Pacemaker shift in the gastric antrum of guinea-pigs produced by excitatory vagal stimulation involves intramuscular interstitial cells.

Intracellular recordings were made from isolated bundles of the circular muscle layer of guinea-pig gastric antrum and the responses produced by stimulating intrinsic nerve fibres were examined. After abolishing the effects of stimulating inhibitory nerve terminals with apamin and L-nitroarginine (NOLA), transmural nerve stimulation often evoked a small amplitude excitatory junction potential (EJP) and invariably evoked a regenerative potential. Neurally evoked regenerative potentials had similar properties to those evoked in the same bundle by direct stimulation. EJPs and neurally evoked regenerative potentials were abolished by hyoscine suggesting that both resulted from the release of acetylcholine and activation of muscarinic receptors. Neurally evoked regenerative potentials, but not EJPs, were abolished by membrane hyperpolarization, caffeine and chloride channel blockers. In the intact antrum, excitatory vagal nerve stimulation increased the frequency of slow waves. Simultaneous intracellular recordings of pacemaker potentials from myenteric interstitial cells (ICC(MY)) and slow waves showed that the onset of each pacemaker potential normally preceded the onset of each slow wave but vagal stimulation caused the onset of each slow wave to precede each pacemaker potential. Together the observations suggest that during vagal stimulation there is a change in the origin of pacemaker activity with slow waves being initiated by intramuscular interstitial cells (ICC(IM)) rather than by ICC(MY).

Selective knockout of intramuscular interstitial cells reveals their role in the generation of slow waves in mouse stomach.

1. Intracellular recording techniques were used to compare the patterns of electrical activity generated in the antral region of the stomachs of wild-type and W/W(V) mutant mice. Immunohistochemical techniques were used to determine the distribution of c-kit-positive interstitial cells of Cajal (ICC) within the same region of the stomach. 2. In wild-type mice interstitial cells were found at the level of the myenteric plexus (ICC(MY)) and distributed within the smooth muscle bundles (ICC(IM)). In these preparations slow waves, which consisted of initial and secondary components, were detected. 3. In W/WV mutant mice ICC(MY) could be identified at the level of the myenteric plexus but ICC(IM) were not detected within smooth muscle bundles. Intracellular recordings revealed that smooth muscle cells generated waves of depolarization; these lacked a secondary component. 4. These results indicate that the secondary regenerative component of a slow wave is generated by ICC(IM). Thus the depolarization arising from the pacemaker cells, ICC(MY), is augmented by ICC(IM), so causing a substantial membrane depolarization in the circular muscle layer. Rather than contributing directly to rhythmical electrical activity, smooth muscle cells appear to depolarize at the command of the two subpopulations of ICC.

Vagal inhibition in the antral region of guinea pig stomach.

The effects of vagal stimulation in the presence of a muscarinic antagonist were examined on three distinct rhythmically active cells located in guinea pig antrum. Vagal stimulation inhibited contractions of the circular muscle layer but did not change their rate of occurrence. With the use of intracellular recording techniques, these stimuli were found to initiate inhibitory junction potentials in the circular layer but produced smaller potential changes in driving and follower cells. Inhibition of the circular muscle layer involved two separate components. The dominant component was independent of changes in membrane potential and was abolished by nitro-L-arginine. After abolishing Ca(2+) entry into smooth muscle cells with a Ca(2+) antagonist, vagal stimulation continued to inhibit the residual contractions associated with each slow wave. When the cyclic changes in intracellular Ca(2+) concentration associated with each slow wave were measured, they were found to be unchanged by vagal stimulation. The observations suggest that vagal inhibition of stomach movements does not alter pacemaker activity in the stomach; rather, it results from a change in the sensitivity of smooth muscle contractile proteins to Ca(2+).

Identification of rhythmically active cells in guinea-pig stomach.

1. When intracellular recordings were made from the antral region of guinea-pig stomach, cells with different patterns of electrical activity were detected. 2. One group of cells, slow-wave cells, generated slow waves which consisted of initial and secondary components. When filled with either Lucifer Yellow or neurobiotin, the cells identified as smooth muscle cells lying in the circular muscle layer. 3. A second group of cells, driving cells, generated large, rapidly rising, potential changes, driving potentials. They had small cell bodies with several processes. With neurobiotin, a network of cells was visualized that resembled c-kit positive interstitial cells of the myenteric region. 4. A third group of cells generated sequences of potential changes which resembled driving potentials but had smaller amplitudes and slow rates of rise. These cells resembled smooth muscle cells lying in the longitudinal muscle layer. 5. When simultaneous recordings were made from the driving and slow-wave cells, driving potentials and slow waves occurred synchronously. Current injections indicated that both cell types were part of a common electrical syncytium. 6. The initial component of slow waves persisted in low concentrations of caffeine, but the secondary component was abolished; higher concentrations shortened the duration of the residual initial component. Driving potentials continued in the presence of low concentrations of caffeine; moderate concentrations of caffeine shortened their duration. 7. Hence three different types of cells were distinguished on the basis of their electrical activity, their responses to caffeine and their structure. These were smooth muscle cells, lying in the longitudinal and circular layers, and interstitial cells in the myenteric region. The observations suggest that interstitial cells initiate slow waves.