K transport and mechanical responses of isolated longitudinal smooth muscle from guinea pig ileum

¹ Departments of Pharmacology and Anatomy, Vanderbilt University School of Medicine, Nashville, Tennessee

The longitudinal smooth muscle layer of the guinea pig ileum was isolated in order to investigate its contractile responses and unidirectional K⁴² fluxes. Pilocarpine (7.5 x 10^–6 m), acetylcholine (6.6 x 10^–6 m), and a modified Tyrode's solution in which potassium ion was substituted for almost all the sodium ion were employed as excitatory agents. Cocaine (8.5 x 10^–4 m) and a calcium-free Tyrode's solution served as inhibitory agents. Smooth muscle tone and potassium efflux of this relatively pure tissue were both increased by all three excitatory substances. Moreover, acetylcholine and pilocarpine produced a decrease in the influx of potassium ion. Bathing the tissue in a calcium-free medium for 1 hour before introducing pilocarpine to the muscle bath eliminated the contractile response that this drug ordinarily produces, but did not diminish appreciably the increase in K⁴² efflux. These observations are qualitatively similar to results previously obtained in analogous experiments on isolated whole ileum. In addition, cocaine (8.5 x 10^–4 m) was found to block the contractile response and about three-quarters of the enhanced K⁴² efflux elicited by the isotonic potassium solution. It is presumed that cocaine acting at the membrane impedes ion fluxes important for smooth muscle contraction.