Concentration-Response Curve for Acetylcholine (ACh) in the Absence and Presence of Atropine and Measurement of the Equilibrium Dissociation Constant (KB) Using Schild Analysis

Introduction: Acetylcholine (ACh) is a fundamental neurotransmitter in the parasympathetic nervous system, mediating smooth muscle contraction through muscarinic receptors. Understanding its interaction with antagonists such as atropine provides insights into pharmacological modulation of smooth muscle activity. Objective: The study aimed to evaluate the competitive interaction between ACh and atropine on muscarinic receptors in guinea pig ileum and to characterize the receptor-binding dynamics, particularly focusing on atropine’s inhibitory effects. Methodology: The research utilized isolated guinea pig ileum tissues, which were exposed to increasing concentrations of ACh in the absence and presence of atropine. Concentration-response curves were generated to determine receptor affinity and inhibitory effects. Equilibrium dissociation constants for both ACh and atropine were calculated to analyze the binding properties and competitive nature of the interaction. Results: Atropine was confirmed as a competitive antagonist of muscarinic receptors, effectively reducing the contractile responses induced by ACh. The data demonstrated that atropine prevents ACh binding, thereby inhibiting smooth muscle contraction, as evidenced by significant shifts in concentration-response curves. These findings reaffirm atropine’s role as a potent therapeutic agent for conditions involving cholinergic overactivity.