Kinetic studies on dose-triphosphoinositide responses and dose-contraction responses in rabbit iris

Abstract

Correlative studies on the dose-triphosphoinositide (TPI) breakdown and -phosphatidic acid (PA) labeling and dose-tension relationship to acetycholine (ACh). carbachol and acetyl-β-methacholine were investigated in the rabbit iris smooth muscle. Concentrations of these agonists between 1 × 10 −8 M and 1 × 10 −4 M caused a concentration-dependent TPI breakdown and PA labeling, and concentrations between 1 × 10 −7 M and 1 × 10 −3 M caused a concentration-dependent contraction of the iris. The ed 50 values for the various cholinergic muscarinic agonists were determined from the dose-response curves. Good correlation between the ed 50 values determined by the biochemical and pharmacological methods, being of the order of 1–8 × 10 −6 M. were observed. The apparent dissociation constants of ACh and atropine were estimated by measuring the effects of these agents on TPI metabolism and iris muscle contraction. Atropine, at concentrations between 1 × 10 −10 M and 1 × 10 −9 M, produced a parallel shift to the right of the ACh dose-response curve in both the biochemical and pharmacological methods. Both biochemical and pharmacological responses were inhibited in a competitive manner by atropine. For the TPI, PA and contraction responses, the K B values (determined with ACh as agonist) were 1.7 × 10 −10, 2 × 10 −9 M and 1.14 × 10 −10 M, respectively, and the corresponding p A 2 values were 10.30, 10.35, and 9.90 respectively. It was concluded that the findings of similar K B values for the atropine-muscarinic receptor, along with similar pA 2 values from the Schild plots, with the TPI, PA and contraction responses in the rabbit iris could suggest a close relationship between the biochemical and pharmacological responses.