THE EFFECT OF PREGNANCY ON MINERALO- AND GLUCO-CORTICOID SECRETION IN THE SHEEP

1. The peripheral blood concentrations of aldosterone, corticosterone and cortisol were measured during pregnancy in conscious, undisturbed sheep. 2. Aldosterone levels did not change during pregnancy and the mean pregnant value, 1·2 s.d. 1·4 ng/100 ml (n= 12) was not significantly different from the non-pregnant value, 2·1 s.d. 1·7 (n= 16). 3. Cortisol levels likewise were unchanged by pregnancy–non-pregnant values were 0·56 s.d. 0·50 μg/100 mi (n= 12) compared with 0·46 s.d. 0·40 μg/100 ml (n= 16) in pregnant sheep. 4. Sheep of 110–140 days gestation had a 400 mmol greater total exchangeable sodium than non-pregnant sheep. Plasma volume and plasma renin concentration tended to be elevated near to term. 5. Very high aldosterone secretion rates and peripheral blood levels could be produced in pregnant sheep by stress, intravenous ACTH or angiotensin II infusions, and by sodium deficiency. It is suggested that the pregnant sheep may show increased sensitivity in contrast to non-pregnant sheep to these stimuli and the enlarged size of their adrenals may be a contributing factor.     

NITRIC OXIDE, THE KIDNEY AND HYPERTENSION

1. According to the renal body fluid feedback mechanism for long-term control, persistent hypertension can only occur as a result of a reduction in renal sodium excretory function or a hypertensive shift in the pressure natriuresis relationship. Although an abnormal relationship between renal perfusion pressure and renal sodium excretion has been identified in every type of hypertension where it has been sought, factors responsible for this effect are still unclear. 2. Nitric oxide (NO) is produced within the kidney and plays an important role in the control of many intrarenal processes that regulate the renal response to changes in perfusion pressure and, thus, help determine systemic vascular volume and blood pressure. Numerous studies have shown that long-term inhibition of NO synthesis results in a chronic hypertensive shift in renal pressure natriuresis. 3. Recent studies have shown that certain animal models of genetic hypertension and forms of human hypertension areas are associated with a decrease in NO synthesis. Reductions in NO synthesis reduce renal sodium excretory function, not only through direct actions on the renal vasculature, but through modulation of other vasoconstrictor processes and through direct and indirect alterations in tubular sodium transport. 4. The causes and consequences of the disregulation of NO in hypertension and other renal disease processes remain an important area of investigation.     

THE ROLE OF THE MACULA DENSA IN RENIN SYNTHESIS

1. The role of the macula densa in renin synthesis was studied using mice with one hydronephrotic kidney. 2. Renin synthesis was assessed by measurement of renal renin, renal mRNA for renin and plasma renin. 3. Sodium depletion stimulated mRNA and renal renin to a similar extent in the hydronephrotic and contralateral kidney. 4. Enalapril stimulated mRNA concentration in both kidneys but renal renin did not rise in the hydronephrotic kidney. 5. Propranolol did not alter the response to sodium depletion in either kidney. 6. The macula densa is not crucial for the stimulation of renin synthesis following sodium depletion. However, it may regulate renin production after mRNA synthesis, possibly by controlling the conversion of prorenin to renin.     

NITRIC OXIDE IN THE MEDIATION OF PRESSURE NATRIURESIS

1. Recent studies have indicated that nitric oxide (NO) production in the kidney contributes to the regulation of renal haemodynamics and excretory function. Inhibition of nitric oxide synthase (NOS) reduces renal blood flow by approximately 25% and markedly reduces sodium excretion without reductions in filtered load. In particular, inhibition of NO synthesis markedly suppresses the slope of the arterial pressure-mediated response in sodium excretion. 2. Further studies have shown that constant intrarenal infusion of a NO donor in dogs treated with a NOS inhibitor produced diuretic and natriuretic responses but failed to restore the slope of the pressure-induced natriuretic response. These data indicate that an alteration in intrarenal NO activity, rather than the simple presence of NO during changes in arterial pressure is required for full expression of pressure natriuretic responses. 3. In support of the hypothesis that NO is involved in the mediation of pressure natriuresis, we also recently demonstrated a direct relationship between changes in arterial pressure and urinary excretion rate of sodium as well as nitrate and nitrite (a marker for endogenous NO activity) in the presence of efficient autoregulation of cortical and medullary blood flow. 4. The direct inhibitory actions of NO on tubular sodium reabsorption have also been observed in cultured tubular cells as well as isolated, perfused cortical collecting duct segments. 5. Thus, the collective data suggest that acute changes in arterial pressure induce changes in intrarenal NO production, which may directly alter tubular reabsorptive function to manifest the phenomenon of pressure natriuresis.     

A COMPARISON OF THE HAEMODYNAMIC EFFECTS OF DIETARY SODIUM RESTRICTION AND ACUTE SODIUM DEPLETION IN THE CONSCIOUS SHEEP

The use of a low Na, low K sorghum grain diet supplemented with intraruminal electrolyte infusions has enabled dietary manipulation of sodium status to be studied in the sheep. Dietary sodium restriction reduced urinary sodium excretion within 24 h with maximal retention after 3 days. There were no other substantial metabolic or haemodynamic changes. A more severe form of sodium deficiency produced by parotid salivary drainage resulted after only 2 days in a sodium deficit 3-4 times that seen with 14 days of sodium restriction. Extracellular fluid volume and cardiac output decreased. Blood pressure was unchanged but there was an increase in peripheral resistance and plasma renin concentration.     

THE EFFECTS OF HIGH Na AND Cl CONCENTRATIONS ON RAT PROXIMAL VOLUME AND Na FLUXES AT ZERO TUBULAR FLOW

1. In vivo micropuncture techniques, with and without peritubular capillary perfusion, were used to study the effects of high extracellular Na and Cl concentrations on transepithelial volume (Jv) and sodium (JNa) fluxes in rat proximal tubules. 2. In a double blind manner, the shrinking drop technique of Gertz was used to measure Jv; JNa was calculated from this and the tubular fluid Na concentration. 3. At both 184 and 279 mmol/l pericellular Na concentrations (both inside and outside the tubular epithelium), net Jv decreased significantly by 15 and 64%, respectively. Net JNa remained constant at 184 but decreased by 29% at 279 mmol/l Na concentration. 4. Thus, at both Na concentrations, when translated to free flow conditions, fractional Na reabsorption must have decreased. These findings, also supported by previous results at these Na concentrations, indicate that active Na transport was inhibited by high pericellular Na concentrations. 5. When intratubular Cl concentration was varied between 108 and 138 mmol/l while peritubular Cl was maintained constant (blood perfusing the capillaries), neither Jv nor JNa changed. Thus, at zero tubular flow, differential Cl/HCO3 concentrations do not provide significant driving forces for net Jv or JNa. 6. When only intratubular but not peritubular Na was elevated to 279 mmol/l, Jv and JNa increased markedly by 50 and 187%, providing evidence that a true solvent drag (solute drag) effect does exist in rat proximal tubules. 7. These findings offer a mechanism to explain why Na reabsorption is not increased when the filtered load of Na is increased with an elevation of plasma Na.(ABSTRACT TRUNCATED AT 250 WORDS)     

EFFECT OF SODIUM NITROPRUSSIDE ON THE BIOPHASE CONCENTRATION OF D-TUBOCURARINE AT THE NEUROMUSCULAR JUNCTION

D-Tubocurarine (DTC) at an intravenous dose of 250 micrograms/kg, or at an intra-arterial dose of 25 micrograms/kg, produced about a 50% block on the twitch tension of the indirectly stimulated tibialis anterior muscle of anaesthetized cats. The DTC-induced neuromuscular block was abolished if it was given intravenously but not intra-arterially, 30 min after the commencement of sodium nitroprusside (SNP) infusion (5 micrograms/kg per min) when there was a maximum decrease in blood pressure. The results indicate that the biophase concentration of DTC at the neuromuscular junction may be decreased as a consequence of the vasodilation induced by SNP leading to a reduced transport of the blocker to the site of action.     

POSSIBLE MECHANISMS INVOLVED IN THE NATRIURETIC RESPONSE TO ATRIAL NATRIURETIC FACTOR (ANF) AND PROANF 31–67 IN THE RAT

1. The present study was conducted to compare the mechanisms involved in the natriuretic response to atrial natriuretic factor (ANF) and pro ANF 31–67. The peptides were infused intravenously into anaesthetized rats at 10 pmol/min for 40 min. 2. Only ANF produced a significant decrease in arterial pressure; the maximum decrease was 11 mmHg (P<0.05). 3. Both peptides produced significant increases in sodium excretion (P<0.05) but only ANF increased the cyclic GMP (cGMP) excretion rate (P<0.01) and neither peptide had a significant effect on plasma renin activity or glomerular filtration rate (GFR). Pro ANF 31–67 did not increase the plasma levels of ANF. 4. These results demonstrate that both ANF and proANF 31–67 have natriuretic effects via a tubular mechanism and suggest that the natriuretic effects of ANF are mediated by cGMP while the effects of pro ANF 31–67 are mediated by a different mechanism, not involving changes in cGMP excretion, changes in GFR or a reduction in renin secretion.     

THE EFFECT OF THE ''HYPERTENSINOGENIC STEROID, 9α-FLUORO-CORTISOL ON BLOOD PRESSURE IN SHEEP WITH ACTH-INDUCED HYPERTENSION

The effect of treatment with 9 alpha-fluorocortisol (9 alpha FF), a steroid which causes hypertension in sheep, was examined in sheep with ACTH-induced hypertension. ACTH treatment alone increased mean arterial pressure (MAP), plasma Na concentration, water intake and urine volume and decreased plasma K concentration. 9 alpha FF treatment, for 3 days during continuing ACTH administration, did not change blood pressure but increased heart rate, water intake and urine volume and decreased urinary K excretion. As 9 alpha FF did not cause a further increment in blood pressure in sheep with ACTH-induced hypertension it is possible that both ACTH and 9 alpha FF may produce hypertension by similar mechanisms.     

EFFECTS OF SODIUM DEPLETION ON TISSUE CONCENTRATIONS OF THE NATRIURETIC HORMONE VASOACTIVE INTESTINAL PEPTIDE

1. Variations in dietary sodium intake have been shown to affect the plasma concentration, the metabolic clearance rate and secretion rate of vasoactive intestinal peptide (VIP). In this study we sought to determine the effect of sodium depletion on the concentration of VIP in plasma and in three tissues, namely heart, lung and kidney. 2. Male Sprague-Dawley rats were placed on low or normal sodium diets and drinking water ad libitum. A third group was placed on a low salt diet and in addition were given frusemide, 1 mg/kg per day in the drinking water. After 7 days the rats were killed, a blood sample collected and tissues harvested. VIP concentrations were determined by radio-immunoassay on unextracted plasma and in tissue after extraction. 3. There were significant differences between the three groups in the Concentration of VIP in the lung (P< 0.0005), kidney (P< 0.005) and plasma (P< 0.025) but not the heart. In the group that received frusemide and the low sodium diet, VIP in the lung was significantly lower than the low sodium (P< 0.005) and normal sodium (P< 0.0001) groups. Similar differences were noted in the kidney (frusemide vs low sodium, P< 0.001; frusemide vs normal, P< 0.01) and plasma (frusemide vs low sodium P< 0.001, frusemide vs normal P< 0.05). 4. We conclude that sodium depletion decreases the concentration of VIP in plasma and in its metabolizing tissues.     

MECHANISMS UNDERLYING THE DECREASE IN CIRCULATING ANGIOTENSIN II CONCENTRATION AFTER SODIUM LOADING

1. Acute sodium loading causes a rapid decrease in the circulating concentration of angiotensin II (AngII), which is apparent from 5 min after sodium administration. This could result from an increase in AngII catabolism and/or a decrease in AngII synthesis/secretion. However, the major determinant of AngII synthesis is thought to be a change in plasma renin activity, which occurs over a longer time frame (15 min). 2. To investigate the mechanisms underlying the rapid decrease in plasma AngII engendered by sodium administration, we performed metabolic clearance studies in male New Zealand white rabbits before and after a hypertonic sodium load of 1.5 mmol/kg as 0.513 mol/L saline i.v. bolus. 3. The metabolic clearance rate of AngII increased significantly from 42.2 ± 9.0 mL/min per kg before sodium to 110.8±33.7 mL/min per kg after sodium administration (P<0.05). The calculated or theoretical secretion rate decreased from 1470.7±404.2 to 573.5 ± 139.5 fmol/min per kg (P<0.025) in response to sodium. 4. We conclude that an increase in AngII metabolism and a decrease in synthesis/secretion contribute to the reduction in circulating AngII, which occurs in the first 60–90 min after sodium loading.     

FUNCTIONS OF THE RENIN-ANGIOTENSIN SYSTEM DURING DEVELOPMENT

1. From studies in chronically catheterized fetal sheep and other species, it can be shown that the renin-angiotensin system (RAS) is active during intra-uterine life. Levels of angiotensin II (AII) in fetal sheep are similar to maternal. 2. The fetal RAS plays a role in maintenance of arterial pressure. The extent to which it does so depends on the level of activity of the system. 3. The distribution of renin within the fetal rat kidney is much more widespread than in the adult. The fetal kidney, like other vascular beds has high levels of the AT₂ angiotensin receptor subtype. With maturation the proportion of the AT₁ receptor subtype increases. 4. Blockade of the fetal RAS with angiotensin converting enzyme (ACE) inhibitors or with the non-peptide AII antagonist (losartan) caused a fall in fetal glomerular filtration rate (GFR) and a rise in renal blood flow (RBF). AII reverses the fall in GFR even though RBF decreases. 5. The fraction of the filtered sodium load reabsorbed by the proximal tubule was not affected when the fetal RAS was blocked by captopril or losartan. High doses of infused AII had no effect on renal reabsorption of sodium, in the short term, but in the long term depressed fractional proximal reabsorption. 6. Only in high doses does AII stimulate the secretion of aldosterone from the fetal adrenal. 7. Since the fetal RAS is responsible for maintenance of GFR and physiological levels of AII do not stimulate either proximal tubular sodium reabsorption nor aldosterone secretion, it is proposed that during intra-uterine life the fetal RAS maintains the renal excretion of sodium and water into the amniotic cavity, thus ensuring an adequate volume of amniotic fluid for normal growth and development.     

NORADRENALINE RELEASE FROM THE RAT HEART DURING ANOXIA: EFFECTS OF CHANGES IN EXTRACELLULAR SODIUM CONCENTRATION AND INHIBITION OF SODIUM UPTAKE MECHANISMS

1. Anoxic perfusion of the isolated rat heart releases noradrenaline in the absence of sympathetic nerve fibre stimulation. 2. Anoxic noradrenaline release is enhanced by reducing the extracellular Na+ concentration, consistent with the proposal that such release occurs by carrier-mediated efflux. 3. Release is also enhanced by lignocaine but inhibited by amiloride and ethylisopropylamiloride, suggesting that sodium entry into adrenergic nerve terminals during anoxia occurs by Na+/H+ (and possibly Na+/Ca2+) exchange.     

SODIUM LOADS ENHANCE THE NATRIURETIC RESPONSES TO ATRIAL NATRIURETIC PEPTIDE AND NEUTRAL ENDOPEPTIDASE INHIBITORS IN CONSCIOUS CYNOMOLGUS MONKEYS

1. The effects of sodium supplements on the renal responses to human atrial natriuretic peptide (hANP 99–126) and to the selective inhibitors of neutral endopeptidase 3.4.24.11 (NEP) SQ 28 603 and candoxatrilat were determined in conscious monkeys. 2. When the monkeys’ diet was changed from 0.55% sodium to 1.1% sodium, the natriuretic response to 100 μmol/kg intravenous of SQ 28603 increased from 665 ± 64 to 1015 ± 224 μEq/3h. An acute oral load of 25 mEq sodium significantly increased the natriuresis stimulated by 300 pmol/kg, P.o., of SQ 28603 from 700 ± 332 μEq/3h in normal monkeys to 2437 ± 841 μEq/3h. Therefore, the non-human primate model was appropriate for investigating the effects of sodium loads on the urinary ANP and cGMP responses to exogenous ANP in the presence and absence of NEP inhibitors. 3. Graded intravenous infusions of saline increased basal urine volume and excretion of sodium and ANP. Salt supplements enhanced the diuretic, natriuretic and ANP responses to 0.3 nmol/kg intravenous of hANP 99–126 in monkeys treated with vehicle or 10 μmol/kg intravenous of candoxatrilat. The sodium and ANP excretions stimulated by hANP 99–126 were positively correlated with each other and with the calculated intravenous sodium load in the presence or absence of candoxatrilat. 4. SQ 28 603 and candoxatrilat (0.3 to 10 μmol/kg intravenous) each produced significant, dose-related potentiation of the natriuretic, cGMP and ANP responses to 0.3 nmol/kg intravenous of hANP 99–126 in monkeys receiving 5 mL/kg + 0.2 mL/min saline. In addition, the highest dose of SQ 28 603 produced significant depressor activity. 5. In conclusion, the increased natriuretic activity of hANP 99–126 in sodium loaded monkeys was mediated, in part, by increased ANP delivery to the guanylate cyclase linked ANP receptors in the distal renal tubules.     

AMYLIN: PHYSIOLOGICAL ROLES IN THE KIDNEY AND A HYPOTHESIS FOR ITS ROLE IN HYPERTENSION

1. There are high-affinity binding sites for amylin in the renal cortex associated with proximal tubules. These appear to represent seven transmembrane (heptatopic) receptors that are known to form ternary complexes with G-proteins and activate second messenger systems. 2.Amylin stimulates sodium/water reabsorption from the basolateral side of the proximal tubules and plays a role in sodium homeostasis. 3. The transient expression of amylin-like mRNA has been detected perinatally, using in situ hybridization, in the sub-nephrogenic zone of the metanephros and is associated with proximal tubules of the developing nephron. There it is thought to play a role as a growth factor for brush border epithelial cells in the developing kidney and in renal regrowth in the adult kidney. 4. In two models of hypertension, the spontaneously hypertensive rat (SHR) and one created surgically by subtotal nephrectomy, renal amylin receptors are activated. In the SHR, activation precedes the rise in blood pressure and suggests that activation of the amylin system may be an important event in the development of hypertension.     

EFFECT OF ATRIAL NATRIURETIC PEPTIDE ON CELLULAR ELEMENT CONCENTRATIONS IN RAT PROXIMAL TUBULES: EVIDENCE FOR INHIBITION OF THE SODIUM PUMP

1. In order to further define the action of atrial natriuretic peptide (ANP) on proximal tubular (PT) transport, combined clearance and electron microprobe X-ray (EMPX) experiments were performed on five male Wistar rats infused with ANP (0.16 nmol/kg per h) and nine control animals. 2. Electron microprobe X-ray analysis of PT cell electrolytes (mmol/ kg wet weight) revealed a similar [Na]_i in both the control and ANP treated groups (16.4 ± 0.4 vs 16.5 ± 0.4; P= 0.894). [Cl]_i was lower in the ANP treated animals (14.8 ± 0.3 vs 12.0 ± 0.3; P<0. 0001) as was [K]_i (131.4 ± 1.4 vs 114 ± 1.7; P<0.0001). The PT cells in the ANP treated group had a significant reduction in dry weight (20.1 ± 0.3 g%vs 19.0 ± 0.3 g%; P<0.024), indicating significant cell swelling. Thus, despite a normal [Na]_i, there was net accumulation of Na_i following ANP treatment. 3. These results are consistent with accumulation of Nai due to inhibition of the Na pump followed by cell swelling and subsequent regulatory volume decrease with exit of K and Cl. These results are the first to show the effect of ANP on PT intracellular electrolytes.     

EFFECTS OF ATRIAL NATRIURETIC FACTOR ON CYCLIC GMP CONTENT IN THE RAT AORTIC SMOOTH MUSCLE: STUDIES ON THE ROLE OF MEMBRANE Na+,K+-ATPase

1. These studies were conducted to determine whether preservation of the functional integrity of the membrane, Na+,K+-stimulated ATPase is essential for the atrial natriuretic factor (r-ANF-8-33) to enhance guanosine 3',5'-monophosphate (cGMP) content in the rat aortic smooth muscle. In freshly dissected rat aortic tissues, levels of cGMP were estimated using radioimmunoassay. 2. ANF (0.1 mumol/L in Krebs-Henseleit media) produced significant elevation in cGMP levels in the aortic smooth muscle when compared with that incubated in the control media, whereas suppression of Na+-pump with ouabain (1.0 mmol/L) and/or K+-free media did not produce any significant changes in the basal cGMP level; these two experimental manoeuvres did not prevent enhancement of cGMP by ANF. 3. Incubation of the tissues in the media containing ouabain plus vasoconstrictor concentrations of norepinephrine (0.3 mumol/L) also did not alter basal cGMP levels and did not prevent the ability of ANF to elevate cGMP. 4. These studies demonstrate that the antagonism by ouabain, of vasorelaxant effects of ANF (as reported in the literature) are not due to the prevention of the ability of ANF to enhance cGMP levels in the arterial smooth muscle. It is proposed that such an antagonism may be related to the actions of ouabain and ANF on diverse, and perhaps independent, mechanisms which affect Ca2+-fluxes across the cell membrane.     

ACUTE EFFECT OF ETHANOL ON RENAL ELECTROLYTE TRANSPORT IN THE RAT

1. Despite human and animal studies, the direct effect of ethanol on renal water and electrolyte transport is poorly understood. The acute effect of increasing plasma concentrations of ethanol was evaluated in a water diuretic anaesthetized rat model which inhibits endogenous arginine vaso-pressin (AVP) release. 2. Ethanol at a plasma concentration of 1.69 ±0.28 mmol/L produced an immediate increase in urine flow (174 ± 11 μL/min pre-ethanol and 189 ± 13 and then 206 ± 12 μL/min during the ethanol infusion; P<0.001) as well as an increase in fractional sodium excretion (0.17 ± 0.04 to 0.28 ± 0.05 and 0.27 ± 0.05%; P<0.001). There was also a brief phosphaturia. These increases in electrolyte excretion had returned to control values by 20 min despite a further increase in the plasma ethanol concentration. 3. The urinary excretion of potassium, calcium and magnesium was not altered nor was glomerular filtration rate or renal plasma flow. 4. Ethanol at a mean concentration of 1.60 mmol/L did not alter the action of a maximal concentration of AVP (75 ng/kg) on water or electrolyte transport. However, the antidiuretic effect of a submaximal concentration of AVP (7.5 ng/kg) was augmented by ethanol at concentrations of 1.63 and 0.98 mmol/L. 5. These studies suggest that the ethanol induced diuresis commonly ascribed to inhibition of AVP secretion may also be due to other intrarenal effects of ethanol, possibly acting within the proximal tubule. These results also confirm recent in vitro findings that while ethanol does not inhibit the action of a maximal concentration of AVP, it does modulate the effects of lower AVP concentrations.