Distribution and properties of excitatory and inhibitory junction potentials in circular muscle of the guinea-pig stomach.

In the presence of guanethidine (10(-6)-5 X 10(-6) M), transmural nerve stimulation evoked an excitatory junction potential (e.j.p.) in the fundus region and an inhibitory junction potential (i.j.p.) in the antrum region of the circular muscle of the guinea-pig stomach. The e.j.p. was blocked by atropine (over 2 X 10(-7) M) while the i.j.p. was blocked by apamin (10(-7) M) but not by adrenergic or cholinergic receptor antagonists. Therefore the i.j.p. may be non-adrenergic and non-cholinergic in nature. In the presence of atropine, nerve stimulation evoked the non-adrenergic, non-cholinergic i.j.p. in both regions of the stomach. In the antrum region, single stimuli enhanced the subsequent slow wave by 1.1-1.3 times, in comparison with that before the stimuli, and this effect was blocked by atropine (over 2 X 10(-7) M). The reversal potential for the e.j.p. was about -18 mV, while that for the i.j.p. was -87 mV in the atropinized fundus and -89 mV in the antrum region. In the fundus region, a pair of nerve stimulations with short intervals (1-4 s) reduced (depression) and with long intervals (5-20 s) enhanced (facilitation) the second e.j.p. or the i.j.p. After inhibition of acetylcholine esterase (AChE) by physostigmine or neostigmine, nerve stimulation evoked an enhanced e.j.p., and then produced a sustained depolarization with a duration of 3-5 s in the fundus region, while the amplitude of slow waves after nerve stimulation was further enhanced in the antrum region. These effects of anticholinesterases were blocked by atropine. Exogenously applied acetylcholine (ACh) depolarized the smooth muscle membrane; the threshold concentration of ACh was about 1000 times higher in the antrum region (10(-6) M) than in the fundus region (10(-9) M). It is concluded that in the guinea-pig stomach, regional differences in junction potentials may be due to different sensitivities of ACh receptor, and that nerve stimulation evokes a cholinergic e.j.p. in a high-sensitivity region (fundus) and a non-adrenergic, non-cholinergic i.j.p. in a low-sensitivity region (antrum).