Sodium excretion in conscious and anesthetized dogs after large saline infusions

The role of the cardiac nerves in regulation of sodium excretion in conscious dogs

Conscious, chronically instrumented dogs, maintained on a high sodium intake, were used to investigate whether surgical cardiac denervation impairs the natriuresis associated with left atrial pressure increase produced in three ways: during an increase in left atrial pressure by means of a reversible mitral stenosis (protocol 1); after an i.v. saline load (1.0 ml 0.9% saline min-1 . kg-1 over 60 min) (protocol 2); after an oral saline load (14.5 mmol Na . kg-1 given with the food as isotonic solution) (protocol 3). During a reversible mitral stenosis, in intact dogs, urine volume and sodium excretion increased markedly (from 34--145 microliters . min-1 . kg-1 and from 3--12 mumol . min-1 . kg-1); mean arterial pressure increased by an average of 2 kPa (15 mm Hg) and heart rate by 53 b/min; plasma renin activity fell from 0.37--0.21 ng AI . ml-1 . h-1 . Cardiac denervation eliminated these effects of left atrial distension except for a small increase in heart rate (12 b/min). This indicates that the natriuresis and diuresis during left atrial distension resulted from stimulation of receptors located in the left atrium. In contrast, during protocol 2 and 3, the same amounts of sodium and water were excreted in the cardiac denervated dogs as compared to the intact dogs. A comparable decrease in plasma renin activity also was observed. -- Apparently the presence of the cardiac nerves is not a prerequisite for maintenance of sodium and water homeostasis.

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.

Changes in interstitial pressure during acute interstitial volume depletion in normally hydrated rats

Interstitial fluid pressure was measured in normally hydrated rats during acute interstitial volume depletion by intravenous hyperoncotic bovine serum albumin infusion. Body fluid volumes, systemic arterial and venous pressure and selected blood and urine variables were also measured. The infusion increased plasma volume twice as much as do iso-oncotic infusions which cause comparable increases in mean central venous pressure. The kidneys responded with a diuresis and natriuresis closely resembling those which follow iso-oncotic infusion in normally hydrated rats. At the end of the elevated renal response plasma volume and plasma protein concentration were not restored to pre-infusion values; total interstitial fluid volume was decreased to one half its control value. Interstitial fluid pressure decreased linearly with volume so that effective interstitial compliance was constant at 0.0717 ml/mm Hg per gram dry tissue weight (1.79 ml/mm Hg per 100 g BW). This was not significantly different from the value 0.0704 previously found in normally hydrated rats but very significantly higher than that in dehydrated rats with comparable interstitial depletion. It is concluded that interstitial compliance is normal over a wide range of interstitial fluid volume in normally hydrated rats but that it can be altered in states of chronic body water depletion.

Excretion of sodium and water by kidneys in situ and by transplanted kidneys following isotonic, hypotonic, iso-oncotic and hyperoncotic intravenous infusions in sodium-loaded and sodium-deprived dogs

The excretion of sodium and water following isotonic, hypotonic, iso-oncotic and hyperoncotic intravenous infusions has been investigated in the kidneys in situ and in transplanted kidneys of narcotized dogs previously submitted to sodium-enriched or-deprived diets. The fractional excretion of sodium depended basically on the cumulative effect on the kidney of the changes in plasma oncotic pressure, plasma sodium concentration, and haematocrit. The differences in excretory responses of sodium-loaded or-deprived animals did not depend on differences in the distribution of infused fluids between intra- and extravascular compartments, but to the sensitivity of the kidney itself to the direct cumulative effect of these non-specific changes in blood composition.

Comparative evaluation of fractional excretion of sodium following saline infusion in transplanted kidneys and in isolated perfused kidneys in conditions of previous high or low dietary sodium intake

The increase of fractional excretion of sodium following the infusion of isotonic saline solution is quantitatively the same in dog kidneys perfused by a whole animal or by a pump-oxygenator machine at identical arterial and venous pressures. The equal response found in both experimental conditions demonstrates that the dilution of the blood, and primarily the decrease of post-glomerular plasma oncotic pressure, represents the only extrarenal humoral factor of significant importance in the reduction of fractional reabsorption of sodium following large saline infusion. The renal response to hemodilution is reduced similarly in both types of experiments when the dogs have been previously submitted to a dietary deprivation of sodium. The decreased sensitivity of the kidney to the stimulus of hemodilution appears as a major determinant of the poor natriuretic response to saline loading of sodium-deprived animals.

On the regulation of the renal response blood volume expansion by vascular parameters in the rat

The dynamic patterns of body fluid volume distribution, of cardiovascular variables and of renal water, sodium and potassium excretion were studied in the anaesthetized rat following acute, iso-oncotic blood volume expansion. The increased renal excretion following expansion was soon reversed and its pattern to termination was not correlated with changes in pulse and mean arterial pressure, in plasma electrolyte concentration or in haematocrit. At the end of the renal response the change in extracellular fluid volume was not usually corrected, the blood volume was always well above its control value and the interstitial fluid was below its control value. In all cases the temporal pattern and the termination of the renal response corresponded closely with the temporal pattern and the return to the control value of the central venous mean and pulse pressure. The findings of thse experiments are not consistent with the view that any of the measured body fluid volumes directly and always determine renal excretion. It is proposed that in response to an acute blood volume expansion an animal may initially regulate neither its blood volume nor its extracellular fluid volume but rather a factor which is reflected in or related to the central venous pressure.