THE CHANGES IN RESPONSE AND IN IONIC CONTENT OF SMOOTH MUSCLE PRODUCED BY ACETYLCHOLINE ACTON AND BY CALCIUM DEFICIENCY.

The initial purpose of this paper is to describe experiments on the responses to acetylcholine of smooth muscle exposed to varying calcium concentrations, in order to throw light on the factors controlling the action of a stimulant drug. The known interaction between calcium ion concentration and the permeability of excitable tissues to sodium and potassium makes it desirable, in addition, to know the changes in ionic content of the smooth muscle when exposed to varying calcium concentration. Further, although it is well known that the action of acetylcholine causes an efflux of potassium from smooth muscle, there is little quantitative evidence as to the net change in ionic content which results. This is of importance in view of the close resemblance between the insensitivity of smooth muscle produced on the one hand by previous exposure to high doses of stimulant drugs and on the other by exposure to potassium-deficient solutions (Paton, 1961). Accordingly, determinations have also been made of the normal sodium, potassium and calcium content of the longitudinal muscle of guinea-pig ileum, and of the changes produced by acetylcholine and by calcium deprivation. addition of acetylcholine, incubations were continued for 5 min. Each muscle was then removed from the bath, rapidly cut from its mountings, blotted between two layers of Whatman No. 1 filter paper, and its wet weight was measured on a torsion balance. It ranged between 15 and 6 mg. To remove organic matter, the muscles were placed in 4-ml. beakers, treated with 2 ml. of 301% hydrogen peroxide, and kept at 960 C overnight (MacIntyre, 1961). The residues were taken up in 2 to 3 ml. of 1 N-nitric acid (containing 0.1% phosphoric acid), stirred, and assayed for sodium, potassium and calcium by flame photometry using a Zeiss model PMQ II instrument. Calibration curves for each cation were made for each run of samples.