CHARACTERISTIC FEATURES OF INHIBITORY JUNCTION POTENTIALS-EVOKED BY SINGLE STIMULI IN THE GUINEA-PIG ISOLATED TAENIA CECI

1. Changes in membrane potential of the guinea-pig isolated taenia cae ci evoked by single stimuli have been investigated using intracellular recording techniques. Nifedipine (10 mu M) was used to arrest spontan eous muscle action potentials. Single stimuli elicited complex junctio n potentials which consisted of both excitatory and inhibitory compone nts. 2. The excitatory component of the compound junction potential wa s unaffected by hexamethonium (100 mu M) but abolished by atropine (1 mu M) and omega-conotoxin GVIA (10-100 nM). 3. In the presence of atro pine, single stimuli elicited fast inhibitory junction potentials (IJP s). IJPs were sometimes biphasic during repolarization with a noticeab le 'slow tail'. Apamin (30-100 nM) potently inhibited the fast IJP and revealed an underlying slow IJP. 4. The fast IJP was also abolished b y omega-conotoxin GVIA (100 nM). However, the slow IJP was insensitive to omega-conotoxin GVIA but was abolished by cadmium (30 mu M). 5. Gu anethidine (3 mu M) and N omega-nitro-L-arginine (10-100 mu M) had no detectable effects on either of the IJPs. The dye Reactive Blue 2 redu ced the amplitude of the fast IJP but this reduction was associated wi th a membrane hyperpolarization. 6. The existence of two distinct IJPs in the guinea-pig taenia caeci has been demonstrated. The ability of omega-conotoxin GVIA to selectively abolish the fast IJP leaving the s low IJP intact suggests that separate nerves are involved in mediating these responses.