Blood pressure and circulating levels of norepinephrine (NE), epinephrine (Epi), and dopamine-beta-hydroxylase (DBH) were measured sequentially in sinoaortic-denervated (SAD) Wistar rats and in sham-operated (SO) rats. Systolic tail pressure, plasma NE and E, and plasma DBH all increased significantly within 2 days in SAD rats. In separate studies of rats with indwelling arterial catheters, arterial pressure and plasma NE and DBH were increased in SAD rats in home cages; restraint caused similar increases of pressure and catecholamines in both groups. Systolic pressure remained increased up to 4 mo and plasma DBH for 10 wk in SAD rats; plasma NE and E, however, declined by 3 wk and thereafter remained close to values of SO rats. Additionally, DBH was increased in heart, mesenteric blood vessels and adrenal glands of SAD rats up to 6 wk; tissue monoamine oxidase activity was also increased up to 4 mo. These findings suggest that activation of the sympathetic nervous system initiates and probably sustains hypertension in SAD rats up to 6 wk after operation; thereafter some other mechanism(s) sustains the hypertension.

tance from the normal of 1 mm. The optimal period for production of hypospadias by the inhibitor corresponds to the early appearance of strong activity of the 3P-enzyme in the fetal testis. The optimal day for the production of adrenal hyperplasia and of clitoral hypertrophy corresponds to the time of maximal adrenal activity of the enzyme. This study thus demonstrates an experimental model for production of the congenital manifestations of this inborn error of metabolism.Recently Guazzi and Sanchetti ( 1) reported that sinoaortic denervated cats had large falls in arterial pressure, about 50 mm Hg, during episodes of deep sleep. Apparently the authors did not observe large pressure falls during light sleep or wakefulness. I n our laboratory during the past several years arterial pressure was recorded from about 50 conscious, normal and renal hypertensive rabbits, before and after carotid sinus and aortic nerves were removed. The postdenervation arterial pressure records were characterized by large, transient pressure falls that occurred as a result of body movements or spontane-*Supported by N I H Grant HE-5543 and Career Development Award 9-KE-HE-73 54 ~ ously without observable external cause. Sometimes basic arterial pressure was below, rather than above, predenervation values as the following results will show.After local infiltration of tissue with 2.0% procaine, either the femoral or central ear artery was cannulated with plastic tubing. One per cent heparin solution was administered to prevent blood clotting. The animal was then placed in a small cage inside a soundattenuated cubicle with a glass window through which the animal could be observed. Systolic and diastolic pressures were measured by a pressure transducer and recorded continuously for 40 minutes. Most of the time record speed was 0.05 cm/sec but intermitat Harvard Libraries on June 27, 2015 ebm.sagepub.com Downloaded from

The degree of reflex bradycardia elicited by a sudden, brief rise in arterial pressure was used as an index of sino-aortic reflex activity in conscious rabbits with early renal hypertension. Change in heart rate was measured every 6 seconds after injected angiotensin which caused a rise in blood pressure of 15 to 60 mm Hg. Ninety-one tests for reflex bradycardia were made in 30 normal rabbits; 14. p i them were tested intermittently during the 1 to 40 day period following unilateral riephrectomy and latex encapsulation of the opposite kidney. A total of 62 tests were made after arterial pressure had. risen 10 to 80 mm Hg, average 35,-above control values. Average decrease in heart rate was significantly less and frequency of negligible reflex bradycardia was much higher in hypertensives than controls. Individual" rabbits showed no tendency for reflex bradycardia to return toward normal magnitude. However, the vasomotor component of the sino-aortic reflex mechanism in renal hypertensive rabbits buffered the pressor response to angiotensin in a normal manner Rej.v setting of sino-aortic reflex activity under hypertensive conditions is dis-_ cussed. ADDITIONAL KEY WORDSheart rate reflex resetting of sino-aortic "reflexes baroreceptors and heart rate angiotensin and hypertension carotid sinus and aortic nerves conscious rabbits• Information concerning function of sinoaortic reflexes in man or in conscious laboratory animals with arterial hypertension is scanty, although it is important in the broader question of baroreflex resetting in hypertension. The term "reset" as applied to the sinoaortic baroreflex system has not been defined precisely in the literature, but is used here to mean that the effector response has returned to normal magnitude or rate under conditions that initially altered it. In this definition sino-aortic receptor activity, afferent or efferent nerve discharge can be substituted for effector response, the effector organs being, of course, the heart and blood vessels.

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Severe constriction of the abdominal aorta in rabbits caused congestive heart failure and death in 4 to 7 days. Expansion of total plasma volume suggested that aortic narrowing increases the vascular bed distal to the stricture. Reduction of total body fluids did not prevent total plasma volume expansion, unless it caused severe dehydration. A large percentage of dietary sodium was retained. Expansion of extraccilular volume occurred even when sodium intake was reduced. The early, gradual post-operative rise in arterial pressure probably depends, in part, on development of an effective circulating plasma volume. A tentative answer is given to the question of what factors determine the exact time that heart failure will occur.