Intracellular microelectrode recording to characterize inhibitory neuromuscular transmission in jejunum of horses

Objective-To evaluate electrical activity of jejunal circular muscle in hor ses and characterize electrical responses to stimulation by intrinsic inhib itory neurons. Sample Population-Portions of jejunum obtained from horses euthanatized for reasons other than gastrointestinal tract disease. Procedure-isolated circular muscle preparations were perfused with oxygenat ed modified Krebs solution. Glass microelectrodes were used for intracellul ar recording of membrane potentials from single smooth muscle cells. Electr ical activity and responses to electrical field stimulation (EFS) of intrin sic neurons in the presence of guanethidine and atropine were recorded. Med iators of responses to nerve stimulation were also evaluated, using N-nitro -L-arginine methyl ester (L-NAME) and apamin. Results-Mean resting membrane potential (RMP) was 41.5 +/- 1.8 mV. Small me mbrane potential oscillations were observed in muscle cells. Single or mult iple action potentials were often superimposed on the peaks of these oscill ations. Spontaneous oscillations and action potentials were blocked by nife dipine. Transient hyperpolarizations of smooth muscle cell membrane potenti als (inhibitory junction potentials [IJP]) were observed in response to ele ctrical field stimulation. The IJP evoked by stimulus trains consisted of a n initial fast component followed by a slow component. The L-NAME did not h ave a significant effect on RMP and did not significantly affect the fast c omponent of IJP at any stimulus frequency tested. In contrast, L-NAME aboli shed the slow component of IJP observed after trains of pulses, in the cont inued presence of L-NAME. apamin had no significant effect on RMP but effec tively reduced the fast component of IJP. Conclusions and Clinical Relevance-Findings suggest that inhibitory neurotr ansmitters supplying equine jejunum act through different ionic mechanisms. Understanding these mechanisms may suggest new therapeutic targets for tre atment of motility disorders.