The effect of nitrate, iodide and bromide on the duration of the active state in skeletal muscle.

I f a frog’s skeletal muscle is soaked in modified R inger’s fluid, containing bromide, n itra te or iodide in place of chloride, the tension and duration of a tw itch, and its heat production, are largely increased, in the order B r < N 0 3 < I. In a tetanic contraction, however, the tension, the heat production and the velocity of shortening under any load are unaffected. The prim ary effect of these anions, acting on the excitable surface of the muscle fibres, is to increase the duration of the active sta te after a shock; the increased tension and heat in a tw itch result from this. The rate a t which the effect comes on after sudden immersion of a muscle in iodide-Ringer depends solely on the rate a t which iodide diffuses into the interspaces between the fibres; the rate of penetration of iodide into the fibres is very slow. The effect is not due to any direct action of the anion on the contractile mechanism, for (1) it passes off rapidly, when a muscle after long soaking in iodide-Ringer is transferred to chloride-Ringer, although iodide rem ains inside for a long tim e, and (2) the tetanic contraction is unaffected. I f the fundam ental * un it ’ of contraction were a cycle of events triggered by excitation and then running its own course to completion, the duration of the active sta te would be an intrinsic property of the contractile mechanism and would not depend on something outside it. B ut if the active contractile sta te is set up and maintained in the interior by some influence tran s­ m itted from the excitable surface outside, its duration would depend on the time course of the responsible process a t the surface. The facts (a) th a t the duration is greatly affected by the presence of, for example, iodide in the interspaces between the fibres, a t a time when there is practically none of it inside the fibres, and (b) th a t the effect rapidly passes off as iodide is removed from the interspaces although a large am ount of it m ay still be inside, is decisive evidence for the second view. These abnormal anions somehow affect the time course of the process, started by excitation, in the surface of the fibres from which the active contractile sta te is transm itted into and m aintained in the interior. This exposes a new link in the chain of events connecting excitation with contraction.