Plasticity of pre- and postsynaptic GABAB receptor function in the paraventricular nucleus in spontaneously hypertensive rats

GABA(B) receptor function is upregulated in the paraventricular nucleus (PVN) of the hypothalamus in spontaneously hypertensive rats (SHR), but it is unclear whether this upregulation occurs pre- or postsynaptically. We therefore determined pre- and postsynaptic GABA(B) receptor function in retrogradely labeled spinally projecting PVN neurons using whole cell patch-clamp recording in brain slices in SHR and Wistar-Kyoto (WKY) rats. Bath application of the GABA(B) receptor agonist baclofen significantly decreased the spontaneous firing activity of labeled PVN neurons in both SHR and WKY rats. However, the magnitude of reduction in the firing rate was significantly greater in SHR than in WKY rats. Furthermore, baclofen produced larger membrane hyperpolarization and outward currents in labeled PVN neurons in SHR than in WKY rats. The baclofen-induced current was abolished by either including G protein inhibitor GDPbetaS in the pipette solution or bath application of the GABA(B) receptor antagonist in both SHR and WKY rats. Blocking N-methyl-d-aspartic acid receptors had no significant effect on baclofen-elicited outward currents in SHR. In addition, baclofen caused significantly greater inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) in labeled PVN neurons in brain slices from SHR than WKY rats. By contrast, baclofen produced significantly less inhibition of GABAergic inhibitory postsynaptic currents (IPSCs) in labeled PVN neurons in SHR than in WKY rats. Although microinjection of the GABA(B) antagonist into the PVN increases sympathetic vasomotor tone in SHR, the GABA(B) antagonist did not affect EPSCs and IPSCs of the PVN neurons in vitro. These findings suggest that postsynaptic GABA(B) receptor function is upregulated in PVN presympathetic neurons in SHR. Whereas presynaptic GABA(B) receptor control of glutamatergic synaptic inputs is enhanced, presynaptic GABA(B) receptor control of GABAergic inputs in the PVN is attenuated in SHR. Changes in both pre- and postsynaptic GABA(B) receptors in the PVN may contribute to the control of sympathetic outflow in hypertension.     

Plasticity of GABAergic control of hypothalamic presympathetic neurons in hypertension

Increased sympathetic outflow contributes to the pathogenesis of hypertension. However, the mechanisms of increased sympathetic drive in hypertension remain unclear. We examined the tonic GABAergic inhibition in control of the excitability of paraventricular (PVN) presympathetic neurons in spontaneously hypertensive rats (SHR) and normotensive controls, including Sprague-Dawley (SD) and Wistar-Kyoto (WKY) rats. Whole cell patch-clamp recordings were performed on retrogradely labeled PVN neurons projecting to the rostral ventrolateral medulla (RVLM) in brain slices. The basal firing rate of PVN neurons was significantly decreased in 13-wk-old SD and WKY rats but increased in 13-wk-old SHR, compared with their respective 6-wk-old controls. The GABA(A) antagonist bicuculline consistently increased the firing of PVN neurons in normotensive controls. Surprisingly, bicuculline either decreased the firing or had no effect in 59.3% of labeled cells in 13-wk-old SHR. In contrast, the GABA(B) antagonist CGP-55845 had no effect on the firing of PVN neurons in normotensive controls but significantly increased the firing of 75% of cells studied in 13-wk-old SHR. Furthermore, the evoked GABA(A) current decreased significantly in labeled PVN neurons of 13-wk-old SHR compared with that in normotensive controls. Both the frequency and amplitude of GABAergic spontaneously inhibitory postsynaptic currents were also reduced in 13-wk-old SHR. This study demonstrates an unexpected functional change in GABA(A) and GABA(B) receptors in regulation of the firing activity of PVN-RVLM neurons in SHR. This change in GABA(A) receptor function and GABAergic inputs to PVN output neurons may contribute to increased sympathetic outflow in hypertension.     

Differential role of cyclooxygenase-1 and -2 on renal vasoconstriction to α1-adrenoceptor stimulation in normotensive and hypertensive rats

Aims

Hypertension is associated with the impairment of renal cyclooxygenase (COX) activity, which regulates vascular tone, salt and water balance and renin release. We aimed to evaluate the functional role of COX isoforms in kidneys isolated from spontaneously hypertensive rats (SHR) after α₁-adrenoceptor (α₁-AR) stimulation.

Main methods

Male six-month-old SHR and normotensive Wistar-Kyoto rats (WKY) were used. The kidneys were isolated to measure perfusion pressure and COX-1- or COX-2-derived prostanoids in response to α₁-AR activation.

Key findings

The basal perfusion pressure was higher in SHR kidneys compared with WKY kidneys (95 ± 11 vs. 68 ± 6 mm Hg, P < 0.05). Phenylephrine induced a greater vasopressor response in SHR kidneys (EC₅₀ of 1.89 ± 0.58 nmol) than WKY kidneys (EC₅₀ of 3.30 ± 0.54 nmol, P < 0.05 vs. SHR). COX-1 inhibition decreased the α₁-AR-induced vasoconstrictor response in WKY but did not affect SHR response, while COX-2 inhibition diminished the response in SHR. Both basal prostacyclin (PGI₂) and thromboxane A₂ (TxA₂) values were higher in SHR kidney perfusates (P < 0.05) and were reduced by COX-1 and COX-2 inhibitors in both strains. Furthermore, phenylephrine increased PGI₂ through COX-2 in WKY and through COX-1 in SHR, but the agonist did not significantly modify TxA₂ in both strains.

Significance

The data suggest that COX-1contributes to vasoconstrictor effects in WKY kidneys and that COX-2 has the same effect in SHR kidneys. The results also suggest that basal release of COX-2-derived vasoconstrictor prostanoids is involved in renal vascular hypersensitivity in SHR.     

Clonidine fails to reduce pressor responsiveness of conscious spontaneously hypertensive rats to vasopressin

Pressor responses and heart rate responses to intravenous injections (3.5-50.0 pmol/kg) of arginine vasopressin (AVP) were recorded in saline- and clonidine-treated spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. Clonidine (20 micrograms/kg, i.v.) caused a marked fall of arterial pressure in SHR but not in WKY rats so that, 20 min after the injection of the alpha 2-adrenoceptor agonist, arterial pressure was similar in the two strains of rats. The curve expressing the relationship between the dose of AVP and the increase of arterial pressure for saline-treated SHR was positioned to the left of that for saline-treated WKY rats. This enhanced pressor responsiveness of SHR to AVP may have been related to impaired reflex activity since heart rate fell much less in SHR than in WKY rats for a given elevation in pressure. Pressure responses to AVP were augmented by clonidine in both SHR and WKY rats so that, similar to saline-treated rats, pressor responsiveness to the peptide was still greater in SHR. Heart rate responses to AVP were not altered significantly by clonidine. The results indicate that clonidine fails to enhance reflex activity and reduce pressor responsiveness of SHR to AVP. The increased pressor responsiveness of both SHR and WKY rats to AVP following clonidine was an unexpected finding and may be related to a peripheral interaction between alpha-adrenergic agonists and AVP.     

Changes in the alpha-adrenoceptors in the medulla oblongata including nucleus tractus solitarii of spontaneously hypertensive rats

The nucleus tractus solitarii (NTS) is a brain stem center mediating depression of blood pressure. In order to elucidate a possible mechanism for the central regulation of blood pressure, we studied noradrenergic indices in the medulla oblongata, a region including the NTS, in spontaneously hypertensive rats (SHR) as compared with normotensive controls of the Wistar Kyoto strain (WKY) at 12 weeks of age. The medulla oblongata was the only brain region showing a significantly low noradrenaline level in the SHR as compared with WKY rats; the level is also significantly decreased at 8 weeks of age. The alpha 1-adrenergic binding sites, as measured with 2-(2, 6-dimethoxy) phenoxyethylamine-methylbenzodioxan [³H]WB4101 showed significant increases inK _D andB _max values in medulla oblongata homogenates from rats of both strains from 4–12 weeks after birth, with no significant interstrain difference. On the other hand, theK _D andB _max of the alpha 2-sites, measured by [³H]yohimbine binding, were reduced in SHR as compared to WKY animals, even at 4 weeks after birth when hypertension was not yet apparent. As expected, the relatively selective alpha 2-antagonist, clonidine, was a potent inhibitor of [³H]yohimbine binding but not of [³H]WB4101 binding in these homogenates. The results suggest that some genetic disorder in the alpha 2-adrenergic transmission system in the NTS region may be involved in the development of hypertension in the SHR rats.Dedicated to Professor Yasuzo Tsukada.     

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.     

Angiotensin II-induced changes in G-protein expression and resistance of renal microvessels in young genetically hypertensive rats

Altered regulation of cAMP may contribute to enhanced renal reactivity to angiotensin II (Ang II) in spontaneously hypertensive rats (SHR). Such a phenomenon may occur in renal preglomerular arterioles and may involve changes in expression of GTP-binding regulatory proteins. We have examined the effects of Ang II on steady state levels of G_i-1,2, G_i-3 G_s and G_q in preglomerular arterioles from young marginally hypertensive SHR and on mean arterial pressure (MAP), renal vascular resistance (RVR) and renal cAMP excretion (UcAMP.V). Young (5-6 week old) SHR and Wistar Kyoto (WKY) rats received Ang II (35 ng/kg/min, s.c.) or vehicle for 7 days via osmotic minipumps. Urine was collected over the last 24 h. On day seven, MAP and renal blood flow were measured in anesthetized rats and RVR was determined. Preglomerular arterioles were isolated by perfusing the kidneys with iron oxide and using a series of mechanical steps coupled with the use of a magnet to retain iron-laden vessels. Membranes were prepared and the expressions of G_i-1,2, G_i-3, G_s and G_q were evaluated by Western immunoblotting. Baseline MAP (124 ± 6 mmHg) was only marginally (p > 0.05) higher in SHR when compared with WKY rats (110 ± 4 mmHg). RBF (3.04 ± 0.16 mL/min) was significantly lower and RVR (41.10 ± 1.37 mmHg.min/mL) was significantly higher in SHR when compared to age-matched WKY rats (4.36 ± 0.30 mL/min and 25.79 ± 1.58 mmHg.min/mL, respectively). Ang II significantly increased MAP in SHR (17 mmHg) but not in WKY rats. These increases in MAP were accompanied by significant increases in RVR in SHR (48% over control) but not in WKY rats. Compared to WKY rats, preglomerular arterioles from SHR exhibited significantly higher basal expression of G_i-1,2 (11- fold), G_i-3 (13-fold) and G_s (3-fold). Chronic infusion of Ang II, however, downregulated the expression of G_s (by 53%; p < 0.05), G_i-1,2 ( by 72%; p < 0.05) and G_i-3 (by 35%; p > 0.05) in SHR preglomerular arterioles but significantly upregulated the expression of these proteins in WKY by 3-, 8- and 15-fold, respectively. Basal levels of G_q were not different in preglomerular arterioles from the two strains but were downregulated by Ang II in both WKY (74% of basal) and SHR (52% of control). Baseline UcAMP.V was significantly lower in SHR (31.22 ± 6.51 nmol/24 h) compared with WKY rats (65.33 ± 3.60 nmol/24 h). Chronic Ang II infusion significantly increased UcAMP.V in SHR as well as WKY rats. These data clearly demonstrate that expressions of G_i isoforms as well as G_s in renal microvessels are elevated during early stages of hypertension and suggest that the elevated levels of G_i proteins may be directly associated with a blunted adenylyl cyclase-cAMP cascade in the renal microvasculature. Furthermore, Ang II appears to directly downregulate the expression of G_s in young SHR but not in young WKY renal microvessels. Such diversity in its effect on G-protein expression may be important for enhanced renal sensitivity to Ang II in SHR.     

Endorphinergic mechanism in the central cardiovascular and analgesic effects of clonidine

In urethane-anesthetized male rats, injection of 5 nmol clonidine into the nucleus of the solitary tract (NTS) causes hypotension and bradycardia. These effects are greater in spontaneously hypertensive rats (SHR) and normotensive Sprague-Dawley (SD) rats than in normotensive Wistar-Kyoto (WKY) rats. The effects of clonidine are stereoselectively inhibited by 100 ng intra-NTS naloxone in SHR and SD but not in WKY rats. In SHR, the effects of clonidine are also inhibited by intra-NTS administration of ICI 174864 (a delta-receptor antagonist) but not by beta-funaltrexamine (a mu-receptor antagonist), while in SD rats only the mu- and not the delta-antagonist was effective. Neonatal treatment of SHR with monosodium glutamate (MSG) reduced the beta-endorphin content of the arcuate nucleus and the NTS, reduced the cardiovascular effects of clonidine, and abolished their naloxone sensitivity. MSG treatment of newborn WKY reduced the beta-endorphin content of the arcuate nucleus but not the NTS and did not affect the responses to clonidine. Measurement of pain sensitivity by the formalin test indicated that clonidine was more potent as an analgesic in SHR and SD than in WKY rats, and its effect was inhibited by naloxone (2 mg/kg i.p.) in the former two strains but not in WKY. It is proposed that a naloxone-sensitive component of the cardiovascular effects of clonidine is due to release of a beta-endorphin-like opioid from the NTS, and that this mechanism is present in SHR and SD but not in WKY rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Age-Related Development of γ-Aminobutyric Acid (GABA)BReceptor Functions in Various Brain Regions of Spontaneously Hypertensive Rats

The age-related development of GABA_Breceptors and their coupling to adenylate cyclase were studied in the brains of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Compared with WKY rats, the specific [³H]GABA binding to GABA_Breceptors showed a significant decrease not only in the posterior hypothalamus, midbrain, hippocampus and striatum of eleven-week-old SHR, which maintain a hypertensive state, but also in the posterior hypothalamus of four-week-old normotensive SHR. Similarly, the GABA_Breceptor agonists (baclofen and DN-2327)-induced suppression of adenylate cyclase activity showed a decrease in the posterior hypothalamus of four-week-old SHR as well as in the posterior hypothalamus and striatum of eleven-week-old SHR. These results suggest that the functions of the GABA_Breceptor in the brain of SHR may be decreased independently from the occurrence of blood pressure elevation and that such changes may even be involved in the pathogenesis of SHR.     

Alterations in a hypothalamic GABA system in the spontaneously hypertensive rat

The density (Bmax) of muscimol and clonazepam binding to hypothalamic membranes from spontaneously hypertensive rats (SHR) was reduced compared to age-matched Wistar Kyoto (WKY) animals in the period 80 – 120 days. There were no significant differences in dissociation constant (Kd) for either ligand at this time. At 30 – 36 days, prior to development of pronounced hypertension, there were no differences in Kd or Bmax for either ligand in SHR and WKY animals. There were also deficits in endogenous hypothalamic GABA concentrations in SHR at 75 and 120 days as compared to WKY. The hypothesis is advanced, that there may be a dysfunction of a hypothalamic GABA system in the SHR rat as hypertension develops.     

Presynaptic Modulation of K+-Evoked [3H]Dopamine Release in Striatal and Frontal Cortical Synaptosomes of Normotensive and Spontaneous-Hypertensive Rats

Regional differences in presynaptic [³H]dopamine ([³H]DA) release and its modulation by D₂ DA-receptors between the frontal cortex and striatum obtained from Wystar-Kyoto (WKY) and spontaneous-hypertensive rats (SHR) have been evaluated using superfused synaptosomes. Synaptosomal tritium content was significantly lower in the frontal cortex than in the striatum in both SHR and WKY (45% and 48%, respectively), but no differences in tritium content were obtained between strains. However, the 15 mM K⁺-evoked [³H]DA overflow was lower in the SHR as compared to WKY rats in both brain regions (striatum 23%, frontal cortex 21). Concentration-response curves for quinpirole (1nM-10 M)-mediated inhibition of 15mM K⁺-evoked [³H]DA release showed no differences between SHR and WKY. These results suggest that SHR has less ability to release [³H]DA as compared to WKY rats, but SHR did not show differences in the autoregulation of such release in both the frontal cortex and striatum.     

β2 Mediated inhibition of [3H]dopamine release from nucleus accumbens slices and monoamine levels in a rat model for attention-deficit hyperactivity disorder

The spontaneously hypertensive rat (SHR) has been proposed as an animal model for attentiondeficit disorder (ADHD). The behavioural problems have been suggested to be secondary to altered reinforcement mechanisms in which nucleus accumbens dopaminergic activity plays an important role. Interaction between the noradrenergic and dopaminergic system in the nucleus accumbens has been implicated in the locomotor hyperactivity and impaire discriminative performance of SHR. The present study therefore investigated whether there was any change in the ₂-adrenoceptor mediated inhibition of dopamine release from nucleus accumbens slices of SHR in comparison with their normotensive Wistar-Kyoto (WKY) controls. The electrically stimulated release of [³H]dopamine (DA) from nucleus accumbens slices was decreased to a similar extent by UK14,304, an ₂-adrenoceptor agonist, in SHR and WKY. Basal norepinephrine (NE) levels were increased in locus coeruleus (LC) and A₂ noradrenergic nuclei, but not in the A₁ nucleus of SHR, while basal serotonin (5-HT) levels were increased in all these pons-medulla nuclei. These results suggest that a primarily dysfunctional LC and A₂ nucleus does not have a secondary effect on dopaminergic transmission in the nucleus accumbens via ₂-adrenoceptor mediated inhibition of DA release. Basal monoamine levels in several brain areas of SHR were significantly different from that of WKY. DA, and 5-HT turnover were decreased in SHR versus WKY suggesting hypofunctional dopaminergic and serotonergic systems in some brain areas of SHR.     

Lipid composition and fluidity in the jejunal brush-border membrane of spontaneously hypertensive rats. Effects on activities of membrane-bound proteins

The lipid composition and fluidity of jejunal brush-border membrane vesicles (BBMV) have been studied in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats. The activities of both Na⁺-dependent D-glucose cotransport and Na⁺-H⁺ antiport have also been determined. A significant increase in the level of free cholesterol was observed in jejunal BBMV from SHR compared to WKY rats. Since phospholipid values did not change in either group of animals, a significant enhancement in the free cholesterol/phospholipid ratio was observed in SHR. A decrease in the levels of phosphatidylethanolamine together with an increase in the values of phosphatidylserine was observed in hypertensive rats. Although the content of phosphatidylcholine (PC) and sphingomyelin (SM) was not singificantly altered in SHR, the ratio PC/SM significantly increased in these animals when compared to WKY rats. The major fatty acids present in bursh-border membranes prepared from SHR and WKY rats were palmitic (160), stearic (180), oleic (181, n-9) and linoleic (182, n-6), and the fatty acid composition was not modified by the hypertension. A decreased fluorescence polarization, i.e., increased membrane fluidity, was observed in SHR, which was not correlated to the increased ratio of cholesterol/phospholipid found in the brush-border membrane isolated from these animals. These structural changes found in SHR were associated to an enhancement in both Na⁺-dependent D-glucose transport and Na⁺-H⁺ antiport activity in the jejunal BBMV of SHR.Abbreviations BBMV brush-border membrane vesicles - DPH 1,6-diphenyl-1,3,5-hexatriene - FC free cholesterol - PC phosphatidylcholine - PE phosphatidylethanolamine - PI phosphatidylinositol - PS phosphatidylserine - SM sphingomyelin - SHR spontaneously hypertensive rat - p steady-state fluoroscence polarization - r_s steady-state fluorescence anisotropy - WKY Wistar Kyoto     

G-proteins and adenylyl cyclase signalling in hypertension

The present studies were undertaken to examine if adenylyl cyclase activity and the levels of G-proteins (Gs and Gi) are altered in cardiovascular tissues in hypertension. Adenylyl cyclase activity and its responsiveness to stimulatory and inhibitory hormones as well as the expression of G-proteins (Gs and Gi) were determined at protein and mRNA levels by using specific antibodies and cDNA probes in hearts and aorta from 12 week old spontaneously hypertensive rats (SHR) and their age-matched control Wistar Kyoto (WKY) rats. The stimulatory effects of guanine nucleotides, isoproterenol, glucagon etc. on adenylyl cyclase activity were decreased in SHR rats as compared to the WKY rats, whereas, the inhibitory hormones inhibited enzyme activity to a grater extent in SHR rats as compared to WKY rats. Furthermore, the levels of Gi-2 and Gi-3 proteins and Gi-2 and Gi-3 mRNA as determined by immunoblotting and Northern blotting techniques respectively were higher in SHR as compared to WKY rats. However, the levels of Gsa were unaltered in SHR. To further investigate if these alterations are the cause or effect of hypertension, the SHRs at various ages of the development of blood pressure (3–5 days, 2, 4 and 8 weeks) and their age-matched WKY were used for G-protein expression and adenylyl cyclase activity. The increased expression of Gi_–2 and Gi_–3 protein and mRNA levels in hearts and aorta were observed as early as in 2-weeks old SHR as compared to WKY, when the blood pressure was still normal. However, the levels of G_s in SHR were not different from WKY rats. In addition, the altered responsiveness of adenylyl cyclase to hormone stimulation and inhibition was also observed as early as in 2 week old SHR. These results suggest that the increased expression of Gi_–2 and Gi_–3 and decreased levels of cAMP precedes the development of blood pressure and may be one of the contributing factors in the pathogenesis of hypertension.Abbreviations NECA N-ethylcarboxamideadenosine - Iso Isoproterenol - Glu Glucagon - ANF atrial natriuretic factor - AII angiotensin II - PT pertussis toxin - CT cholera toxin - FSK forskolin - GTPS guanosine 5-[-thio]triphosphate - Gs stimulatory guanine nucleotide regulatory protein - Gi inhibitory guanine nucleotide regulatory protein - WKY WistarKyoto rats - SHR spontaneously hypertensive rats The work presented in this report was supported by grants from Medical Research Council of Canada and Quebec Heart FoundationM.B.A-S is a recipient of the Medical Research Council Scientist Award from the Medical Reserch Council of Canada.     

Catecholamine Content Changes in Brain Regions of Spontaneously Hypertensive Rats Under Immobilization Stress

We compared the effect of immobilization stress on noradrenaline (NA) and 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) content in two brain regions--diencephalon and pons-medulla oblongata--in young and adult spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). In SHR, NA content decreased with time after the onset of the stress, whereas levels of its metabolite MHPG increased. In WKY, NA and MHPG showed no change. The MHPG/NA ratio in both regions increased relative to the duration of the stress in SHR, whereas it remained almost constant in WKY. The rate of increase in the ratio was much higher in the diencephalon of adult (12-week-old) than of young (4-week-old) SHR. In SHR, NA turnover in the brain is readily affected by environmental stress, and these changes in the noradrenergic system may induce or sustain hypertension.     

Heterogeneity in Arterial Remodeling among Sublines of Spontaneously Hypertensive Rats

Objectives

Spontaneously hypertensive rats (SHR) have been used frequently as a model for human essential hypertension. However, both the SHR and its normotensive control, the Wistar Kyoto rat (WKY), consist of genetically different sublines. We tested the hypothesis that the pathophysiology of vascular remodeling in hypertension differs among rat sublines.

Methods and Results

We studied mesenteric resistance arteries of WKY and SHR from three different sources, at 6 weeks and 5 months of age. Sublines of WKY and SHR showed differences in blood pressure, body weight, vascular remodeling, endothelial function, and vessel ultrastructure. Common features in small mesenteric arteries from SHR were an increase in wall thickness, wall-to-lumen ratio, and internal elastic lamina thickness.

Conclusions

Endothelial dysfunction, vascular stiffening, and inward remodeling of small mesenteric arteries are not common features of hypertension, but are subline-dependent. Differences in genetic background associate with different types of vascular remodeling in hypertensive rats.     

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.     

Impairment of L-arginine metabolism in spontaneously hypertensive rats.

Plasma L-arginine concentrations were determined in spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) before and after water immersion stress. There was no difference in the plasma levels of L-arginine before stress loading between SHR and WKY rats. A significant decrease in the L-arginine level was found in the adult SHR rats after the stress stimuli. However, there was no change in plasma levels of L-arginine in the adult WKY rats before and after water immersion stress. In the weanling rats, significant increases were observed in the plasma L-arginine levels after stress loading in both strains. These findings indicate that there may be an impairment of the L-arginine metabolism in the SHR rats with age and that it may involve in the genesis of hypertension in the SHR rat through the L-arginine-EDRF system.     

Some disturbances in the function of ryanodine receptors of spontaneously hypertensive rat cardiomyocytes detected using 4-chloro<Emphasis Type="Italic">m</Emphasis>-cresol

The rate of Ca²⁺ release from the sarcoplasmic reticulum in response to the activation of ryanodine receptors with 4-chlorom-cresol in the cardiomyocytes of three rat strains—spontaneously hypertensive (SHR), normotensive (WKY), and Wistar rats—during 5 weeks of their growth and development was studied aiming to detect the functional differences in the operation of these receptors at various stages of arterial hypertension. In response to 4-chlorom-cresol, a drastic increase in the rates of [Ca²⁺]_i accumulation in SHR myocytes after 17 days of development is recorded versus a decrease in the rates of Ca²⁺ efflux from the sarcoplasmic reticulum of Wistar and WKY rat cardiomyocytes. A correlation between this phenomenon and a genetic defect of ryanodine receptors in SHR rats seems rather unlikely, as the newborn WKY and SHR rats exposed to 4-chlorom-cresol at a concentration range of 0.5–2.0 mM did not display any differences in the rate of sarcoplasmic reticulum Ca²⁺ release. On the other hand, it is possible that the pathological changes in the function of ryanodine receptors manifest themselves later in the ontogenesis. The connection of this phenomenon with an increase in the role of ryanodine receptors in the excitation-contraction coupling in muscle cells and an increase in the calpain expression in SHR rats (absent in the WKY rats) by the age of 3 weeks is discussed. It is assumed that the cleavage of ryanodine receptor subunits by calpain can noticeably intensify the sarcoplasmic reticulum Ca²⁺ release after activation of these receptors without influencing the receptor binding characteristics.     

Enhanced vasoconstriction to α_1 adrenoceptor autoantibody in spontaneously hypertensive rats

Autoimmune activities have been implicated in the pathogenesis of hypertension.High levels of autoantibodies against the second extracellular loop of α1-adrenoceptor(α1-AR autoantibody,α1-AA) are found in patients with hypertension,and α1-AA could exert a α1-AR agonist-like vasoconstrictive effect.However,whether the vasoconstrictive effect of α1-AA is enhanced in hypertension is unknown.Using aortic rings of spontaneously hypertensive rats(SHR) and normotensive Wistar-Kyoto(WKY) rats,we observed the vasoconstrictive responses to α1-AA with phenylephrine(α1-AR agonist) as a positive control drug.Aortic nitrotyrosine levels were also measured by ELISA and immunohistochemistry.The results showed that the aortic constrictive responses to α1-AA and phenylephrine(both 1 nmol L-1-10 μmol L-1) were greater in SHR than in WKY rats.Endothelial denudation or L-NAME(a non-selective NOS inhibitor)(100 μmol L-1) increased α1-AA- or phenylephrine-induced vasoconstrictions both in SHR and WKY.However,selective iNOS inhibitor 1400W(10 μmol L-1) enhanced the α1-AA-induced aortic constriction in WKY,but not in SHR.The aortic nitrotyrosine level was significantly higher in SHR than WKY,as shown by both ELISA and immunohistochemistry.These results indicate that the vasoconstrictive response to α1-AA is enhanced in SHR,and this altered responsiveness is due to endothelial dysfunction and decreased NO bioavailability.The study suggests an important role of α1-AR autoimmunity in the pathogenesis and management of hypertension especially in those harboring high α1-AA levels.