Abstract

Serotonin-induced contractions of isolated feline bronchial rings were relaxed rapidly and reversibly by increased CO₂ concentrations in the bath. However, acetylcholine-induced contractions were relatively insensitive to CO₂. This selective effect of CO₂ was not altered by physostigmine. Decreasing pH with constant CO₂, mimicked the effect on tension of changing C0₂, suggesting that the effect of CO₂ was due to a change in extracellular pH. Increased CO₂ decreased tension reversibly during washout of serotonin without altering time rate of relaxation with 5% CO₂. In the presence of increased extracellular potassium, or with very low calcium concentrations in the bath, the selective effect of CO₂, persisted. Thus, the effect of CO₂ does not appear to depend on the membrane potential or on alteration of the availability of calcium in the external medium. Blockade of tissue nervous elements with propranolol, sotalol, atropine or tetrodotoxin did not influence the effect of CO₂. Whereas potassium-induced contractions were relatively insensitive to CO₂, contractions produced by methacholine, bethanechol and carbachol were relaxed by CO₂. Thus, sensitivity to CO₂ was not a unique property of the serotonin receptor system. CO₂ could act by altering the binding of drug to receptor or by influencing the specific transmission process from the receptor to the contractile elements.