Effect of variations of plasma sodium concentration on the adrenal response to angiotensin II

The secretion rate of aldosterone was increased by infusion of valine-5-angiotensin II into the adrenal arterial blood supply of sheep. The dimensions of the increase in aldosterone output were inversely related to the control plasma concentration. This high aldosterone secretion was reduced substantially by concurrent adrenal infusion of concentrated NaCl solution which increased plasma sodium concentration by approximately 10 meq/liter. The reduction of aldosterone secretion occurred within 20 minutes. Angiotensin II infusion did not increase the secretion rates of cortisol or corticosterone.

The significance of the finding that environmental sodium concentration has different effects on the aldosterone-stimulating action and the pressor action of angiotensin II is discussed.

Effects of infusion of catecholamines and angiotensin II on renin release in anesthetized dogs

Renin release was studied during the infusion of catecholamines (.7–5.6 µg/min) and angiotensin II (.25–1.05 µg/min) into the renal artery, and also during and following iv infusion of 16 µg/min noradrenaline. Filtration rate (Ccr), RPF, and RBF were estimated from the renal clearance of creatinine and the extraction ratio. Marked increases in renal vein renin titer and V-A renin difference occurred within 1 min of the onset of infusion of catecholamine into the renal artery and continued for 15 min beyond the infusion period. This infusion was always accompanied by diminished renal function. During iv infusion of noradrenaline, renin release did not usually occur, but on stopping the iv infusion, the ensuing fall in blood pressure was coincident with a marked release of renin apparently unaccompanied by decreased renal function. Infusion of angiotensin into the renal artery, in spite of causing renal vasoconstriction and marked decrease in urine volume, was not accompanied by renin release. The difficulties of measuring renin release under these conditions are discussed.

The relationship of the renal vascular activity of angiotensin II to the autonomic nervous system

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In vitro demonstration of vascular hyper-responsiveness in experimental hypertension

The ventral caudal artery from DCA hypertensive and control rats was excised, mounted in a plastic chamber, and perfused with Krebs-Henseleit solution. Diameter of the artery and flow through the artery were measured at perfusion pressures ranging from 0 to 150 mm Hg. During these perfusions, constriction of the arterial segment was produced by adding norepinephrine, vasopressin, angiotensin II or rat serum in concentrations known to produce submaximal contractions.

Samples of each experimental artery before and after artificial perfusion were fixed, sectioned, and stained. The tunica media of hypertensive arteries was thickened. Also, degenerative changes were observed in the internal elastic lamina and adjacent tunica media. Structural thickening of the tunica media was due primarily to expansion of the extracellular space and also due possibly to muscle fibre hypertrophy.

In the absence of contraction, flow through the hypertensive artery was moderately reduced at all pressures because its lumen was narrowed by the thickened wall.

Total wall tension, muscle tension, and elastic tension of the artery were estimated from the pressure-flow-radius data. Strength of muscular contraction was then measured in terms of work performed during contraction at a constant pressure. From this analysis, the muscle fibres in hypertensive arteries were found to be unequivocally hyper-responsive to norepinephrine, vasopressin, and rat serum. The possible cause of this hyper-responsive property is discussed.

Serum was collected both from normotensive and from hypertensive donor rats and its pressor activity tested on the in vitro preparation. The pressor activity in hypertensive serum was found to be significantly elevated.

The elasticity of the hypertensive artery was increased in the absence of contraction and unchanged during contraction. No hypertensive artery was found to be stiffer than normotensive arteries. These changes are probably related to degeneration of elastin.

Arterial hypertension elicited by subpressor amounts of angiotensin

Long-term infusion of amounts of angiotensin insufficient at the beginning to raise arterial pressure results, after several days in sustained arterial hypertension in unanesthetized dogs. This hypertension is to a large degree dependent on environmental stimuli, and results chiefly from increase in peripheral resistance. As in dogs with renal hypertension, there is increased pressor responsiveness to tyramine. This indirect action of angiotensin to increase total peripheral resistance and arterial pressure by an action on the sympathetic nervous system, along with an upward resetting of the carotid sinus buffering mechanism, might logically account for the neural component of chronic renal hypertension. Such a proposal integrates the humoral and neural elements of the mosaic describing the mechanisms of tissue perfusion.

A new and sensitive bioassay for catecholamines

The isolated central artery of the rabbit ear is highly sensitive to catecholamines when perfused with Krebs solution containing 5-hydroxytryptamine. The preparation responds to 1 ng of noradrenaline and is extremely long lasting. It does not discriminate between noradrenaline and adrenaline.

Mechanisms of cardioaccelerator action of angiotensin

The cardioaccelerator influence of synthetic angiotensin observed in dogs having baroreceptors denervated or blood pressure buffered was studied. In dogs which had received morphine (3 mg/kg) and chloralose (90 mg/kg) and which had the sinoaortic zones denervated, angiotensin, in doses of 1 µg/kg intravenously, still caused cardiac acceleration after transection of the spinal cord at C6 or C7 or after removal of sym-pathetic chains bilaterally from the stellate through T4 or T5. In intact dogs under morphine-chloralose which had received full ganglionic blocking doses of either tetraethylammonium chloride or pentolinium tartrate, angiotensin produced marked cardiac acceleration. Administration of bretylium tosylate, in doses of 10 mg/kg intravenously, largely or completely prevented the cardioaccelerator action of angiotensin. The beta receptor blocking agents nethalide and Inderal had effects similar to those of bretylium tosylate. The studies indicate that there is a peripheral adrenergic basis for much or all of the cardiac acceleration produced by these doses of angiotensin. In the intact animal this influence would be expected to counteract much of the cardiac inhibition elicited reflexly from baroreceptors.

Absence of direct chronotropic action of angiotensin infused into the sinus node artery

Angiotensin was shown to have no significant direct chronotropic action when perfused into the canine sinus node through its artery in situ. A slight cardiac acceleration which occurs during the systemic effect (aortic hyper-tension) from angiotensin is not due to sensitization of the sinus node to norepinephrine. Since it can be reversed and blocked with naphthylisoproterenol, this sinus tachycardia is presumably mediated by catecholamines delivered to the heart from an extracardiac source. Slowing of the sinus node during hypertension from angiotensin in the nonatropinized dog is due to a vagal reflex and is not caused by a direct negative chronotropic effect on the sinus node.

Effect of angiotensin on maximal tubular transport of PAH and glucose in dogs

Although the physiological actions of angiotensin have attracted considerable interest, its effects on the maximal renal tubular transport rates for p-aminohippuric acid (PAH) and glucose (Tmpah and Tmg) have not previously been described in any species. This study therefore comprised an examination of Tmpah at three dose levels and Tmg at two dose levels of angiotensin. The Tm of neither substance was reduced. In the course of the experiments, glucose was found to elevate control PAH clearance measurements and to offer some protection against the renal vasoconstrictive effect of angiotensin. It was concluded that angiotensin does not cause focal renal cortical deprivation of blood flow, nor does it interfere with the tubular transport of PAH or glucose. Other aspects of the renal response to angiotensin are altered by glucose.

Compliance of the apex of the left ventricle in unanesthetized dogs

A method has been developed for determining the compliance of the left ventricle of the unanesthetized dog through the chronic sealing of an oncometer to the cardiac apex. The volume of the enclosed apex could be increased by a known amount and the end-diastolic transmural pressure measured. The heart rates of the unanesthetized dogs were slow and the volume of the apex was held constant, hence rate-dependent nonelastic factors were minimized. It is concluded that under the conditions of these experiments, end-diastolic compliance either decreased during infusions of l-epinephrine and angiotensin or did not change. Acetylstrophanthidin administration had no effect on ventricular compliance.

distensibility; elasticity; effects of catecholamines on ventricular compliance; oncometer

Submitted on June 8, 1964