Chronic hypertension in ANP knockout mice: contribution of peripheral resistance

Atrial Natriuretic Peptide (ANP) exerts a chronic hypotensive effect which is mediated by a reduction in total peripheral resistance (TPR). Mice with a homozygous disruption of the pro-ANP gene (-/-) fail to synthesize ANP and develop chronic hypertension in comparison to their normotensive wild-type (+/+) siblings. In order to determine whether alterations in basal hemodynamics underlie the hypertension associated with lack of endogenous ANP activity, we used anesthetized mice to measure arterial blood pressure (ABP) and heart rate (HR), as well as cardiac output (CO) by thermodilution technique. -/- (n = 7) and +/+ (n = 10) mice of comparable weight and age were used. Stroke volume (SV) and TPR were derived from CO, HR, and ABP by a standard formula. ABP (mm Hg) was significantly higher in -/- (132+/-4) (P < 0.0001) than in +/+ mice (95+/-2). CO (ml min(-1)), HR(beats min(-1))and SV (microl beat(-1)) did not differ significantly between -/- and +/+ mice (CO -/- = 7.3+/-0.5, +/+ = 8.3+/-0.6; HR -/- = 407+/-22, +/+ = 462+/-21; SV -/- = 17.6+/-1.1, +/+ = 17.6+/-1.7). However, TPR (mm Hg ml(-1) min(-1)) was significantly elevated in -/- mice (18.4+/-0.7) compared to +/+ mice (12.3+/-1) (P = 0.0003). Autonomic ganglion blockade with a mixture of hexamethonium and pentolinium was followed by comparable percent reductions in CO (-/- = 28+/-4, +/+ = 29+/-3), HR (-/- = 9+/-4, +/+ = 16+/-4) and SV(-/- = 21+/-4, +/+ = 15+/-6) in both genotypes. However, the concomitant decrease in ABP (%) in -/- (41+/-2) was significantly greater than in +/+ (23+/-4) mice (P = 0.0009) and was accompanied by a significant reduction in TPR. We conclude that the hypertension associated with lack of endogenous ANP is due to elevated TPR, which is determined by an increase in cardiovascular autonomic tone.     

Hypotension and bradycardia during caloric restriction in mice are independent of salt balance and do not require ANP receptor

We hypothesized that caloric restriction (CR)-induced hypotension would correlate with increased sodium excretion through an atrial natriuretic peptide (ANP)-dependent mechanism. To test this hypothesis, the cardiovascular parameters of c57/Bl mice were measured with radiotelemetry while urine was collected. The 23-h mean blood pressure (BP) dropped from 108.6 +/- 1.8 to 92.7 +/- 2.4 mmHg, and 23-h heart rate dropped from 624 +/- 5 to 426 +/- 13 beats/min over 7 days of CR at 29 degrees C. Contrary to our hypothesis, urine sodium excretion decreased by 55% by day 7 of CR. Consistent with decreased sodium excretion was the drop in plasma ANP (from 82.4 +/- 4.3 to 68.0 +/- 5.8 pg/ml). To explore the possibility that CR lowers BP through an ANP receptor-dependent mechanism that is independent of its effect on sodium retention, we measured the cardiovascular parameters of mice deficient in the ANP receptor (NPR1(-/-)) or the ANP clearance receptor (NPR3(-/-)). Mean BP fell from 117.1 +/- 3.9 to 108.0 +/- 4.7 mmHg in the NPR1(-/-) mice and from 87.0 +/- 2.4 to 78.4 +/- 1.7 mmHg in the NPR3(-/-) mice during CR. These data indicate that the hypotension induced by CR does not depend on increased sodium excretion. Rather, it appears that the mouse responds to the low BP induced by CR with an increase in sodium reabsorption. Furthermore, circulating ANP levels and data from NPR1(-/-) and NPR3(-/-) mice suggest that the ANP pathway may not be involved in the cardiovascular response to CR.     

Effects of increased perfusion pressure on medullary collecting duct function

The role of the medullary collecting duct in pressure natriuresis has not been established. In vivo microcatheterization was used to study the effect of an acute increase in blood pressure induced by bilateral carotid artery and vagal nerve ligation on medullary collecting duct function in anaesthetized rats. Increased fluid and electrolyte excretion during pressure natriuresis were accompanied by increased delivery of water, sodium, chloride, and potassium to the beginning of the medullary collecting duct, a change that was significantly greater than in a second series of time-control animals. These increases in delivery were within the range for which constant fractional NaCl reabsorption had been found previously. However, during increased perfusion pressure, reabsorption of both sodium and chloride in the medullary collecting duct as a fraction of delivered load were reduced from 81 +/- 4.1 to 51 +/- 9.3% (p less than 0.01) and from 65.7 +/- 6.0 to 42.7 +/- 9.1% (p less than 0.01), respectively. No significant changes in medullary collecting reabsorption were seen in the time controls. We conclude that increased perfusion pressure, in addition to increasing delivery to the medullary collecting duct, also inhibits sodium chloride reabsorption in this nephron segment.     

Attenuated renovascular constrictor responses to angiotensin II in adenosine 1 receptor knockout mice

In the present experiments we examined the renovascular constrictor effects of ANG II in the chronic and complete absence of A1 adenosine receptors (A1AR) using mice with targeted deletion of the A1AR gene. Glomerular filtration rate (GFR) was not different between A1AR +/+ and A1AR -/- mice under control conditions (450.5 +/- 60 vs. 475.2 +/- 62.5 microl/min) but fell significantly less in A1AR -/- mice during infusion of ANG II at 1.5 ng/min (A1AR +/+: 242 +/- 32.5 microl/min, A1AR -/-: 371 +/- 42 microl/min; P = 0.03). Bolus injection of 1, 10, and 100 ng of ANG II reduced renal blood flow and increased renal vascular resistance significantly more in A1AR +/+ than in A1AR -/- mice. Perfused afferent arterioles isolated from A1AR +/+ mice constricted in response to bath ANG II with an EC50 of 1.5 +/- 0.4 x 10(-10) mol/l, whereas a right shift in the dose-response relationship with an EC50 of 7.3 +/- 1.2 x 10(-10) mol/l (P < 0.05) was obtained in arterioles from A1AR -/- mice (P < 0.05). The expression of AT1A receptor mRNA was not different in kidney RNA from A1AR +/+ or A1AR -/- mice. We conclude that chronic A1AR deficiency diminishes the effectiveness of ANG II to constrict renal resistance vessels and to reduce GFR.     

Influence of the adenosine A1 receptor on blood pressure regulation and renin release

The present study was performed to investigate the role of adenosine A1 receptors in regulating blood pressure in conscious mice. Adenosine A1-receptor knockout (A1R-/-) mice and their wild-type (A1R+/+) littermates were placed on standardized normal-salt (NS), high-salt (HS), or salt-deficient (SD) diets for a minimum of 10 days before telemetric blood pressure and urinary excretion measurements in metabolic cages. On the NS diet, daytime and nighttime mean arterial blood pressure (MAP) was 7-10 mmHg higher in A1R-/- than in A1R+/+ mice. HS diet did not affect the MAP in A1R-/- mice, but the daytime and nighttime MAP of the A1R+/+ mice increased by approximately 10 mmHg, to the same level as that in the A1R-/-. On the SD diet, day- and nighttime MAP decreased by approximately 6 mmHg in both A1R-/- and A1R+/+ mice, although the MAP remained higher in A1R-/- than in A1R+/+ mice. Although plasma renin levels decreased with increased salt intake in both genotypes, the A1R-/- mice had an approximately twofold higher plasma renin concentration on all diets compared with A1R+/+ mice. Sodium excretion was elevated in the A1R-/- compared with the A1R+/+ mice on the NS diet. There was no difference in sodium excretion between the two genotypes on the HS diet. Even on the SD diet, A1R-/- mice had an increased sodium excretion compared with A1R+/+ mice. An abolished tubuloglomerular feedback response and reduced tubular reabsorption can account for the elevated salt excretion found in A1R-/- animals. The elevated plasma renin concentrations found in the A1R-/- mice could also result in increased blood pressure. Our results confirm that adenosine, acting through the adenosine A1 receptor, plays an important role in regulating blood pressure, renin release, and sodium excretion.     

Reduced baroreceptor sensitivity during hypotension in ANP-knockout mice

We studied baroreflex gain in inactin-anesthetized mice that had been genetically modified to be depleted of atrial natriuretic peptide (ANP -/-). Wild-type mice (ANP +/+) served as controls. ANP -/- mice had a significantly higher basal arterial blood pressure (ABP) than ANP +/+ mice [112+/-7 vs. 80+/-5 mmHg (mean +/- SEM)]. Their basal heart rates were not different (491+/-13 vs. 446+/-19 bpm). A third group, composed of ANP +/+ mice only, was rendered acutely hypertensive by an intravenous infusion of arginine vasopressin acetate (0.3 pg bolus followed by 0.3 pg/h) so as to serve as a control for the elevated ABP in the ANP -/- mice. Transient changes in ABP were caused by bolus injections of oxymetazoline hydrochloride (1.5-3 ng) or sodium nitroprusside (20-100 ng). Baroreflex gain was calculated as the ratio of the peak heart rate change that followed the peak change in mean ABP resulting from injection of oxymetazoline or nitroprusside. There were no significant differences among the groups in their responses to transient hypertension. On the other hand, the ANP -/- mice showed a significantly depressed tachycardic response to transient hypotension when compared with the other two groups. We conclude that the ANP -/- mice are unable to increase efferent sympathetic nervous activity adequately above the high basal activity that is a feature of this animal model.     

Collecting duct function in cis-platinum nephrotoxicity

Microcatheterization was used to study the effect of cis-platinum nephrotoxicity on inner medullary collecting duct function in anaesthetized rats. Osmolality of collecting duct fluid increased from the beginning to the end (papillary tip) of the collecting duct by only 69 +/- 11 mosmol/kg in cis-platinum treated rats (at 5-6 days) compared with 306 +/- 75 mosmol/kg in sham controls (p less than 0.01). Tubular fluid to plasma inulin concentration ratio was reduced at the beginning and end of the collecting duct. Tubular fluid sodium, chloride, and potassium concentrations were lower at the papillary tip in cis-platinum treated rats (p less than 0.01). The results indicate that collecting duct water reabsorption is reduced, but electrolyte reabsorption is normal (or even increased) in cis-platinum nephrotoxicity. Papillary tissue sodium chloride concentration was reduced in cis-platinum treated rats. We conclude that the characteristic decrease in urine concentrating ability in cis-platinum nephrotoxicity is not primarily the result of an intrinsic abnormality in collecting duct function but is secondary to decreased papillary hypertonicity resulting from impaired function in more proximal nephron segments, presumably the pars recta of the proximal tubule and the loop of Henle where previous studies have demonstrated abnormal function.     

Prostaglandins that increase renin production in response to ACE inhibition are not derived from cyclooxygenase-1

It is well known that nonselective, nonsteroidal anti-inflammatory drugs inhibit renal renin production. Our previous studies indicated that angiotensin-converting enzyme inhibitor (ACEI)-mediated renin increases were absent in rats treated with a cyclooxygenase (COX)-2-selective inhibitor and in COX-2 -/- mice. The current study examined further whether COX-1 is also involved in mediating ACEI-induced renin production. Because renin increases are mediated by cAMP, we also examined whether increased renin is mediated by the prostaglandin E(2) receptor EP(2) subtype, which is coupled to G(s) and increases cAMP. Therefore, we investigated if genetic deletion of COX-1 or EP(2) prevents increased ACEI-induced renin expression. Age- and gender-matched wild-type (+/+) and homozygous null mice (-/-) were administered captopril for 7 days, and plasma and renal renin levels and renal renin mRNA expression were measured. There were no significant differences in the basal level of renal renin activity from plasma or renal tissue in COX-1 +/+ and -/- mice. Captopril administration increased renin equally [plasma renin activity (PRA): +/+ 9.3 +/- 2.2 vs. 50.1 +/- 10.9; -/- 13.7 +/- 1.5 vs. 43.9 +/- 6.6 ng ANG I x ml(-1) x h(-1); renal renin concentration: +/+ 11.8 +/- 1.7 vs. 35.3 +/- 3.9; -/- 13.0 +/- 3.0 vs. 27.8 +/- 2.7 ng ANG I x mg protein(-1) x h(-1); n = 6; P < 0.05 with or without captopril]. ACEI also increased renin mRNA expression (+/+ 2.4 +/- 0.2; -/- 2.1 +/- 0.2 fold control; n = 6-10; P < 0.05). Captopril led to similar increases in EP(2) -/- compared with +/+. The COX-2 inhibitor SC-58236 blocked ACEI-induced elevation in renal renin concentration in EP(2) null mice (+/+ 24.7 +/- 1.7 vs. 9.8 +/- 0.4; -/- 21.1 +/- 3.2 vs. 9.3 +/- 0.4 ng ANG I x mg protein(-1) x h(-1); n = 5) as well as in COX-1 -/- mice (SC-58236-treated PRA: +/+ 7.3 +/- 0.6; -/- 8.0 +/- 0.9 ng ANG I x ml(-1) x h(-1); renal renin: +/+ 9.1 +/- 0.9; -/- 9.6 +/- 0.5 ng ANG I x mg protein(-1) x h(-1); n = 6-7; P < 0.05 compared with no treatment). Immunohistochemical analysis of renin expression confirmed the above results. This study provides definitive evidence that metabolites of COX-2 rather than COX-1 mediate ACEI-induced renin increases. The persistent response in EP(2) nulls suggests involvement of prostaglandin E(2) receptor subtype 4 and/or prostacyclin receptor (IP).     

Molecular mechanisms of ANP inhibition of renal sodium transport.

ANP, a hormone secreted by the atria of mammalian hearts in response to volume expansion, increases urinary sodium excretion in part by inhibiting sodium reabsorption across the inner medullary collecting duct. A number of nephron segments may contribute to the ANP-induced natriuresis; however, this review will focus on the cellular mechanisms of ANP inhibition of electrogenic sodium reabsorption by the inner medullary collecting duct. Patch-clamp studies conducted on rat inner medullary collecting duct cells in primary culture revealed that ANP, via its second messenger cGMP, inhibits electrogenic sodium reabsorption by reducing the open probability of a cation channel located in the apical membrane. Cyclic GMP inhibits the cation channel and thereby sodium reabsorption by two mechanisms. First, cGMP inhibits the channel by a phosphorylation-independent mechanism, by binding either to an allosteric modifier site on the channel or to a regulatory subunit. Second, cGMP inhibits the channel by activating cGMP-dependent protein kinase, which by a sequential pathway involving the GTP-binding protein, Gi, inhibits the channel. These cGMP-dependent mechanisms inhibiting sodium reabsorption across the inner medullary collecting duct account for a substantial component of the natriuresis following a rise in ANP levels.     

A potential role for the endothelin ET A receptor in salt-sensitive hypertension of the proANP gene-disrupted mouse

We have previously shown that the partial disruption of the gene for atrial natriuretic peptide (ANP) results in a salt-sensitive phenotype. The present study examined the possibility that alterations in either the ANP natriuretic pathway or endothelin (ET) system in the kidney of the salt-challenged ANP +/− mouse was responsible for its salt-sensitive phenotype. Plasma ANP levels and renal cGMP activity were increased in response to a salt load in both ANP +/+ and +/− mice. However, the mRNA expression of proANP was found to be increased only in the ANP +/− kidney along with its guanylyl cyclase-linked receptor, NPRA; the upregulation of NPRA mRNA was limited to the renal medulla. This suggests that the renal ANP pathway remains capable of responding to a salt load in the ANP +/− animal, but may be compensating for other dysfunctional pathways. We also report a significant increase in renal ET-1 mRNA and ET_A receptor protein expression in medulla and cortex of the salt-treated, ANP +/− mouse, but not its wild-type counterpart. In fact, ET_A expression decreased in the renal cortex of the ANP +/+ salt-treated animal. The ET_B receptor expression was not affected by diet in either genotype. We hypothesize that the salt-sensitive hypertension in the ANP +/− mouse is exacerbated, and possibly driven by the vasoconstrictive effects resulting from an upregulated ET-1/ET_A pathway.     

Sympathetic-renal interaction in chronic arterial pressure control

The effects of neonatal sympathectomy of donors or recipients on posttransplantation arterial pressure were investigated in spontaneously hypertensive rats (SHR) by renal transplantation experiments. Conscious mean arterial pressure (MAP) and renal vascular resistance were 136 +/- 1 mmHg and 15.5 +/- 1.2 mmHg x ml(-1) x min x g in sympathectomized SHR (n = 8) vs. 158 +/- 4 mmHg (P < 0.001) and 20.8 +/- 1.1 mmHg x ml(-1) x min x g (P < 0.05) in controls (n = 10). Seven weeks after transplantation of a kidney from neonatally sympathectomized SHR donors, MAP in SHR recipients (n = 10) was 20 mmHg lower than in controls transplanted with a kidney from hydralazine-treated SHR (n = 10) (P < 0.05) associated with reduced sodium sensitivity of MAP. Neonatal sympathectomy also lowered MAP in F1-hybrids (F1H; SHR x Wistar-Kyoto rats). Within 6 wk after transplantation, renal grafts from untreated SHR increased MAP by 20 mmHg in sympathectomized F1H (n = 10) and by 35 mmHg in sham-treated F1H (n = 8) (P < 0.05). Neonatal sympathectomy induces chronic changes in SHR kidney function leading to a MAP reduction even when extrarenal sympathetic tone is restored. Generalized reduction in sympathetic tone resets the kidney-fluid system to reduced MAP and blunts the extent of arterial pressure rise induced by an SHR kidney graft.     

Rapid natriuretic action of aldosterone in the rat.

Rapid, nongenomic actions of aldosterone have been demonstrated in a number of cell types in vitro, including renal cell lines, but there remains little direct evidence that it is able to exert rapid effects on the kidney in the whole animal. Accordingly, the aim of this study was to determine whether aldosterone induces rapid changes in the renal handling of electrolytes or acid-base balance in the anesthetized rat. With the use of a servo-controlled fluid replacement system, spontaneous urine output by anesthetized male Sprague-Dawley rats was replaced with 2.5% dextrose. After a 3-h equilibration and a 1-h control period, rats were infused with aldosterone (42 pmol/min) or vehicle for 1 h. Aldosterone infusion induced a rapid (within 15 min) increase in sodium excretion that peaked at 0.24 +/- 0.08 compared with 0.04 +/- 0.01 micromol x min(-1) 100 x body weight(-1) (P = 0.041) in the vehicle-infused rats. This natriuresis was not associated with changes in glomerular filtration rate; urine flow rate; potassium, chloride, or bicarbonate excretion; or urine pH. The mechanisms involved are unclear, but because we have previously shown that aldosterone stimulates a rapid (4 min) increase in cAMP generation in the rat inner medullary collecting duct (IMCD) (Sheader EA, Wargent ET, Ashton N, and Balment RJ. J Endocrinol 175: 343-347, 2002), they could involve cAMP-mediated activation of the cystic fibrosis transmembrane conductance regulator chloride channel, which drives sodium secretion in the IMCD.     

Age differences in renal response to atrial natriuretic peptide in rabbits

Plasma levels of atrial natriuretic peptide (ANP) and the effect of exogenous ANP on renal function have been studied in newborn and adult rabbits. In order to investigate an age difference in responsiveness to ANP, we studied the renal effects of alpha-human ANP (1-28) administered at the same dose per kg body weight in adult and neonatal rabbits. Plasma basal ANP levels were similar in 18 newborn (4- to 11-day-old) compared to 7 adult rabbits (150 +/- 16 and 151 +/- 28 pg/ml, resp.). Eleven newborn and 11 adult rabbits were anesthetized and mechanically ventilated. After a control period, each animal received an hANP loading dose (3 micrograms/kg i.v.), followed by an infusion of 0.3 micrograms/kg/min. Blood gases remained stable throughout the experiment in both groups. Mean blood pressure decreased in newborn (28.5 +/- 0.8 to 26.2 +/- 1.0 mmHg) and adult (92 +/- 3 to 84 +/- 3 mmHg) animals. Percent hANP-induced changes in renal functions in newborn and adult rabbits were, respectively: urine flow rate: -21 +/- 4% and +57 +/- 8%; urinary sodium excretion: +4 +/- 7% and +81 +/- 11%; glomerular filtration rate (GFR): -19 +/- 4% and -4 +/- 6%; renal blood flow (RBF): -22 +/- 4% and -11 +/- 5%. As expected, diuresis and natriuresis increased in adult rabbits. Failure of hANP to increase natriuresis and diuresis in newborn rabbits could be related to the marked decrease in GFR, receptor immaturity and/or interactions with other hormonal systems.     

Role of EP(2) and EP(3) PGE(2) receptors in control of murine renal hemodynamics

The kidney plays a central role in long-term regulation of arterial blood pressure and salt and water homeostasis. This is achieved in part by the local actions of paracrine and autacoid mediators such as the arachidonic acid-prostanoid system. The present study tested the role of specific PGE(2) E-prostanoid (EP) receptors in the regulation of renal hemodynamics and vascular reactivity to PGE(2). Specifically, we determined the extent to which the EP(2) and EP(3) receptor subtypes mediate the actions of PGE(2) on renal vascular tone. Renal blood flow (RBF) was measured by ultrasonic flowmetry, whereas vasoactive agents were injected directly into the renal artery of male mice. Studies were performed on two independent mouse lines lacking either EP(2) or EP(3) (-/-) receptors and the results were compared with wild-type controls (+/+). Our results do not support a unique role of the EP(2) receptor in regulating overall renal hemodynamics. Baseline renal hemodynamics in EP(2)-/- mice [RBF EP(2)-/-: 5.3 +/- 0.8 ml. min(-1). 100 g kidney wt(-1); renal vascular resistance (RVR) 19.7 +/- 3.6 mmHg. ml(-1). min. g kidney wt] did not differ statistically from control mice (RBF +/+: 4.0 +/- 0.5 ml. min(-1). 100 g kidney wt(-1); RVR +/+: 25.4 +/- 4.9 mmHg. ml(-1). min. 100 g kidney wt(-1)). This was also the case for the peak RBF increase after local PGE(2) (500 ng) injection into the renal artery (EP(2)-/-: 116 +/- 4 vs. +/+: 112 +/- 2% baseline RBF). In contrast, we found that the absence of EP(3) receptors in EP(3)-/- mice caused a significant increase (43%) in basal RBF (7.9 +/- 0.8 ml. min(-1). g kidney wt(-1), P < 0.05 vs. +/+) and a significant decrease (41%) in resting RVR (11.6 +/- 1.4 mmHg. ml(-1). min. g kidney wt(-1), P < 0.05 vs. +/+). Local administration of 500 ng of PGE(2) into the renal artery caused more pronounced renal vasodilation in EP(3)-/- mice (128 +/- 2% of basal RBF, P < 0.05 vs. +/+). We conclude that EP(3 )receptors mediate vasoconstriction in the kidney of male mice and its actions are tonically active in the basal state. Furthermore, EP(3) receptors are capable of buffering PGE(2)-mediated renal vasodilation.     

Hormonal, renal, hemodynamic responses to acute neutral endopeptidase inhibition in heart transplant patients.

We investigated the hemodynamic, renal, and hormonal responses to neutral endopeptidase (NEP) inhibition during a 6-h, double-blind, randomized, placebo-controlled study in seven chronic, stable heart transplant patients. Baseline characteristics were similar during both experiments, and no significant changes were observed after placebo. NEP inhibition increased circulating endothelin-1 (from 2.01 +/- 0.1 to 2.90 +/- 0.2 pmol/l; P < 0.01), atrial natriuretic peptide (ANP; from 21.5 +/- 2.7 to 29.6 +/- 3.7 pmol/l; P < 0.01), and the ANP second messenger cGMP. Noteworthy, systemic blood pressure did not increase. Renal plasma flow and glomerular filtration rate remained unmodified after NEP inhibition. Filtration fraction (33 +/- 13%), diuresis (196 +/- 62%), and natriuresis (315 +/- 105%) increased significantly in relation to ANP and cGMP. A strong inverse relationship was observed between excreted cGMP and sodium reabsorption (r = -0.71, P < 0.0001). Thus, despite significantly increasing endothelin-1, NEP inhibition did not adversely influence systemic or renal hemodynamics in transplant patients. ANP, possibly through a tubular action, enhances the natriuresis observed after NEP inhibition.     

Diminished baroreflex control of heart rate responses in otoconia-deficient C57BL/6JEi head tilt mice

The maintenance of stable blood pressure during postural changes is known to involve integration of vestibular and cardiovascular central regulatory mechanisms. Sensory activity in the vestibular system plays an important role in cardiovascular regulation. The purpose of this study was to determine the role of vestibular gravity receptors in normal baroreflex function. Baroreflex heart rate (HR) responses to changes in blood pressure (BP) in otoconia-deficient head tilt (het) mice (n = 8) were compared with their wild-type littermates (n = 12). The study was carried out in conscious male mice chronically implanted with arterial and venous catheters for recording BP and HR and for the infusion of vasoactive drugs. Resting HR was higher in the het mice (661 +/- 13 beats/min) than in the wild-type mice (579 +/- 20 beats/min). BP was comparable in the het (113 +/- 4 mmHg) and wild-type mice (104 +/- 4 mmHg). The slopes of reflex decreases in HR in response to phenylephrine (PE) were blunted in the het mice (-5.5 +/- 1.5 beats x min(-1) x mmHg(-1)) compared with the wild-type mice (-8.5 +/- 0.9 beats x min(-1) x mmHg(-1)). Likewise, reflex tachycardic responses to decreases in BP with sodium nitroprusside (SNP) were significantly blunted in the het mice (-0.8 +/- 0.3 beats x min(-1) x mmHg(-1)) versus the wild-type mice (-2.2 +/- 0.6 beats x min(-1) x mmHg(-1)). Frequency-domain analysis of the HR variability suggests that under resting conditions, parasympathetic contribution was lower in the het versus wild-type mice. Mapping of the expression of immediate-early gene product, c-Fos, in forebrain and brain stem nuclei in response to a BP challenge showed no differences between the wild-type and het mice. These results suggest that tonic activity of gravity receptors modulates and is required for normal function of the cardiac baroreflexes.     

Effect of crude extract of Stevia rebaudiana on renal water and electrolytes excretion.

To evaluate the effect of crude extract of Stevia rebaudiana on renal water, Na+ and K+ excretion, male Wistar rats (250-350 g each) under antidiuresis or water diuresis conditions, were evaluated. During intravenous infusion of the extract (0.05 mg/min/100 g) no significant differences were detected in mean arterial pressure or renal hemodynamics parameters. In contrast, fractional water and sodium excretion and solute clearance increased significantly, in both groups of animals. In antidiuresis rats the extract significantly increased reabsorption of water by the collecting duct and in water diuresis animals the extract significantly increased free water clearance. The data suggest preferential action of the extract in the proximal tubular cells involved with salt transport mechanism.     

Abolished tubuloglomerular feedback and increased plasma renin in adenosine A1 receptor-deficient mice

The hypothesis that adenosine acting on adenosine A1 receptors (A1R) regulates several renal functions and mediates tubuloglomerular feedback (TGF) was examined using A1R knockout mice. We anesthetized knockout, wild-type, and heterozygous mice and measured glomerular filtration rate, TGF response using the stop-flow pressure (P(sf)) technique, and plasma renin concentration. The A1R knockout mice had an increased blood pressure compared with wild-type and heterozygote mice. Glomerular filtration rate was similar in all genotypes. Proximal tubular P(sf) was decreased from 36.7 +/- 1.2 to 25.3 +/- 1.6 mmHg in the A1R+/+ mice and from 38.1 +/- 1.0 to 27.4 +/- 1.1 mmHg in A1R+/- mice in response to an increase in tubular flow rate from 0 to 35 nl/min. This response was abolished in the homozygous A1R-/- mice (from 39.1 +/- 4.1 to 39.2 +/- 4.5 mmHg). Plasma renin activity was significantly greater in the A1R knockout mice [74.2 +/- 14.3 milli-Goldblatt units (mGU)/ml] mice compared with the wild-type and A1R+/- mice (36.3 +/- 8.5 and 34.1 +/- 9.6 mGU/ml), respectively. The results demonstrate that adenosine acting on A1R is required for TGF and modulates renin release.     

Regulation of renal NaCl and water transport by the ATP/UTP/P2Y2 receptor system

Extracellular nucleotides (e.g., ATP) activate ionotropic P2X and metabotropic P2Y receptors in the plasma membrane to regulate and maintain cell function and integrity. This includes the renal tubular and collecting duct system, where the locally released nucleotides act in a paracrine and autocrine way to regulate transport of electrolytes and water and maintain cell volume. A prominent role has been assigned to Gq-coupled P2Y(2) receptors, which are typically activated by both ATP and UTP. Studies in gene knockout mice revealed an antihypertensive activity of P2Y(2) receptors that is linked to vasodilation and an inhibitory influence on renal salt reabsorption. Flow induces apical ATP release in the thick ascending limb, and first evidence indicates an inhibitory influence of P2Y(2) receptor tone on the expression and activity of the Na-K-2Cl cotransporter NKCC2 in this segment. The apical ATP/UTP/P2Y(2) receptor system in the connecting tubule/cortical collecting duct mediates the inhibitory effect of dietary salt on the open probability of the epithelial sodium channel ENaC and inhibits ENaC activity during aldosterone escape. Connexin 30 has been implicated in the luminal release of the ATP involved in the regulation of ENaC. An increase in collecting duct cell volume in response to manipulating water homeostasis increases ATP release. The subsequent activation of P2Y(2) receptors inhibits vasopressin-induced cAMP formation and water reabsorption, which facilitates water excretion and stabilizes cell volume. Thus recent studies have established the ATP/UTP/P2Y(2) receptor system as a relevant regulator of renal salt and water homeostasis and blood pressure regulation. The pathophysiological relevance and therapeutic potential remains to be determined, but dual effects of P2Y(2) receptor activation on both the vasculature and renal salt reabsorption implicate these receptors as potential therapeutic targets in hypertension.     

Effect of angiotensin II infusion in rats on Na,K-ATPase activity in renal cortical microsomal preparations

Direct dose-dependent effects of angiotensin II on renal tubular sodium reabsorption have been demonstrated. Alterations in tubular sodium reabsorption may occur via modulation of renal Na,K-ATPase activity. Thus, these experiments were undertaken to ascertain whether angiotensin II could influence renal cortical Na,K-ATPase activity. Angiotensin II, 495 ng/microliters/h, or vehicle (controls) was infused for 24 h via miniosmotic pumps 48 h after rats were adrenalectomized and implanted with osmotic pumps containing 12.5 micrograms/microliters corticosterone (Treatment I) or both corticosterone and 0.2 microgram/microliter aldosterone (Treatment II), and in rats receiving 3% NaCl in their food (sodium loaded, Treatment III). Rats receiving Treatments I and III received saline to drink. Renal cortical microsomal membranes were prepared, and the effects of angiotensin II infusion on the K1/2 and Vmax for Na, K, and ATP determined. Angiotensin II infusions were associated with (i) a decrease (P less than 0.001) in the K1/2 for Na activation of Na,K-ATPase from 14 +/- 3 to 6 +/- 1 (n = 4 experiments), 16 +/- 1 to 12 +/- 1 (n = 5), and 12 +/- 3 to 7 +/- 1 (n = 5) mM (means +/- SE) for treatments I, II, and III, respectively; (ii) no changes in the K1/2 for K activation or the Km for ATP; (iii) no changes in the Vmax for Na, K, or ATP; and (iv) no change in Mg-ATPase activity. We conclude that angiotensin II infusion is associated with a decrease in the K1/2 of renal cortical Na,K-ATPase activity for sodium. This action of angiotensin II on the enzyme activity may contribute to the regulation of tubular sodium transport.