RESUMEN
Experiments were designed to determine the receptor subtype(s) involved in the contraction of the rat proximal colon to NPY. In this tissue, mRNA of Y2 and Y4 NPY receptor subtypes were highly expressed, whereas Y5 mRNA levels were very low and Y1 mRNA levels were intermediate. NPY analogues induced contractions with the following order of potency: rPP > hPP = PYY = NPY = [Leu31,Pro34]NPY > NPY(2-36) = [D-Trp32]NPY > NPY(33-36). Responses to NPY, PYY and NPY(13-36) were not or partially affected by tetrodotoxin, in contrast to the responses to [Leu31,Pro34]NPY, rPP, hPP and [D-Trp32]NPY which were fully blocked. Atropine did not inhibit the contractions to NPY, PYY and [Leu31,Pro34]NPY but significantly affected those to NPY(13-36), [D-Trp32]NPY, rPP and hPP. The specific Y1 receptor antagonist BIBP 3226 was ineffective but JCF 104 and JCF 105 (two compounds with preferential affinity toward the hY5 receptor versus the hY1 or hY2 receptor) abolished the contractions provoked by the NPY analogues. These results suggest that NPY activates three receptor subtypes, a Y2 subtype possibly by a direct action on the smooth muscle cells, as well as a Y4 and a Y5 (or 'Y5-like') subtype which, respectively, release acetylcholine and an unknown neurotransmitter.