Dietary Salt Loading Research Articles

Dietary salt loading exacerbates the increase in sympathetic nerve activity caused by intravenous insulin infusion in rats

Obesity and type 2 diabetes mellitus frequently produce chronic elevations in blood insulin levels. Importantly, hyperinsulinemia stimulates increases in sympathetic nerve activity that may predispose to hypertension, atherosclerosis, and end-organ damage. Because depletion of dietary salt (NaCl) increases angiotensin II levels, which has been shown to enhance sympathetic responses to excitatory stimuli such as thermal stimulation and bicuculline in the hypothalamus, we predicted that insulin-induced elevations in lumbar sympathetic activity would be augmented by low NaCl and suppressed by high dietary NaCl. Adult male Sprague-Dawley rats were randomized into groups receiving low (0.0 mEq/d, n = 10), normal (2.0 mEq/d, n = 10), and high (5.7 mEq/d, n = 10) NaCl for a period of 8 days. After this, the animals were anesthetized for measurement of heart rate, mean arterial pressure, and lumbar sympathetic nerve activity during 110 minutes of intravenous insulin infusion (15 mU/kg per minute) with euglycemic clamp. Insulin administration caused modest blood pressure decreases accompanied by heart rate increases that were similar across the 3 dietary groups. Unexpectedly, sympathetic increases to insulin were lowest in the low-NaCl group (100%-135% ± 24%), moderate in the normal-NaCl group (100%-170% ± 23%), and greatest in the high-NaCl group (100%-252% ± 39%). Dietary NaCl level did not affect baseline blood glucose or insulin sensitivity as assessed by euglycemic clamp. These findings indicate that dietary salt loading exacerbates the lumbar sympathoexcitatory response to intravenous insulin infusion in rats.

Effects of Dietary Salt Load and Salt Depletion on the Course of Hypertension and Angiotensin II Levels in Male and Female Heterozygous Ren-2 Transgenic Rats

Background: In the present study we evaluated plasma and kidney angiotensin II (ANG II) levels in female and male Ren-2 transgenic rats (TGR) in comparison to age-matched female and male normotensive Hannover Sprague-Dawley rats. Methods: The rats were maintained on a normal sodium (NS) diet (0.6% NaCl) or fed a high sodium (HS) diet (2% NaCl) for 4 days or were sodium depleted by administration of 40 mg furosemide per liter drinking water overnight followed by 3 days of low sodium diet (0.01% NaCl) (LS + F). ANG II levels were determined by radioimmunoassay. Results: Female TGR at the age of 38 days were already hypertensive and had developed cardiac hypertrophy, whereas male TGR at this age still exhibited a normotensive phenotype. HS diet increased the blood pressure (BP) but did not alter the ANG II levels in TGR at any age. LS + F decreased the BP without significant change in ANG II concentrations in TGR. Female TGR responded to salt loading and salt depletion by more pronounced changes in BP than male TGR. Conclusions: Female TGR develop hypertension more rapidly and the salt-sensitive component of hypertension is more pronounced in female than in male TGR.

Salt loading produces severe renal hemodynamic dysfunction independent of arterial pressure in spontaneously hypertensive rats

We have previously shown that salt excess has adverse cardiac effects in spontaneously hypertensive rats (SHR), independent of its increased arterial pressure; however, the renal effects have not been reported. In the present study we evaluated the role of three levels of salt loading in SHR on renal function, systemic and renal hemodynamics, and glomerular dynamics. At 8 wk of age, rats were given a 4% (n = 11), 6% (n = 9), or 8% (n = 11) salt-load diet for the ensuing 8 wk; control rats (n = 11) received standard chow (0.6% NaCl). Rats had weekly 24-h proteinuria and albuminuria quantified. At the end of salt loading, all rats had systemic and renal hemodynamics measured; glomerular dynamics were specially studied by renal micropuncture in the control, 4% and 6% salt-loaded rats. Proteinuria and albuminuria progressively increased by the second week of salt loading in the 6% and 8% salt-loaded rats. Mean arterial pressure increased minimally, and glomerular filtration rate decreased in all salt-loaded rats. The 6% and 8% salt-loaded rats demonstrated decreased renal plasma flow and increased renal vascular resistance and serum creatinine concentration. Furthermore, 4% and 6% salt-loaded rats had diminished single-nephron plasma flow and increased afferent and efferent arteriolar resistances; glomerular hydrostatic pressure also increased in the 6% salt-loaded rats. In conclusion, dietary salt loading as low as 4% dramatically deteriorated renal function, renal hemodynamics, and glomerular dynamics in SHR independent of a minimal further increase in arterial pressure. These findings support the concept of a strong independent causal relationship between salt excess and cardiovascular and renal injury.

Expression of endothelial nitric oxide synthase is suppressed in the renal vasculature of angiotensinogen-gene knockout mice

We have attempted to elucidate the mechanism by which endothelial-type nitric oxide synthase (eNOS) is regulated in the kidney, with special reference to the role of renal hemodynamics and angiotensin II (Ang II). We compared angiotensinogen gene knockout (Atg-/-) mice, which lacked Ang II (resulting in sodium/water depletion and severe hypotension), with wild-type (Atg+/+) mice. Using Western blot analysis and the NADPH diaphorase histochemical reaction, we found that the expression and activity of eNOS were markedly lower in the renal vessels of Atg-/- mice compared with wild-type (Atg+/+) mice. Dietary salt loading significantly enhanced renal eNOS levels and increased blood pressure in Atg-/- mice, but severe hypotension almost abolished the effects of salt loading. In contrast, in Atg+/+ mice, altered salt intake or hydralazine had no effect on renal eNOS levels. These results suggest that perfusion pressure plays an essential role in maintaining renal vascular eNOS activity, whereas Ang II plays a supportive role, especially when renal circulation is impaired.

Effects of dietary salt loading on the responses of isolated rat mesenteric arteries to leptin

Leptin induces relaxation of vascular smooth muscle through an endothelium-dependent release of nitric oxide (EDNO) and administration of a high-salt diet reduces the relaxation of vessels to EDNO. We would, therefore, predict that salt loading would reduce the leptin-induced dilatation. However, in salt-loaded animals the relaxation to acetylcholine is maintained through an endothelial-dependent hypopolarizing factor instead of EDNO. These experiments were, therefore, designed to examine whether in salt-loaded animals the response to leptin would be reduced or whether, as for acetylcholine, an alternative mechanism would be substituted. Weanling rats were given diets containing either 0.4% (n = 10) or 8% (n = 9) sodium chloride for 4 weeks. They were then killed and a length of second order mesenteric artery was mounted in a perfusion myograph with diameter changes measured using a microscope-video tracking system. The vessels were preconstricted with norepinephrine and then the effects of graded concentrations of leptin were determined. In vessels from the low salt animals leptin caused a dose-dependent dilatation (maximum change 31.4% +/- 5.8% of the initial norepinephrine-induced constriction) but in the high salt animals the change was only 3.4% +/- 1.1%. The nitric oxide synthase blocker Nomega-nitro-L-arginine methyl ester (L-NAME) abolished the responses, although responses could still be obtained in vessels from both groups to the NO donor, sodium nitroprusside. These results indicate that salt loading to rats almost completely abolishes the vasodilatation to leptin. This implies endothelial disruption and, unlike the response to acetylcholine, no other vasodilator mechanism is implicated. This could provide a link between high salt intake and hypertension because the known increase in sympathetic activity caused by leptin would not be countered by a direct vasorelaxation.

Hypertension in mice lacking the proatrial natriuretic peptide convertase corin

Atrial natriuretic peptide (ANP) is a cardiac hormone that regulates blood pressure. In cardiomyocytes, the hormone is synthesized as a precursor, proatrial natriuretic peptide (pro-ANP), which is proteolytically converted to active ANP. Corin is a cardiac transmembrane serine protease that has been shown to process pro-ANP in vitro, but its physiological importance had not been established. Here, we show that corin-deficient (Cor-/-) mice develop normally during embryogenesis and survive to postnatal life. Cor-/- mice have elevated levels of pro-ANP but no detectable levels of ANP as compared with WT littermates. Infusion of an active recombinant soluble corin transiently restores pro-ANP conversion, resulting in the release of circulating biologically active ANP. Using radiotelemetry to assess blood pressure, we find that Cor-/- mice have spontaneous hypertension as compared with WT mice, and it is enhanced after dietary salt loading. Pregnant Cor-/- mice demonstrate late-gestation proteinuria and enhanced high blood pressure during pregnancy. In addition, Cor-/- mice exhibit cardiac hypertrophy resulting in a mild decline in cardiac function later in life. Thus, our data establish corin as the physiological pro-ANP convertase and indicate that corin deficiency may contribute to hypertensive heart disease.

Salt, Blood Pressure and Cardiovascular Changes in Human and Experimental Studies – A Review

The present review describes the effects of ingestion of high salt diet on blood pressure and attempts to elucidate some of the cardiovascular changes that give rise to elevated blood pressure. High dietary salt loading especially in experimental animals tend to result in elevated levels of arterial blood pressure. Studies in humans have also linked the high intake of salt to increase in systolic and diastolic blood pressures. Some of the pathophysiological changes include cardiac hypertrophy and enhanced cardiac contractility, enhanced contraction of blood vessels and veins in response to constrictor agonists and diminished relaxation of arterial resistance vessels to vasodilators. The combination of these factors are bound to contribute to the pressor effects of high dietary salt. In humans, there appears to be racial differences in the response to salt with blacks showing more pressor response than caucasians. The peculiar response in blacks has been attributed in part to enhanced vascular response to vasoconstrictors as well as genetic defects of the epithelial sodium channel (ENaC). More work at molecular level is required to further unwrap the genetic basis of salt hypertension especially in black populations. Key Words: Salt, Blood pressure, Blood Vessels Nigerian Medical Practitioner 46(2) 2004: 22-26

The role of sodium in hypertension is more complex than simply elevating arterial pressure

Excessive salt intake exacerbates hypertension and further increases left-ventricular mass in clinical essential and experimental hypertension. Additionally, a growing body of evidence strongly suggests that high dietary salt loading exerts detrimental cardiac effects independently of its hemodynamic load. The clinical evidence of cardiac structural and functional alterations associated with salt is, however, scarce. In order to explore the purported beliefs in humans, in this review we draw on our experimental studies in naturally occurring hypertension and discuss the clinical implications of the nonhemodynamic mechanisms underlying these salt-related changes.

Carotid chemoreceptor reflexes following dietary salt loading in rats

There is a strong association between salt intake and hypertension. Alterations in baroreceptor activity, which precede and contribute to the elevation in blood pressure, have also been shown to affect chemoreceptor reflex response. Dietary salt loading with 8% sodium chloride was carried out in Sprague Dawley rats aged 8 weeks for a period of 5-6 weeks. Blood pressure was thereafter recorded under anaesthesia from the common carotid artery with a Grass Polygraph 7D model, whereas serum Na[Formula: See Text] and K[Formula: See Text] concentrations were measured using a flame photometer. Salt loading resulted in elevated arterial blood pressure as well as hypokalaemia. Stimulation of the carotid chemoreceptor by injection of sodium dithionite resulted in elevated arterial blood pressure, decreased heart rate and hyperventilation in both control and salt-loaded rats. However, the bradycardic response as estimated by the difference in percentage reduction in heart rate was significantly higher in salt rats (36%) than in the control rats (10%). The results indicate that a high-salt diet results in enhanced bradycardic response to carotid chemoreceptor stimulation and that this observation may be related to the attendant hypokalaemia.

Role of the alpha1D-adrenergic receptor in the development of salt-induced hypertension.

In an attempt to elucidate whether there is a specific alpha1-adrenergic receptor (alpha1-AR) subtype involved in the genesis or maintenance of hypertension, the alpha1D-AR subtype was evaluated in a model of salt-induced hypertension. The alpha1D-AR-deficient (alpha1D-/-) and control (alpha1D+/+) mice (n=8 to 14 in each group) were submitted to subtotal nephrectomy and given 1% saline as drinking water for 35 days. Blood pressure (BP) was monitored by tail-cuff readings and confirmed at the end point by direct intraarterial BP recording. The alpha1D-/- mice had a significantly (P=0.0004) attenuated increase in BP response in this protocol (baseline 94.6+/-2.8 versus end point 107.4+/-4.5 mm Hg) compared with that of their wild-type counterparts (alpha1D+/+), from a baseline 97.4+/-2.9 to an end point 139.4+/-4.5 mm Hg. Seven of 15 alpha1D+/+ mice died with edema, probably owing to renal failure, whereas 14 of 15 alpha1D-/- mice were maintained for 35 days. Body weight, renal remnant weight, and residual renal function were similar in the 2 groups, whereas the values of plasma catecholamines (epinephrine, norepinephrine, and dopamine) were higher in alpha1D+/+ than in the alpha1D-/- mice. These data suggest that alpha1D-AR plays an important role in developing a high BP in response to dietary salt-loading, and that agents having selective alpha1D-AR antagonism could have significant therapeutic potential in the treatment of hypertension.

Effects of antisense oligodeoxynucleotide targeting of the alpha(2B)-adrenergic receptor messenger RNA in the central nervous system.

The results of previous studies with genetically engineered mice have suggested that an intact central alpha(2B)-adrenergic receptor (alpha(2B)-AR) subtype mediates the development and maintenance of salt-induced hypertension. In the present study, we sought to further define the role of this receptor by injecting antisense oligodeoxynucleotides (AS-ODNs), targeting a selected sequence of the alpha(2B)-AR mRNA, into the lateral cerebral ventricle of rats that had undergone prior subtotal nephrectomy and dietary salt loading. Cell culture studies showed that these AS-ODNs could block alpha(2B)-AR protein generation. Before AS-ODN injection, blood pressure (BP) averaged 133+/-5 mm Hg during the daytime and rose to 165+/-4 mm Hg during the nighttime activity hours (P<0.001 versus baseline average of 120+/-2 mm Hg). The injection of AS-ODNs during the early afternoon prevented the BP rise and was associated with a significant fall in heart rate (from 385+/-12 to 306+/-15 bpm, P<0.05) and symptoms of sedation that lasted for several hours, with a peak at 3 to 6 hours and full recovery by 24 hours. At that time, a second injection produced identical effects in all rats (n=9). Control rats (n=10) that received scrambled ODN injections had no changes in BP or heart rate patterns, and neither group had evidence of neurotoxicity, indicating that these effects are specifically due to translational inhibition of central alpha(2B)-AR. We conclude that a fully functional central alpha(2B)-AR is necessary for the induction of salt-dependent hypertension.

Elevating dietary salt exacerbates hyperpnea-induced airway obstruction in guinea pigs

Previous studies have indicated that increased dietary salt consumption worsens postexercise pulmonary function in humans with exercise-induced asthma (EIA). It has been suggested that EIA and hyperpnea-induced airway obstruction (HIAO) in guinea pigs (an animal model of EIA) are mediated by similar mechanisms. Therefore, the purpose of this study was to determine whether altering dietary salt consumption also exacerbated HIAO in guinea pigs. Furthermore, the potential pathway of action of dietary salt was investigated by blocking leukotriene (LT) production during HIAO in guinea pigs. Thirty-two male Hartley strain guinea pigs were split into two groups. One group (n = 16) of animals ingested a normal-salt diet (NSD) for 2 wk; the other group (n = 16) ingested a high-salt diet (HSD) for 2 wk. Thereafter, animals were anesthetized, cannulated, tracheotomized, and mechanically ventilated during a baseline period and during two dry gas hyperpnea challenges. After the first challenge, the animals were administered either saline or nordihydroguaiaretic acid, a LT inhibitor. Bladder urine was analyzed for electrolyte concentrations and urinary LTE(4). The HSD elicited higher airway inspiratory pressures (Ptr) than the NSD (P < 0.001) postchallenge. However, after infusion of the LT inhibitor and a second hyperpnea challenge, HIAO was blocked in both diet groups (P < 0.001). Nonetheless, the HSD group continued to demonstrate slightly higher Ptr than the NSD group (P < 0.05). Urinary LTE(4) excretion significantly increased in the HSD group compared with the NSD group within treatment groups. This study has demonstrated that dietary salt loading exacerbated the development of HIAO in guinea pigs and that LT release was involved in HIAO and may be moderated by changes in dietary salt loading.

Differential gene regulation of renal salt entry pathways by salt load in the distal nephron of the rat.

The aim of the present study was to determine the molecular responses of the main salt-reabsorbing systems in the distal nephron to changes of salt load of the organism. For this purpose we analysed messenger ribonucleic acid (mRNA) levels for the bumetanide-sensitive Na+K+2Cl- cotransporter (BSC1), the thiazide-sensitive Na+Cl- cotransporter (TSC), the kidney-specific inwards rectifier K+ channel (ROMK), the amiloride-sensitive epithelial Na+ channel (ENaC) and the kidney-specific Cl- channel ClC-K2, in the cortex and inner and outer medulla of kidneys from male Sprague-Dawley rats fed a high- (8% w/w), normal- (0.6%) or low-(0.02%) salt diet or treated chronically with subcutaneous infusions of furosemide (12 mg/kg per day). BSC1 and ROMK mRNA levels did not differ between the four treatment groups. TSC mRNA increased during furosemide treatment 1.75-fold versus control but was not affected by a high- or a low-salt diet. The mRNA for the alpha-subunit of ENaC increased with the low-salt diet (about 1.5-fold) and with furosemide (about 2.1-fold) in all kidney zones, but did not change with the high-salt diet. Dietary salt loading down-regulated CIC-K2 mRNA in the outer medulla 0.6-fold versus control whilst furosemide treatment, but not the low-salt diet, increased ClC-K2 mRNA in the outer (1.6-fold) and inner medulla (2.0-fold). These findings suggest that gene expression of Na+ and Cl- entry pathways in the distal nephron are at least partly regulated by the salt load of the organism, such that salt-reabsorbing systems are stimulated by salt deficiency and suppressed by salt overload.

DIETARY SALT CONSUMPTION AND LEUKOTRIENE-DEPENDENT HYPERPNEA-INDUCED AIRWAY OBSTRUCTION IN GUINEA PIGS

The purpose of this study was to determine if altering dietary salt consumption moderates hyperpnea-induced airway obstruction (HIAO) in guinea pigs (an animal model of exercise-induced asthma), and to determine whether blocking leukotriene (LT) biosynthesis alters the airway response to hyperpnea on the different salt diets. Thirty-two male Hartley strain guinea pigs were split into two groups. One group (n = 16) of animals ingested a normal salt diet (NSD) for 2 weeks, while the other group (n = 16) ingested a high salt diet (HSD) for two weeks. Thereafter, animals were anesthetized, cannulated, tracheotomized, and mechanically ventilated during a baseline period and during a dry gas hyperpnea challenge. After the hyperpnea challenge, baseline ventilator settings were resumed and animals were administered either saline [NSD- CON (n = 8) and HSD-CON (n = 8)], or NDGA, a LT inhibitor [NSD-BLO (n = 8) and HSD-BLO (n = 8)]. Subsequently, a second hyperpnea challenge was followed by a second post-hyperpnea recovery period. Airway inspiratory pressures (Ptr) were monitored continiously Bladder urine was collected by needle aspiration after euthanasia, and analyzed for urinary concentrations of sodium, chloride, potassium and creatinine. There were no significant differences between the NSD and HSD diet for Ptr prior to the first hyperpnea challenge (p > 0.05). However, the HSD elicited higher Ptr than the NSD (p < 0.001) post-challenge. Following infusion of saline (CON), and a second hyperpnea challenge, the HSD-CON group demonstrated higher Ptr than the NSD diet group (p < 0.001). However, following infusion of the LT-inhibitor, and a second hyperpnea challenge, HIAO was attenuated in both the NSD-BLO and HSD-BLO groups compared to the CON groups respectively (p < 0.001). Nonetheless, the HSD-BLO group continued to demonstrate higher Ptr than the NSD-BLO group (p < 0.05). Conclusion: This study has demonstrated that dietary salt loading exacerbates the development of HIAO in guinea pigs, and that LT release is involved in HIAO and may be moderated by changes in dietary salt loading. (Supported by the College Research Council, College of Veterinary Medicine and Biomedical Sciences, Colorado State University)

Targeted Disruption of Soluble Epoxide Hydrolase Reveals a Role in Blood Pressure Regulation

Renal microsomal cytochrome P-450 monooxygenase-dependent metabolism of arachidonic acid generates a series of regioisomeric epoxyeicosatrienoic acids that can be further metabolized by soluble epoxide hydrolase to the corresponding dihydroxyeicosatrienoic acids. Evidence exists that these metabolites affect renal function and, in particular, blood pressure regulation. To examine this possibility, blood pressure and renal arachidonic acid metabolism were examined in mice with a targeted disruption of the soluble epoxide hydrolase gene. Systolic blood pressure of male soluble epoxide hydrolase-null mice was lower compared with wild-type mice in both the absence and presence of dietary salt loading. Both female soluble epoxide hydrolase-null and wild-type female mice also had significantly lower systolic blood pressure than male wild-type mice. Renal formation of epoxyeicosatrienoic and dihydroxyeicosatrienoic acids was markedly lower for soluble epoxide hydrolase-null versus wild-type mice of both sexes. Although disruption of soluble epoxide hydrolase in female mice had minimal effects on blood pressure, deletion of this gene feminized male mice by lowering systolic blood pressure and altering arachidonic acid metabolism. These data provide the first direct evidence for a role for soluble epoxide hydrolase in blood pressure regulation and identify this enzyme as a novel and attractive target for therapeutic intervention in hypertension.

Role of the Bradykinin B 2 Receptor in Salt-Induced Hypertension

P189 The kallikrein-kinin system may contribute to blood pressure (BP) regulation via its vasodilatory, natriuretic and diuretic activity. In order to explore the role of bradykinin in this respect, we studied knockout mice (n=16) with deleted gene for the B 2 receptor of bradykinin (BKB 2 R -/-), submitted to subtotal nephrectomy (SN) and dietary salt-loading, in comparison to their wildtype counterparts (BKB 2 R +/+, n=10). Systolic BP mmHg (by tail cuff), heart rate (HR b/min), plasma catecholamines, (NEPI, EPI ng/ml) and heart weight/body weight ratio (mg/g) are shown in the Table below. *p&lt;0.05 knockouts vs wildtype. Thus, the BKB 2 R gene knockout mice had higher BP at baseline and developed more severe hypertension. Moreover, they became hypertensive in 19±2 days versus 28±2 days for the wildtype (p&lt;0.05). We conclude that bradykinin, acting via its B 2 receptor, contributes to maintenance of normotension and protects against salt-induced hypertension.

Pressor Responses to Central Sodium and Ouabain Are Attenuated in Transgenic Rats Deficient in Brain Angiotensinogen

P195 Studies using AT 1 -blockers suggest that the brain renin-angiotensin system (RAS) contributes to sympatho-excitation and hypertension by high dietary salt or central sodium loading. To more specifically examine this role of the brain RAS, [TGR (AsrAOGEN)] transgenic rats were used. These rats express an antisense RNA against angiotensinogen mRNA specifically in the brain, and the brain angiotensinogen level is reduced by more than 90%. In freely moving TGR and SD controls, BP and HR responses to intracerebroventricular (icv) infusion (3.8 μl/min for 10 min) of artificial CSF (aCSF) and Na + -rich aCSF (containing 0.2, 0.3 and 0.45 M Na + ) as well as icv injection of ouabain (0.3 and 0.6 μg/2 μl) were assessed. The vasopressin antagonist [d(CH 2 ) 5 Tyr(Me)]AVP (30 μg/kg) was given iv before each icv infusion. Data are means±SEM (n=6, for each).* p + -rich aCSF support the concept that the brain RAS plays an important role in the sympatho-excitatory effects of ouabain and CSF sodium.

Pressor Responses to Central Sodium and Ouabain Are Attenuated in Transgenic Rats Deficient in Brain Angiotensinogen

P195 Studies using AT 1 -blockers suggest that the brain renin-angiotensin system (RAS) contributes to sympatho-excitation and hypertension by high dietary salt or central sodium loading. To more specifically examine this role of the brain RAS, [TGR (AsrAOGEN)] transgenic rats were used. These rats express an antisense RNA against angiotensinogen mRNA specifically in the brain, and the brain angiotensinogen level is reduced by more than 90%. In freely moving TGR and SD controls, BP and HR responses to intracerebroventricular (icv) infusion (3.8 μl/min for 10 min) of artificial CSF (aCSF) and Na + -rich aCSF (containing 0.2, 0.3 and 0.45 M Na + ) as well as icv injection of ouabain (0.3 and 0.6 μg/2 μl) were assessed. The vasopressin antagonist [d(CH 2 ) 5 Tyr(Me)]AVP (30 μg/kg) was given iv before each icv infusion. Data are means±SEM (n=6, for each).* p&lt;0.05, vs SD rats. Somewhat surprisingly, angiotensin I and II levels of not only hypothalamus but also plasma were lower in TGR vs. SD rats. The clearly attenuated sympatho-excitatory and pressor effects to ouabain and Na + -rich aCSF support the concept that the brain RAS plays an important role in the sympatho-excitatory effects of ouabain and CSF sodium.

Hemolymph homeostasis in relation to diel feeding activity and microclimate in the prototypal land isopodLigia pallasii

Twenty-four-hour variability in hemolymph osmolality was assessed in a field population of the semiterrestrial isopod Ligia pallasii and related to feeding activity and microenvironmental conditions of relative humidity (RH) and temperature in the species' habitats. Hemolymphs were also analyzed for levels of ammonia and glucose as indicators of digestive and absorptive activities. Comparative data on hemolymph osmolality and microhabitat conditions were also obtained for several ligiid species in Hawai'i. Late-morning peaks in hemolymph osmolality in L. pallasii were coincidental with peaks in ammonia and glucose and with predicted times of digestion/absorption based on observed feeding activity in the population, suggesting that the elevations in osmolality were due to salt-loading from the species' seaweed diet. Uniformity of microenvironmental conditions of RH and temperature in Ligia's supralittoral habitat, whether temperate or tropical, and regardless of time of day or localized weather conditions, suggests that dietary salt-loading is likely the greatest challenge to osmotic homeostasis in ligiids. This feature and other aspects of hemolymph osmolality in ligiids are discussed in the context of colonization of land by prototypal terrestrial isopods.

Increases in intracellular calcium of arterial smooth muscle cells in transgenic mice overexpressing Na+/H+ exchanger.

We produced transgenic mice overexpressing Na+/ H+ exchanger as a model of salt-sensitive hypertension and reported that dietary salt loading elevates blood pressure in these transgenic mice. We speculate that this blood pressure elevation may be attributed to the elevation of intraarterial smooth muscle Ca2+ concentration through Na+/Ca2+ exchange. To test this hypothesis, we measured the isometric tension of aortic rings and intracellular free calcium ([Ca2+]i) of cultured smooth muscle cells. In the transgenic mice, the aortic ring contraction induced by 5 mM caffeine (percentage of 60 mM K-induced contraction) was significantly greater than control mice (60.1 +/- 5.5% vs. 44.8 +/- 3.1%). The mean [Ca2+]i in vascular smooth muscle cells (VSMCs) of transgenic mice (123.1 +/- 19.7 nM) was higher than those in VSMCs of control mice (66.6 +/- 7.2 nM). These observations suggest that dietary salt loading increases the concentration of calcium in arterial smooth muscle cells in this transgenic mice. These findings are helpful in tracing the causes of salt-sensitive hypertension.