Receptors for tachykinins in canine intestine circular muscle.
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125I-Tyr8-substance P binding was examined in a plasma membrane enriched fraction from the circular muscle of canine small intestine. The binding was quick in onset, reversible and saturable to a single high affinity binding site; KD and maximum receptor concentration were 0.50 = 0.06 nM and 742 +/- 173 fmol/mg of protein, respectively. KD values from kinetic and saturation studies were in agreement. The rank order of potency of the tachykinins and related compounds in displacing this binding was consistent with that of a neurokinin (NK)-P (NK-1) type binding site. 125I-eledoisin did not bind specifically to these membranes and eledoisin was relatively ineffective in displacing the 125I-Tyr8 substance P. Kassinin was totally ineffective. Based on relative agonist potencies, the tachykinins appeared to contract the circular muscle of canine small intestine in vitro by a mechanism which also could be considered to be a NK-P (NK-1) type receptor. However, a comparison of data obtained by these two techniques suggests that the NK-P (NK-1) type binding site and the receptor(s) responsible for contractile responses are not identical. There were discrepancies in relative orders of potency and in absolute potencies. The simplest explanation is that there is also an NK-A (NK-2) receptor which is functional but not labeled by our ligands. Possible reasons for these discrepancies are discussed.
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