[Behavior of blood pressure and renal blood flow during the carotid sinus reflex in unanesthetized dogs. Comparative studies on the influence of Na-pentobarbital anesthesia and surgical preparation upon the reflex response]

Renal renin secretion as regulator of body fluid homeostasis

The renin-angiotensin system is essential for body fluid homeostasis and blood pressure regulation. This review focuses on the homeostatic regulation of the secretion of active renin in the kidney, primarily in humans. Under physiological conditions, renin secretion is determined mainly by sodium intake, but the specific pathways involved and the relations between them are not well defined. In animals, renin secretion is a log-linear function of sodium intake. Close associations exist between sodium intake, total body sodium, extracellular fluid volume, and blood volume. Plasma volume increases by about 1.5 mL/mmol increase in daily sodium intake. Several lines of evidence indicate that central blood volume may vary substantially without measurable changes in arterial blood pressure. At least five intertwining feedback loops of renin regulation are identifiable based on controlled variables (blood volume, arterial blood pressure), efferent pathways to the kidney (nervous, humoral), and pathways operating via the macula densa. Taken together, the available evidence favors the notion that under physiological conditions (1) volume-mediated regulation of renin secretion is the primary regulator, (2) macula densa mediated mechanisms play a substantial role as co-mediator although the controlled variables are not well defined so far, and (3) regulation via arterial blood pressure is the exception rather than the rule. Improved quantitative analyses based on in vivo and in silico models are warranted.

The influence of cardiopulmonary receptors on long-term blood pressure control and plasma renin activity in conscious dogs

The isolated and combined influence of cardiopulmonary and arterial baroreceptor denervation on long-term blood pressure (MAP), heart rate (HR), plasma volume (PV) and plasma renin activity (PRA) was studied in 10 conscious, chronically instrumented foxhounds receiving a normal sodium diet. Cardiopulmonary denervation was achieved by surgically stripping both thoracic vagi. Near complete arterial baroreceptor denervation, leaving most cardiopulmonary fibres intact, was made by left vagal deafferentiation which has been shown to eliminate most aortic baroreceptor afferents, and a carotid sinus denervation. Five groups were studied: (I) control (n = 9), (II) cardiopulmonary denervation (n = 5), (III) aortic baroreceptor denervation (n = 5), (IV) arterial baroreceptor denervation (n = 4) and (V) total denervation (n = 6). No changes in PV were observed. Only group V revealed significantly higher levels of MAP (119.5 +/- 5.4 vs. 100.1 +/- 1.6 mmHg; P less than 0.05), HR (118.1 +/- 4.4 vs. 87.8 +/- 3.7 beats min-1; P less than 0.001) and PRA (3.0 +/- 0.8 vs. 0.9 +/- 0.2 ng AI m-1 h-1; P less than 0.05). It is suggested that the isolated function of either cardiopulmonary or arterial baroreceptors is sufficient to maintain these variables at a normal level. Contrary to the results of other reports the cardiopulmonary receptors do not seem to regulate MAP at a level about which the arterial baroreceptors operate. When both groups of afferents were interrupted MAP, HR and PRA rose to significantly higher levels, implying that cardiopulmonary and arterial baroreceptor afferents interact in a sense of a non-additive attenuation.

Basal vascular tone in the kidney. Evaluation from the static pressure-flow relationship under normal autoregulation and at maximal dilation in the dog

In conscious dogs static pressure-flow relationships (I-P curves) were obtained both for the normal autoregulating kidney vasculature and under conditions of maximal vasodilation induced by infusion of papaverine or acetylcholine into the renal artery. Concentration-response curves for the substances infused showed the typical S-shape. Control I-P curves exhibited an autoregulatory plateau; under maximal vasodilation the I-P curves were pressure-passive up to a perfusion pressure of 50 mm Hg, following a power function (1 = a-Pn) with an exponent greater than 1. Under the influence of acetylcholine, renal blood flow was significantly higher than under control conditions even at a perfusion pressure of 20 mm Hg. This indicates that there is an appreciable vascular tone even at a low pressure. The I-P curves under acetylcholine showed a break at about 50 mm Hg, above which the I-P curves were straight lines.

A possible sympathetic cholinergic mechanism in the renal reflex elicited by stimulation of abdominal viscera in the dog

1. Isolated blood-bathed tissues which were superfused with renal venous blood during various manipulations of the visceral organs of dogs showed no detectable pharmacologically active substances, except a noradrenaline-like substance which was liberated under some conditions of manipulation. 2. An acetylcholine-like substance was liberated into renal venous blood during experimental manipulation of visceral organs in dogs during infusion of physostigmine; the acetylcholine activity was detected with the dorsal muscle of the leech. 3. The amount of liberated acetylcholine-like substance depended on the strength of visceral stimulation as judged by the changes in the hepatic portal venous pressure. Strong reflex stimulation produced the release not only of acetylcholine but also of noradrenaline. 4. Physostigmine augmented and prolonged the reflexly induced renal vasodilator and diuretic responses, and these were abolished by atropine. It was suggested that the reflex involves a cholinergic nervous mechanism. 5. In some dogs there was a high output of acetylcholine from the kidney during visceral stimulation, and this was accompanied by a decrease in urine flow. The reason was not known. 6. Prostaglandin E1 administered into the renal artery produced similar vasodilator and diuretic responses to acetylcholine, but the onset of the effects were slower and they persisted longer than those produced by acetylcholine.