THE DETERMINATION OF DISSOCIATION-CONSTANTS FOR SUBSTANCE-P AND SUBSTANCE-P ANALOGS IN THE GUINEA-PIG ILEUM BY PHARMACOLOGICAL PROCEDURES

  • 1 January 1983
    • journal article
    • research article
    • Vol. 23  (3) , 558-562
Abstract
The Kd values of substance P (SP), physalaemin, kassinin, and SP analogs acting on SP receptors in guinea-pig ileal longitudinal muscle strips were determined by the pharmacological procedures of Furchgott. This method involves analysis of the concentration-response data before and after fractional inactivation of receptors with phenoxybenzamine (2 .times. 10-5 M). Estimations of the Kd values for SP were similar when phenoxybenzamine was incubated for 10, 13 or 15 min. Coincubation with high concentrations of SP protected against receptor inactivation with phenoxybenzamine, but bradykinin and serotonin did not cross-protect SP receptors. Kd values for SP were similar when trypsin was substituted for phenoxybenzamine [Kd = 8.1 .+-. 4 nM (n [no.] = 9) vs. 10 .+-. 6 nM (n = 5)]. In atropinized preparations the Kd values obtained for physalaemin were similar to those obtained for untreated preparations [Kd = 8.0 .+-. 3.6 nM (n = 5) and 12.6 .+-. 3 nM (n = 4), respectively]. The effects of phenoxybenzamine on concentration-response curves for kassinin showed greater shifts to the right with phenoxybenzamine. This indicated that kassinin may interaction with another population of receptors, in addition to the sites that SP and other analogs bind. A direct correlation was found between EC50 [median effective concentration] values and Kd values for SP and SP analogs. It was estimated that, for SP, a 20% receptor occupancy is required to elicit a 50% response.

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