Intracellular electrical activity of muscularis mucosae of the dog stomach

Mechanical and intracellular electrical activity was recorded simultaneously in vitro from smooth muscle of the muscularis mucosae of the canine antrum. The intracellularly recorded membrane potential averaged -51 ± 1.4 mV (mean ± SE). Spontaneous electrical activity consisted of spike-shaped potentials that were 20-40 mV in amplitude. The rate of rise of the spike potential was slow (<0.2 V/s) and the half-time duration was long (0.5-5.0 s). Phasic contractions were often but not always coupled with spike potentials. Ion substitution studies suggested that the spike potential had a greater dependence on Na⁺ than on Ca²⁺. Field stimulation of intramural nerves hyperpolarized the membrane potential and abolished spikes or reduced their amplitude and frequency. These changes were associated with a reduction in tone and phasic contractile activity. The response to stimulation of inhibitory nerves was mimicked by epinephrine, neurotensin, and vasoactive intestinal polypeptide. The resistance to adrenergic blocking agents ruled out the possibility of norepinephrine as the transmitter. The tetrodotoxin sensitivity of the response to neurotensin suggests that neurotensin acts indirectly through the inhibitory nerves. Mimicry between the action of applied vasoactive intestinal polypeptide (VIP) and field stimulation provides support for the hypothesis that VIP may be an inhibitory neurotransmitter. These studies indicate that smooth muscle in the canine gastric muscularis mucosae generates spontaneous electrical and mechanical activity and receives a noncholinergic, nonadrenergic inhibitory innervation.