Neural modulation of the cyclic electrical and mechanical activity in the rat colonic circular muscle: putative role of ATP and NO

1 The rat colonic circular muscle displays cyclic episodes of myenteric pot ential oscillations (MPOs), each of them associated with a spontaneous cont raction. Nifedipine 1 mu M abolished both MPOs and their associated contrac tions. TTX (1 mu M) increased the amplitude and frequency of spontaneous co ntractions. 2 Electrical field stimulation (EFS) induced a non-adrenergic non-cholinerg ic (NANC) inhibitory junction potential (IJP), with two phases: an initial fast hyperpolarization (characterized by IJP amplitude) and a sustained hyp erpolarization (characterized by IJP duration). 3 Sodium nitroprusside (10 mu M) hyperpolarized and abolished spontaneous c ontractions even in presence of TTX or I mu M apamin. ATP (100 mu M) also h yperpolarized and abolished spontaneous contractions but its effects were d ecreased by TTX and abolished by apamin. 4 Suramin (100 mu M) or apamin reduced the amplitude of the IJPs, but did n ot affect their duration. Incubation with L-NOARG (1 mM) reduced the durati on but not the amplitude of the IJPs. In presence of L-NOARG plus suramin o r L-NOARG plus apamin, both duration and amplitude of the IJPs were reduced but a residual IJP could still be recorded. 5 We conclude that the mechanical and electrical cyclic activity of the rat colonic circular muscle is modulated but not originated by the enteric ner vous system and involves L-type calcium channel activity. EFS induces relea se of NANC inhibitory neurotransmitters which hyperpolarize and relax smoot h muscle cells. Both ATP and NO are involved in IJP generation: ATP is resp onsible for the first phase of the IJPs involving activation of apamin-sens itive potassium channels, whereas NO initiates the second phase which is in dependent of the activation of such channels.