Intracerebroventricular metformin attenuates salt-induced hypertension in spontaneously hypertensive rats.

The aim of this study was to examine the effects of long-term continuous intracerebroventricular (icv) infusion of metformin on blood pressure (BP) in spontaneously hypertensive rats (SHR). To accelerate the development of hypertension, SHR were fed a 8% NaCl diet during the 3-week study period. Metformin was given in the following doses: 0 (isotonic saline; n = 7), 25 (n = 8), 50 (n = 6), 100 (n = 6), and 200 microg/day icv (n = 5). Mean arterial pressure (MAP) and heart rate (HR) were measured by radiotelemetry, and as a measure of the contribution of sympathetic nerve activity to BP, the decrease in MAP in response to ganglionic blockade with hexamethonium, 30 mg/kg iv, was determined once weekly. In vehicle treated rats, MAP increased by 27+/-4 mm Hg, whereas in rats treated with a low dose of metformin (25 microg/day), MAP increased only by 7+/-3 mm Hg (P < .01). The hypotensive response to hexamethonium was attenuated by all doses of metformin suggesting that chronic icv metformin decreased central sympathetic outflow. The highest doses of metformin (100 and 200 microg/day) also prevented development of hypertension, but these doses were highly neurotoxic as demonstrated by histologic evaluation post mortem. Fast-Fourier transformation of MAP revealed increased variability within the 0.15 to 0.6 Hz frequency range in rats treated with neurotoxic doses of metformin, suggesting impaired sympathetic control of BP in these animals. In conclusion, long-term icv infusion with apparently nontoxic doses of metformin attenuates hypertension and decreases the hypotensive responses to ganglionic blockade in SHR, suggesting a centrally elicited sympathoinhibitory action.

Inhibition of nitric oxide synthesis attenuates insulin-mediated sympathetic activation in rats.

BACKGROUND AND OBJECTIVES: Infusion of insulin produces sympathoexcitation, nitric oxide (NO) generation and NO-mediated vasodilation. Because central nervous system NO may inhibit sympathetic outflow, the present study was designed to determine whether NO synthase blockade would enhance insulin-mediated sympathetic activation. We additionally aimed to determine whether augmented sympathoexcitation and reduced NO-mediated vasodilation, during combined NO synthase blockade and hyperinsulinemia, would result in a blood pressure increase. DESIGN AND METHODS: We infused vehicle (Control; n = 7) or insulin (10 mU/min) in anaesthetized rats receiving either no pretreatment (Insulin; n = 7) or after pretreatment with the NO blocker, NG-monomethyl-L-arginine (L-NMMA-insulin; 0.25 mg/kg per min; n = 7), while measuring mean arterial pressure (MAP), heart rate and lumbar sympathetic nerve activity (SNA) during euglycemic clamp. An additional control group received L-NMMA (L-NMMA; n = 7). RESULTS: Insulin rats had large SNA increases (190 +/- 22% from 100% baseline), contrasting with small increases in the Control (136 +/- 10%) and L-NMMA (135 +/- 20%) groups. Unexpectedly, NO blockade abolished insulin-induced SNA increases in the L-NMMA-insulin group (96 +/- 12%). In agreement with the SNA findings, Insulin rats had heart rate increases while no heart rate changes were observed in the L-NMMA-insulin, Control, or L-NMMA groups. In addition, there was an unexpected was a lack of MAP increase in L-NMMA-insulin rats. MAP also did not change in the Control, L-NMMA or Insulin groups. CONCLUSIONS: These findings suggest that NO is necessary for insulin to exert its sympathoexcitatory effects, and that insulin-induced NO release may play a role in activating increases in lumbar SNA.

Converting enzyme inhibition with captopril abolishes sympathoexcitation to euglycemic hyperinsulinemia in rats.

Converting enzyme inhibition and angiotensin II receptor antagonism attenuate elevations in heart rate and plasma norepinephrine in response to insulin, suggesting that integrity of the renin-angiotensin system is necessary for insulin-induced sympathoexcitation. To test this, we infused vehicle (saline) in control experiments, or insulin (40 mU/min) during euglycemic clamp in captopril-pretreated (intravenously 2.5 mg/kg, then 1 mg/kg/h) and in nonpretreated urethane-anesthetized Wistar rats while measuring mean arterial pressure, heart rate, and lumbar sympathetic nerve activity. Although euglycemic hyperinsulinemia produced similar blood pressure (BP) increases in insulin-infused rats (change in mean arterial pressure: +9 +/- 3 mm Hg) compared with vehicle-controls (+6 +/- 2 mm Hg), insulin decreased blood pressure (BP) in captopril-pretreated insulin-infused rats (-8 +/- 3 mm Hg). Control rats developed mild heart rate increases (change in heart rate: +28 +/- 15 beats/min), contrasting with a marked tachycardia in insulin-infused rats (+82 +/- 13 beats/min) and a bradycardia in captopril-treated insulin-infused rats (-16 +/- 18 beats/min). As with heart rate, insulin-infused rats experienced large increases in lumbar sympathetic nerve activity (+ 127 +/- 29%), whereas small and equivalent elevations were observed in vehicle-treated rats (+39 +/- 24%) and in captopril-pretreated insulininfused rats (+61 +/- 13%). These observations demonstrate an attenuation of insulin-induced sympathetic activation by renin-angiotensin blockade with captopril in Wistar rats, and suggest that the renin-angiotensin system is critical for insulin to exert its sympathoexcitatory effects.

Metformin attenuates salt-induced hypertension in spontaneously hypertensive rats.

Metformin, an antihyperglycemic agent used for treatment of type 2 diabetes mellitus, lowers blood pressure in humans and experimental animals. We recently demonstrated that short-term administration of metformin may lower blood pressure by reducing sympathetic neural outflow. The present studies were initiated to determine whether long-term administration of metformin blunts salt-induced hypertension, a condition characterized by elevated sympathetic activity. Male spontaneously hypertensive rats, in which radiotelemeters had been implanted for continuous monitoring of heart rate and blood pressure, were randomly assigned to groups that received vehicle (drinking water) or metformin (500 mg/kg per day) and ate a normal 0.3% NaCl diet and to groups that received vehicle or metformin and ate a high 8.0% NaCl diet for a period of 4 weeks. Although metformin did not affect blood pressure in the animals that ate the normal-salt diet (vehicle, 130+/-3 mm Hg; metformin, 133+/-5 mm Hg; mean+/-SEM), drug treatment blunted the rise in pressure caused by a high-salt diet (vehicle, 153+/-4 mm Hg; metformin, 140+/-5 mm Hg; P<0.001). In agreement, during direct pressure recordings in anesthetized rats, the animals that ate the high-salt diet had higher pressures (136+/-13 mm Hg) than those in the control (98+/-5 mm Hg, P<0.01), metformin (100+/-7 mm Hg, P<0.01), and metformin/high-salt groups (92+/-3 mm Hg, P<0.01). Finally, metformin lowered heart rate in rats that ate the normal- and high-salt diets (310+/-3 and 305+/-4 bpm) compared with rats that ate normal- and high-salt diets given vehicle (332+/-3 and 324+/-2 bpm, P<0.01). These data indicate that the chronic depressor actions of metformin are enhanced in animals with hypertension exacerbated by a high-salt diet.

High dietary salt enhances acute depressor responses to metformin.

The antidiabetic drug metformin lowers blood pressure (BP) more in spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto rats (WKY), and the hypotensive effect is enhanced by high dietary salt. To determine whether enhanced hypotension is secondary to greater decreases in sympathetic nerve activity (SNA), we placed WKY and SHR on normal salt (0.3%), and SHR on high salt (8.0%) for 2 weeks and then measured anesthetized BP and lumbar SNA to metformin (0, 10, 50, and 100 mg/kg, given intravenously). Baseline BP were similar in SHR groups but lower in WKY. Although metformin decreased BP more in high salt SHR (50 mg/kg: deltaBP: -23+/-1 mm Hg) than in normal salt SHR (-14+/-1 mm Hg, P< .01) and less in WKY (-10+/-1 mm Hg, P<.05), equivalent decreases in SNA were observed. We conclude that both strain and high salt potentiate acute depressor responses to metformin through mechanisms that are independent of SNA.

Effects of adrenergic, cholinergic and ganglionic blockade on acute depressor responses to metformin in spontaneously hypertensive rats.

Metformin lowers blood pressure in humans and in experimental animal models. To determine the mechanism of acute metformin-induced hypotension, we measured changes in mean arterial pressure (MAP) and heart rate (HR) during metformin alone (0, 10, 50, 100 mg/kg i.v.; n = 10) and during concomitant alpha adrenergic (phentolamine, 5 mg/kg; n = 5), beta adrenergic (propranolol, 3 mg/kg; n = 6), muscarinic (atropine, 200 micrograms/kg; n = 7), ganglionic (hexamethonium, 30 mg/kg; n = 11), nitric oxide synthase (NG-methyl-L-arginine acetate salt, 15 mg/ kg; n = 9) and combination ganglionic plus alpha adrenergic plus beta adrenergic (n = 6) blockade in spontaneously hypertensive rats (SHR). Responses to metformin alone were also assessed in normotensive Wistar-Kyoto rats (n = 6). In SHRs, metformin elicited depressor responses accompanied by tachycardia (100 mg/kg; delta MAP, -26 +/- 3 mm Hg; delta HR, +49 +/- 12 bpm). Depressor responses in Wistar-Kyoto rats were significantly attenuated (100 mg/kg; delta MAP, -9 +/- 4 mm Hg; P < .01). Hypotensive actions of metformin in SHRs were abolished and reversed into pressor responses by hexamethonium (100 mg/kg; delta MAP, +24 +/- 6 mm Hg), phentolamine (100 mg/kg; delta MAP, +62 +/- 10 mm Hg) and by combination ganglionic plus adrenergic (100 mg/kg; delta MAP, +62 +/- 10 mm Hg) blockade. Neither propranolol, atropine nor NG-methyl-L-arginine acetate salt affected hypotensive responses to metformin. We conclude that acute intravenous metformin administration decreases MAP by causing withdrawal of sympathetic activity. The increase in MAP uncovered by hexamethonium and phentolamine suggests that the original depressor response to metformin is buffered by mechanisms unrelated to the autonomic nervous system.

Effects of subfornical organ lesions on sympathetic nerve responses to insulin.

Although insulin exerts potent excitatory effects on the sympathetic nervous system, the mechanisms of insulin-induced activation remain unclear. To demonstrate a central nervous system site of sympathoexcitation, we recently found that destruction of tissues surrounding the anteroventral third ventricle region abolishes elevations in sympathetic nerve activity to intravenous insulin administration. Anteroventral third ventricle lesions may eliminate sympathoexcitation by destroying cell bodies in the lesioned area or by interrupting fibers of passage from the subfornical organ. To determine whether the lesions abolish sympathetic increases by disrupting efferent fibers from the subfornical organ, we measured lumbar sympathetic activity in anesthetized anteroventral third ventricle-lesioned (n = 4) and subfornical organ-lesioned (n = 12) rats before and during intravenous insulin at 0.13 U/h while maintaining euglycemia. Additional sham-lesioned rats received infusion of insulin (n = 10) and the vehicle for insulin (n = 10). Insulin administration in sham-lesioned rats elevated lumbar activity from 100% to 171 +/- 14% (+/-SE), whereas vehicle infusion did not alter sympathetic activity (100% to 113 +/- 11%). In anteroventral third ventricle-lesioned rats, insulin failed to increase sympathetic nerve activity (100% to 119 +/- 14%). Importantly, rats with subfornical organ lesions had increases in nerve activity that were indistinguishable from increases observed in insulin-infused sham-lesioned rats (100% to 163 +/- 21%). These findings indicate that whereas the anteroventral third ventricle region itself is crucial for sympathoexcitation to insulin, the subfornical organ and fibers originating from the subfornical organ traversing the anteroventral third ventricle area are not essential in mediating elevations in lumbar sympathetic nerve activity to hyperinsulinemia.

Anteroventral third ventricle lesions attenuate pressor responses to serotonin in anesthetized rats.

When administered intravenously, serotonin (5-hydroxytryptamine; 5-HT) evokes a triphasic blood pressure response, consisting of the Bezold-Jarisch-associated depressor response, a pressor action, and long-lasting depressor response. Because the pressor response may, in part, be caused by central nervous system (CNS) activation by 5-HT, we predicted that destruction of the anteroventral third ventricle (AV3V) region, an area rich in 5-HT receptors, would attenuate increases in blood pressure to intravenous 5-HT. In anesthetized sham-lesioned and AV3V-lesioned Sprague-Dawley rats, we measured mean arterial pressure (MAP), heart rate (HR), and lumbar sympathetic nerve activity (SNA) to increasing bolus doses of intravenous 5-HT (1, 2.5, 5, 10, 25 mu g/kg), before and after blockade of bradycardia using methylatropine (200 mu g/kg). In all rats, bolus injections of 5-HT elicited bradycardia accompanied by a fall in lumbar SNA and an initial hypotension followed by a pressor response and a longer lasting hypotensive response. The bradycardia, reduction in lumbar SNA, and both depressor responses were equivalent in sham-lesioned and AV3V-lesioned groups. Importantly, AV3V lesions attenuated pressor responses to increasing doses of 5-HT (3 +/- 1, 6 +/- 4, 6 +/- 4, 17 -/+ 4 35 +/- 3 mmHg) compared to sham-lesioned controls (6 +/- 3, 16 +/- 7, 33 +/- 5, 54 +/- 4, 51 +/- 6 mmHg; P < 0.0001). This attenuation was conserved following blockade of bradycardia with methylatropine (P < 0.01). In summary, pressor responses to intravenous 5-HT are diminished by AV3V lesions. These data indicate that the pressor component of the blood pressure response to intravenous 5-HT is partly dependent upon interaction with the CNS.

Mechanisms of insulin action on sympathetic nerve activity.

Insulin resistance and hyperinsulinemia may contribute to the development of arterial hypertension. Although insulin may elevate arterial pressure, in part, through activation of the sympathetic nervous system, the sites and mechanisms of insulin-induced sympathetic excitation remain uncertain. While sympathoexcitation during insulin may be mediated by the baroreflex, or by modulation of norepinephrine release from sympathetic nerve endings, it has been shown repeatedly that insulin increases sympathetic outflow by actions on the central nervous system. Previous studies employing norepinephrine turnover have suggested that insulin causes sympathoexcitation by acting in the hypothalamus. Recent experiments from our laboratory involving direct measurements of regional sympathetic nerve activity have provided further evidence that insulin acts in the central nervous system. For example, administration of insulin into the third cerebralventricle increased lumbar but not renal or adrenal sympathetic nerve activity in normotensive rats. Interestingly, this pattern of regional sympathetic nerve responses to central neural administration of insulin is similar to that seen with systemic administration of insulin. Further, lesions of the anteroventral third ventricle hypothalamic (AV3V) region abolished increases in sympathetic activity to systemic administration of insulin with euglycemic clamp, suggesting that AV3V-related structures are critical for insulin-induced elevations in sympathetic outflow.

Intracerebroventricular insulin produces nonuniform regional increases in sympathetic nerve activity.

Insulin has been shown to increase sympathetic nerve activity (SNA). Although it has been proposed that insulin acts within the central nervous system (CNS) to increase sympathetic neural outflow, there is little evidence for direct central neural sympathoexcitatory effects of insulin. To determine whether intracerebroventricular insulin elicits increases in peripheral SNA, we infused insulin (0.1 microU/min, low; 10 microU/min, medium; and 100 microU/min, high doses) or artificial cerebrospinal fluid (aCSF) into the third cerebral ventricle of chloralose-anesthetized Wistar rats while recording lumbar SNA. In separate animals, 10 microU/min of insulin were infused while recording adrenal SNA and renal SNA. Blood glucose and plasma insulin levels did not significantly change during intracerebroventricular infusion of insulin. Lumbar SNA, expressed as percentage of baseline, did not change in rats infused with aCSF (+ 13% +/- 8%) but increased significantly in rats infused with low (+ 72 +/- 17%), medium (+ 119 +/- 30%), and high (+ 113 +/- 25%) doses of insulin (P < 0.05). Intracerebroventricular insulin failed to significantly increase adrenal SNA or renal SNA. Blood pressure and heart rate did not change during insulin infusion. The results indicate that administration of insulin into the third cerebral ventricle produces regionally nonuniform increases in sympathetic neural outflow in the absence of changes in blood glucose or plasma insulin.

Anteroventral third ventricle lesions abolish lumbar sympathetic responses to insulin.

Insulin has been shown to increase sympathetic nerve activity. Because evidence shows that insulin acts within the central nervous system, we hypothesized that lesions of the anteroventral third ventricle region, an area rich in insulin receptors, would abolish sympathetic responses to hyperinsulinemia. We measured mean arterial pressure and lumbar sympathetic nerve activity in fasted, anesthetized sham-lesioned (n = 8) and lesioned (n = 8) rats before and after intravenous insulin infusion at 0.13 U/h during euglycemic clamp. Additional sham-lesioned (n = 10) and lesioned (n = 5) rats received vehicle infusion. Insulin-infused sham-lesioned rats had substantially greater increases in lumbar sympathetic nerve activity (+83 +/- 18%) than vehicle-infused sham-lesioned rats (+27 +/- 4%). Most importantly, insulin-infused lesioned rats had increases in sympathetic activity (+32 +/- 11%) that were no greater than lesioned rats receiving vehicle (+23 +/- 16%). Blood pressure was not altered by insulin or vehicle. To test the possibility that lesions of the anteroventral third ventricle region nonspecifically suppress sympathetic excitatory responses, we evaluated reflex increases in lumbar sympathetic activity to nitroglycerin in sham-lesioned (n = 5) and lesioned (n = 8) rats. Rats with lesions and sham lesions showed comparable increases in lumbar nerve activity during nitroglycerin-induced hypotension. In summary, increases in sympathetic nerve activity to intravenous insulin infusion are abolished by anteroventral third ventricle lesions. These data indicate that the integrity of this brain region is necessary for activation of lumbar sympathetic nerve activity by systemic administration of insulin.

[The value of a new digital laser card for image/report documentation and transmission]. / Wertigkeit einer neuen digitalen Laserkarte zur Bild-/Befunddokumentation und Ubermittlung.

The capabilities of a patient-oriented digital optical laser-card for the documentation of the image/report unit and for image transmission were assessed. 150 conventional X-rays covering the fields of urology (n = 50), traumatology (n = 50) and orthopaedics (n = 50) were digitised using a CCD scanner and subsequently transmitted to an Image-Transfer Medium (ITM) and to an optical laser-card. The image quality for the detection of relevant diagnostic parameters was evaluated by 4 radiologists and one clinician of the corresponding specialty. Based upon a total of 4740 decision readings for each method, it was found that the optical laser-card reduced the image quality significantly (p < 0.01) in comparison to the digitised ITM images in all fields. Thus, a primary diagnostic statement cannot be made based upon the images of the optical card. However, concomitant documentation of the image and opinion on the card may be used for the transmission of the radiological report, especially to external referring institutions.

Physiological correlates of attenuated salt appetite in Fischer 344 rats.

Dietary NaCl deprivation stimulates a robust salt appetite in Wistar rats but has little influence on this behavior in rats of the Fischer 344 (F344) strain. To examine physiological substrates of attenuated salt appetite in F344 rats, several pertinent measures of renal function and fluid homeostasis were made in Wistar and F344 rats eating normal and NaCl-deplete diets. Physiological adjustments to NaCl deprivation were similar between the two strains; however, F344 rats showed smaller increases in plasma renin activity (PRA) than their Wistar counterparts. In addition, F344s decreased urinary sodium excretion more rapidly than Wistar rats in response to deprivation. The present studies also revealed several strain differences in baseline fluid and electrolyte regulation. Relative to the Wistar strain, F344 rats were characterized by high baseline PRA, increased arginine vasopressin (AVP) excretion, decreased urine volume, and diminished thirst. We propose that AVP and oxytocin activation may reduce salt preference and suppress the development of salt appetite in F344 rats.

Erythropoietin increases blood pressure in normotensive and hypertensive rats.

Treatment with recombinant human erythropoietin (rHuEPO) successfully reverses anemia in uremic patients. Of major concern, however, are blood pressure increases during rHuEPO therapy, observed particularly in patients with a history of hypertension. The present study was designed to determine whether high-dose rHuEPO elevates blood pressure in nonuremic rats, and if so, whether preexisting hypertension enhances this response. We examined blood pressure responses to high (100 IU/kg) and very high (200 IU/kg) doses of rHuEPO or placebo, given subcutaneously every other day for 3 weeks to male spontaneously hypertensive rats (SHR) and their normotensive genetic controls (Wistar-Kyoto rats, WKY). The high and very high doses of rHuEPO stimulated equivalent increases in hematocrit, and this increase was always larger in SHR than in WKY. In contrast to the pattern of hematocrit changes, blood pressure did not change following high-dose rHuEPO but was elevated in both strains after the very high dose of the drug. Although the rise in blood pressure tended to be greater in SHR than in WKY, this difference was not significant. The data indicate that very high-dose rHuEPO raises blood pressure comparably in normotensive and hypertensive rats and this increase is relatively independent of the increase in hematocrit.

Long-term effects of a somatostatin analogue on renal haemodynamics and hypertrophy in diabetic rats.

1. To determine whether treatment with octreotide, a somatostatin analogue, may diminish or prevent long-term diabetic renal hypertrophy and nephropathy, uninephrectomized streptozotocin-diabetic rats maintained under moderate glycaemic control (approximately 300 mg/dl) were treated with either placebo (n = 10 rat/group) or octreotide for 14 weeks. Uninephrectomized non-diabetic rats given either placebo or octreotide served as controls. 2. Average body weight was diminished and kidney weight, daily urinary protein excretion, glomerular filtration rate and renal plasma flow were elevated in both diabetic groups relative to controls. 3. Administration of octreotide reduced average body weight and packed cell volume in non-diabetic and diabetic rats compared with their respective controls, but did not affect glomerular hyperfiltration or the increase in urinary protein excretion. 4. Histological examination at 14 weeks disclosed unequivocal glomerular hypertrophy and mild glomerular and tubulointerstitial lesions consistent with early diabetic renal alterations in all diabetic rats, but there was no independent effect of octreotide treatment. 5. Thus, long-term treatment with octreotide did not afford protection against the development of renal hypertrophy-hyperfiltration and the evolution of early diabetic nephropathy in rats.

Effect of erythropoietin on hematocrit and blood pressure in normotensive and hypertensive rats.

Treatment with recombinant human erythropoietin (rHuEPO) successfully reverses anemia in uremic patients. Of major concern, however, are blood pressure (BP) increases during rHuEPO therapy, observed particularly in persons with a history of hypertension. To determine whether preexisting hypertension enhances BP increases to rHuEPO, BP responses to 2 wk of rHuEPO or placebo were observed in spontaneously hypertensive rats (SHR) and their normotensive genetic controls (Wistar-Kyoto [WKY] rats. In addition, the role of endothelial-released nitric oxide (NO) in BP alterations caused by rHuEPO through i.v. infusions of endothelium-dependent and independent vasoactive agents were indirectly examined. At trial completion, rHuEPO elevated hematocrit, hemoglobin, and mean cell volume more in SHR than in WKY rats (P less than 0.001). Despite the considerable increase in hematocrit, rHuEPO did not alter BP in either strain. An infusion of NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of NO formation, elevated BP more in rHuEPO-treated SHR than in identically treated WKY rats (P less than 0.05). Further, the administration of L-arginine caused a greater decrease in blood pressure in SHR than in WKY rats, independent of treatment condition (P less than 0.01). Because changes in BP with endothelium-independent agents were similar across groups, responses to L-NMMA and L-arginine were specific to the endothelium and probably independent of basal BP. Thus, rHuEPO provoked greater erythropoiesis in SHR than in WKY rats but did not elevate BP. L-NMMA stimulated higher BP in SHR treated with rHuEPO, suggesting a compensatory increase in vasodilatory NO synthesis to protect against a hypertensive effect of the drug in SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

A comprehensive review of the salt and blood pressure relationship.

Salt has played an important role in the human diet since earliest times. However, increases in the availability and consumption of dietary salt have raised concerns that excessive intakes may cause hypertension. Although recent research has linked salt intake to variations in blood pressure, definitive conclusions have been lacking. Uncertainties in this area are due to the complex effects of salt on the cardiovascular system and on blood pressure regulation. Nevertheless, many of these complexities are now well understood and have been summarized in this review. Among the topics we examine are the effects of salt on fluid and electrolyte homoeostasis; potential mechanisms of salt-induced hypertension; the epidemiology of salt intake and blood pressure; the effects of salt restriction and supplementation on blood pressure regulation; the potential roles of sodium and chloride ions, as well as interactions with dietary potassium, calcium, and magnesium; current theories of salt sensitivity; the clinical risks of dietary salt depletion; and the dietary sources of salt.

Selective effects of sodium and chloride depletion on salt appetite in rats.

Findings from hypertension research indicate that dietary sodium chloride (NaCl), Na+, and Cl- independently influence blood pressure, electrolyte metabolism, and hormone secretion. In this context, we examined the effects of NaCl, Na+, and Cl- depletion, respectively, on the development of saline preference (salt appetite) in rats. Male Wistar rats were given a normal diet (1% NaCl) for 15 days and tested for salt appetite using a two-bottle choice test, one bottle containing water and the other 0.3 M saline. The animals were then divided into three groups (n = 11/group): one group received low NaCl, another received a Na(+)-deficient, normal-Cl-diet (low Na+), and a third received a Cl(-)-deficient, normal-Na+ diet (low Cl-). Salt appetite was again tested after 19 days on these diets. Both NaCl and Na+ depletion stimulated saline intake (P less than 0.01), whereas salt appetite did not change in the low-Cl- group. Water intake was not influenced by the regimens. In addition, no alterations were noted for weight, systolic blood pressure, plasma Na+ concentration, or blood pH. Dietary Cl-depletion, however, significantly reduced plasma Cl- concentration (P less than 0.05), and reduced plasma potassium in relation to rats depleted in Na+ (P less than 0.05). Plasma renin activity and urinary aldosterone were elevated in low-NaCl and Na(+)-depleted rats relative to the Cl(-)-depleted group (P less than 0.05). These results suggest that salt appetite is increased by dietary Na+ deficiency but not by Cl- deficiency. Salt appetite may be controlled by central or peripheral systems specifically sensitive to Na+ or by hormonal changes characteristic of Na+ depletion, such as the activation of renin and aldosterone observed in the low-NaCl and low-Na+ groups.

Dietary calcium and iron: effects on blood pressure and hematocrit in young spontaneously hypertensive rats.

Supplemental dietary calcium in spontaneously hypertensive rats (SHRs) aged 21-28 d produces a decrease in blood pressure and hematocrit. The simultaneous fall in hematocrit and blood pressure suggests that the changes in blood pressure may be, in part, a consequence of the decrease in hematocrit and reduction in viscosity. To examine this possibility, SHRs aged 21 d were placed on one of four diets varying in iron content. At age 28, the animals showed iron-induced variations in hematocrit (P less than 0.001) but no difference in blood pressure. Subsequent manipulation of the ratio of calcium and iron in the diets of additional groups of animals resulted in variations in hematocrit that were independent of the calcium-induced alterations in blood pressure. We conclude that the effects of calcium on blood pressure are relatively independent of its effects on hematocrit.