Sodium intake is increased by social stress and the Y chromosome and reduced by clonidine

The objectives were to determine 1) if female rats have higher Na intake than males and if social stress increases Na intake, 2) if the sympathetic nervous system (SNS) mediates the stress effects and the gender effect, and 3) if the Y chromosome (Yc) from a hypertensive father increases Na intake. Four rat strains (n = 10/group) of both sexes were used: 1) Wistar Kyoto normotensive (WKY), 2) an F(16) backcross with a Yc from a hypertensive father (SHR/y), 3) spontaneously hypertensive rat (SHR), and 4) an F(16) backcross with a Yc from a normotensive father (SHR/a). Females showed greater baseline Na intake than males (hypertensive strains), intruder stress increased Na intake, and clonidine decreased Na intake, but not in WKY or SHR females. SHR/y males had higher baseline Na intake compared with WKY males. In conclusion, the higher Na intake in females during baseline and stress was partially mediated through the SNS in hypertensive strains and the SHR Yc was partially responsible for the increased Na intake in SHR/y and SHR males compared with WKY.     

Hypertensinogenic factors and renal alpha-adrenoceptors in young SHR and WKY rats

The effects of high sodium intake (drinking 1% NaCl), DOCA and DOCA + 1%NaCl for 6 weeks on renal alpha 1- and alpha 2-adrenoceptors and on systolic blood pressure (SBP) were examined in young spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). On normal sodium intake, SHR rats had higher renal alpha 1 (p less than .001) and alpha 2-adrenoceptor densities (p less than .001) and SBP (p less than .001) than WKY rats. Although, WKY rats given either 1% NaCl, DOCA, and DOCA + 1% NaCl developed hypertension after 6 weeks of treatment, only 1% NaCl administration for the same period produced an increase in the alpha 1- and alpha 2-adrenoceptor densities when compared to the control (p less than .01 and p less than .001, respectively). In the SHR rats, to the contrary, ingestion of 1% NaCl and DOCA + 1% NaCl increased the already elevated alpha 2-adrenoceptor density (p less than .001) and SBP even more in this strain after 6 weeks of treatment. Equilibrium dissociation constants (KD), however, were similar for both classes of receptors in experimental and control rats. This study indicates that postweaning exposure of the WKY and SHR rats to a high salt treatment and DOCA can influence the renal alpha-adrenoceptor densities. Although the functional significance of the changes is unclear, it is reasonable to speculate that postweaning exposure to a hypertensinogenic stimuli such as a 1% NaCl and/or DOCA may ultimately interfere with the functional development of the kidney differently in rats genetically predisposed to hypertension (SHR) from normotensive (WKY) rats.     

Effect of vasopressin and V1 receptors blockade on hypotensive action of ANP in normotensive (WKY) and spontaneously hypertensive rats.

The aim of the study was to find out whether vasopressin (AVP) modifies hypotensive and heart rate accelerating effects of atrial natriuretic peptide (ANP) in normotensive (WKY) and spontaneously hypertensive (SHR) conscious rats. The effect of i.v. administration of 1; 2 and 4 micrograms of ANP on blood pressure (MP) and heart rate (HR) was compared during i.v. infusion of 0.9% NaCl (NaCl), NaCl+AVP (1.2 ng kg-1 min-1) and NaCl+dEt2AVP (V1 receptors antagonist, 0.5 microgram kg-1 min-1). AVP increased MP in SHR and WKY and decreased HR in SHR. V1 antagonist decreased MP and increased HR only in SHR. In SHR ANP decreased MP and increased HR during NaCl, AVP and V1 antagonist infusion. In WKY these effects were observed only during AVP administration. In each experimental situation hypotension and tachycardia induced by ANP were greater in SHR than in WKY. In both strains ANP induced changes in MP and HR were enhanced during AVP in comparison to NaCl infusion. V1 antagonist did not modify effects of ANP in WKY and SHR. The results indicate that ANP abolishes hypertensive response induced by blood AVP elevation and that the basal levels of endogenous vasopressin acting through V1 receptors does not interfere with hypotensive action of ANP neither in WKY nor in SHR.     

A new inbred Wistar-Kyoto rat substrain exhibiting apparent salt sensitivity and borderline hypertension

The normotensive Wistar-Kyoto (WKY) rat strain is a traditional control for the spontaneously hypertensive rat (SHR). We found trait differences between two inbred normotensive WKY strains, derived originally from different vendors, and compared these two strains from La Jolla-Taconic Farms (WKY/lj-tf) and La Jolla-Charles River (WKY/lj-cr) with the inbred SHR/lj-cr for cardiovascular, diurnal, and activity traits under normal and high (8%) NaCl diets. Marked genetic diversity was found between the two vendor-derived WKY. By using an extended study design and radiotelemetry, we compared WKY/lj-cr, WKY/lj-tf, and SHR/lj-cr with the following results: systolic pressure (120 +/- 1, 133 +/- 1, 168 +/- 3 mmHg, respectively); diurnal variation in heart rate (DeltaHR: 46 +/- 3, 71 +/- 4, 57 +/- 2 beats/min, respectively); and salt sensitivity of arterial pressure (Deltasystolic: 10 +/- 1, 21 +/- 1, 20 +/- 1 mmHg, respectively). The WKY/lj-tf genotype apparently results in compromised control of arterial pressure and heart rate, especially during high NaCl intake, and greater susceptibility to high pressure (i.e., high NaCl-induced secondary changes). WKY/lj-tf thus constitutes a new inbred borderline hypertensive WKY substrain offering unique opportunities for genomic studies into the development of genetic hypertension.     

Antihypertensive effect of low ethanol intake in spontaneously hypertensive rats

Light to moderate drinking in humans lowers the risk of coronary heart disease and may lower blood pressure. We examined the effect of chronic low daily alcohol consumption on blood pressure, platelet cytosolic free calcium [Ca²⁺]_i, tissue aldehyde conjugates and renal vascular changes in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). We also examined the effects of the same weekly amount of alcohol consumption over a one day period each week simulating weekend drinking in humans. Animals, age 7 weeks, were divided into six groups of six animals each and were treated as follows: WKY and SHR control, normal drinking water; WKY and SHR, 0.5% ethanol in drinking water; WKY and SHR, 3.5% ethanol in drinking water one day/week. After 14 weeks systolic blood pressure, platelet [Ca²⁺]_i, liver, kidney and aortic aldehyde conjugates were significantly higher (p < 0.05) in untreated SHRs as compared to untreated WKYs. Daily 0.5% ethanol consumption in SHRs significantly (p < 0.05) attenuated these changes and also attenuated smooth muscle cell hyperplasia and narrowing of the lumen in small arteries and arterioles of the kidney. WKY rats treated with 0.5% ethanol had lower aldehyde conjugates without any significant effect on blood pressure and platelet [Ca²⁺]_i as compared to WKY controls. Consumption of 3.5% ethanol one day/week did not affect blood pressure and associated changes in normotensive WKY rats or hypertensive SHRs as compared to their respective controls. These results suggest that chronic daily low ethanol intake lowers blood pressure in SHRs by lowering tissue aldehyde conjugates and cytosolic free calcium.     

Nonshivering thermogenesis and brown fat in spontaneously hypertensive rats

Oxygen consumption was measured before and during infusion of the catecholamine isoproterenol in age-matched spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats. Mass-independent rates of oxygen consumption of anesthetized 7-week-old rats were similar in the WKY and SHR rats (11.08 +/- 0.74 and 11.33 +/- 0.82 ml O2 min-1 kg-.67, respectively). Catecholamine infusion elicited increased total oxygen consumption in both WKY and SHR animals (15.0 +/- 1.0 and 14.9 +/- 1.2 ml O2 min-1 kg-.67, respectively), and the magnitude of these increases did not significantly differ. To assess whether there were changes in the metabolic state of brown adipose tissue, the major site of catecholamine-induced thermogenesis in rats, enzymes whose activity is proportional to aerobic capacity were assayed in vitro. In both the interscapular and cervical brown fat depots, maximal citrate synthase and maximal HOAD (beta-hydroxyacyl-CoA dehydrogenase) activities were similar in SHR and WKY rats. There were also no significant differences in brown fat protein content, suggesting no differential growth of this tissue in the two rat strains. Our results indicate that the nonshivering thermogenic capacity of the hypertensive SHR rats does not differ from that of the normotensive WKY animals.     

Noradrenergic content and turnover rate in kidney and heart shows gender and strain differences.

The objective of this study was to compare strain and gender differences in kidney and heart norepinephrine (NE) content and turnover rate in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR, SHR/a, and SHR/y). Our laboratory has shown that the Y chromosome has a significant effect on blood pressure in the SHR model of hypertension through the use of two new rat stains, SHR/a and SHR/y, to study the Y chromosome. SHR/a have a SHR autosomal genetic background with a WKY Y chromosome, whereas the SHR/y rats have a WKY autosomal genetic background with a SHR Y chromosome. Tissues were homogenized after alpha-methyl-DL-p-tyrosine injection and analyzed for NE. The male kidney NE content was significantly lower in the WKY compared with the SHR, SHR/y, and SHR/a. Kidney and heart NE content was significantly higher in females compared with males in all strains except the SHR/y. The WKY and SHR/y females had significantly lower kidney NE turnover rates, and the SHR and SHR/a females had significantly higher kidney NE turnover rates than strain-matched males. This study suggests both a strain and gender difference in sympathetic nervous system activity through noradrenergic neurotransmission.     

Prenatal and early postnatal dietary sodium restriction sensitizes the adult rat to amphetamines

Acute sodium deficiency sensitizes adult rats to psychomotor effects of amphetamine. This study determined whether prenatal and early life manipulation of dietary sodium sensitized adult offspring to psychomotor effects of amphetamine (1 or 3 mg/kg ip) in two strains of rats. Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) dams were fed chow containing low NaCl (0.12%; LN), normal NaCl (1%; NN), or high NaCl (4%; HN) throughout breeding, gestation, and lactation. Male offspring were maintained on the test diet for an additional 3 wk postweaning and then fed standard chow thereafter until testing began. Overall, blood pressure (BP), total fluid intake, salt preference, and adrenal gland weight were greater in SHR than in WKY. WKY LN offspring had greater water intake and adrenal gland weight than did WKY NN and HN offspring, whereas WKY HN offspring had increased BP, salt intake, and salt preference compared with other WKY offspring. SHR HN offspring also had increased BP compared with other SHR offspring; all other measures were similar for SHR offspring. The low-dose amphetamine increased locomotor and stereotypical behavior compared with baseline and saline injection in both WKY and SHR offspring. Dietary sodium history affected the rats' psychomotor response to the higher dose of amphetamine. Injections of 3 mg/kg amphetamine in both strains produced significantly more behavioral activity in the LN offspring than in NN and HN offspring. These results show that early life experience with low-sodium diets produce long-term changes in adult rats' behavioral responses to amphetamine.     

Cold-restraint induced gastric lesions in normotensive and spontaneously hypertensive rats

Spontaneously hypertensive (SHR) rats and normotensive Wistar-Kyoto (WKY) rats were subjected to 2 hr of cold-restraint stress at 2–6°C following a 24 hr fast. WKY rats had a significantly greater incidence and degree of ulceration of the gastric glandular mucosa than did SHR rats. Mean arterial pressure, obtained from a chronic arterial cannula, fell during 2 hr of cold-restraint stress in both SHR and WKY rats. Heart rate was unchanged in WKY but fell significantly in SHR. Plasma norepinephrine (NE) and epinephrine (E), determined by radioenzymatic assay, increased significantly following stress. Increased levels of NE remained similar for both SHR and WKY rats, while post-stress levels of E for the SHR rats greatly exceeded E levels for WKY rats. A greater degree of hypothermia was also noted in SHR rats. Decreased stress induced ulcerogenesis in the SHR may be due to the well-known altered hemodynamic and autonomic nervous system reactivity in this strain or other factors not yet discovered.     

Effects of endomorphin-2 on arterial blood pressure and pain threshold in spontaneously hypertensive rats and modification of these effects by beta-funaltrexamine and nor-binaltorphimine

The effects of intracerebroventricular (icv) administration of endomorphin-2 (E2) on arterial blood pressure and pain threshold in spontaneously hypertensive rats (SHR) and modification of these effects by K [OP2] and mu [OP3] opioid receptors antagonists were investigated. Endomorphin-2 administrated icv in doses of 8, 16 and 32 mcg produced dose-dependent analgesic and hypotensive effect. In SHR decrease in blood pressure amounted 2.667, 4.0 and 6.534 kPa, respectively. Pain threshold increased by 1.7, 3.6 and 8.9 (g x 10). In Wistar Kyoto (WKY) strain, being the normotensive controls, E2 in doses of 8 and 16 mcg decrease in blood pressure was less pronounced and amounted 1.200 and 1.467 kPa, respectively, whereas the pain threshold increased by 7.2 and 10.4 (g x 10), respectively. Both E2 effects were antagonized by equimolar icv doses of beta-funaltrexamine (beta-FNA). Equimolar doses of nor-binaltorphimine (nor-BNI) attenuated analgesic action of E2, but were without hypotensive action produced by E2. A strong correlation between drop in blood pressure and increase in pain threshold observed in the SHR and WKY strains after icv administration of E2, indicate close interaction between systems responsible for pain perception and blood pressure control.     

Pressor responsiveness to vasopressin in spontaneously hypertensive rats

The present study was designed to find out whether pressor responsiveness to vasopressin (AVP) is altered in spontaneously hypertensive rats (SHR) in comparison with their normotensive controls (WKY). Blood pressure and heart rate changes after injection of graded doses of 2.5, 5.0 and 10.0 ng of AVP (Calbiochem) i.v. were compared in 9 conscious, unrestrained spontaneously hypertensive (SHR) and 11 normotensive Wistar Kyoto (WKY) rats, chronically instrumented with venous and arterial catheters. The threshold dose necessary to elicit a significant increase in blood pressure and reduction of heart rate was lower in WKY than in SHR. At each dose level the blood pressure elevation persisted for a longer period in WKY than in SHR. Bradycardia was greater in WKY than in SHR both in absolute terms and in relation to the blood pressure increase. Thus, the results reveal diminished pressor responsiveness to moderate doses of AVP in SHR in spite of suppressed reflex bradycardia. It is suggested that the peripheral action of AVP on the vascular system is attenuated in SHR.     

Sodium homeostasis in transplanted rats with a spontaneously hypertensive rat kidney

Recipients of a kidney from spontaneously hypertensive rats (SHR) but not from normotensive Wistar-Kyoto rats (WKY) develop posttransplantation hypertension. To investigate whether renal sodium retention precedes the development of posttransplantation hypertension in recipients of an SHR kidney on a standard sodium diet (0.6% NaCl), we transplanted SHR and WKY kidneys to SHR x WKY F1 hybrids, measured daily sodium balances during the first 12 days after removal of both native kidneys, and recorded mean arterial pressure (MAP) after 8 wk. Recipients of an SHR kidney (n = 12) retained more sodium than recipients of a WKY kidney (n = 12) (7.3 +/- 10 vs. 4.0 +/- 0.7 mmol, P < 0.05). MAP was 144 +/- 6 mmHg in recipients of an SHR kidney and 106 +/- 5 mmHg in recipients of a WKY kidney (P < 0.01). Modest sodium restriction (0.2% NaCl) in a further group of recipients of an SHR kidney (n = 10) did not prevent posttransplantation hypertension (MAP, 142 +/- 4 mmHg). Urinary endothelin and urodilatin excretion rates were similar in recipients of an SHR and a WKY kidney. Transient excess sodium retention after renal transplantation may contribute to posttransplantation hypertension in recipients of an SHR kidney.     

Ion transport in erythrocytes of Prague hypertensive rat

The Prague hypertensive rat is a unique strain exhibiting genetic hypertension in which a hypertensive line (PHR) was bred in parallel with a normotensive one (Prague normotensive rat--PNR) from the same parental pair. Sodium efflux from Na(+)-loaded erythrocytes into Mg2(+)-sucrose medium was measured in these two strains as well as in spontaneously hypertensive rats (Okamoto-Aoki, SHR) and in normotensive outbred Wistar rats. Kinetic parameters--maximal velocity and apparent dissociation constant (reflecting the affinity for internal sodium)--were calculated. It was found that PHR as well as SHR had a higher Na+ leak and a decreased activity of the ouabain-sensitive Na+ transport as compared to Wistar rats. Furosemide-sensitive Na+ transport was substantially lower in erythrocytes of both hypertensive strains (PHR and SHR) than in the respective normotensive strains (PNR and Wistar).     

Lifelong hyperarousal in the spontaneously hypertensive rat indicated by operant behavior

Instrumental conditioning techniques were used to obtain objective evidence of differences in behavioral arousal between the spontaneously hypertensive rat (SHR) and the normotensive ancestral Wistar Kyoto (WKY) strain. Subjective emotionality ratings previously indicated that the genetically hypertensive rats were more active and aggressive than their normotensive cousins. In a lengthy series of operant conditioning sessions using a small number of adult female SHR and WKY rats, hyperarousal in the SHR was confirmed by their significantly higher response outputs on either response contingent or time contingent schedules of reinforcement. Conditioned emotionality tests during this series of experiments also suggested hyperarousal and aggressiveness in the SHR, since the fear-conditioned stimulus suppressed bar-pressing in the SHR much less than in the WKY. Further experiments with young prehypertensive SHR rats provided the same evidence of hyperresponsivity in the SHR compared to the WKY strain. Furthermore, these young SHR failed to develop hypertension by the end of the study (14 weeks of age), while their nonconditioned SHR cousins had become clearly hypertensive by the same age. This suggests that factors related to the conditioning methods modified the development of high blood pressure in this animal model of essential hypertension.     

Long-term effects of betamethasone on blood pressure and hypothalamo-pituitary-adrenocortical function in spontaneously hypertensive and normotensive rats

An enhanced hypothalamo-pituitary-adrenocortical (HPA) activity has been described during onset of elevated blood pressure in spontaneously hypertensive rats (SHR). An instability of the HPA axis could thus contribute to the development of hypertension in these animals. Glucocorticoid effects on blood pressure and HPA function were studied therefore in SHR and normotensive Wistar-Kyoto (WKY) and Wistar rats. Beginning at 4 weeks of age, the rats were treated with 0.1 and 0.5 microgram betamethasone per milliliter drinking water for 7 weeks. SHR and WKY responded with a significant elevation in average blood pressure. In SHR, mean blood pressure rose from 181.4 +/- 3.9 (mean +/- SEM) to 203.1 +/- 2.8 mm Hg in response to the lower dose of betamethasone and to 209.2 +/- 4.0 mm Hg in response to 0.5 microgram betamethasone per milliliter drinking water. In WKY, blood pressure increased from 134.4 +/- 3.3 to 148.2 +/- 3.0 and 157.9 +/- 4.5 mm Hg in response to the lower and higher dose of betamethasone, respectively. No significant effect was seen in Wistar rats, where the mean blood pressure values changed insignificantly from 133.8 +/- 2.1 to 136.3 +/- 3.2 and 135.6 +/- 2.4 mm Hg. Stress-induced secretion of corticosterone was significantly suppressed in a dose-dependent manner in all three strains. Stress-induced secretion of adrenocorticotropin was markedly reduced by 0.5 microgram betamethasone per milliliter in SHR and by both doses in WKY. No significant effect, however, was seen in Wistar rats. A predisposition to the hypertensiogenic actions of glucocorticoids was found therefore in SHR and WKY, but not in Wistar rats.     

Drinking to intracerebroventricularly infused angiotensin II, III, and IV in the SHR

Spontaneously hypertensive rats (SHR) revealed exaggerated water consumption to the intracerebroventricular (ICV) infusion of angiotensin II (AII), and angiotensin III (AIII), as compared with Wistar-Kyoto (WKY) and Sprague-Dawley (SD) normotensive rat strains, in agreement with an earlier report (30) that employed ICV bolus injections of AII and AIII. However, the ICV infusion of AII(3-8) (AIV) did not yield reliable drinking. A second hypothesis that the infusion of AII and AIII would yield equivalent drinking within members of each strain, as previously observed with bolus ICV injections in SD rats, was not confirmed. In contrast, ICV infusion of AII yielded greater water intake than AIII in members of each strain tested. These results suggest that the slow infusion of these ligands allowed endogenous aminopeptidases to adequately keep pace with the degradation of these peptides in contrast with bolus injections that could temporarily saturate the available aminopeptidases thus extending the half-life of the ligand.     

Spontaneously hypertensive and Wistar Kyoto rats are genetically disparate.

Spontaneously hypertensive rats (SHR) are one of the most common animal models used to study essential hypertension in humans. Because SHR and normotensive Wistar Kyoto (WKY) rats were both established from the same parental, normotensive Wistar stock, WKY animals have been used almost exclusively as control animals in studies of SHR. Recently, the suitability of WKY rats as normotensive controls for SHR has been challenged. To establish whether or not SHR and WKY rats share the same immunologic backgrounds, we initially performed a series of skin grafting experiments on these animals. In all cases, grafts of SHR donor skin to WKY recipients and of WKY donor skin to SHR recipients resulted in complete rejection within 7 to 10 days. In addition, grafts of WKY donor skin to other WKY recipients resulted in graft rejection. By contrast, skin grafts between SHRs were always accepted. To further characterize the genetic distinctions between SHR and WKY rats, allelic profiles based on a series of immunologic and biochemical markers were established for each strain. These findings clearly establish that SHR and WKY rats differ at the major histocompatibility complex, in specific blood group antigens, and in a panel of isozymic markers. Moreover, whereas SHRs have the same genetic profiles irrespective of source, some colonies of WKY rats are outbred, as judged by their variant allelic profiles.     

Chronobiologic Evaluation of Angiotensin-Converting Enzyme Activity in Serum and Lung Tissue from Normotensive and Spontaneously Hypertensive Rats

Angiotensin-converting enzyme (ACE) activity in serum and lung tissue from both normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) was determined at six different circadian times. In WKY rats serum ACE varied significantly within 24 h, mainly due to reduced enzyme activity at 12:00 h. In SHR the 24-h profile of serum ACE did not exhibit time-dependent differences. Mean serum ACE activity over 24 h was significantly higher in WKY than in SHR. In lung tissue ACE activity did not depend on the circadian time in either strain. Mean enzyme activity in lung tissue was not different between WKY and SHR. We conclude that circadian changes in the activity of serum and tissue ACE are unlikely to play an important role in the regulation of the circadian blood pressure profile in both normotensive and spontaneously hypertensive rats.     

Differential size distribution of atrial dense granules in spontaneously hypertensive, Wistar-Kyoto and Wistar rats

The differential size distribution of atrial dense granules (ADGs) in spontaneously hypertensive rats (SHR) and two normotensive controls, Wistar and Wistar-Kyoto (WKY) rats, was investigated. The ADGs in SHR were smaller than those in Wistar rats. The ADGs of WKY rats were of intermediate size. It is possible that ADGs are more rapidly secreted in SHR compared with WKY and Wistar rats or that the smaller-diameter granules may contain more atriopeptinogen than the larger granules. The intermediate size of the ADGs in WKY suggests that the WKY variant is morphologically intermediate between the SHR and Wistar strains.     

Chronobiologic Evaluation of Angiotensin-Converting Enzyme Activity in Serum and Lung Tissue from Normotensive and Spontaneously Hypertensive Rats

Angiotensin-converting enzyme (ACE) activity in serum and lung tissue from both normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) was determined at six different circadian times. In WKY rats serum ACE varied significantly within 24 h, mainly due to reduced enzyme activity at 12:00 h. In SHR the 24-h profile of serum ACE did not exhibit time-dependent differences. Mean serum ACE activity over 24 h was significantly higher in WKY than in SHR. In lung tissue ACE activity did not depend on the circadian time in either strain. Mean enzyme activity in lung tissue was not different between WKY and SHR. We conclude that circadian changes in the activity of serum and tissue ACE are unlikely to play an important role in the regulation of the circadian blood pressure profile in both normotensive and spontaneously hypertensive rats.