STUDIES ON EXPERIMENTAL HYPERTENSION

Abstract

Constriction of the aorta just above the origin of both main renal arteries invariably resulted in elevation of the carotid systolic and carotid mean pressure. The hypertension was not immediate, but developed in about the same time as after constriction of the main renal arteries (3). Constriction of the aorta just below the origin of both main renal arteries had no significant effect on the carotid systolic or carotid mean pressure. Since these results were first reported (1), Rytand (88, 89) has shown by an indirect method, namely, the demonstration of the development of cardiac hypertrophy, that hypertension in the upper part of the body can be produced in the rat by constriction of the aorta just above the origin of both main renal arteries. The immediate effect of constriction of the aorta either below or above the main renal arteries is a fall of blood pressure (femoral mean pressure) below the site of the clamp, the extent of the fall being directly dependent upon the degree of constriction of the aorta. Of particular interest is the eventual elevation of the femoral mean pressure above the normal in some animals with the aorta constricted or even occluded above the origin of the main renal arteries. This was most pronounced and persistent in those animals in which, in addition, the aorta below the origin of the renal arteries, and, in some animals, the main renal arteries, also were constricted. The most important factors which determined this elevation of blood pressure in the lower part of the body were probably increased flow of blood into the vascular bed below the clamp and peripheral vasoconstriction of renal and humoral origin, as in the case of the hypertension produced by constriction of the main renal arteries alone (2–86). Although elevation of the carotid systolic or carotid mean pressure occurred invariably within 24 to 48 hours after the constriction of the aorta above the site of origin of both main renal arteries, yet there was a tendency, after a variable period, for the elevated blood pressure to become lower or even to drop to the original level. Increased constriction, and finally occlusion of the aorta, above the origin of the main renal arteries, and even constriction or occlusion of the aorta below the renal arteries, in addition, failed to induce hypertension that persisted for a long time at a high level. In order to produce this effect, it was necessary to constrict the main renal arteries as well. The possible explanation of the failure of the hypertension to persist for a long time after constriction of the aorta alone, is that the initial ischemia of the kidneys disappeared due to the improvement of the blood flow through the kidneys as a result of (a) the increase of the natural accessory circulation to the kidneys; (b) the increased blood pressure above the site of the clamp and consequent increased flow of blood into the part of the aorta below the clamp; (c) increased pressure below the site of the clamp due, in great part, to peripheral vasoconstriction, and in part to the increased inflow of blood into the lower part of the body through the aorta and collateral channels. For the dog, this method is not necessary for the production of persistent hypertension. Constriction of the main renal arteries is easily performed and is effective for the production of generalized hypertension (2–11). However, constriction of the aorta in addition to constriction of the renal arteries results in greatly elevated persistent hypertension. Constriction of the aorta alone above the origin of the main renal arteries would be useful in the dog only for the production of relatively short periods of hypertension in the upper part of the body. For small animals it may be a more effective and useful method. In the dog, the only technical difficulty encountered was the erosion of the wall of the aorta by the clamp. This may not occur in small animals. In previous studies (2–11) that have dealt with the constriction of the main renal arteries, this accident rarely occurred. When the constriction of the aorta above the origin of the main renal arteries was of moderate degree, or was gradually made very great, the resultant hypertension was not accompanied by impairment of renal excretory function, as determined by urea clearance or by the quantity of urea, creatinine or non-protein nitrogen in the blood, the benign phase of hypertension (3). When the constriction of the aorta was suddenly made very great, impairment of the renal excretory function usually followed, and the animal developed fatal convulsive uremia and characteristic vascular lesions, the malignant phase of hypertension (9). These facts, are all indicative of the renal origin of the hypertension which results from the constriction of the aorta just above the origin of both main renal arteries. Hypertension did not persist for a sufficiently long time to permit any conclusive comparison between the effect of the high and low pressures on the structure of the vascular system, above and below the site of the clamp, respectively. During the period of survival of these animals, no significant differences were observed between the appearance of the vascular system of the upper part of the body and that of the lower part of the body, and significant cardiac hypertrophy did not develop. In the aorta and large arteries, intimal arteriosclerosis was not observed. In the aorta of one old animal several small plaques of calcification were found in the media, but these were present in the portion of the aorta below, as well as above the clamp, and they were no larger or more abundant than were observed in some old dogs with normal blood pressure. Dogs 3–50 and 3–83, that are still alive, with very high blood pressure above the site of the aortic clamps, and relatively low pressure (though greater than normal) below the site of the aortic clamps, will be valuable for the determination of possible differences between the effects of the two levels of blood pressure in the large and small blood vessels. In these dogs also, it will be possible to determine the effect of the persistently high blood pressure on the myocardium. The possible application of the results of this study to the problem of the pathogenesis of human eclampsia is mentioned here for consideration. Since this condition occurs in pregnancy only at a time when the uterus is greatly enlarged, it is at least possible that the mass may press on the aorta or both main renal arteries sufficiently to produce renal ischemia. The suddenness with which the uremic convulsive phase of eclampsia develops is in keeping with this idea. In the dog, an aggravating effect of pregnancy on an already established hypertension has not been noted. As a matter of fact, most of the hypertensive dogs that have become pregnant, have shown a slight or moderate fall, rather than an increased rise of pressure. Since the dog stands with the body in a horizontal position, and does not lie on its back, pressure of the pregnant uterus on the aorta and blood vessels is less than in human beings who stand erect and frequently lie on their backs. The soundness of this suggestion could be tested by placing pregnant women, in the early stage of eclampsia, in a position which could relieve possible pressure on the aorta and main renal arteries. A possible explanation of the fall of pressure in the pregnant hypertensive dogs is the compensatory effect of the normal kidneys of the pups, as in the case of an animal with one main renal artery constricted and the other kidney normal. As has been shown (3, 31, 72), the presence of one normal kidney in an animal hypertensive due to constriction of the other main renal artery, results, after a variable period, in a return of the blood pressure to normal. How the normal kidney acts to produce this effect is not known.