Site of the action of a synthetic atrial natriuretic peptide evaluated in humans

Natriuretic Peptides and Normal Body Fluid Regulation

Natriuretic peptides are structurally related, functionally diverse hormones. Circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are delivered predominantly by the heart. Two C-type natriuretic peptides (CNPs) are paracrine messengers, notably in bone, brain, and vessels. Natriuretic peptides act by binding to the extracellular domains of three receptors, NPR-A, NPR-B, and NPR-C of which the first two are guanylate cyclases. NPR-C is coupled to inhibitory proteins. Atrial wall stress is the major regulator of ANP secretion; however, atrial pressure changes plasma ANP only modestly and transiently, and the relation between plasma ANP and atrial wall tension (or extracellular volume or sodium intake) is weak. Absence and overexpression of ANP-related genes are associated with modest blood pressure changes. ANP augments vascular permeability and reduces vascular contractility, renin and aldosterone secretion, sympathetic nerve activity, and renal tubular sodium transport. Within the physiological range of plasma ANP, the responses to step-up changes are unimpressive; in man, the systemic physiological effects include diminution of renin secretion, aldosterone secretion, and cardiac preload. For BNP, the available evidence does not show that cardiac release to the blood is related to sodium homeostasis or body fluid control. CNPs are not circulating hormones, but primarily paracrine messengers important to ossification, nervous system development, and endothelial function. Normally, natriuretic peptides are not powerful natriuretic/diuretic hormones; common conclusions are not consistently supported by hard data. ANP may provide fine-tuning of reno-cardiovascular relationships, but seems, together with BNP, primarily involved in the regulation of cardiac performance and remodeling. © 2017 American Physiological Society. Compr Physiol 8:1211-1249, 2018.

Role of the sodium-hydrogen exchanger in mediating the renal effects of drugs commonly used in the treatment of type 2 diabetes

Diabetes is characterized by increased activity of the sodium-hydrogen exchanger (NHE) in the glomerulus and renal tubules, which contributes importantly to the development of nephropathy. Despite the established role played by the exchanger in experimental studies, it has not been specifically targeted by those seeking to develop novel pharmacological treatments for diabetes. This review demonstrates that many existing drugs that are commonly prescribed to patients with diabetes act on the NHE1 and NHE3 isoforms in the kidney. This action may explain their effects on sodium excretion, albuminuria and the progressive decline of glomerular function in clinical trials; these responses cannot be readily explained by the influence of these drugs on blood glucose. Agents that may affect the kidney in diabetes by virtue of an action on NHE include: (1) insulin and insulin sensitizers; (2) incretin-based agents; (3) sodium-glucose cotransporter 2 inhibitors; (4) antagonists of the renin-angiotensin system (angiotensin converting-enzyme inhibitors, angiotensin receptor blockers and angiotensin receptor neprilysin inhibitors); and (5) inhibitors of aldosterone action and cholesterol synthesis (spironolactone, amiloride and statins). The renal effects of each of these drug classes in patients with type 2 diabetes may be related to a single shared biological mechanism.

ANP-mediated inhibition of distal nephron fractional sodium reabsorption in wild-type and mice overexpressing natriuretic peptide receptor

Atrial natriuretic peptide (ANP) elicits natriuresis; however, the relative contributions of proximal and distal nephron segments to the overall ANP-induced natriuresis have remained uncertain. This study was performed to characterize the effects of ANP on distal nephron sodium reabsorption determined after blockade of the two major distal nephron sodium transporters with amiloride (5 microg/g body wt) plus bendroflumethiazide (12 microg/g body wt) in male anesthetized C57/BL6 and natriuretic peptide receptor-A gene (Npr1) targeted four-copy mice. The lower dose of ANP (0.1 ng x g body wt(-1) x min(-1), n = 6) increased distal sodium delivery (DSD, 2.4 +/- 0.4 vs. 1.6 +/- 0.2 mueq/min, P < 0.05) but did not change fractional reabsorption of DSD compared with control (86.3 +/- 2.0 vs. 83.9 +/- 3.6%, P > 0.05), thus limiting the magnitude of the natriuresis. In contrast, the higher dose (0.2 ng x g body wt(-1) x min(-1), n = 6) increased DSD (2.8 +/- 0.3 mueq/min, P < 0.01) and also decreased fractional reabsorption of DSD (67.4 +/- 4.5%, P < 0.01), which markedly augmented the natriuresis. In Npr1 gene-duplicated four-copy mice (n = 6), the lower dose of ANP increased urinary sodium excretion (0.6 +/- 0.1 vs. 0.3 +/- 0.1 mueq/min, P < 0.05) and decreased fractional reabsorption of DSD compared with control (72.2 +/- 3.4%, P < 0.05) at similar mean arterial pressures (91 +/- 6 vs. 92 +/- 3 mmHg, P > 0.05). These results provide in vivo evidence that ANP-mediated increases in DSD alone exert modest effects on sodium excretion and that inhibition of fractional reabsorption of distal sodium delivery is requisite for the augmented natriuresis in response to the higher dose of ANP or in Npr1 gene-duplicated mice.

Calcium entry blocker nicardipine inhibits sodium and inorganic phosphate reabsorption independent of renal circulation in dogs

The effects of nicardipine on renal function were studied in anesthetized dogs. The changes in the tubular sodium (Na) and inorganic phosphate (PO(4)) reabsorption caused by the drug infusion into the renal artery without altered systemic and real circulation were especially evaluated. In dogs receiving a smaller dose of nicardipine (5 ng.kg(-1).min(-1)) into the left renal artery the blood pressure and renal circulation did not change, but urine volume and urinary Na and PO(4) excretion increased significantly. In dogs receiving a larger dose of nicardipine (50 ng.kg(-1).min(-1)) into the renal artery, renal plasma flow, urine volume and urinary Na and PO(4) excretion increased significantly, but creatinine clearance did not. The fractional distal Na reabsorption did not change with nicardipine infusion in either group. PO(4) reabsorption is considered to occur mainly in the proximal renal tubule, so its appearance in urine in increased quantities without the changes of systemic and renal circulation suggests proximal activity of the drug.

Acute symptomatic hyponatremia and cerebral salt wasting after head injury: an important clinical entity

Hyponatremia is a well known complication of traumatic and nontraumatic cerebral injury, often related to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Nonetheless, it also can be associated with a different entity, the syndrome of cerebral salt wasting (CSW). The authors report the case of a 4.5-year-old boy presenting with major head injury who at day 6 after admission had generalized tonic-clonic seizures caused by severe acute hyponatremia (serum sodium level, 119 mmol/L) and signs of dehydration. Despite initial isotonic rehydration, hyponatremia persisted because of excessive renal salt losses and concomitant enormous water losses, necessitating increasing amounts of sodium, up to 160 mmol/kg/d, and large amounts of intravenous fluids, up to 27 L/d. Highly increased levels of atrial natriuretic peptide (ANP) confirmed the diagnosis of CSW. The occurrence of a CSW has to be recognized early in the clinical course for adequate treatment and remains one of the important differential diagnosis of SIADH in hyponatremic states in patients with cerebral disorders, especially after head injury.

Regulation of water channel activity of aquaporin 1 by arginine vasopressin and atrial natriuretic peptide

Aquaporin 1 (AQP1), a six-transmembrane domain protein that functions as a water channel, is present in many fluid secreting and absorbing tissues such as kidney, brain, heart, and eye. It is believed that among the five known mammalian aquaporins, kidney aquaporin (AQP2) is the only water channel that is regulated by arginine vasopressin (AVP). The present data suggest that AQP1 may also be regulated by AVP. The application of AVP to Xenopus oocytes injected with AQP1 cRNA increased the membrane permeability to water. In addition, our data reveal that atrial natriuretic peptide (ANP), a peptide hormone that plays an important role in the regulation of body fluid homeostasis, blocks the AQP1-mediated increase in water permeability. Incubation with 8-bromo-cAMP or direct 8-bromo-cAMP injection into oocytes expressing AQP1 cRNA significantly increased membrane permeability to water, suggesting that stimulation of AQP1 activity by AVP may involve a cAMP-dependent mechanism. Regulation of water permeability by AVP and ANP has potential relevance to active water transport in a variety of tissues that express AQP1 including kidney, brain, and eye.

Effects of high dose atrial natriuretic peptide on renal haemodynamics, sodium handling and hormones in cirrhotic patients with and without ascites

To elucidate and to try to reverse the antinatriuretic mechanisms in liver cirrhosis, atrial natriuretic peptide (ANP) was given as a pharmacological bolus dose (2 micrograms per kg body weight) to 14 cirrhotic patients, and as a control to 14 healthy subjects. The nine patients with ascites had baseline values of glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and blood pressure (BP) similar to controls. Their distal tubular fractional reabsorption of sodium (DFRNa), estimated by the lithium clearance technique, was higher than in controls, and so were plasma values of aldosterone (564 vs. 119 pmol l-1 medians), endothelin (1.23 vs. 0.63 pmol l-1), ANP (7.5 vs. 3.6 pmol l-1) and cyclic GMP (8.8 vs. 4.6 nmol l-1); p < 0.01 for all. The five patients without ascites had higher GFR and ERPF, and lower plasma angiotensin II than controls. After ANP injection, similar plasma levels of ANP and cyclic GMP were reached in all groups. Urinary sodium excretion rate increased in controls (0.23 to 0.52 mmol min-1, p < 0.01), while GFR increased (108 to 117 ml min-1, p < 0.05), and DFRNa decreased (93 to 89%, p < 0.01). In cirrhotics with ascites sodium excretion was unaltered (0.12 to 0.11 mmol min-1), and so was GFR (84 to 83 ml min-1). Proximal tubular fractional reabsorption of sodium increased after 90 min, whereas DFRNa decreased immediately (97 to 96%, p < 0.01) though less markedly than in controls. Sodium excretion increased in four of five patients without ascites (0.23 to 0.27 mmol min-1, medians). In patients with ascites, endothelin in plasma decreased after ANP (p < 0.05). Plasma levels of angiotensin II, aldosterone and vasopressin were unchanged in all groups. In conclusion, although hyper-reabsorption of sodium occurred in the distal rather than the proximal part of the nephron in cirrhotic patients with ascites, ANP had no natriuretic effect. This was most probably due primarily to the lack of increase of GFR and blunted inhibition of DFRNa, attributed to high aldosterone. The effect of ANP in suppressing the high endothelin did not seem to improve sodium excretion.

Pressure-dependent, enhanced natriuretic response to low-dose, atrial natriuretic peptide infusion in essential hypertension

OBJECTIVE

To examine whether the effect of atrial natriuretic peptide (ANP) on renal glomerular and tubular segmental handling of sodium in patients with essential hypertension is pressure dependent.

DESIGN

Part 1. The renal effects of a low-dose continuous infusion (10 ng kg-1 min-1) with ANP for 1 h were compared in 10 untreated essential hypertensives (EH) and 13 normotensive control subjects (CS). Part 2. The hypertensives were studied on another day with ANP infusion during preceding acute BP reduction with sodium nitroprusside infusion (NP). The results were compared with those obtained during infusion with ANP+placebo (Part 1).

METHODS

Lithium clearance was used to estimate the proximal tubular reabsorption of sodium.

RESULTS

Part 1. Atrial natriuretic peptide caused an exaggerated increase in urinary sodium excretion (+102 vs. +38%: P < 0.05), fractional excretion of sodium (+80 vs. +37%: P < 0.05), and urinary output (+56 vs. +8.3%; P < 0.05) in EH compared with CS. Glomerular filtration rate and filtration fraction increased to the same degree in both groups. Absolute lithium clearance (CLi) increased and FELi tended to increase (P = 0.061) in EH, but these were unchanged in CS. The increase in plasma cyclic guanosine 5'-phosphate (cGMP) and urinary excretion of cGMP and the decrease in plasma aldosterone during ANP infusion were the same in the two groups. Part 2. During NP infusion the natriuresis caused by ANP in EH was reduced (+51 vs. +99%; P < 0.05). The relative changes in GFR, CLi, and FELi during ANP infusion were not affected by the preceding BP reduction with NP. Mean arterial pressure was reduced from 122 to 101 mmHg during NP infusion. The relative increase in sodium excretion in EH was significantly correlated to mean arterial pressure.

CONCLUSIONS

Low-dose ANP infusion causes an exaggerated natriuresis in untreated essential hypertensives due to a more pronounced reduction in tubular reabsorption. After BP reduction, the natriuresis induced by ANP in essential hypertensives is decreased, probably due to a less pronounced reduction in tubular reabsorption beyond the proximal tubules. We suggest that the enhanced natriuretic response to ANP in EH in secondary in some degree to the elevated systemic pressure.

Extra-natriuretic effects of atrial peptide in humans

To evaluate extra-natriuretic effects of atrial natriuretic peptide (ANP), plasma ANP (pANP) levels were assessed in seven healthy men on low-sodium diet (80 mEq NaCl/day), in basal conditions and during stepwise infusion of human ANP (2, 4, 8 and 16 ng/min/kg). To determine the individual physiological (PHY) pANP level, we measured pANP in the same subjects after a high-salt diet (400 mEq NaCl/day), that is, in a physiological stimulation of ANP. We then compared the effects of the PHY levels of ANP to the effects of pharmacological (PHA) pANP levels. Neither PHY nor PHA pANP levels modified creatinine clearance or blood pressure. The progressive rise in pANP levels was associated with increases in urinary excretion of Na+, K+ and urea. ANP alone respectively accounted for 41%, 30% and 92% of the increase in natriuresis, kaliuresis and urea excretion that occurred after changing salt intake from 80 to 400 mEq/day. Pharmacological ANP levels raised CH2O and reduced UOsm. Interestingly, PHA levels were associated with significant decrease in serum K+ (from 4.5 +/- .1 to 4.0 +/- .1 mEq/liter) and plasma urea (from 31.9 +/- 5 to 24.2 +/- 4 mg/dl). The mean cumulative urinary potassium and urea losses corresponded to the theoretical body losses of potassium and urea; moreover, the individual cumulative urinary losses of potassium and urea significantly correlated with the corresponding decrement in their plasma levels. In conclusion, ANP has both physiological and pharmacological significance in the control of potassium and urea metabolism by decreasing plasma levels of K+ and urea through effects on the renal excretory function.

Dose-response study of atrial natriuretic peptide bolus injection in healthy man

The effects of human atrial natriuretic peptide (ANP) on glomerular filtration rate (GFR), renal plasma flow (RPF), urinary flow rate, urinary sodium excretion, tubular function estimated by lithium clearance, and plasma levels of sodium and water homeostatic hormones were studied in a dose-response study with 50 healthy subjects. Placebo or ANP 0.5, 1.0, 1.5, or 2.0 micrograms kg-1 bwt was given as an intravenous bolus injection to five different groups. GFR rose after ANP, whereas no immediate change in RPF was observed. Significant increases with no distinct additional effect of ANP doses higher than 1.0 microgram kg-1 were detected in filtration fraction, urinary flow rate and urinary excretion rate of sodium. Both proximal and distal fractional reabsorption of sodium was reduced and the effect seemed to flatten out at doses higher than 1.0 microgram kg-1. Dose-dependent increases in cyclic guanosine monophosphate in urine and plasma were found after ANP bolus injection, and the rise in both was correlated with the increase in urinary sodium excretion. ANP caused a dose-dependent decrease in blood pressure and an increase in pulse rate. Plasma concentrations of angiotensin II and arginine vasopressin did not change after ANP. In summary, we found that ANP bolus injection caused a natriuresis and diuresis in healthy man with a threshold at a dose of 1.0 microgram kg-1. No distinct further renal effects were observed with higher doses despite dose-dependent increases in urinary cGMP excretion and plasma cGMP. Inhibition of both proximal and distal tubular fractional sodium reabsorption by ANP contributed to the natriuretic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic factor: its (patho)physiological significance in humans

The first human studies using relatively high-doses of ANF revealed similar effects as observed in the preceding animal reports, including effects on systemic vasculature (blood pressure fall, decrease in intravascular volume), renal vasculature (rise in GFR, fall in renal blood flow), renal electrolyte excretion (rises in many electrolytes), and changes in release of a number of different hormones. Whether all these changes are the result of direct ANF effects or secondary to a (single) primary event of the hormone remains to be determined. Certainly, it has been proven that more physiological doses of ANF fail to induce short-term changes in many of these parameters leaving only a rise in hematocrit, natriuresis and an inhibition of the RAAS as important detectable ANF effects in humans. This leads us to hypothesize that ANF is a "natriuretic" hormone with physiological significance. The primary function in humans is to regulate sodium homeostasis in response to changes in intravascular volume (cardiac atrial stretch). Induction of excess renal sodium excretion and extracellular volume shift appear to be the effector mechanisms. The exact mechanism of the natriuresis in humans still needs to be resolved. It appears however, that possibly a small rise in GFR, a reduction in proximal and distal tubular sodium reabsorption, as well as an ensuing medullary washout, are of importance. The pathophysiological role of ANF in human disease is unclear. One may find elevated plasma irANF levels and/or decreased responses to exogenous ANF in some disease states. Whether these findings are secondary to the disease state rather than the cause of the disease remains to be resolved. Therapeutic applications for ANF, or drugs that intervene in its production or receptor-binding, seem to be multiple. Most important could be the antihypertensive effect, although areas such as congestive heart failure, renal failure, liver cirrhosis and the nephrotic syndrome cannot be excluded. Although the data that have been gathered to date allowed us to draw some careful conclusions as to the (patho)physiological role of ANF, the exact place of ANF in sodium homeostatic control must still be better defined. To achieve this, we will need more carefully designed low-dose ANF infusion, as well as ANF-breakdown inhibitor studies. Even more promising, however, is the potential area of studies open to us when ANF-receptor (ant)agonists become available for human use.

Renal and hormonal effects and tolerance of an ANP analogue in healthy man

The effect of an analogue of atrial natriuretic peptide (P-ANP) on glomerular filtration rate (GFR), renal plasma flow (RPF), urinary flow rate, urinary sodium excretion, tubular function estimated by the lithium clearance technique, and plasma levels of sodium and water homeostatic hormones, has been studied in 40 healthy males. Placebo or P-ANP 0.3, 1.5, or 3.0 micrograms.kg-1 bwt were given as an intravenous bolus injection to different groups. P-ANP did not cause any immediate change in GFR or RPF, but significant dose-dependent increases in filtration fraction, urinary flow rate and urinary excretion rate of sodium were detected during the first 30 min after administration. Proximal absolute and fractional tubular reabsorption and distal absolute tubular reabsorption of sodium did not change after injection of P-ANP, while the distal fractional reabsorption of sodium was reduced in a dose dependent manner during the first 30 min. Plasma angiotensin II and aldosterone were significantly increased 30 and 150 min after dosage, whereas plasma atrial natriuretic peptide, plasma arginine vasopressin, and urinary excretion of prostaglandin E2 were unchanged. Cyclic guanosine monophosphate both in plasma and urine were increased in a dose-dependent manner. P-ANP cause a significant reduction in diastolic blood pressure and an increase in pulse rate. Two subjects had vasovagal syncope 30-60 min after injection of P-ANP. It is concluded that P-ANP has natriuretic, diuretic and hypotensive properties in healthy man.

Attenuated renal excretory response to atrial natriuretic peptide in congestive heart failure in man

The renal and hormonal effects of atrial natriuretic peptide given as a bolus injection (2.0 micrograms/kg) were studied in 12 patients with congestive heart failure before and after treatment with captopril for 4 weeks and in 13 healthy control subjects. Atrial natriuretic peptide caused a rise in urinary excretion of sodium and urinary flow in the controls, whereas no increases were observed in the patients. Both proximal and distal fractional reabsorption of sodium, as evaluated by the lithium clearance technique, decreased less in the patients than in the controls. Basal plasma concentrations of atrial natriuretic peptide and cyclic guanosine monophosphate (cGMP), and the basal urinary excretion of cGMP, were elevated in the patients. The increases in both plasma and urinary cGMP after administration of atrial natriuretic peptide were blunted in heart failure. Basal glomerular filtration rate and renal plasma flow were reduced, and filtration fraction increased, in the patients. A positive correlation (r = 0.958, P less than 0.01) was found between renal plasma flow and the relative increase in urinary excretion of sodium in the patients with heart failure. Treatment with captopril did not improve the natriuretic and diuretic effect of exogenous atrial natriuretic peptide, but resulted in an increase in filtration fraction after administration of atrial natriuretic peptide not present before captopril.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle sympathetic nerve activity and renal responsiveness to atrial natriuretic factor during the development of hepatic ascites

PURPOSE

Sodium retention in cirrhosis has been attributed to an imbalance between vasoconstrictive antinatriuretic forces such as the sympathetic nervous system and vasodilatory natriuretic agents such as atrial natriuretic factor (ANF). With the development of refractory ascites, cirrhotic patients become unresponsive to the natriuretic effect of ANF. Animal data suggest that the sympathetic nervous system plays a key role in mediating the refractoriness to ANF. We therefore studied the relationship between sympathetic nerve activity (SNA) and the natriuretic response to ANF in normal subjects and cirrhotic patients. We also attempted to localize the intrarenal site of refractoriness to ANF by lithium clearance.

PATIENTS AND METHODS

Twenty-six patients with biopsy-proven cirrhosis and seven age- and sex-matched normal volunteers were studied after a week of 20 mmol/day sodium intake and no diuretics. Muscle SNA was recorded from the peroneal nerve (microneurography) and correlated with responsiveness to a 2-hour ANF infusion. Lithium clearance was used as a marker of sodium reabsorption proximal to the intramedullary collecting duct, the main site of ANF action. Plasma norepinephrine, renin, and aldosterone levels were also determined. Patients were categorized into three groups: nine patients free of ascites (by ultrasonography), five ascitic patients who responded to a 2-hour ANF infusion (i.e., had a natriuretic response to ANF above 0.83 mmol/hour), and 12 ascitic patients who did not respond.

RESULTS

Muscle SNA was greatly increased in the ascitic nonresponder patients compared with the normal subjects (64 +/- 4 versus 27 +/- 7 bursts/minute, p less than 0.001), moderately increased in ascitic responders (47 +/- 6 bursts/minute, p less than 0.05), but not significantly increased in nonascitic patients with cirrhosis (34 +/- 5 bursts/minute). SNA was positively correlated with plasma norepinephrine levels (r = 0.69; p less than 0.005) and inversely correlated with peak sodium excretion during the ANF infusion (r = -0.63; p less than 0.001). Plasma renin activity and aldosterone were markedly elevated in ascitic nonresponders, and normal in ascitic responders and nonascitic patients. Lithium clearance was reduced in ascitic patients compared with nonascitic patients, did not change after the ANF infusion, and correlated inversely with SNA (r = -0.61; p less than 0.01).

CONCLUSION

These results support the concept that the sympathetic nervous system is a factor in renal sodium handling in cirrhosis, especially in the initiation of sodium retention and the development of refractory ascites. Refractoriness to ANF might be explained, at least in part, by increased neurally mediated sodium reabsorption proximal to the intramedullary collecting duct, the main site of ANF action.

Abnormal tubular handling of sodium and water induced by atrial natriuretic peptide in essential hypertension

Atrial natriuretic peptide (ANP) was given as an intravenous bolus injection (2.0 micrograms kg-1) to 12 essential hypertensive patients (EH) and 13 normotensive control subjects (C) in order to study the effect of ANP on renal glomerular and tubular function using the lithium clearance technique. Urinary sodium excretion (EH, + 370% vs. C, + 120%; P less than 0.001) and urine volume (EH, + 137% vs. C, + 62%; P less than 0.01) increased significantly more in EH than in controls after ANP injection. Glomerular filtration rate, renal plasma flow, and plasma concentrations of angiotensin II, aldosterone and arginine vasopressin remained almost unchanged after ANP injection, whereas the filtration fraction increased to the same extent in both groups. Both proximal (EH, - 15% vs. C, - 5%; P less than 0.01) and distal fractional reabsorption (EH, - 12% vs. C, - 5%; P less than 0.01) of sodium decreased more markedly after ANP in EH than in controls. The increase in plasma cGMP and urinary excretion of cGMP was the same in the two groups. Mean blood pressure decreased and heart rate increased to the same extent in both groups. It is concluded that the increase in urinary sodium excretion and urine volume induced by ANP bolus injection is exaggerated in EH due to a more pronounced reduction in the reabsorption of sodium and water in both the proximal and the distal tubule.

Enhanced urinary excretion of albumin and beta 2 microglobulin in essential hypertension induced by atrial natriuretic peptide

Atrial natriuretic peptide (ANP) was given as an intravenous bolus injection (2.0 micrograms kg-1) to 12 essential hypertensive patients (EH) and 10 normotensive control subjects (C) in order to study the effect of ANP on urinary excretion of albumin and beta 2-microglobulin, and on glomerular filtration rate (GFR), renal plasma flow (RPF), and filtration fraction (FF). After the ANP injection, urinary excretion of albumin increased significantly (p less than 0.01) in EH from 7.3 micrograms min to 125 micrograms min (medians) and in C from 2.9 micrograms min-1 to 8.1 micrograms min-1 (p less than 0.05). Urinary excretion of beta 2-microglobulin increased in EH from 70 ng min-1 to 1022 ng min-1 (p less than 0.01) and in C from 118 ng min-1 to 170 ng min-1 (p less than 0.01). The increase in urinary excretion of both albumin (p less than 0.01) and B2-microglobulin (p less than 0.01) was significantly more pronounced in EH than in C. GFR and RPF were almost unchanged in both groups. FF rose to the same degree in the two groups. The increase in fractional excretion of sodium and in urine volume after ANP was enhanced in EH. It is concluded that ANP in pharmacological doses increased urinary excretion of albumin and beta 2-microglobulin to a considerably larger extent in essential hypertensive patients than in normotensive control subjects.

Does urinary atrial natriuretic peptide reflect plasma levels in humans?

1. Saline infusion produced concurrent mean increases over basal in plasma and urinary atrial natriuretic peptide (ANP) of approximately 100%. 2. Subpressor calcium infusion also produced concurrent increases in plasma and urinary ANP of 38 and 60% respectively. 3. Urinary ANP appeared to be as responsive to the dynamic stimulation of saline and calcium infusion as plasma levels. 4. Urinary ANP may be useful when an integrated estimation of ANP secretion and metabolism is required. Further studies are required to assess clinical utility in a variety of settings, for example during endopeptidase 3.4.24.11 inhibition.

The influence of posture on renal tubular function in man

The acute effects of changes in posture on renal function were investigated in healthy volunteers. Proximal tubular function was assessed using two methods simultaneously: lithium clearance and urine flow rate during maximal water diuresis. Changing from the standing to the supine posture was associated with significant increases in urine flow rate and in the clearances of sodium, potassium, creatinine and lithium (p less than 0.01 in each case). Increases in fractional water excretion and fractional lithium excretion (using creatinine clearance as an estimate of glomerular filtration rate) were also observed (p less than 0.01). The increases in lithium clearance and urine flow rate, both absolute and fractional, were significantly correlated. These findings suggest that the acute increase in sodium excretion following assumption of the supine posture results largely from an increase in end-proximal fluid delivery, due partly to an increase in glomerular filtration and partly to a decrease in fractional reabsorption in the proximal tubule. The results also provide circumstantial support for the use of lithium clearance as a marker of end-proximal delivery in man.

The pharmacology and therapeutics of diuretics in the pediatric patient

Selection of appropriate diuretic therapy in children is hampered by a lack of age-specific pharmacokinetic and pharmacodynamic data, especially in premature neonates. Well-designed clinical trials in neonates, infants, and younger children are necessary prerequisites to safer and more efficacious diuretic therapy.

Acute effects of physiological increments of alpha-atrial natriuretic peptide in man

Seven dehydrated volunteers received three hour infusions of 0.8 pmol kg-1 min-1 of human alpha-atrial natriuretic peptide (h-alpha ANP) or vehicle alone (Ve) in a single-blind, randomized cross-over design. H-alpha ANP infusion increased plasma h-alpha ANP from 4.2 +/- 0.4 to 20.3 +/- 6.4 pm. H-alpha ANP suppressed plasma renin activity from 3.30 +/- 0.48 to 1.37 +/- 0.35 ng ml-1 hr-1 (P less than 0.001 vs. Ve). Plasma aldosterone was unaltered by h-alpha ANP. Fractional excretion of filtered sodium (FENa) changed from 0.92 +/- 0.09 to 1.13 +/- 0.16% with h-alpha ANP, and from 1.02 +/- 0.09 to 0.69 +/- 0.11% with Ve (P less than 0.01 h-alpha ANP vs. Ve). FEK was unchanged. FEpo4 increased from 7.2 +/- 1.2 to 9.2 +/- 1.2% and FELi from 22.1 +/- 1.4 to 24.9 +/- 3.0% with h-alpha ANP (both P less than 0.05 vs. Ve). H-alpha ANP decreased mean urinary osmolality by approximately 150 mOsmol kg-1 compared to Ve (P less than 0.01). GFR, RPF and filtration fraction were unchanged by h-alpha ANP, H-alpha ANP was associated with a significant tachycardia (P less than 0.01 vs. Ve) but with no significant change in arterial pressure. These results suggest that small increments of plasma h-alpha ANP, mimicking physiological changes, are natriuretic at least partly by reducing proximal tubular reabsorption of sodium, and also impair urinary concentration.

Atrial natriuretic peptide and urinary prostaglandins in man

1. In order to assess the effects of atrial natriuretic factor on the renal biosynthesis of prostaglandins (PG), the urinary excretion of PGE2, PGF2 alpha, 6-keto-PGF1 alpha and thromboxane (Tx)B2 were followed in eight salt-loaded healthy volunteers infused for 2 h with a non hypotensive dose of human atrial natriuretic peptide (hANP, 0.7 nmol min-1). 2. Within 1 h, hANP, infusion produced a marked increase in the urinary PG output, especially of PGE2 and 6-keto-PGF1 alpha (188 +/- 21% and 202 +/- 24% of the pre-infusion values respectively), followed by a significant decrease during the recovery period. 3. No correlations could be uncovered between the urinary excretion of sodium and that of any of the PGs. In contrast, during the infusion of hANP, the urinary output of PGE2 and of 6-keto-PGF1 alpha was found positively related to the urinary flow rate (r = 0.42; P less than 0.05; n = 32 and r = 0.43; P less than 0.05; n = 32 respectively) as well as during the recovery period (r = 0.66, P less than 0.001; n = 32 and r = 0.55; P less than 0.01; n = 32 respectively). 4. It was concluded that, in man, infusion of a non hypotensive dose of hANP is followed by a rise in urinary PG excretion presumably reflecting enhanced renal PG biosynthesis. This increased urinary PG excretion does not seem to be involved in the natriuretic action of hANP but might participate to its diuretic effect.

Blunted natriuretic response and low blood pressure after atrial natriuretic factor in early cirrhosis

We compared the natriuretic response to a standard dose of atrial natriuretic factor in nine patients with early cirrhosis (no ascites or edema) with the response in normal subjects displaying a range of baseline sodium excretions due to different sodium intakes (20 mmoles per day, n = 9; 100 mmoles per day, n = 9, and 200 mmoles per day, n = 9). In these normal subjects, sodium output rose, in the same order, from 49 +/- 12 to 177 +/- 26, from 116 +/- 21 to 365 +/- 106 and from 228 +/- 29 to 901 +/- 85 mumoles per min in the first 20 min after 100 micrograms atrial natriuretic factor (human atrial natriuretic factor 99-126). Thus, irrespective of basal excretion, natriuresis rose by at least 2-fold. In the cirrhotic patients, natriuresis rose from 173 +/- 42 to 305 +/- 77 mumoles per min, that is by hardly 1-fold, significantly less than in the normal subjects (p less than 0.01). Renal function studies indicated that atrial natriuretic factor caused less rise in glomerular filtration rate and in fractional sodium excretion. Atrial natriuretic factor induced a fall in blood pressure only in the cirrhotic group, from 130 +/- 4/81 +/- 2 to 108 +/- 4/68 +/- 3 mmHg (p less than 0.001). Plasma atrial natriuretic factor was not low in the cirrhotic patients. Although these data are compatible with a primary disturbance of sodium excretion in early cirrhosis without ascites, such an explanation is complicated by the concomitant drop in blood pressure after atrial natriuretic factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide in patients with obstructive uropathy

Renal response to release of bilateral ureteral obstruction resembles that to intravenous administration of atrial natriuretic peptide. In a prospective study we measured plasma atrial natriuretic peptide levels before and serially after relief of obstruction in 9 patients (mean age 65 +/- 2 years old) with bilateral ureteral obstruction and azotemia. Obstruction was documented by renal ultrasonography. Before relief of obstruction blood urea nitrogen and serum creatinine levels were 85 +/- 18 (mean +/- standard error) and 8.2 +/- 1.3 mg. per dl., respectively, accompanied by metabolic acidosis but not hyperkalemia. Mean plasma atrial natriuretic peptide (measured by radioimmunoassay) was 129 +/- 28, which was markedly elevated compared to 46 +/- 7 pg. per ml. in 7 age-matched control subjects (p less than 0.01). After relief of obstruction, prominent post-obstructive diuresis and natriuresis ensued; the plasma atrial natriuretic peptide level progressively decreased to that noted in the control group, accompanied by improvement in renal function, and diminishing diuresis and natriuresis. These findings were associated with a significant weight loss and an increase in plasma renin activity (from a mean of 1.57 +/- 0.68 to 5.27 +/- 1.82 ng. per ml. per hour, p less than 0.01). These results suggest that atrial natriuretic peptide release is augmented in patients with bilateral ureteral obstruction and azotemia, probably due to hypervolemia, and may contribute to post-obstructive diuresis and natriuresis.

Low dose atrial natriuretic factor in primary aldosteronism: renal, hemodynamic, and vascular effects

Whether atrial natriuretic factor (ANF) plays a physiological role in primary aldosteronism has yet to be determined. In the present study, the renal, hemodynamic, humoral, and vascular effects of a synthetic (WY-47663) human analogue were studied in five water-loaded (15 ml H2O/kg) patients with adenomatous primary aldosteronism, a salt-sensitive, low renin, volume-expanded syndrome. ANF was infused for 3 hours at a low rate (0.005 micrograms/kg/min), which approximately doubled circulating immunoreactive ANF. Glomerular filtration rate and renal blood flow (inulin and para-aminohippurate clearance) remained stable, but sodium excretion increased significantly suggesting a dissociation between renal hemodynamics and natriuresis as well as a direct inhibitory effect on tubular sodium reabsorption by ANF. Intra-arterial diastolic blood pressure, heart rate, forearm blood flow (plethysmographic method), and arterial plasma norepinephrine did not change, but systolic blood pressure declined and hematocrit rose suggesting plasma volume contraction by ANF. Plasma aldosterone levels were unchanged indicating a loss of ANF-mediated aldosterone inhibition, possibly related to qualitative or quantitative alterations of ANF receptors in tumoral adrenal tissue. Infusion of the analogue into the brachial artery was at a rate of 0.005 micrograms/dl forearm tissue/min x 30 minutes, which also doubled local immunoreactive venous ANF concentrations and vasodilated forearm arterioles. These data suggest a physiological role for ANF in modulating body fluid volume even in human primary aldosteronism.

Small intra- and large inter-individual variability in lithium clearance in humans

We report the inter- and intra-individual variability in fractional lithium clearance (CLi), an alleged quantitative index of Na and water delivery from the proximal tubules, in humans (N = 91). The inter-individual variability was large, the variation coefficients at various Na excretion rates ranging between 11% and 19%. The intra-individual variability was small, the relative intra-individual standard deviation for duplicate measurements (N = 33) being 5%. These observations suggest large inter-individual differences in proximal tubular Na reabsorption. To confirm this, we also studied the inter- and intra-individual variability in the maximum urine flow during water diuresis (Vmax), an index of Na delivery to the diluting segment. They were found to be almost identical to the inter- and intra-individual variability in CLi, and fractional CLi and Vmax correlated strongly (r = 0.83, P less than 0.001). In addition, the inter-individual variability in the fractional clearance of uric acid (CUA), a directional marker of Na reabsorption in the proximal tubules, was large, but the intra-individual variability small. The correlation between fractional CUA and CLi, however, was relatively weak (r = 0.40, P less than 0.01). Although our results do not prove the exact, quantitative validity of the lithium clearance concept, we conclude that the variability in CLi reflects large inter-individual differences in Na handling in the proximal segments of the nephron. Our observations also have implications for the use of the lithium clearance method. The large inter-individual variability in CLi makes the method less suitable to detect subtle differences in CLi in small, unpaired groups of subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal response to infusion versus repeated bolus injections of atrial natriuretic factor in man

In 7 healthy humans consuming a 170 mmol sodium diet the effect of the mode of administration of atrial natriuretic factor (human ANF 99-126) on renal function has been investigated, using conventional clearance studies during maximal water diuresis. ANF was administered as four repeated bolus (0.4 microgram/kg) injections and, after a 2-day interval, as a one-hour infusion (0.02 microgram/kg/min) preceded by a 0.4 microgram/kg bolus injection. In the two experiments ANF caused comparable elevations in glomerular filtration rate, free water clearance, and lithium excretion. No change in blood pressure or heart rate was observed in either study, and plasma renin activity and aldosterone fell by a similar extent. As expected, the time course of plasma ANF concentrations was markedly different during the two studies. It is concluded that with those doses of ANF the changes in renal haemodynamics and sodium handling were essentially similar after bolus injections and a constant infusion.

Opposite effects of enalapril and nitrendipine on natriuretic response to atrial natriuretic factor. Renal function evaluated with clearance studies in humans

In clearance studies, we analyzed the effect of Ca2+ entry blockade with nitrendipine treatment (20 mg b.i.d. for 4 days) and of converting enzyme inhibition with enalapril treatment (20 mg b.i.d. for 4 days) on renal response to atrial natriuretic factor (ANF) (25 micrograms bolus followed by an infusion of 0.03 microgram/kg/min for 90 minutes) in six healthy volunteers who were taking 300 mmol sodium daily. In a control study ANF was administered without Ca2+ entry blockade or converting enzyme inhibition. Natriuresis rose from 239 +/- 38 to 605 +/- 137 mumol/min in the control study (p less than 0.05), from 330 +/- 53 to 943 +/- 152 mumol/min with Ca2+ entry blockade (p less than 0.05), and from 236 +/- 22 to 344 +/- 39 mumol/min with converting enzyme inhibition (NS). ANF induced a rise in maximal free water clearance, inulin clearance, and in the excretion of multiple electrolytes except potassium. Fractional lithium reabsorption fell. In general, these effects were stronger during Ca2+ entry blockade and blunted during converting enzyme inhibition. p-Aminohippurate clearance tended to decrease during the control study (NS), remained constant during Ca2+ entry blockade, and decreased significantly when ANF was infused during converting enzyme inhibition (p less than 0.05 vs. control and vs. Ca2+ entry blockade study). Blood pressure was lowered by Ca2+ entry blockade and, to a somewhat greater extent, by converting enzyme inhibition, but ANF administration induced no additional fall except for a short-term drop during Ca2+ entry blockade.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal interaction of atrial natriuretic peptide with angiotensin II: glomerular and tubular effects

1. The possible interactions between the renal effects of atrial natriuretic peptide (ANP) and angiotensin II (AII) were studied in normal sodium-replete human subjects. Recent investigations have suggested that ANP inhibits the pressor and volume-retaining effects of activation of the renin-angiotensin system. Thus, ANP may attenuate the effects of AII on renal haemodynamics or tubular transport. 2. ANP (0.1 micrograms/kg per min, 60 min) was intravenously infused into eight normal human subjects with and without pretreatment with enalapril (20 mg, per oral), an inhibitor of the converting enzyme, and during infusion of AII (10 mg/kg per min). 3. ANP infusion alone caused increases in the urine volume (from 96 +/- 23 to 229 +/- 44 mL/h, P less than 0.05) and urinary sodium excretion (from 11.5 +/- 1.6 to 20.9 +/- 4.2 mEq/h, P less than 0.05). These changes were accompanied by an increase in the glomerular filtration rate (from 127 +/- 9 to 158 +/- 9 mL/min, P less than 0.05). ANP infusion after enalapril administration lowered the mean blood pressure (from 76 +/- 2 to 71 +/- 3 mmHg, P less than 0.05) to a level similar to that observed during ANP infusion alone (from 84 +/- 2 to 74 +/- 2 mmHg, P less than 0.01), but did not result in a significant diuresis (from 139 +/- 23 to 174 +/- 51 mL/h) or natriuresis (from 19.7 +/- 2.5 to 14.3 +/- 3.4 mEq/h, P less than 0.05). This combined treatment with a converting enzyme inhibitor and ANP reduced both the glomerular filtration rate (160 +/- 9 to 141 +/- 10 mL/min) and the renal plasma flow (from 775 +/- 49 to 570 +/- 45 mL/min, P less than 0.01). 4. The antinatriuretic effects of exogenous AII were reversed by superimposed ANP infusion (urinary sodium excretion: from 4.8 +/- 1.0 to 24.3 +/- 5.2 mEq/h, P less than 0.01). Under these conditions, the glomerular filtration rate increased (from 114 +/- 6 to 156 +/- 7 mL/min, P less than 0.05) to levels similar to those observed with ANP infusion alone. In addition the increased tubular sodium reabsorption induced by AII was inhibited by concomitant ANP infusion (fractional proximal tubular sodium reabsorption: from 90.7 +/- 3.5 to 80.3 +/- 16.6%, P less than 0.05, fractional post-proximal tubular sodium reabsorption: from 91.5 +/- 9.8 to 87.6 +/- 8.8%, P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Atrial natriuretic peptide in man

The heart is the major source of atrial natriuretic peptides (ANP). A propeptide is stored in atrial myocytes. In normal humans, atrial distension secondary to volume overload and/or increased atrial pressures are thought to stimulate the secretion of biologically active alpha-ANP (ANF-[99-126], 28 amino residues) into the circulation. Plasma immunoreactive ANP (irANP) rises in response to acute sodium-volume loading, the central shift of volume produced by lying down or by immersion, acute increases in blood pressure (BP), dynamic exercise, or the administration of glucocorticoids or mineralocorticoids. Plasma irANP also rises with aging. Synthetic alpha-ANP infused acutely i.v. can lower BP, reduce plasma volume by an extravascular shift, cause baroreflex-mediated sympathetic activation, directly inhibit adrenal steroidogenesis and lower plasma aldosterone and cortisol, directly inhibit renal renin release, elevate plasma insulin; diuresis, free water clearance and natriuresis increase already in response to low alpha-ANP doses that raise plasma irANP within the physiological-pathological range. It follows that in addition to direct influences on cardiovascular and renal function, the ANP system may comprise a cardio-adrenal feedback mechanism and perhaps also modulate insulin and the release of ADH. The major although yet unproven physiological role of the ANP system may be the protection of the heart against volume and/or pressure overload. The pathophysiological, diagnostic and therapeutic aspects of elevated plasma irANP values, ANP measurements, or administration of synthetic ANP, respectively, in various diseases are currently under intense study and of great potential interest.

Noradrenaline attenuates the natriuretic effect of atrial natriuretic factor in man

1. The effects of noradrenaline (0.025 micrograms kg-1 min-1) and atrial natriuretic factor (0.04 micrograms kg-1 min-1) alone, and in combination, were studied in eight healthy salt replete men. 2. Atrial natriuretic factor increased urinary sodium excretion and flow rate without changing glomerular filtration rate or systemic haemodynamics. 3. Noradrenaline decreased urinary sodium excretion without changing glomerular filtration rate, urinary flow rate or systemic haemodynamics. 4. When atrial natriuretic factor was administered against a background infusion of noradrenaline the natriuretic response to the peptide was significantly attenuated. 5. Further analysis showed that this attenuation was due to the additive antinatriuretic effect of noradrenaline rather than to a specific interaction between atrial natriuretic factor and noradrenaline. 6. The possible significance of this interplay between noradrenaline and atrial natriuretic factor is discussed in the context of experimental evidence for an important role of the sympathetic nervous system in modulating the renal effects of atrial natriuretic factor in animals.

Enhanced natriuretic effect of atrial natriuretic factor during mineralocorticoid escape in humans

We examined the question of whether escape from the sodium-retaining effect of mineralocorticoid involves an increased natriuretic effect of atrial natriuretic factor (ANF). Seven healthy volunteers taking a 170 mmol Na/100 mmol K diet received an intravenous bolus (25 micrograms) followed by a 1-hour infusion (0.02 micrograms/kg/min) of ANF (human ANF-[99-126]) before and after 10 days of 9-fludrocortisone acetate, 0.5 mg b.i.d. Escape was accompanied by an increase in body weight (from 72.2 +/- 12.9 to 74.0 +/- 12.6 kg; p less than 0.05), mean arterial pressure (from 95 +/- 4 to 109 +/- 3 mm Hg; p less than 0.01), plasma ANF (from 9 +/- 2 to 24 +/- 4 pmol/L; p less than 0.01), and inulin clearance (from 124 +/- 9 to 137 +/- 7 ml/min; p less than 0.05). Indexes for renal sodium handling (lithium and free water clearance) were compatible with a decreased "proximal" and an increased "distal" tubular reabsorption fraction. ANF infusion raised inulin clearance comparably before and after escape to 138 +/- 10 and 152 +/- 7 ml/min, respectively, but the natriuretic effect was much larger (p less than 0.05) after escape (from 366 +/- 34 to 1294 +/- 278 mumol/min) than before (from 248 +/- 48 to 630 +/- 124 mumol/min). Indexes for tubular reabsorption were consistent with greater suppression of both "proximal" and "distal" tubular sodium reabsorption by ANF after versus before mineralocorticoid expansion. These results indicate that escape is accompanied not only by a rise in plasma ANF but also by potentiation of the natriuretic effect of ANF.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding sites of atrial natriuretic peptide in human renal tissue-quantification by in vitro receptor autoradiography

Specific binding sites for atrial natriuretic peptide (99-126) in different areas of normal human renal tissue were quantified by in vitro autoradiography. Our data represent the first characterization of ANP binding sites in different structures of the human kidney. Characterization of ANP binding revealed by Scatchard plot analysis a single class of high affinity binding sites in the glomeruli (Kd 0.53 +/- 0.11 nM; BMax 74.4 +/- 17.86 fmol/mg protein), the vasculature (Kd 0.18 +/- 0.014 nM; BMax 91.6 +/- 25.02 fmol/mg protein), and the medulla (Kd 0.34 +/- 0.13 nM; BMax 106.0 +/- 30.61 fmol/mg protein). These sites may play a key role in the actions of the cardiac hormone in human kidney and in the ameliorating effects of ANP in the recovery from acute renal failure.

Frusemide pretreatment blunts the inhibition of renal tubular sodium reabsorption by ANF in man

The effects of atrial natriuretic factor (ANF) 15 pmol/kg/min on renal function were studied in 7 normal male volunteers during maximal water diuresis. Subjects were studied in neutral salt balance either before, or after, seven days treatment with 40 mg oral frusemide. The post-frusemide state was associated with activation of the renin-angiotensin system (RAAS) and generally higher noradrenaline levels; this state was also associated with sodium retention, mainly due to enhanced distal nephron reabsorption. Without diuretic pretreatment ANF produced a natriuresis and diuresis associated with inhibition of both proximal and distal nephron sodium reabsorption. In contrast, after frusemide pretreatment, ANF caused an increase in water excretion (urinary flow rate) but no change in sodium excretion. In the post-diuretic condition ANF did not affect renal tubular handling of sodium. The enhanced tubular reabsorption of sodium post-frusemide, and the failure of ANF to suppress this, could be due to activation of the RAAS and SNS.

Binding sites for atrial natriuretic peptide in the kidney and aorta of the hagfish (Myxine glutinosa)

1. In the atlantic hagfish (Myxine glutinosa) binding sites for atrial natriuretic peptide (ANP) were visualized by autoradiography in contractile structures of the renal system (glomeruli, neck segment, and archinephric duct) and in the aorta. 2. Since the location of binding sites is comparable to that in higher vertebrates including man, these results suggest that ANP may act as a hormone already in cylcostomata.