NONADRENERGIC NONCHOLINERGIC (NANC) EXCITATORY RESPONSE OF THE CHANNEL CATFISH INTESTINE

1 Optimal parameters for electrical field stimulation (EFS) of catfish pyloric and middle intestinal segments were determined (15 Hz, 60 V) from a range of frequencies (5-45 Hz) and voltages (40-120 V) using a modified Magnus' method. Contractile responses were produced by EFS wh ich were reproducible and showed no significant difference between the tissues. 2 The contractile cholinergic responses of the tissues to ca rbachol and acetylcholine (ACh) were blocked by atropine on an equimol ar concentration, whereas, these responses were enhanced in the presen ce of neostigmine, an acetylcholinesterase inhibitor. 3 Adrenergic res ponses were examined with noradrenaline (NA). NA produced contraction of the segments only, at a concentration of 10(-4) M. Among the variou s adrenoceptors, beta-adrenoceptor stimulation produced a weak relaxat ion whereas, both alpha(1)- and alpha(2)-adrenoceptor stimulation prod uced contractions, of which alpha(2)-induced contraction was of greate r magnitude. The beta, alpha(1) and alpha(2) responses were blocked by their respective blocking agents propranolol, prazosin and yohimbine. 4 The autonomic components of the response to EFS were determined by using selected cholinergic and adrenergic antagonists separately or co llectively. Cholinergic blockade with atropine did not produce a signi ficant blockade of the EFS-induced response. Similarly, blockade of be ta-adrenoceptors with propranolol did not modulate the contractile res ponse to EFS to any significant level. Blockade by prazosin or yohimbi ne did not significantly change the contractile response to EFS. After a complete blockade of the adrenergic and cholinergic divisions, the intestinal segments still showed a contractile response to EFS which w as not significantly different from the control response. This indicat ed the presence of a non-adrenergic non-cholinergic (NANC) response. 5 Tetrodotoxin, at 10(-6) M, significantly blocked the EFS-induced NANC response suggesting a neurogenic origin for the response. 6 The prese nt study indicated that the EFS-induced response of the catfish intest inal segments is predominantly NANC-e in nature suggesting an importan t role for it in the regulation of intestinal motility.