Effects of a nitric oxide synthase inhibitor on non-cholinergic junction potentials in the circular muscle of the guinea pig ileum

Intracellular microelectrodes were used to record junction potentials from the circular muscle cells of the guinea pig ileum in vitro at 37°C in a modified Krebs solution containing nifedipine (1–2 μM) and hyoscine (1 μM). Transmural nerve stimulation, using volleys of three pulses at 50 Hz, produced a complex response consisting of an inhibitory junction potential (IJP) followed by a prolonged depolarization. Following the addition of the nitric oxide synthase inhibitor N G-nitro- l-arginine (NOLA, 100 μM) the amplitude of the IJP (recorded 10 mm aboral to the stimulating electrodes) was increased by approx. 10% ( n = 4). The further addition of apamin (250 nM) abolished the IJP revealing a non-cholinergic excitatory junction potential (EJP). In other experiments ( n = 8), preparations were treated with apamin then subjected to substance P desensitization (500 nM, > 20 min). Transmural nerve stimulation now produced a triphasic response (recorded 1 mm aboral to the stimulating electrodes) consisting of: (a) an initial hyperpolarization (approx. 5 mV) lasting about 1 s; followed by (b) a depolarization reaching a peak (approx. 7 mV less negative than the RMP) approx. 2 s after nerve stimulation; and finally (c) a small (approx. 3 mV) hyperpolarization. The addition of NOLA reduced all three phases by 80–90% ( n = 8). The subsequent addition of l-arginine (5 mM) partially reversed these effects ( n = 3). Conditioning hyperpolarization up to 20 mV increased the amplitude of the NOLA-sensitive IJP and EJP. Further conditioning hyperpolarization reduced the amplitude of the IJP and enhanced the amplitude of the EJP. Large conditioning hyperpolarizations (> 60 mV) reduced the amplitude of both the IJP and EJP. An estimation of the membrane conductance changes occurring during the initial hyperpolarization and depolarization suggest that it was either unchanged or increased. During large conditioning hyperpolarizations in the absence of nerve stimulation, the membrane potential was unstable and began to show spontaneous oscillations (up to 30 mV, every 4–5 s) resembling slow waves. These experiments indicate that NO, or a related compound, appears to mediate the nerve induced apamin-resistant IJP and substance P- and hyoscine-resistant EJP in the circular muscle of the guinea pig ileum.