MODIFICATION OF THE TONUS AND DRUG-INDUCED CONTRACTION OF THE ISOLATED RAT ILEUM BY REMOVAL OF Na FROM BATHING MEDIA

Abstract

Effect of removal of Na from bathing media on the tonus and contractile response to drugs of strips of the rat ileum was investigated. Removal of Na from the bathing media by replacing NaCl with sucrose or choline Cl produced a transient contraction, which was not altered by treatment with atropine. This contraction was attenuated when preparations were exposed to Ca-free media. After a 30 min exposure to Na+-free media, contractions caused by ACh and Ba and the phasic contraction by K were decreased, whereas the tonic contraction by K was increased. In preparations in which contractions by removal of Na+ were abolished by repeated treatments with Na+-removal in Ca-free media, the contractions were restored when the preparations were incubated with Ca++ and then with Na+-containing solutions. This restoration was not obtained when preparations were soaked in Na-free media after incubation with Ca++. It appears that removal of Na from bathing media inhibits uptake of Ca by intracellular storage sites and increases the influx of Ca++, resulting in an increase in the amount of cellular active Ca++, and that Na+ plays an important role in binding Ca++ in intracellular storage sites.