During the painful phase of attacks of vascular headache of the migraine type there is dilatation of the large and small blood vessels of the head both intra-and extracranially, often more on one side than the other.1-3 In most attacks, however, the pain stems chiefly from the large subsurface cranial arteries and their branches. Aching pain is experienced when these vessels are distended, pulled upon, or displaced. Yet dilatation of these vessels, for instance that induced by immersion of the body in hot water, is not usually painful and does not induce other focal features of the migraine attack, i.e., edema, tenderness on pressure, and heightened vulnerability of tissue to injury. These observations led to the hypothesis that in addition to vasodilatation, a local sterile inflammation occurs. Also a substance (or substances) accumulates in the walls of the arteries and in the adjacent perivascular, areolar, and supporting tissues as well that lowers pain thresholds, increases capillary permeability, and heightens vul¬ nerability to injury.4-6
MethodThe observations of Keele and his associates7 demonstrating that the pain-producing substance in blister fluid had many of the properties of the group of vasodilator polypeptides referred to as bradykinin,8 Kallidin,9 or plasma kinnins,10 led us to assay on the isolated rat uterus specimens of subsurface tissue fluid collected during head¬ ache. The rat uterus contracts in the presence of minute amounts of these polypeptides. Preliminary analysis demonstrated that specimens collected dur¬ ing headache contracted the rat uterus.6 To char¬ acterize the active substance, the rat uterus, rat duodenum, and rat blood pressure were used for quantitative assay.Bioassay Method.\p=m-\(A) Smooth Muscle: Vir¬ gin female rats (150-200 gm.) were killed by a stunning blow on the head. The bicornuate uterus and the first 8 or 10 cm. of duodenum proximate to the pylorus were removed and placed in modi¬ fied Ringer-Locke solution. In most experiments this solution contained, per liter, 9.0 gm. NaCl, 0.42 gm. KC1, 0.02 gm. CaCla, 0.50 gm. NaHCO», 0.50 gm. dextrose, and distilled water to vol¬ ume.In some experiments the CaCU concen¬ tration was increased to 0.06 gm. per liter to increase the sensitivity of the uterus to "head¬ ache stuff." A single horn of the uterus and a segment of duodenum were then cut to suitable length, and all adherent connective tissue blood vessels and fat were carefully dissected away. After rinsing the interior of the tubular muscles with the saline solution they were tied at both ends with cotton thread. The muscles were then mounted in a single 3 cc. chamber, one end of each being secured to the bottom of the chamber and