicardipine (2 µM) eliminated the Ca2+ transient generated by smooth muscle, but did not affect the fast upstroke associated with ICC-MY. These results indicate that slow waves follow a sequence of activation, beginning with the ICC-MY and ICC-IM network, followed later by a sustained Ca2+ transient in the muscle layers that is responsible for contraction.
(Received 8 December 2003;
accepted after revision 29 January 2004;
first published online 30 January 2004)
Corresponding author T. K. Smith: Department of Physiology and Cell Biology, University of Nevada School of Medicine, Reno, Nevada, 89557, USA. Email: tks{at}physio.unr.edu
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