BOTH ATP AND THE PEPTIDE-VIP ARE INHIBITORY NEUROTRANSMITTERS IN GUINEA-PIG ILEUM CIRCULAR MUSCLE

1. Intracellular membrane potential recordings were made from circular smooth muscle cells of the guinea-pig ileum in the presence of atropine (1-mu-M) and nifedipine (0.1-mu-M) at 30-degrees-C. 2. Perfusion with adenosine triphospate (ATP, 100-mu-M) and vasoactive intestinal peptide (VIP, 2-mu-M) resulted in membrane hyperpolarizations of 6.4 +/- 0.3 and 6.8 +/- 0.3 mV, respectively. Picospritzes of ATP (10 mM in pipette) and VIP (100-mu-M in pipette) resulted in membrane hyperpolarizations of 6.9 +/- 0.4 and 6.3 +/- 0.4 mV, respectively. 3. The ATP-induced hyperpolarizations were antagonized by alpha,beta-methylene ATP desensitization (100-mu-M for 30 min) and the ATP antagonist Reactive Blue 2 (200-mu-M), but were unaffected by the VIP antagonist VIP 10-28 (1-mu-M). 4. The VIP-induced hyperpolarizations were antagonized by VIP 10-28, but unaffected by alpha, beta-methylene ATP desensitization and Reactive Blue 2. 5. A single pulse of transmural nerve stimulation (2 ms, 15 mA) resulted in an inhibitory junction potential (IJP) that reached a maximal amplitude of 12.9 +/- 0.5 mV at 378 +/- 20 ms from the stimulus. This fast IJP was abolished by apamin (2-mu-M) or tetrodotoxin (1-mu-M), antagonized by alpha, beta-methylene ATP desensitization or Reactive Blue 2, but unaffected by VIP 10-28. 6. In the presence of apamin (1-mu-M), four pulses of transmural stimulation (2 ms, 20 Hz, 15 mA) resulted in an IJP that reached a maximal amplitude of 4.8 +/- 0.2 mV at 1.4 +/- 0.1 s from the stimulus. This slow IJP was antagonized by tetrodotoxin (1-mu-M) or VIP 10-28 (1-mu-M), augmented by Reactive Blue 2 (200-mu-M), and unaffected by alpha, beta-methylene ATP desensitization. 7. These findings provide evidence that both ATP and VIP are inhibitory neurotransmitters in the circular muscle layer of the ileum and that ATP may be the neurotransmitter responsible for the fast IJP and VIP the neurotransmitter responsible for the slow IJP.