Untersuchungen an wachen Hunden über die Einstellung der Natriumbilanz

Mechanisms of sodium balance: total body sodium, surrogate variables, and renal sodium excretion

The classical concepts of human sodium balance include 1) a total pool of Na+ of ≈4,200 mmol (total body sodium, TBS) distributed primarily in the extracellular fluid (ECV) and bone, 2) intake variations of 0.03 to ≈6 mmol·kg body mass−1·day−1, 3) asymptotic transitions between steady states with a halftime (T½) of 21 h, 4) changes in TBS driven by sodium intake measuring ≈1.3 day [ΔTBS/Δ(Na+ intake/day)], 5) adjustment of Na+ excretion to match any diet thus providing metabolic steady state, and 6) regulation of TBS via controlled excretion (90–95% renal) mediated by surrogate variables. The present focus areas include 1) uneven, nonosmotic distribution of increments in TBS primarily in “skin,” 2) long-term instability of TBS during constant Na+ intake, and 3) physiological regulation of renal Na+ excretion primarily by neurohumoral mechanisms dependent on ECV rather than arterial pressure. Under physiological conditions 1) the nonosmotic distribution of Na+ seems conceptually important, but quantitatively ill defined; 2) long-term variations in TBS represent significant deviations from steady state, but the importance is undetermined; and 3) the neurohumoral mechanisms of sodium homeostasis competing with pressure natriuresis are essential for systematic analysis of short-term and long-term regulation of TBS. Sodium homeostasis and blood pressure regulation are intimately related. Real progress is slow and will accelerate only through recognition of the present level of ignorance. Nonosmotic distribution of sodium, pressure natriuresis, and volume-mediated regulation of renal sodium excretion are essential intertwined concepts in need of clear definitions, conscious models, and future attention.

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 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.

Postprandial changes of renal blood flow. Studies on conscious dogs on a high and low sodium intake

Postprandial renal blood flow was studied in 14 conscious dogs on a chronic high and low sodium intake on 72 days after implantation of an electromagnetic flow transducer around the left renal artery. Fasting renal blood flow was 11.7 plus or minus 3.2 ml/min with kg on high socium intake (43 days) and 11.5 plus or minus 3.3 ml/min with kg on low sodium intake (29 days). During ingestion no change of renal blood flow occurred; mean arterial pressure rose transiently. During digestion renal blood flow increased always and was, like the fasting renal blood flow of dogs on a high sodium intake was 41 plus or minus 23%, and of dogs on a low sodium intake 35 plus or minus 15% referring to fasting controls; peak increase mostly occurred between 60 and90 min postprandially and was due to a decrease of renal vascular resistance. Renal blood flow also increased after augmentation of intravascular volume by an intravenous infusion; volume receptors may be involved.

Impaired autoregulation of blood flow and glomerular filtration rate in the isolated dog kidney depleted of renin

The effect of renin depletion on autoregulation of renal blood flow and glomerular filtration rate was examined in an isolated perfused kidney preparation. Group 1 dogs received a normal-sodium diet, group 2 dogs received deoxycorticosterone acetate (DOCA) and a high-sodium diet, group 3 dogs received DOCA and a sodium-deficient diet, group 4 dogs received a high-sodium diet without DOCA, and dogs in groups 5 and 6 were treated just like dogs in groups 1 and 2, respectively. Renin release was stimulated by decreasing renal arterial pressure to 50 mm Hg. An increase in renal arterial pressure from 100 to 160 mm Hg was associated with an impaired autoregulatory response in kidneys from group 2 dogs and in two kidneys from group 4 dogs; these kidneys exhibited suppressed or absent renin release in response to a decrease in renal arterial pressure. A separate group of experiments demonstrated that DOCA plus a high-sodium diet completely depleted the kidney of renin stores, indicating that decreased renin secretion reflected depletion of renal renin content. Infusing angiotensin II (0.22 ± 0.06 µg/min) into the renal artery of group 2 kidneys did not normalize the autoregulatory response. A decrease in renal arterial pressure from 150 to 50 mm Hg was associated with impaired autoregulation of glomerular filtration rate and renal blood flow in kidneys from group 6 dogs; renin secretion was undetectable in these kidneys. All kidneys from groups 1, 3, and 5 and the remaining six kidneys from group 4 exhibited a normal autoregulatory response and normal renin release. In groups 1-3, zonal blood flow was measured using the radiolabeled microsphere technique. Only in group 2 did a significant redistribution of fractional blood flow from the inner to the outer cortex occur. These experiments demonstrate that renin depletion impairs the capacity of the kidney to autoregulate blood flow and glomerular filtration rate; thus the data are consistent with the hypothesis that the renin-angiotensin system participates in the autoregulatory response.