The responses of atrial natriuretic factor concentrations to acute volume changes in conscious rats

A rapid and sensitive radioimmunoassay has been developed for measurements of atrial natriuretic factor (ANF) in rat plasma. The antiserum, raised to rat ANF (99-126), cross-reacts with rat ANF (103-123), ANF (103-125), ANF (103-126) but not with smaller fragments, human ANF (99-126), angiotensin II, bradykinin or vasopressin. The plasma ANF concentration is 181 +/- 24 pg/ml (N = 24) in the unstressed conscious rats (Charles River CD, male). The ANF immunoreactivity in the plasma extracts was verified by HPLC analysis, which displayed one major immunoreactive peak of ANF corresponding to rat ANF (99-126) and some smaller fragments. Intravenous injection of saline elevated circulating ANF, whereas acute volume depletion by hemorrhage, water deprivation and furosemide diuresis greatly lowered plasma ANF. The prompt response of plasma ANF levels to acute changes in volume status is consistent with the proposed role of ANF as a volume-regulatory hormone.     

Endopeptidase-24.11 in human plasma degrades atrial natriuretic factor (ANF) to ANF(99-105/106-126)

We previously demonstrated the presence of ANF(99-126), and ANF(99-126) cleaved between Cys105 and Phe106 (cleaved ANF), in human coronary sinus plasma. We now report that cleaved ANF is formed when synthetic ANF(99-126) is added to human plasma. When synthetic ANF(99-126) was incubated in heparinized human plasma, HPLC analysis showed two degradation products. The main product was shown by amino acid and sequence analysis to be cleaved ANF. Degradation of ANF was inhibited by EDTA and phosphoramidon. These findings are consistent with the action of endopeptidase EC 3.4.24.11, which may play an important part in the biological inactivation of ANF.     

Degradation of atrial natriuretic factor in the rat.

The biologically active circulating form of atrial natriuretic factor (ANF) in the rat is the 28-amino-acid peptide ANF-(Ser-99-Tyr-126). Degradation of this peptide in vivo as well as in vitro, in whole blood, in plasma and by the isolated mesenteric artery was investigated. Studies in vivo in the rat demonstrated that the elimination and degradation of ANF was extremely fast: within 3 min more than 95% of the injected immunoreactive material was eliminated from circulation. The production of a short C-terminal peptide was detected on injection of 125I-ANF-(Ser-99-Tyr-126) into the rat. This peptide increased proportionately with incubation time. Experiments in vitro in the presence of whole blood or plasma did not cause any major destruction of ANF even after incubation for 60 min. After this prolonged incubation in plasma, ANF-(Ser-99-Tyr-126) was partially converted into ANF-(Ser-103-Tyr-126), a less potent peptide. Isolated mesenteric-artery preparation appeared to degrade ANF in a manner very similar to the system in vivo. These results suggest that degradation of ANF may occur either after internalization in the vascular cells or by a membrane-bound enzyme in the vasculature.     

Disappearance of atrial natriuretic factor from circulation in the rat

The rate of disappearance of radioiodinated forms of 3 different atrial natriuretic factors (ANF (Ser 99-Tyr 126), ANF (Arg 101-Tyr 126), ANF (Ser 103-Tyr 126)) from circulation in the rat was studied. Before proceeding to study the half-life of these peptides, the biological activity of their cold iodinated forms was examined. Upon incorporation of iodine into the ANF molecule, there was a 2 to 5-fold loss in their binding affinities to mesenteric arteries and adrenal capsules as compared to their respective uniodinated forms. A similar loss in their potency to inhibit basal aldosterone release from adrenal zona glomerulosa cells was observed. The rate of disappearance of the radioiodinated peptides from plasma was very fast; the half-life of ANF (Ser 99-Tyr 126) was 16.8 +/- 0.9 sec. Similar values were also obtained for ANF (Arg 101-Tyr 126) and ANF (Ser 103-Tyr 126). The in vivo disappearance of ANF from plasma is probably due to the binding to receptors in the cells since in vitro incubation of ANF (Ser 99-Tyr 126) with rat plasma caused only a slight loss in its immunoreactivity in the first 5 minutes. Hepatectomy and nephrectomy did not cause any major prolongation of the disappearance rate suggesting that these two organs may not be the primary sites involved in the removal of this peptide from circulation.     

Increased release of the N-terminal and C-terminal portions of the prohormone of atrial natriuretic factor during immersion-induced central hypervolemia in normal humans

The role of peptides from the N terminus and C terminus of the 126 amino acid atrial natriuretic factor (ANF) prohormone in modulating renal sodium and water handling has not been defined. Since water immersion to the neck (NI) provides an acute central volume expansion identical to that produced by 2 liters of saline but without plasma compositional change, immersion to the neck was used to assess the N-terminal and C-terminal portions of the ANF prohormone response to acute central blood volume expansion in seven seated sodium-replete normal subjects. Both the C terminus, which contains amino acids 99-126 and is identical to ANF, and the whole N terminus (i.e., amino acids 1-98) increased promptly with NI and peaked after 1 hr of immersion. A Mr 3900 peptide from the midportion of the N terminus consistent with amino acids 31-67 (i.e., pro-ANF-31-67) also increased with NI and followed a pattern of increasing circulating concentration nearly identical to that of the whole N terminus of the prohormone, except that its maximal concentration was at the second hour of the 3 hr of NI. With cessation of immersion, ANF decreased to preimmersion levels within 1 hr whereas the N terminus and pro-ANF-31-67, although their circulating concentrations were decreasing, were still significantly elevated at 1 hr. These findings suggest that the increase in plasma ANF, the N terminus of the ANF prohormone, and pro-ANF-31-67 from the midportion of the N terminus, with natriuretic properties similar to ANF, contribute to the natriuretic response to NI, implying a physiologic role for these atrial peptides in modulating volume homeostasis in humans.     

Three distinct forms of atrial natriuretic factor receptors: kidney tubular epithelium cells and vascular smooth muscle cells contain different types of receptors

Three distinct ANF receptor subtypes have been identified and characterized from cultured canine kidney tubular (MDCK) cells and rat thoracic aortic smooth muscle (RTASM) cells. These three ANF receptor subtypes include; (1) a disulfide-linked 140 kDa protein found in RTASM cells which was reduced by sulfhydryl reagent dithiothreitol (DTT) to a 70 kDa band, (2) a disulfide-unlinked 120 kDa protein, specific to MDCK cells whose Mr was not reduced by DTT and (3) a 68-70 kDa protein prevalent in both RTASM and MDCK cells whose Mr was not reduced by DTT. The non-reducible 68-70 kDa and the reducible 140 kDa proteins showed strong affinities to the full-length ANF (99-126) and truncated ANF (103-123) peptides, however, non-reducible 120 kDa protein showed strong affinity only to the full length ANF (99-126) but negligible or very weak affinity to truncated ANF (103-123). These findings suggest that distinct ANF receptor subtypes are present in renal and vascular cells which might be linked to diverse physiological functions of ANF such as natriuresis and diuresis in kidney and vasorelaxation in vascular smooth muscle cells.     

The secretion of atrial natriuretic factor-(99-126) by cultured cardiac myocytes is regulated by glucocorticoids

Atrial natriuretic factor (ANF) is stored in mammalian atria primarily as ANF-(1-126), the precursor to the known circulating form of the hormone ANF-(99-126). When primary cultures of atrial myocytes were maintained in a complete serum-free medium, they contained and secreted an ANF-(1-126)-like peptide. The addition of dexamethasone to the culture medium, however, resulted in the secretion of a molecule with chromatographic characteristics identical to ANF-(99-126), although the intracellular storage form of ANF was unchanged. Radiosequencing and amino acid analysis confirmed that the cultures maintained in dexamethasone secreted authentic ANF-(99-126). Chronic exposure of the cells to dexamethasone also resulted in a significant increase in the quantity of immunoreactive ANF both contained and secreted by the cultures. Dexamethasone stimulated ANF processing and secretion by atrial cultures in a dose-dependent manner, with an approximate EC50 of 10 nM. This stimulation could be reversed by removing the glucocorticoid from the culture medium. ANF processing was also stimulated by the specific glucocorticoid receptor agonist RU 28362, and both DEX- and RU 28362-stimulated ANF processing was inhibited by the specific glucocorticoid receptor antagonist RU 38486. Ventricular cells, which possess few granules and release ANF in a constitutive fashion, were also capable of processing ANF in a glucocorticoid-dependent fashion. Medium freshly removed from atrial cultures did not convert ANF-(1-126) to ANF-(99-126) nor was exogenous ANF-(1-126) efficiently processed when added to the medium of actively processing cultures. These results indicate that the post-translational processing of ANF-(1-126) to ANF-(99-126) likely occurs within or in close association with the cardiac myocytes and is not dependent on the presence of large quantities of secretory granules. Furthermore, it is apparent that both the expression and the post-translational processing of ANF by cultured cardiac myocytes is specifically regulated by glucocorticoids.     

Secretion of atrial natriuretic factor-(1-98) by primary cardiac myocytes

Previous studies have demonstrated that primary cultures of cardiac myocytes maintained in a complete serum-free medium contain a precursor to atrial natriuretic factor (ANF-(1-126]. The cultured cells secrete this precursor unless maintained in the presence of glucocorticoids wherein the known circulating form derived from the C-terminal of ANF (ANF-(99-126] is secreted. The present study was designed to determine the fate of the N-terminal region of the ANF precursor during secretion from myocytes maintained in glucocorticoids. A radioimmunoassay (RIA) was developed using synthetic ANF-(1-16); the antiserum demonstrated cross-reactivity toward ANF-(1-126) and ANF-(1-98)-like peptides but did not cross-react with ANF-(99-126). Coupling this RIA with an ANF-(99-126)-specific RIA and reversed phase, size exclusion, and ion exchange high performance liquid chromatography (HPLC), it was shown that primary cultures of atrial myocytes maintained in dexamethasone contained ANF-(1-126) and secreted ANF-(99-126) and a peptide that was chromatographically indistinguishable from ANF-(1-98). Isolated perfused rat hearts were also shown by RIA and HPLC to secrete similar peptides. The primary cells were labeled with [35S]methionine, and the secreted N-terminal ANF-related material was immunoprecipitated with the ANF-(1-16) antiserum. HPLC, tryptic peptide mapping, and radiosequencing demonstrated that this peptide possessed an N-terminal structure identical to that of ANF-(1-126). When the cells were labeled with [3H] leucine and the secreted N-terminal ANF-related material was immunoprecipitated and analyzed by tryptic mapping, it was shown to possess labeled tryptic peptides consistent with the structure of ANF-(1-98). Tryptic mapping of [3H]arginine-labeled N-terminal ANF-related material demonstrated the presence of all peptides consistent with the ANF-(1-98) structure, including ANF-(92-98). These studies demonstrate that primary atrial myocytes contain ANF-(1-126) and in the presence of dexamethasone secrete both ANF-(1-98) and ANF-(99-126), the two major circulating forms of the hormone.     

Identification and characterization of three distinct atrial natriuretic factor receptors. Evidence for tissue-specific heterogeneity of receptor subtypes in vascular smooth muscle, kidney tubular epithelium, and Leydig tumor cells by ligand binding, photoaffinity labeling, and tryptic proteolysis

Three distinct atrial natriuretic factor (ANF) receptors have been identified and characterized from rat thoracic aortic cultured vascular smooth muscle (RTASM) cells, kidney tubular epithelium (MDCK), and Leydig tumor (MA-10) cells. These include 1) a disulfide-linked 140-kDa protein found in RTASM cells, which was reduced by dithiothreitol (DTT) to 70 kDa, 2) a 120-135-kDa single polypeptide protein, specific to MDCK and MA-10 cells whose Mr was not reduced by DTT, and 3) a 66-70-kDa protein prevalent in both RTASM and MDCK cells, which was not reduced by DTT. After incubation of RTASM cells with 4-azidobenzoyl 125I-ANF, labeling of the 140-kDa protein was blocked by both full-length ANF(99-126) and truncated ANF103-123. In contrast, the labeling of the 120-kDa receptor in MDCK cells was blocked only by full-length ANF(99-126). However, labeling of the 68-70-kDa receptor in both RTASM and MDCK cells was blocked by full-length ANF(99-126) and truncated ANF(103-123). Binding of 125I-ANF(99-126) to RTASM and MDCK cells was rapid, specific, and saturable with a Kd of 1.5 x 10(-10) M and binding capacity (Bmax) of 2.1 x 10(5) sites/RTASM cell and Kd 4.5 x 10(-10) M and Bmax 5 x 10(4) sites/MDCK cell, respectively. Binding of 125I-ANF(99-126) to RTASM cells was displaced with both full-length ANF(99-126) and truncated ANF(103-123), however, binding to MDCK cells was efficiently displaced only with full-length ANF. Both ANF(99-126) and ANF(103-123) stimulated cGMP in RTASM cells but only ANF(99-126) elicited cGMP in MDCK cells. Tryptic proteolysis of the high Mr single chain receptor produced only a 68-kDa fragment, whereas disulfide-linked 140-kDa receptor yielded 52-, 38-, 26-, and 14-kDa fragments. These data provide direct biochemical evidence for three distinct ANF receptors which might be linked to diverse physiological functions of ANF such as natriuresis in the kidney, vasorelaxation in vascular smooth muscle, and steroidogenic responsiveness in Leydig cells.     

Immunoreactive atrial natriuretic factor (IR-ANF) in human plasma

A direct radioimmunoassay for ANF in human plasma was developed. A synthetic alpha-human atrial peptide (Ser 99-Tyr 126) was used for preparation of the iodinated tracer and the standards. The sensitivity of the method is 1.9 pg/ml. Concentration of immunoreactive ANF (IR-ANF) in plasma of 59 clinically normal subjects was 65.3 +/- 2.5 pg/ml (mean +/- SE). In two patients who underwent atrial pacing an increase of about 100 percent in circulating IR-ANF was observed. IR-ANF was extracted from human plasma by Vycor glass and purified by HPLC. The main immunoreactive isolated peak contained a low molecular weight peptide.     

Degradation and clearance of atrial natriuretic factors (ANF)

Atrial natriuretic factor, the first well defined natriuretic hormone is synthesized in the human heart as 151 aminoacid (AA) preprohormone and stored as 126 AA prohormone in atrial granules. Upon appropriate stimulation, the prohormone is cleaved into a 98 AA N-terminal fragment and a 28 AA C-terminal fragment, the biological active ANF(99-126), both circulating in plasma. Circulating ANF(99-126) is cleared by various organs, such as lung, liver and intestine, kidney and upper and lower limbs. Reported arterial-venous extraction ratios vary greatly, but are not much different between organs, the average extraction ratio being about 35%. Due to marked differences of organ blood flow, the contribution of various organs to total body ANF clearance differs considerably. Major mechanisms for ANF clearance are uptake by clearance receptors and degradation by an endoprotease (EC 3.4.24.11.). Clearance receptors, distinct from the receptors mediating the biological actions of ANF, have been demonstrated in various organs. Characterization of the ANF degrading enzyme activity has been performed in kidney tissue. Whether and how pathophysiological states affect ANF clearance is still poorly understood. Inhibition of clearance by ANF analogues binding to clearance receptors and by inhibitors of degrading peptidase can increase the biological action of circulating ANF. This may prove to be a therapeutic approach in diseases with smooth muscle contraction or volume overload.     

The post-translational processing of rat pro-atrial natriuretic factor by primary atrial myocyte cultures

Primary cultures of neonatal rat atrial myocytes were maintained in two different serum-free media for up to 25 days. Reversed-phase high performance liquid chromatography coupled with atrial natriuretic factor (ANF)-specific radioimmunoassay demonstrated that the cultures maintained in our previously described serum-free medium (Glembotski, C.C., and Gibson, T. R. (1985) Biochem. Biophys. Res. Commun. 132, 1008-1017) secreted primarily ANF-(1-126)-like material, whereas those cultures maintained in a different formulation of medium secreted mostly ANF-(99-126)-like material. Cultures that secreted ANF(99-126)-like material were biosynthetically labeled with [35S]cysteine followed by immunoprecipitation of secreted ANF and analysis by reversed-phase, size exclusion, and ion-exchange high performance liquid chromatography. The labeled ANF-(99-126)-like peptide was shown to be chromatographically indistinguishable from other synthetic peptides related to ANF-(99-126). Labeled ANF purified from extracts of the cultured cells was chromatographically indistinguishable from authentic ANF-(1-126), and could be cleaved specifically by thrombin into labeled ANF-(99-126)-like material. These results indicate that primary atrial myocytes maintained under certain serum-free conditions are capable of secreting ANF-related material that is chromatographically indistinguishable from ANF-(99-126), the known circulating form of the hormone. Additional preliminary studies suggest that the presence of glucocorticoids in the culture medium may confer ANF processing ability on cultured myocytes.     

Atrial natriuretic peptide, ANP(99-126), receptors in rat thymocytes and spleen cells

A single class of saturable, specific binding sites for the circulating form of atrial natriuretic peptides, ANP(99-126), was identified in rat thymus and spleen and in isolated thymocytes and spleen cells using quantitative autoradiographic techniques. In the thymus, the relative potency of ANP analogs to inhibit [125I] ANP(99-126) binding was ANP(99-126) = ANP(103-126) greater than ANP(111-126) greater than ANP(103-125). ANP(103-123) could not displace [125I]ANP(99-126) binding. Addition of ANP(99-126) stimulated the formation of cyclic GMP in isolated thymocytes and spleen cells in a dose-dependent manner. Our results indicate that immune cells have specific ANP receptors which could be coupled to guanylate cyclase activation and may play a role in the regulation of the immune response.     

Comparative biological activities of ANF (Arg 101-Tyr 126) and the synthetic form of circulating ANF (Ser 99-Tyr 126)

The biological activities of ANF (Arg 101-Tyr 126) and of the circulating form, ANF (Ser 99-Tyr 126), were compared in the following assays: precontracted rabbit aortic strip and chick rectum, rat natriuresis, inhibition of aldosterone secretion and receptor affinity in bovine and rat adrenal zona glomerulosa cells, and receptor affinity in rabbit aorta and rat mesenteric artery cells. The results demonstrate that both peptides share the same biological activities. It is concluded that the addition of two amino acids to the N-terminal of ANF (Arg 101-Tyr 126) does not modify its biological characteristics, validating thus previous research employing this peptide.     

Bivariate assay for cleaved atrial natriuretic factor (ANF(101–105/106–126)) in sheep plasma

A bivariate assay has been developed to measure both ANF, and cleaved ANF (ANF(101–105/106–126)) the primary product of endopeptidase-24.11 action on rat ANF(101–126). The assay employed two different antisera directed to the carboxy-terminal and ring regions of ANF to enable discrimination and measurement of intact endogenous ANF(99–126) and infused cleaved ANF (rat ANF(101–105/106–126)) in sheep plasma extracts. The assay which was validated by HPLC analysis of plasma extracts, had sufficient accuracy and precision to measure the metabolic clearance rate and half-life of cleaved ANF when infused in normal sheep.     

Interactions between endothelin and atrial natriuretic factor in the rat aortic ring preparation.

The potential interaction (s) between atrial natriuretic factor (ANF) and porcine--human endothelin (ET-1) was investigated in the endothelium-denuded rat aortic ring preparation. ET-1 produced a sustained contraction of aortic rings with an ED50 of 3.6 +/- 0.49 x 10(-9) M. Within the concentration range of 10(-9) to 10(-7) M, both rat ANF 103-126 and rat ANF 99-126 when preincubated with tissues reduced the contractile efficacy of ET-1 especially at low concentrations resulting in a small but significant rightward shift of the dose--response curve to ET-1. In contrast, at a concentration of 10(-10) M, rANF 99-126 but not rANF 103-126 produced a significant leftward shift of the dose--response curve to ET-1 and an increase in the maximal developed tension for the dose--response curve to ET-1. For tissues incubated in the absence of extracellular calcium or in the presence of the calcium channel blocker nifedipine (5 x 10(-7) M), both ANF derivatives produced a dose-dependent decrease in the maximum contraction, but no change in potency to ET-1. Addition of either rANF 103-126 or rANF 99-126 to tissues maximally contracted with ET-1 resulted in relaxation, reaching a maximum of 70%. The ED50 values for relaxation were 2.7 +/- 0.51 x 10(-8) and 3.5 +/- 0.60 +/- 10(-8) M for rANF 103-126 and rANF 99-126, respectively. ET-1 did not interact with biologically responsive and clearance receptors for ANF.(ABSTRACT TRUNCATED AT 250 WORDS)     

The circadian rhythm of the N-terminus and C-terminus of the atrial natriuretic factor prohormone

Circadian variation in the circulating concentrations of the N-terminal and C-terminal portions of the atrial natriuretic factor prohormone (pro ANF) was evaluated in 8 men, ages 41-47, who have been followed for 19 years with respect to circadian variation in physiological variables including blood pressure and clinical chemistries. The N-terminus of the ANF prohormone contains two peptides consisting of amino acids 1-30 and 31-67 while the C-terminus contains 1 peptide (amino acids 99-126) of this 126 amino acid prohormone which lower blood pressure and have natriuretic properties. To determine if either the N-terminus and/or the C-terminus of the prohormone have a circadian variation in their circulating plasma concentrations these 8 men had blood samples obtained for radiommunoassay every 3 hr during a 24-hr period. Three radiommunoassays which immunologically recognize (1) the whole N-terminus (i.e. amino acids 1-98), (2) the midportion of the N-terminus (amino acids 31-67) and (3) the C-terminus (amino acids 99-126) of the ANF prohormone were utilized. The whole N-terminus, the midportion of the N-terminus which circulates after being proteolytically cleaved from the rest of the N-terminus, and the C-terminus each had a peak circulating concentration between 0400 and 0700 which were significantly (P less than 0.001) higher than their concentrations at any other time throughout the 24-hr period.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of a long-acting recombinant human serum albumin-atrial natriuretic factor (ANF) expressed in Pichia pastoris

The cardiac hormone atrial natriuretic factor (ANF) combines pharmacological properties of drugs used to treat essential hypertension (EH), congestive heart failure (CHF) and acute myocardial infarction (AMI). Treatment of CHF or AMI patients with an intravenous (iv) infusion of the circulating form of ANF (ANF(99-126)) produces significant clinical improvement. The short half-life (5 min) and peptide nature of ANF impose logistic restrictions for chronic administration. To increase its half-life, we fused ANF and human serum albumin (HSA) mini-genes by recombination in Pichia pastoris. The activity of three configurations of the fusion protein was tested in vitro and in vivo. The fusion protein that comprised of C-terminus HSA connected to N-terminus ANF via a hexaglycine linker showed the best outcome; it increased cGMP production in vitro. In vivo an iv bolus of HSA-ANF into mice increased significantly plasma cGMP levels and lowered blood pressure (BP) for up to 6 h hence successfully extended ANF half-life in plasma while retaining its biological activity. HSA-ANF represents the basis for development in the chronic therapeutic use of ANF.     

Atrial natriuretic factor-induced cGMP accumulation in rat anterior pituitary cells in culture is not coupled to hormonal secretion

While atrial natriuretic factor (ANF) does not influence ACTH secretion, it was reported to have a marked stimulatory effect on the intracellular accumulation of cGMP in rat anterior pituitary cells in culture. Since many biological actions of ANF appear coupled to its excitatory action on target cell guanylate cyclase, the current study was designed to characterize the ANF-induced cGMP response in anterior pituitary with a view to determining whether the nucleotide plays a regulatory role in the secretory function of this gland. A 3 min exposure of cells in primary culture to 300 nM ANF (99-126) or 100 microM sodium nitroprusside (SNP), a stimulator of guanylate cyclase, causes maximal 10- and 3-fold elevations of cGMP levels, respectively. Following a progressive decrease, 6- and 2-fold increases over basal cGMP levels are still observed after 180 min of incubation with ANF (99-126) and SNP, respectively. The half-maximal stimulation of cGMP accumulation induced by a 10 min exposure to ANF (99-126), or rat atriopeptin II (ANF 103-125) is observed at 9 +/- 2 and 125 +/- 22 nM, respectively. ANF fragments (99-109) and (111-126), as well as human cardiodilatin (hANF 1-16), do not alter cGMP levels. Basal and ANF-induced cGMP levels are at least 10-fold higher in cell populations enriched in gonadotrophs compared to gonadotroph-impoverished preparations. A 3 h incubation of cells with ANF (0.1-1000 nM), however, fails to modify spontaneous or LHRH-induced LH secretion. Similarly, ANF does not alter spontaneous release of GH, TSH or PRL. The data suggest indirectly that gonadotrophs represent a principal site at which ANF acts to stimulate cGMP synthesis, but that the nucleotide is not a specific regulator of the LH secretory process; nor is it generally involved as a second messenger in the secretory function of any cell type of the anterior pituitary gland.     

The effect of hypovolemia on the renal and cardiovascular responses to atrial natriuretic factor (ANF) infusion.

The renal and cardiovascular effects of ANF infusion have been examined in separate series of experiments; in conscious instrumented sheep following either hemorrhage (10 mL/kg body weight) or removal of 500 mL of plasma by ultrafiltration. Renal arterial infusion of hANF (99-126) at 50 micrograms/h increased sodium excretion from 99 +/- 30 to 334 +/- 102 (p less than 0.05) in normal animals, and from 77 +/- 31 to 354 +/- 118 mumol/min in hemorrhaged animals. Similarly in sheep following ultrafiltration, cardiac output and stroke volume were reduced by intravenous infusion of ANF (100 micrograms/h), although these effects were less marked than those observed in normal animals. The rapid modulation of natriuretic responses to ANF observed in volume expanded animals is not seen in this model of acute volume depletion suggesting that the mechanism through which the renal response to ANF is modulated in low sodium or volume states is not simply the reverse of that which produces rapid enhancement of response following blood volume expansion.