Comparison of slow waves in multiple and single bundle smooth muscle preparations from the guinea-pig gastric pylorus D.F. van Helden and M.S. Imtiaz, School of Biomedical Sciences, University of Newcastle, N.S.W. 2308, Australia.

Print Abstract Van Helden DF Imtiaz MS

Recently we presented evidence that slow waves (SWs) in single smooth muscle bundles were the consequence of Ca²⁺ phase waves. These arise through Ca²⁺ stores interacting as strongly coupled oscillators, which entrain with a phase delay across the syncytium of cells, such that the oscillatory Ca²⁺ release appears to propagate along the array of cells (van Helden et al., 2003). This mechanism and the resultant generation of regenerative SW depolarizations result through Ca²⁺ release causing depolarization that feeds back to cause further inositol 1,4,5-trisphosphate receptor-mediated Ca²⁺ release. The present study presents comparative data on the properties of SWs in large multi-bundle pyloric tissues.

In vitro experiments were conducted on multi-bundle preparations of circular smooth muscle dissected from the antral side of the gastric pylorus from young guinea-pigs (5-20 days) killed by overexposure to the inhalation anaesthetic halothane (5-10% in air) followed by exsanguination. Tissues were dissected free of the mucosa and excess connective tissue, longitudinal muscle and submucous plexus and pinned onto sylgard in a small (~0.5 ml) volume bath. The tissue was superfused with a physiological saline solution at 35^oC and viewed with an inverted microscope. Intracellular recordings were made with up to 4 microelectrodes with contractions inhibited using nifedipine (1 µM) throughout. Data is presented as mean ± s.e.m.

Recordings normally exhibited regular slow waves though some were less regular demonstrating subthreshold pacemaker potentials and spontaneous transient depolarizations (STDs). SWs were abolished by the SR Ca²⁺ pump inhibitor cyclopiazonic acid (CPA) applied at 10-20 µM for 5-20 min (membrane potential-Vm -63 ± 2 mV in control, -58 ± 3 in CPA, n=4) and by chelation of intracellular Ca²⁺ using BAPTA/AM (20-30 µM, 15 - 50 min application; n=3; no significant change in Vm). The initial effect of both CPA and BAPTA was to transiently expose the underlying pacemaker potentials. These maintained their spatial synchrony and often showed little if any spatial decrement, with longer exposure disrupting all rhythmicity leaving asynchronous STD activity. Multiple microelectrode recordings indicated that SWs exhibited "CVs" of 18 ± 2 mm.s^-1 when recorded along muscle bundles (n=3 tissues, 10 SWs/tissue). SW synchronicity could be markedly enhanced by application of ACh, to tissues that exhibited weak (i.e. intermittent) SWs, with the largest ACh-induced enhancement in "CV" yet recorded changing from 0.9 ± 0.1 (n=10) to 7 ± 1 mm.s^-1 (n=6 SWs) by 60 nM ACh. These findings closely parallel observations made on single muscle bundles, which exhibit very similar characteristics, pharmacology and "CVs" and large ACh-induced enhancement in "CVs" (van Helden et al., 2003). The data are consistent with the interpretation that Ca²⁺ phase waves underlie slow waves both in single bundle and large multi-bundle smooth muscles.

Van Helden DF et al. (2003) J. Physiol. 548.1, 271-296.