[Homeostasis of extraarterial blood circulation; volume regulation as an independent physiologic parameter]

Historical perspective of cardiorenal integration

1. Early research on blood volume as an independent parameter affecting kidney function took the approach that the sensors must be located in the most compliant, that is the reservoir portion, of the cardiovascular system. This encompasses the great veins and the cardiac atria. 2. Small changes in volume were shown not to affect the compliance of this reservoir and messages from the atrial receptor network were shown to travel in the vagus nerve and to control urine volume by antidiuretic hormone. 3. Although greatly affected by the water immersion stimulus, sodium excretion was not as dependent on vagus integrity. The ensuing search for the natriuretic arm of the blood volume mechanism persisted for the next 20 years. 4. Finally, one aspect of the elusive natriuretic factor was found exactly where theory had suggested, namely the most distensible part of the system, in specialized granules in the cardiac atria.

The control of sodium metabolism to maintain osmo- and volumehomeostasis

Chronically instrumented female beagles were maintained in standardized environmental and dietary conditions allowing careful examination of the mechanisms governing sodium homeostasis. The experimental increase in left atrial pressure (obtained by a reversible mitral stenosis) is accompanied by an increase in sodium excretion (atrial natriuresis, AN). AN served as an experimental manoeuvre from which the mechanisms governing sodium homeostasis could be elucidated. The results allow the following conclusions: (1) The 'signals' arising from distension of the left atrium (e.g. expansion of the extracellular fluid volume) appear not to be a necessary prerequisite for the maintenance of sodium homeostasis. (2) The control mechanisms seem to be very sensitive to changes in total body sodium (TBS). A small reduction in TBS abolishes sodium eliminating processes e.g. saline diuresis, osmotic diuresis, AN. (3) It is probable that a natriuretic factor exists for sodium elimination. In summary, total body sodium appears to be controlled by a series of 'redundant' mechanisms which guarantee an appropriate strategy for the comfort and ultimate survival of the organism. At the moment it is impossible to quantitate the contributions made by the various mechanisms in the control of sodium metabolism.

Left atrial pressure and sodium balance in conscious dogs on a low sodium intake

10 conscious chronically prepared dogs were used. After recovery from thoracotomy (catheter into the left atrium, nylon purse string around the mitral annulus) they were kept chronically on a low sodium intake (less than 0.5 meq Na/kg bw daily). On 51 days left atrial pressure (LAP) was increased for 60 min about 10 cm H2O once daily by tightening the purse string (distension period: DP). During (DP) urine volume (V) increased about threefold, and sodium excretion (ENa) about sixfold. The amount of renal sodium loss on the days when LAP was increased exceeded the daily intake considerably. The application of DOCA (15 mg i.m.) did not diminish ENa during DP and 60 min therere increased by about 15 mm Hg. The data suggest that stimulation of intrathoracic receptors by a reversible mitral stenosis augments renal sodium excretion even in a state of a highly stimulated tubular sodium resorption.

Left atrial pressure and postprandial diuresis in conscious dogs on a high sodium intake

5 conscious, well trained, female dogs kept on a high sodium intake (14 meq Na/kg bw) were used to measure left atrial pressure (LAP), urine volume (V), sodium and potassium excretion (UNaV, UKV) as well as plasma osmolality (Posm) before and up to 180 min after food intake. The dogs were fitted with a catheter in the left atrium (thoracotomy). In all experiments (n=23) LAP increased postprandially (pp) above fasting controls. The mean peak increase ranged from 4 to 6 cm H2O and was observed as early as 61-80 and as late as 161-180 min pp. Increase in LAP was closely correlated to V which rose from 36+28 to 160+51/ul/min. kg. pp V was also correlated to pp UNa V, which increased from 4.8 +/- 3.3 to 34.O+/-8.5/ueq/min-kg. The pp increase in LAP and its close relation to pp V and pp UNav emphasize the assumption that intrathoracic receptors are involved in the regulation of body fluids.

Evidence for an indirect sympathetic control of atrial stretch receptor discharge in the dog

Experiments were performed to determine the influence of cardiac sympathetic efferent nerve stimulation on the discharge rate of atrial type B stretch receptors in the anesthetized, open-chest dog. In all experiments, the left stellate ganglion was stimulated following volume expansion. To determine the effects of stellate stimulation, the responses of atrial receptors were observed during the withdrawal of blood in steps following intravascular volume expansion. Stimulation of the stellate ganglion decreased receptor discharge and left atrial pressure in all experiments. A change from a control left atrial pressure of 2.5 cm H 2 O (in peak left atrial v -wave pressure) resulted in a mean increase in receptor discharge of 6.6 ± 1.4 spikes/cardiac cycle during stellate stimulation; however, a change from a control pressure of 0.0 cm H 2 O resulted in a mean decrease in receptor discharge of 1.5 ± 1.5 spikes/cardiac cycle. The curves relating the change in atrial receptor discharge to the change in left atrial pressure during hemorrhage and the curve for these parameters during stellate stimulation were not significantly different from each other. Apparently, the decreased discharge during sympathetic stimulation was the result of a decline in left atrial pressure rather than a result of any direct effect on the receptor per se.