A Probable Mechanism of Action of Serotonin.

Although the hormone serotonin (5-hydroxytryptamine) is known principally because it causes smooth muscles to contract, the reason for this action is quite unknown. The purpose of this paper is to present evidence for the view that serotonin exerts its physiological effects by carrying calcium ions across the membrane into animal cells. It is now well established that the isolated uterus of an estrogenized rat is a desirable test object in which to observe the muscle-contracting action of serotonin. 1-3 Very small amounts of the hormone (0.003-0.01 Atg/cc.) cause this isolated tissue to contract strongly. We have observed that calcium ions or potassium ions will also make this tissue respond in the way it does to serotonin. The ions, however, are far less potent than the hormone. Under the conditions to be described below, calcium chloride was 70,000 times less potent (on a molar basis) than serotonin. Potassium chloride was only one-fourth as potent as calcium chloride. Magnesium chloride was inactive, although, when it had been present in the bath in high concentration, repeated contractions of the muscle always occurred after washing. The first point of evidence, therefore, which linked serotonin to these ions was that calcium ions (and to a lesser extent potassium ions) had the qualitative activity of serotonin on the muscle. The activity of calcium ions was discovered during the assay of tissue extracts for serotonin. Some of these extracts contained considerable quantities of calcium ions. When control solutions containing only these ions but no other components of tissue were assayed, the serotonin-like activity was found. The second point of evidence which suggested that metallic ions were concerned in the action of serotonin was that, when an isolated uterus was treated with versene (ethylenediaminetetraacetate), serotonin no longer was able to cause a contraction. When the versene was washed away or neutralized by calcium ions, serotonin again caused the muscle to contract. This demonstrated that the versene had not brought about irreversible damage to the tissue. These experiments were done in the following way. Virgin female rats were estrogenized with a single intraperitoneal injection of 0.01 mg. of diethylstilbestrol. Twenty hours later the animals were killed, and a horn of the uterus was removed and suspended in oxygenated salt solution of the following composition: NaCl, 9 gm.; KCl, 420 mg.; CaCl2, 20 mg.; NaHCO3, 0.5 gm.; glucose, 0.5 gm.; water, 1 liter. One end of the tissue was attached to the lever of a kymograph. The preparation was washed by rapid emptying of the bath and prompt refilling. Temperature was 300 C. Additions of test compounds were made in small volume to the saline solution while it was in contact with the tissue. The record shown in Figure 1 indicates that, qualitatively, the contractions caused by serotonin and by calcium chloride were similar but that there were signifi-