Age- and Phenotype-Dependent Changes in Circulating MMP-2 and MMP-9 Activities in Normotensive and Hypertensive Rats

Matrix metalloproteinases (MMPs) are important in the pathogenesis of numerous diseases. The present study aimed to monitor the activation of MMP-2 and MMP-9 in spontaneously hypertensive rats (SHR) and their normotensive counterparts-Wistar-Kyoto rats (WKY). The animals were divided according to age (7, 20, and 52 weeks) and phenotype into: WKY-7, WKY-20, WKY-52, SHR-7, SHR-20 and SHR-52 groups. MMP plasma activities were determined by gelatine zymography. We monitored selected parameters of oxidative stress and antioxidant status. N-terminal pro-brain natriuretic peptide (NT-proBNP) was determined as a marker of heart function and neurohumoral activation. SHR-7 showed higher MMP-2 activity compared with WKY-7, while SHR-52 showed lower MMP-2 and MMP-9 activities compared with WKY-52. Examining age-dependent changes in MMP activities, we found a decrease in MMP-2 activity and increase in MMP-9 activity with increasing age in both phenotypes. Parameters of oxidative stress and antioxidant status as well as NT-proBNP levels were not significantly worsened due to aging in SHR. Our results suggest that hypertension is accompanied by varying MMP activation during aging. The results of our study may indicate that MMP-2 inhibition is therapeutically applicable during the development of hypertension, while in developed, stabilized and uncomplicated hypertension, systemic MMP-2 and MMP-9 inhibition may not be desirable.

The effect of multivitamin-multimineral supplementation on the health status of inbred Wistar and spontaneously hypertensive rat strains

The nutraceutical industry has proliferated in recent years, with the most popular form of supplementation being the multivitamin-multimineral (MVMM) supplement. In the animal health sector, supplement use has also expanded. The objective of this study was to determine the effects of MVMM supplementation, beneficial or otherwise, on the general health status of the spontaneously hypertensive rat (SHR) strain, an animal model used in hypertension research. A commercially prepared MVMM supplement was given tri-weekly via oral dosing for 8 weeks to two groups of seven adult female SHR and Wistar rats. Their corresponding control groups were dosed with deionised water only. Systolic and diastolic blood pressure, fasting blood glucose, growth rate and food and water intake were measured weekly. At the end of 8 weeks, the animals were euthanased and a full blood profile, urine sodium to potassium ratio, blood urea nitrogen levels and total plasma cholesterol was measured for all groups. The results indicated that growth rate was higher for the SHR supplemented group. Supplementation also decreased diastolic blood pressure in both Wistar and SHR groups and increased red blood cell count and decreased total cholesterol in the SHR group. No adverse effects on the general health status of the animals were observed. MVMM supplementation may therefore be useful in aiding growth and delaying the onset of hypertension and its effects. It may also assist in the longevity of the breeding stock of SHR rats.

Antihypertensive effect of Eucommia ulmoides Oliv. extracts in spontaneously hypertensive rats

AIM OF THE STUDY

To investigate the antihypertensive fractions of Eucommia ulmoides Oliv. and their underlying mechanisms in spontaneously hypertensive rats (SHRs).

MATERIALS AND METHODS

In vivo study, Eucommia ulmoides lignans (EuL) and Eucommia ulmoides iridoids (EuI) were administrated to Sprague-Dawley (SD) rats and SHRs, and their blood pressures were measured. Plasma level of nitric oxide (NO) was measured by colorimetric method, and renin activity (RA) and plasma concentration of angiotensin II (Ang II) were measured by radioimmunoassay. In vitro study, rat mesenteric artery was treated with EuL and the vessel relaxation responses were determined.

RESULTS

EuL could lower blood pressures of both SD rats and SHR dose-dependently by either intravenous (i.v.) or intragastric (i.g.) administration, but EuI failed to affect blood pressure in the two kinds of rats. Meanwhile, no synergistic effect was observed with the combination of EuL and EuI. The plasma level of NO in SHR treated with EuL 300 mg/kg twice a day was markedly increased. Both plasma RA and Ang II level were decreased with long-term oral treatment of EuL 150 and 300 mg/kg twice a day. In perfusion experiment, EuL relaxed mesenteric artery quickly and dose-dependently and the effect on the artery with and without endothelium was the same.

CONCLUSIONS

EuL may be the effective fraction to lowering blood pressure and its antihypertensive effect is probably associated with regulating NO and renin-angiotensin system (RAS) and directly relaxing artery.

Differences in home cage behavior and endocrine parametres in rats of four strains

OBJECTIVE

The majority of studies that involve behavioral testing in different rat strains is based on measuring behavioral responses to stressful situations in a novel environment. Very little is known on the spontaneous behavior in rat strains. The aim of the present study was to compare home cage behavior and basal hormone levels in two outbred rat strains, Sprague-Dawley (SD) and Wistar as well as two inbred strains, Lewis and Spontaneously Hypertensive Rats (SHR).

METHODS

Twenty-eight male rats from four strains of rats (n=7/strain) were used in this study. Behavior of each rat in his home cage was recorded by a video camera for 45 minutes during the dark phase of the day. The parameters considered included rearing, jumps, ambulation, grooming, feeding/drinking and no movements. Blood plasma was analyzed for aldosterone, plasma renin activity (PRA), adrenocorticotropin hormone (ACTH) and corticosterone by specific radioimmunoassays.

RESULTS

One-way ANOVA revealed significant inter-strain differences in counts of jumps and rearing. In comparison with Sprague-Dawley and Wistar rats, Lewis rats displayed significantly more jumps and rearing. Statistical analysis showed significant inter-strain differences in the levels of aldosterone and of plasma renin activity. The highest levels of aldosterone were found in Lewis rats. Plasma renin activity was significantly lower in SHR than in Sprague-Dawley rats. Correlation analysis failed to reveal any relationships between behavioral and endocrine parameters. Significant strain differences were observed also in relative weights of the spleen, adrenals and thymus.

CONCLUSION

Plasma renin activity and basal mineralocorticoid secretion did not show parallel pattern if compared among different rat strains. Locomotor activity in the home cage, which could represent general activity to be considered in evaluating emotional responses, was highest in the Lewis rats.

Adverse interactions of rofecoxib with lisinopril in spontaneously hypertensive rats

BACKGROUND

Hypertension and arthritis are frequent comorbidities. Nonsteroidal anti-inflammatory drugs (NSAIDs) are well known to produce hypertension or attenuate the effects of antihypertensive agents in a few patients. The influence of selective NSAIDs on blood pressure and the cardiovascular and renal effects of coxibs have still to be investigated. The purpose of this study was to test the hypothesis that rofecoxib interferes with antihypertensive activity and cardiorenal protective effects of lisinopril in spontaneously hypertensive rats (SHRs).

METHODS

Twenty-one unanaesthetised, male spontaneously hypertensive rats (SHRs), 16 weeks old, were randomized to receive lisinopril (LS) 15 mg/kg/d or rofecoxib (RF) 20 mg/kg/d or combination of lisinopril (LS) and rofecoxib (RF) for 2 weeks. The arterial blood pressure changes were recorded each week. The Sodium Hydrogen Exchange (NHE) activity of erythrocytes was determined 2 weeks after the study. The surviving animals were sacrificed 24 h after the last dose, and the sections of their hearts and kidneys were assessed histologically for injury by a pathologist masked to the treatment.

RESULTS

RF completely prevented the hypotensive effects of LS during the first week of treatment but the antihypertensive efficacy of LS was restored during the second week of treatment. The NHE in erythrocytes of 18-week-old SHRs was found to be significantly lower than the age-matched Wistar rats (P < 0.05), and LS treatment reversed these values to Wistar control in SHRs. RF was devoid of any effect on NHE of erythrocytes. The histological examination revealed that the myocardial and renal protection induced by LS was attenuated by concomitant RF therapy.

CONCLUSIONS

These results indicate that COX-2 inhibitors should be used judiciously in patients with history of hypertension, ischemic heart disease, or chronic renal failure.

IGF-I, 17beta-estradiol and progesterone in SHR and in rats treated with L-NAME: fetal-placental development

METHODS

The relationship between progesterone (P4), 17beta-estradiol, insulin-like growth factor (IGF-I) and embryonic and fetal development, were examined. Female Sprague Dawley rats were divided into two groups: control untreated (n=60) and treated (n=63) with L-NAME (N(omega)-nitro-L-arginine methyl ester), 15 mg/day in drinking water from the first day after mating to day 18 of gestation. A further group was formed (n=63) of spontaneously hypertensive rats (SHR). Mean systolic blood pressure was recorded daily. On days 6, 11 and 18 of pregnancy, the number of sites of implantation, litter and placenta weight was examined. In addition, serum levels of P4, 17beta-estradiol and IGF-I were determined.

RESULTS

Systolic blood pressure (SBP) (mmHg) increased significantly in L-NAME and SHR rats over the course of the experiment. On day 6 of pregnancy, in the L-NAME group the number of implantation sites, levels of IGF-I and 17beta-estradiol were significantly lower than in the control group. In SHR, only the concentration of IGF-I was low (p<0.05). In contrast, on day 11 of pregnancy no variation was found in the parameters under study. On day 18 of gestation, a significant decrease in litter and placenta weight, concentration of P4 and IGF-I was observed in the experimental groups.

CONCLUSION

The data further suggest that nitric oxide might regulate IGF-I production, indicating that growth factors may play an important role in fetal-placental development.

Potassium control of extrarenal renin secretion in transgenic (mRen-2)27 and normal rats

Plasma active renin and prorenin were followed for 12 h after bilateral, unilateral, and sham nephrectomy (BNx, UNx, and SNx) in anesthetized transgenic (mRen-2)27 rats to compare them with Sprague-Dawley and spontaneously hypertensive rats (SDR and SHR). In Ren-2 rats, active renin and prorenin increased with plasma potassium post-BNx and were augmented by potassium infusion. The increase in prorenin but not active renin was abolished by bilateral adrenalectomy (BADRx). However, this did not reduce prorenin below normal, indicating that the high plasma prorenin Ren-2 phenotype is not only of adrenal origin. SNx and UNx also raised plasma active renin and prorenin in Ren-2 rats, with positive correlations to plasma potassium. In SDR and SHR, active renin fell below prorenin post-BNx, and adrenal ablation and potassium loading (in SDR) modified the decreasing active renin profile consistent with low levels of regulated extrarenal secretion. In Ren-2 rats, adrenal but not extra-adrenal prorenin secretion is potassium sensitive and stress related. The unidentified source of active renin in BNx+BADRx Ren-2 rats is also potassium and stress related.

Decrease in circulating and urine adrenomedullin concentrations in stroke-prone spontaneously hypertensive rats

Adrenomedullin (AM) is a peptide with potent vasodilatory and hypotensive properties. Plasma AM levels in rats with experimentally induced hypertension, such as Dahl salt-sensitive rats and two-kidney, one-clip hypertensive rats, are higher than those in normotensive rats. We previously noted, however, that plasma AM levels in spontaneously hypertensive rats (SHR) are similar to those in Wistar-Kyoto rats. To define the role of AM in rats with severe hypertension, we investigated changes in circulating and tissue AM levels in stroke-prone spontaneously hypertensive rats (SHRSP/Izm). The immunoreactive rat AM levels in plasma, urine, and tissue measured with a sensitive radioimmunoassay, and the AM mRNA levels in various tissues in 15-wk-old SHRSP/Izm were compared with those in age-matched Wistar-Kyoto rats (WKY/Izm). The plasma and urinary AM levels in SHRSP/Izm were significantly lower than those in WKY/Izm [plasma AM, 2.14+/-0.06 (SE) vs. 3.24+/-0.16 fmol/ml, p< 0.001; urinary AM, 16.36+/-3.21 vs. 36.12+/-6.09 fmol/ml, p< 0.01]. A negative correlation was found between the plasma AM level and the systolic blood pressure in both SHRSP/Izm and WKY/Izm. Reverse-phase high-performance liquid chromatography showed that the molecular components of plasma immunoreactive AM in SHRSP/Izm were similar to those in WKY/Izm. Furthermore, tissue AM levels in various organs in SHRSP/Izm were not lower than those in WKY/Izm. In conclusion, low levels of circulating AM may contribute to the maintenance of high blood pressure in 15-wk-old SHRSP/Izm. These low plasma AM levels may be caused by accelerated metabolism of circulating AM in SHRSP/Izm.

Neutrophil-mediated damage to vascular endothelium in the spontaneously hypertensive rat

Lysis of aortic endothelial cells (EC) by neutrophils from spontaneously hypertensive rats (SHR) was investigated using a nonradioactive cytotoxicity assay. Interleukin-1-activated EC, but not unstimulated EC, were effective target cells for lysis by SHR neutrophils. Supernatants from activated neutrophil did not exert a cytotoxic effect on EC. Inhibitors of reactive oxygen species did not affect the cytotoxicity of neutrophils on EC. In contrast, inhibitors of serine protease and elastase markedly inhibited the cytotoxicity of neutrophils on EC. Antibodies against the endothelial cell surface ligands ICAM-1 (CD54) and E-selectin (CD62E) inhibited the adhesion and cytotoxicity of activated neutrophils on EC. The cytotoxicity of neutrophils required direct cell-to-cell contact because separating them with a microporous membrane abrogated the neutrophil-mediated cytotoxic activity. These results demonstrate that SHR neutrophils possess potent cytotoxicity against cytokine-activated EC. Neutrophil-mediated damage of EC could contribute to organ damage in hypertension under conditions of local or systemic activation of neutrophils.

High potassium diets reduce vascular and plasma lipid peroxides in stroke-prone spontaneously hypertensive rats

We examined the effect of high potassium (K) diet on oxidative stress to endothelium in hypertensive rats. Five-week-old stroke-prone spontaneously hypertensive rats (SHRsp) were fed a 5% high NaCl diet containing either 0.5% normal K (n = 28) or 2.1% high K (n = 19) for 6 weeks, and lipid peroxides in the aortic intima and plasma were measured. Lipid peroxides were extracted into an organic solvent to avoid the interference of carbohydrates or glycoproteins, and malondialdehyde (MDA) produced from lipid peroxides by acid-heating was measured by its reaction to thiobarbituric acid. The antioxidant butylated hydroxytoluene prevented spurious lipid peroxide formation during the whole procedure, and optimum Fe3+ allowed a maximum MDA production from lipid peroxides. The high K SHRsp showed lower lipid peroxide levels than the normal K SHRsp both in the intima (5.6 +/- 0.3 vs. 7.2 +/- 0.4 nmol MDA/mg fatty acids, p < 0.003) and plasma (0.91 +/- 0.08 vs. 1.46 +/- 0.10 nmol MDA/ml, p < 0.001). Mean arterial pressure was slightly lower by 13 mmHg in the high K SHRsp, but these differences were still obvious even when we compared groups of rats with precisely matching blood pressures. These results indicate that high K diets reduce oxidative stress on the endothelium of high NaCl-fed SHRsp independently of blood pressure changes. This effect may be involved in the mechanism by which high K diets protect endothelium and reduce stroke incidence in hypertensive animals. Thus, we improved the method of lipid peroxide measurement and propose the protective effects of high K diet against oxidative stress to endothelium in hypertension animals.

Platelet lipoxygenase in spontaneously hypertensive rats

We have previously reported that the nonselective lipoxygenase inhibitor phenidone is a potent hypotensive agent in the spontaneously hypertensive rat (SHR). In the present study, we examined the relationship between production of platelet 12-hydroxyeicosatetraenoic acid (12-HETE) and intra-arterial blood pressure in SHR and Wistar-Kyoto rats (WKY) using both a cross-sectional analysis and an acute pharmacological intervention. Basal generation rate of 12-HETE by platelets collected from the SHR was approximately 3.7-fold higher than in the WKY (0.86 +/- 0.24 versus 0.23 +/- 0.05 nmol/mL per 10 minutes, respectively; P < .01). Systolic arterial pressure was positively related to platelet 12-HETE formation rate when the entire rat population was considered (r = .70, P < .001). The specific 12-lipoxygenase inhibitor cinnamyl-3,4-dihydroxycyanocinnamate induced lowering of both arterial blood pressure and platelet 12-lipoxygenase activity in SHR. At 15 mg/kg, cinnamyl-3,4-dihydroxycyanocinnamate elicited a marked hypotensive effect in SHR but not in WKY. This reduction in arterial pressure was accompanied by an approximate 70% inhibition in platelet 12-HETE production rate. The return of high blood pressure to basal levels was associated with a significant rise in the production of platelet 12-HETE toward control values (baseline, 0.97 +/- 0.33 nmol/mL per 10 minutes; nadir of blood pressure, 0.19 +/- 0.03; resumption of basal pressure, 0.42 +/- 0.14). In contrast, captopril (15 mg/kg) induced a quantitatively similar decrease in blood pressure but had no effect on platelet 12-HETE generation rate. Thus, hypertension in SHR is linked to increased production rate of platelet 12-HETE. Acute blood pressure reduction attained during lipoxygenase inhibition but not by angiotensin converting enzyme inhibition leads to a concomitant reduction in the production of platelet 12-HETE. We speculate that since rat arterial tissue produces 12-HETE, increased 12-lipoxygenase activity in SHR may contribute to the maintenance of elevated arterial pressure in this strain.

Cerebral prothrombotic state in spontaneously hypertensive rats of different breeds

The thrombotic tendency in two breeds of spontaneously hypertensive rats (SHRs) was assessed by inducing occlusive thrombosis in pial blood vessels using a helium-neon laser method. A prothrombotic state was observed in the SHR of one breed as compared to the corresponding control WKY, but not in the other. Circulating platelet and erythrocyte counts and haematocrit values were significantly negatively correlated in arterioles with the number of laser pulses, which is an index of the thrombotic tendency, suggesting that the prothrombotic state was partly dependent on the counts of blood components. However, there was no correlation between the measured blood parameters and prothrombotic state in venules. The difference observed in arterioles and venules might be due to their differential haemodynamics and endothelial properties. Prothrombotic state in the SHR of one breed would be partly due to the much higher difference in blood components from its corresponding control compared to the comparatively less difference in the other breed.

Relationship between hypotensive effects and plasma concentrations of clonidine in spontaneously hypertensive rats: continuous treatment and sudden termination of clonidine infusion

1. Clonidine was administered subcutaneously (62.5, 125 and 250 micrograms/kg/day) for 8 days using an osmotic infusion pump in spontaneously hypertensive rats (SHR). Clonidine, administered at 125 and 250 micrograms/kg/day at 48 hr after infusion, respectively, and thereafter were maintained at this level throughout the infusion period. 3. After terminating clonidine infusion, a rapid drug elimination from the plasma was manifested in both groups (125 and 250 micrograms/kg/day) with plasma clonidine levels, resulting in a decrease below 0.5 ng/ml at 4 and 6 hr, respectively. Four hours after terminating clonidine infusion at 125 and 250 micrograms/kg/day, transient but not remarkable increases in blood pressure and heart rate were observed only in the latter group compared with the values before termination. 4. These findings reveal that marked hypotensive effects were induced by relatively high doses (125 and 250 micrograms/kg/day) of clonidine in SHR, but no remarkable withdrawal symptoms after termination of clonidine infusion were observed. Therefore, unwanted withdrawal symptoms probably occur when an extremely high dose (250 micrograms/kg/day or more) of clonidine was infused for a long period (8 days or more) in SHR.

[Ectacytometry of the erythrocytes in rats of the SHR, WKY and Wistar strains]

Deformation properties of erythrocytes have been studied using osmotic gradient ectacytometric procedure in rats with spontaneous hereditary hypertension (SHR), normal tension rats from Wistar-Kyoto strain and albino rats from Wistar strain. It was shown that high intrinsic viscosity and, to a less extent, changes in their form account for the increased rigidity of the erythrocytes from SHR rats. No significant changes in visco-elastic properties of the membrane were found. The degree of hydration of haemoglobin significantly increases in the order SHR-WKY-Wistar.

Elevated plasma adrenaline in spontaneously hypertensive rats

Having found that circulating adrenaline (AD) is selectively elevated in stroke-prone spontaneously hypertensive rats (SHRSP) compared with Wistar-Kyoto rats (WKY), we extended the comparison to include other normotensive and hypertensive rat strains. Aortic catheters were implanted in young (5-7 weeks) and old (7-9 months) WKY, Black-Hooded Wistar (BHW), Sprague Dawley (SD), spontaneously hypertensive rats (SHR) and SHRSP for repeated measurement of mean arterial pressure (MAP) and blood sampling under conscious resting conditions. In the young SHR and SHRSP, MAP was already significantly higher than in age-matched WKY but MAP in the SD rats was similar. Plasma AD was significantly higher in SHR and lower in SD rats when compared with WKY. There was no difference in plasma noradrenaline (NA) between strains at this age. At the older age, MAP was 40-60 mmHg higher in SHR and SHRSP than in WKY and BHW but was significantly lower in the SD strain. Circulating AD did not differ between the normotensive strains but was 3-4 times higher in the hypertensive strains. Plasma NA was elevated in SHR only. The acute reduction of MAP caused by ganglion blockade (an index of the sympathetically mediated component of resting blood pressure) was greater in SHR and SHRSP than in WKY at the older age only. However, the residual MAP after ganglion blockade was significantly higher in the hypertensive strains at both ages. Regression analysis showed that in the older rats, plasma AD was correlated with resting MAP, the reduction in MAP with ganglion blockade, the residual MAP and plasma NA.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in Ca2+ mobilization in platelets from stroke-prone spontaneously hypertensive rats

Intracellular free Ca2+, [Ca2+]i, levels were measured in platelets from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY) using fura-2AM. In the presence of extracellular Ca2+ (1 mM), [Ca2+]i levels in unstimulated platelets of 2- and 9-month-old SHRSP were both significantly higher than those of the age matched WKY. In the absence of extracellular Ca2+, the levels in platelets from 9-month-old SHRSP were also higher than any other groups examined. Receptor-linked Ca2+ influxes of old SHRSP were smaller when thrombin or collagen was given to the platelets. Phorbol 12-myristate 13-acetate (TPA) enhanced more prominently the Ca2+ influx into old SHRSP platelets than into old WKY platelets. These results strongly suggest that the Ca2+ permeability across plasma membrane is increased in young as well as old SHRSP platelets, where the resting [Ca2+]i level is highly sustained because of an impaired Ca2+ uptake mechanism and possible enhancement of protein kinase C activity.

Platelets from spontaneously hypertensive rats exhibit decreased expression of inhibitory guanine nucleotide regulatory protein. Relation with adenylyl cyclase activity

We have recently demonstrated an enhanced expression of inhibitory guanine nucleotide regulatory protein (Gi) in the heart and aorta from spontaneously hypertensive rats (SHR) as compared with control Wistar-Kyoto (WKY) rats; this enhanced Gi expression was associated with an increased inhibition of adenylyl cyclase by inhibitory hormones and decreased stimulation of adenylyl cyclase by stimulatory hormones. In the present studies, we have determined the levels of stimulatory and inhibitory guanine nucleotide regulatory proteins (Gs and Gi, respectively) in platelets from SHR by cholera toxin- and pertussis toxin-catalyzed ADP-ribosylations, respectively, as well as by immunoblotting techniques using specific antibodies for Gs and Gi. Cholera toxin catalyzed the ADP-ribosylation of a single protein of M(r) 45,000 in rat platelets from SHR and WKY rats, and the labeling of this band was not altered in SHR as compared with WKY rats. Pertussis toxin, on the other hand, catalyzed the ADP-ribosylation of a single protein band of M(r) 41,000 in platelets from SHR and WKY rats, and unlike the response in heart and aorta, the labeling of this band was significantly decreased in SHR as compared with WKY rats. Furthermore, immunoblotting experiments using AS/7 antibody, which is specific for Gi alpha-1 and Gi alpha-2, showed a decrease in Gi alpha-2 in platelets from SHR as compared with WKY rats. In addition, the inhibitory effects of angiotensin II and atrial natriuretic factor on adenylyl cyclase and cAMP levels were completely abolished in SHR platelets, whereas the stimulatory effects of GTP, N-ethylcarboxamide adenosine, prostaglandin E1, and forskolin on adenylyl cyclase and cAMP levels were enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular conversion of angiotensin I in stroke-prone spontaneously hypertensive and Wistar-Kyoto rats

OBJECTIVE

Linkage studies have shown that the gene locus for angiotensin converting enzyme (ACE) is associated with the expression of hypertension in stroke-prone spontaneously hypertensive rats (SHRSP). We tested the hypothesis that the conversion of angiotensin I (Ang I) to angiotensin II (Ang II) in blood vessels is elevated in SHRSP.

DESIGN

We measured the conversion rate of Ang I to Ang II during one pass through an isolated resistance vessel bed. We used the same substrains of SHRSP and Wistar-Kyoto control rats (WKY) that had been employed in the earlier linkage studies.

METHODS

Isolated hindquarters from young and adult (10- to 12- and 36- to 38-week-old) rats were perfused with an artificial medium and then infused with Ang I at 0.5 and 2 pmol/ml. Ang I and II were measured with high-performance liquid chromatography and radioimmunoassay in hindquarter effluent and in blank control channels. Conversion and extraction rates were calculated from angiotensin levels in hindquarter and blank perfusion channels, respectively.

RESULTS

The conversion rates of Ang I to Ang II did not differ between SHRSP and WKY in young or in adult rats. Captopril completely abolished the formation of Ang II in all groups of rats. During infusion at the higher dose of Ang I, the extraction of Ang I was significantly decreased in SHRSP compared with WKY.

CONCLUSIONS

Our results are consistent with the notion that the metabolism of angiotensin is decreased in spontaneously hypertensive rats. However, we found no support for the hypothesis that vascular ACE is responsible for high blood pressure in SHRSP. These findings suggest that other genes close to the ACE locus or the hyperexpression of the enzyme in other areas may contribute to hypertension in these rats.

Evidence from comparative investigations that impaired platelet activation is not specific for stroke-prone spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Platelet behavior of Sprague Dawley (SD), Wistar (WI), Wistar-Kyoto (WKY), spontaneously hypertensive (SHR), and stroke-prone spontaneously hypertensive rats (SHRSP) was studied in vivo to evaluate the importance of hypertension-related hemostatic disorders.

METHODS

The study was based on the model of stimulus-induced pulmonary microembolization of labeled platelets. After injection of 51Cr-labeled homologous platelets into urethane-anesthetized rats, the organ distribution of the platelets was continuously monitored by gamma detectors. Count rates of two detectors--one placed above the animals' thoraxes (C1), the other above their abdomens (C2)-and the ratio of C1:C2 were calculated. The following platelet activators were applied intravenously: adenosine diphosphate (ADP; 50 micrograms/kg), collagen (100 micrograms/kg), and thrombin (50 IU/kg).

RESULTS

All three substances caused a reversible pulmonary accumulation of the labeled platelets and hence an increase in C1/C2 (delta C1/C2%). ADP induced a shift of 75% in SD, 52% in WI, 32% in WKY, 30% in SHR, and 31% in SHRSP. Thrombin-mediated shift was 79% in SD, 64% in WI, 58% in WKY, 48% in SHR, and 54% in SHRSP. Collagen induced a shift of 85% in SD, 96% in WI, 84% in WKY, 56% in SHR, and 62% in SHRSP.

CONCLUSIONS

Because indistinguishable results were observed in both hypertensive strains, we conclude that impaired platelet aggregation is not specific for SHRSP. Hence, it may not primarily be responsible for the increased occurrence of stroke in these animals.

[The activity and properties of K-pNPphosphatase in membrane preparations of erythrocytes with different polypeptide compositions in Wistar, Wistar-Kyoto and SHR strain rats]

A higher K-pNPPase activity was found in erythrocyte membrane preparations from the SHR as compared with the Wistar rats. Removal of peripheral proteins from the membrane skeleton depressed the K-phosphatase activity and eliminated the difference between the SHR and Wistar rats. No activating effect of the ATP and Ca on the enzyme was found. The data obtained suggest that the proteins of the erythrocyte membrane skeleton are connected with regulation of the enzyme transport in the erythrocyte membranes of the SHR via a modulating effect of intracellular ATP and Ca.

Elevation of plasma immunoglobulin A in the spontaneously hypertensive rat

Prior studies describe deficiencies of T-cell-mediated immunity in the spontaneously hypertensive rat (SHR) strain of Okamoto and Aoki. This report describes an alteration of humoral immunity: elevation of the plasma concentration of immunoglobulin (Ig) A and of circulating IgA autoantibodies to single-stranded DNA, double-stranded DNA, and thyroglobulin. The increased plasma IgA levels are evident in prehypertensive SHR, hence not secondary to the hypertension, and they result mainly from increments in polymeric IgA. Plasma IgA content also varied concordantly with the level of systolic blood pressure as influenced by age (older > younger) and gender (male > female) in both the SHR and control Wistar-Kyoto rat strains. Strain differences in plasma IgG or IgM were not observed. Studies of peripheral blood lymphocytes indicate that increased production of IgA is one mechanism for the increment in plasma content. The number of blood lymphocytes capable of producing IgA in vitro in response to the mitogen lipopolysaccharide is increased in SHR. When cultured in the absence or presence of lipopolysaccharide, peripheral blood lymphocytes of SHR secrete more IgA in vitro than do cells of the control strain. No significant strain differences in biliary or renal excretion of IgA were observed. The observed alterations of IgA in the SHR either are causative factors in the development of the hypertension or are the products of an epiphenomenon in which IgA and blood pressure are affected separately, but in parallel, by causative factors related to rat strain, age, and gender.

Increased plasma level of endothelin-1 in the Okamoto spontaneously hypertensive rat

The objectives of this study were to determine plasma levels of endothelin (ET) in a genetic model of hypertension and in control rats during control conditions and in response to short-term volume expansion with saline. Okamoto spontaneously hypertensive rats (SHR) and control Wistar-Kyoto (WKY) rats were used in this study. One group of each strain served as control animals, and another group of each strain underwent volume expansion with saline (5% of body weight infused during a period of 30 minutes). The levels of ET-1 and ET-3 were measured in plasma by using a double-antibody radioimmunoassay. In the control groups of SHR and WKY rats, plasma ET-1 levels were 72.5 +/- 14.9 pg/ml (N = 8) and 40.2 +/- 7.5 pg/ml (N = 12), respectively (P < 0.05). In the volume-expanded SHR group (N = 8), the plasma ET-1 level was 45.5 +/- 11.1 pg/ml (approximately 37% less than that of the control SHR group), whereas it was 40.6 +/- 10.2 pg/ml in the volume-expanded group of WKY rats (N = 10) (almost identical to that of the control WKY group). Plasma levels of ET-3 were similar in control and in volume-expanded groups of SHR and WKY rats. These data show that basal levels of plasma ET-1 are significantly higher in the SHR than in the WKY rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin resistance in the conscious spontaneously hypertensive rat: euglycemic hyperinsulinemic clamp study

To determine whether spontaneously hypertensive rats (SHR) are insulin resistant when compared with their genetic control, Wistar-Kyoto rats (WKY), insulin-stimulated glucose utilization was studied in both strains with the euglycemic hyperinsulinemic clamp technique. This methodology can determine if insulin resistance is present and whether it is due to ineffective stimulation of peripheral glucose utilization, or to incomplete suppression of (hepatic) endogenous glucose production (EGP) by insulin, or both. Twelve WKY and 15 SHR (all male) had long-term catheters surgically placed. After surgical recovery, fasting metabolic parameters were measured in the conscious, unstressed state. Clamp studies were then performed on nine WKY and eight SHR. EGP was measured before and during euglycemic hyperinsulinemia with the tracer-dilution technique (6-3H-glucose). Indices of fasting metabolism (plasma glucose, insulin, and hepatic EGP) were not different between WKY and SHR. During the clamp studies, the glucose infusion rate (GIR) required to maintain euglycemia was significantly lower in SHR (SHR, 0.055 +/- 0.003 v WKY, 0.106 +/- 0.001 mmol/kg.min-1; P < .001). EGP was completely suppressed during euglycemic hyperinsulinemia in all WKY and in six of eight SHR. We conclude that conscious, nonstressed SHR are insulin resistant when compared with WKY. Attenuated insulin-stimulated peripheral glucose utilization implicates skeletal muscle, and not liver, as the primary site of insulin resistance in SHR.

Calcium suppresses central angiotensin II pressor response less in SHR

To determine whether calcium alters central cardiovascular regulation, cardiovascular responses to intracerebroventricular (ICV) injection of calcium were recorded in conscious Wistar rats. Calcium injection consistently produced dose-dependent decreases in mean parterial pressure and heart rate. Pretreatment with a calcium channel blocker, diltiazem, attenuated cardiovascular responses to calcium. Decreases in plasma norepinephrine indicated the contribution of sympatho-inhibition to vasodepression by calcium. Preceding calcium injection reduced pressor responses to ICV-injected angiotensin II. These findings suggest that there is a pharmacological interaction between calcium and angiotensin II in the central nervous system. In spontaneously hypertensive rats (SHR), cardiovascular responses to calcium was larger than Wistar Kyoto rats (WKY). By contrast, calcium reduced pressor responses to angiotensin II only in WKY but not in SHR. Because the central interaction between calcium and angiotensin II has been different in SHR, our results imply that this difference may be related to the maintenance of high blood pressure in SHR.

Effect of a diet rich in linseed oil on complex viscosity and blood pressure in spontaneously hypertensive rats (SHR)

The influence of a diet rich in linseed oil (10% in weight) with a content of 61.2% of alpha-linolenic fatty acid on blood pressure and complex blood viscosity was investigated in spontaneously hypertensive rats. A decrease in blood pressure by 59 mm Hg was found compared to the age-matched, untreated control group. The viscous (eta') and elastic (eta") components of viscosity were also reduced at various shear rates (gamma). The same applies to the aggregation index, which is a measure of the aggregation tendency of red blood cells (RBC). Of course, the effects on blood viscosity cannot explain the observed degree of blood-pressure-lowering. Nevertheless, a decrease in the viscosity can improve the flow conditions in the microcirculation, which may lead to a better oxygen supply.

Inverse changes in erythroid cell volume and number regulate the hematocrit in newborn genetically hypertensive rats

Erythrocytosis and microcytosis have been described in strains of genetically hypertensive rats and in essentially hypertensive humans. Published discussion of these phenomena has centered around their relationship to observed alterations in ionic transport and the pathogenesis of hypertension. In presenting data for another strain of spontaneously hypertensive rats in which these findings are exhibited, we note that erythroid cell size decreases concurrently with the increase in cell numbers so that the hematocrit and the mean corpuscular hemoglobin concentration remain constant. Data from the literature support the hypothesis that erythroid cell size is inversely proportional to cell count in a large number of species. Erythrocytosis, as it develops in the neonatal rat, is a consequence of the marked immaturity of this species at birth. Erythrocytosis in the spontaneously hypertensive rat is not due to a difference in the affinity of its hemoglobin for oxygen or to significant tissue anorexia. Microcytosis in the spontaneously hypertensive rat is the consequence of a continuation of the linear volume decrease with age of its erythroid cells seen in the normotensive animals and may be accounted for by the production of smaller cells with concomitant regulation of individual cell volume.

A greater stimulation of Na-H antiport in cultured vascular smooth muscle cells by serum from spontaneously hypertensive rats

We investigated the influence of serum from spontaneously hypertensive rats (SHR) on the Na-H antiport and Na-K pump activity of cultured vascular smooth muscle cells (VSMC). Subjecting the VSMC to rat serum stimulated the ouabain-sensitive 86Rb uptake. This activation of the Na-K pump was significantly greater with the SHR serum than the Wistar-Kyoto rat (WKY) serum. Furthermore, the serum from SHR exhibited a larger Na uptake in the presence of ouabain than that from WKY. A greater 5-(N,N-hexamethylene) amiloride-sensitive Na uptake accounted for this difference. When the Na-K pump was active, serum from either of the two strains did not cause an increase in intracellular Na concentrations. We concluded that serum from SHR produced a more pronounced activation of both the Na-H antiport and the Na-K pump. The study suggests that there are humoral factors in SHR serum which elicit the stimulation of the Na-H antiport, leading to the activation of the Na-K pump of VSMC.

Purification of parathyroid hypertensive factor from plasma of spontaneously hypertensive rats

Parathyroid hypertensive factor (PHF) is a newly described hypertensive factor that may be related to elevation of blood pressure in 30-40% of North American essential hypertensive patients. PHF is also found in several animal models of hypertension, including spontaneously hypertensive rats, and deoxycorticosterone acetate salt hypertensive rats. Plasma collected from spontaneously hypertensive rats (SHR) was used in the present study for purification of PHF. Plasma was dialyzed at a molecular mass cutoff of 1 kDa, and then ultrafiltered at a molecular mass cutoff of 5 kDa. PHF activity, as determined by bioassay (characteristic delayed hypertensive response in normotensive rat) was retained in the fraction that was greater than 1 kDa and less than 5 kDa. Dialyzed and ultrafiltered SHR plasma was fractionated by molecular-exclusion chromatography, either with Bio-Gel P-6 liquid chromatography, or TSK 2000 SW HPLC. The biological activity was detected in a discrete region corresponding to a molecular mass of 2.5-3 kDa. When the molecular-exclusion fraction was subsequently fractionated by reverse-phase HPLC, biological activity was located in a single discrete peak, which did not occur in plasma from normotensive rats prepared in a similar manner. The biologically active fraction of PHF was inactivated by trypsin; this and its UV spectrum indicate the presence of a peptide structure.

Reduced calcium binding capacity is an intrinsic abnormality of red blood cell membrane in spontaneously hypertensive rats

Reduced calcium (Ca) binding capacity is a widespread and primary abnormality in SHR. It was reported that in red blood cell (RBC) membranes it is detectable only in membrane preparations containing sealed inside-out vesicles, the formation of which implies the partial removal of cytoskeletal proteins from the RBC membrane. The present study compares the Ca binding capacity of RBC membrane preparations with normal (+CS) and reduced (-CS) cytoskeleton content in SHR and normotensive WKY control rats. In addition to Ca binding capacity and protein content, the cholesterol content and the acetylcholinesterase (ACE) activity were measured as having a quantitative measure of integral membrane components in the different membrane preparations. In both strains the cholesterol/protein ratio, the ACE activity per mg of membrane protein, and the Ca binding capacity were all significantly higher in -CS compared to +CS membrane preparations (P less than .001). A statistically significant difference in Ca binding capacity between SHR and WKY was observed only using -CS membranes preparation. The results support the concept of a reduced membrane Ca binding capacity in rat genetic hypertension: this abnormality is detectable only in membrane preparations with reduced cytoskeletal content.

Effects of a fructose-enriched diet on plasma insulin and triglyceride concentration in SHR and WKY rats

Significant increases (P less than 0.001) in plasma insulin and triglyceride concentrations and in blood pressure were seen when SHR and WKY rats ate a fructose-enriched diet for 14 days. However, all of the changes were significantly accentuated (P less than 0.02-0.001) in SHR rats. Specifically the increment in plasma insulin concentration following the fructose-enriched diet was 42 +/- 4 microU/ml in SHR as compared to 25 +/- 4 microU/ml in WKY rats (P less than 0.001). Plasma triglyceride concentrations also increased to a greater degree in response to fructose in SHR rats (260 +/- 24 vs. 136 +/- 20 mg/dl, P less than 0.001). Finally, the fructose-induced increase in blood pressure of 29 +/- 4 mm of Hg in SHR rats was greater (P less than 0.02) than that seen in WKY rats (19 +/- 2 mm of Hg). There was no change in plasma glucose concentration in response to the fructose diet. WKY rats gained more weight than did the SHR rats. Thus, although plasma triglyceride and insulin concentration and blood pressure increased when either WKY or SHR rats consumed a fructose enriched diet, the magnitude of these changes was greater in SHR rats.

Maternal influences on sympathetic-adrenal medullary system in spontaneously hypertensive rats

The technique of reciprocal cross fostering was used to assess the influence of the maternal environment on the functioning of the sympathetic-adrenal medullary system in the spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) normotensive rats. Control, in-fostered, and cross-fostered rats were tested in adulthood to assess 1) the neural contribution to resting mean arterial blood pressure (MAP) and 2) sympathetic-adrenal medullary responses to acute footshock stress. Adult resting MAP was significantly lower in cross-fostered SHRs (139.6 mmHg) compared with control or in-fostered SHRs (162.1 and 159.3 mmHg). In addition, the decrease in MAP after sympathetic blockade (40.6 mmHg) was significantly less in cross-fostered SHRs compared with controls (50.1 mmHg). Sympathetic-adrenal medullary responses to foot-shock were greater in SHR than WKY rats; however, cross-fostered SHRs showed exaggerated responses compared with control and in-fostered SHRs. Altering the maternal environment did not produce any measurable effects on the neural contribution to resting MAP or sympathetic-adrenal medullary responsivity to acute stress in the WKY strain. These results indicate that the blood pressure-lowering effect of cross fostering in the SHR strain is caused in part by a dampening of the neural contribution to resting MAP; however, these animals retain their strain's characteristic adrenergic hyperreactivity to stressful stimulation.

LH and PRL secretion in ovariectomized spontaneously hypertensive rats

The pulsatile patterns of LH and PRL secretion, and effects of stress on these patterns were examined in ovariectomized normotensive Wistar, spontaneously hypertensive, and genetically matched normotensive Wistar Kyoto rats. Judged by the overall mean of PRL concentrations, PRL levels in spontaneously hypertensive rats were not different from those in Wistar Kyoto rats, but the pulse amplitudes as well as the overall mean concentrations were significantly greater in both Wistar Kyoto and spontaneously hypertensive rats than in Wistar rats. Increases in PRL release during immobilization stress in spontaneously hypertensive rats occurred earlier than in Wistar rats, but the peak levels were not different between the two animal groups, though significantly lower in Wistar Kyoto rats than in Wistar rats. Overall mean of LH concentrations was higher, but pulse amplitude was smaller in spontaneously hypertensive rats than in Wistar Kyoto rats, and in both groups significantly smaller than in Wistar rats. The decrease in LH release owing to stress was similar in all animal groups. A significant association between LH and PRL peaks was evident in all animals, although the rate of association was lowest in Wistar Kyoto rats. In conclusion, the central nervous system mechanisms for both LH and PRL secretion differ markedly in both spontaneously hypertensive and Wistar Kyoto rats from those in Wistar rats, and no specific difference was found in spontaneously hypertensive rats compared with Wistar Kyoto and Wistar rats as controls.

Plasma concentrations of endothelin-1 in spontaneously hypertensive rats and DOCA-salt hypertensive rats

To investigate the possible involvement of endothelin-1 (ET-1), an endothelium-derived potent vasoconstrictor peptide, in the pathophysiology of hypertension, plasma ET-1 levels in 15-week-old spontaneously hypertensive rats (SHR) and DOCA-salt hypertensive rats were measured with a sandwich-type enzyme immunoassay. The vasocontractile effect of ET-1 in aortic helical preparations was significantly more sensitive in DOCA-salt hypertensive rats than in control sham-operated rats, but plasma levels of ET-1 did not differ between them. Plasma ET-1 levels in genetically hypertensive rats (SHR and stroke-prone SHR) were significantly lower than those in age-matched normotensive Wistar-Kyoto (WKY) rats. The plasma concentrations of big ET-1, a precursor of ET-1, in both SHR and SHR-SP were significantly lower than those of WKY, suggesting that the production of ET-1 is decreased in rats of genetic hypertension. Although the vascular reactivity to ET-1 increased in both DOCA-salt hypertensive and genetically hypertensive rats, present findings of the plasma ET-1 levels suggest that the role of ET-1 in the vascular control system may be different in DOCA-salt hypertensive rats and genetically hypertensive rats.

Chronic antihypertensive drug treatment decreases protein kinase C activity in platelets from SHR

There is evidence that protein kinase C activity in platelets from adult SHR is significantly higher than this activity in age-matched WKY. In the present study, protein kinase C activity in the SHR was measured following antihypertensive drug treatment. Chronic administration of enalapril to SHR for 2 weeks decreased both systolic blood pressure and protein kinase C activity to the levels seen in the WKY. Similar results were obtained in case of chronic treatment of SHR with hydralazine or nifedipine. These results suggest that enhanced protein kinase C activity of SHR can be suppressed by lowering blood pressure by antihypertensive drugs.

Plasma and pituitary thyroid-stimulating hormone and their responses to bromocriptine in the Japanese strains of hypertensive and normotensive rats and the F2 generation of the hybrid

Plasma thyroid-stimulating hormone (TSH) concentration and pituitary TSH content were examined in bromocriptine (BRC)- and vehicle-treated male Spontaneously Hypertensive Rat (SHR), its normotensive control, the Wistar-Kyoto (WKY) rat and the F2 generation of the SHR/WKY hybrid. The hybrid rats were divided into three groups according to their systolic blood pressures (BP) as follows--group I: BP less than 140 mmHg; group II: BP = 140-160 mmHg and group III: BP greater than 160 mmHg. Plasma TSH concentrations were not significantly different between the vehicle-treated SHR and WKY nor between the three groups of vehicle-treated hybrid rats. The pituitary TSH content was significantly higher in the vehicle-treated SHR than in the vehicle-treated WKY while it was not significantly different between the three groups of vehicle-treated hybrid rats. Plasma TSH concentrations in the BRC-treated SHR, WKY and three groups of hybrid rats were significantly lower than the concentrations in the corresponding vehicle-treated rats. Pituitary TSH content were significantly lower in the BRC-treated SHR, group I and group III rats but not in the BRC-treated WKY and group II rats when compared to their respective vehicle-treated controls. The results of this study suggest that TSH is not involved in the maintenance of systolic BP in the SHR, WKY and hybrid rats.

Parathyroid origin of a new circulating hypertensive factor in spontaneously hypertensive rats

The parathyroid gland has been causally associated with some forms of hypertension, yet parathyroid hormone (PTH) has been shown to possess hypotensive properties. Recently, we described the presence of a circulating hypertensive factor in spontaneously hypertensive rat (SHR) plasma, the expression of which is associated with a novel cell type in the parathyroid gland. In order to determine whether this hypertensive factor might be produced by the parathyroid gland, the effects of parathyroidectomy and parathyroid transplantation on mean arterial pressure (MAP) and hypertensive factor activity were studied. Parathyroidectomy, but not sham operation, decreased MAP in SHR and resulted in the disappearance of the hypertensive factor from the plasma. Transplantation of SHR parathyroid glands into Sprague-Dawley (SD) rats resulted in an increase in MAP and the appearance of the factor in the plasma, whereas transplantation of SD parathyroid glands into SHR had exactly the opposite effect. SD to SD transplantation had no effect on MAP or hypertensive factor activity. Infusions of PTH gave opposite effects to hypertensive factor infusions. These results suggest that the hypertensive factor is parathyroid gland-dependent but is not PTH. On this basis, we have proposed that the factor be referred to as "Parathyroid Hypertensive Factor," or "PHF."

Taurine lowers blood pressure in the spontaneously hypertensive rat by a catecholamine independent mechanism

Taurine, a sulfur aminoacid, has been studied for a role in blood pressure regulation since it functions as a generalized inhibitory neurotransmitter and is found in high concentrations in the myocardium. We reinvestigated the magnitude of the hypotensive effect of chronic taurine administration to the spontaneously hypertensive rat (SHR) and the role of catecholamines in such an action. The SHR received either a 1% taurine solution or tap water to drink for 16 weeks. Taurine treatment caused a significant persistent reduction in blood pressure by 4 weeks that was maximal at 16 weeks (146 +/- 6 [exp.] v 182 +/- 5 [control] mm Hg, P less than .01). While this taurine-induced decline in blood pressure in the SHR was not accompanied by alterations in plasma epinephrine levels, there was a steady 235% increment in the norepinephrine concentration from 231 +/- 31 pg/mL initially to 542 +/- 126 pg/mL at completion of the study, P less than .02. The reduction in blood pressure was associated with decreased proteinuria in the taurine-treated SHR (9.6 +/- 4 [exp.] v 21.5 +/- 7 [control] mg/24 h, P less than .02) and less cardiac and renal hypertrophy. We conclude that taurine administration results in a 20 to 25% reduction in blood pressure in the SHR. The mechanism of this hypotensive action requires further study but is independent of changes in plasma catecholamine levels. The vasodepressor effect of taurine leads to less hypertensive injury to the kidney and heart in the SHR.

Congenital changes of platelet functions in stroke-prone SHR: aggregability of gel-filtered platelets, PRP and whole blood, and effects of hypotensive treatment

Platelet aggregation in whole blood, platelet rich plasma, and gel-filtered platelets were markedly attenuated in SHRSP compared with those in age-matched normotensive WKY. The result was consistent with the previous report of washed platelets. Despite prevention of high blood pressure, a long duration of hypotensive treatment only slightly improved aggregability of washed platelets but did not restore it to the range of age-matched WKY platelets. Blood pressure, heart ratios and thrombin-induced washed platelet aggregation were examined in SHRSP, WKY, and the cross (F1: WKY x SHRSP). The higher blood pressure and heart ratios the lower platelet aggregability was observed in the three strains, and there was no overlapping distribution of these values. F1 progeny exhibited intermediate values in blood pressure, heart ratio and platelet aggregability between the parental values. These results suggested that hypofunctions of SHRSP platelet were not secondary changes due to high blood pressure, but primary changes which are genetically linked to high blood pressure.

Impaired reflex response to volume expansion in NaCl-sensitive spontaneously hypertensive rats

Abnormal baroreceptor reflex function that antedates or is a consequence of NaCl loading could contribute to the NaCl-induced exacerbation of hypertension in NaCl-sensitive spontaneously hypertensive rats (SHR-S). The current study tested the hypothesis that an impairment in cardiopulmonary baroreceptor reflex function exists in SHR-S before NaCl loading. The reflex response to volume expansion was compared in SHR-S, NaCl-resistant SHR (SHR-R), and normotensive Wistar-Kyoto (WKY) and Sprague-Dawley rats maintained on a normal NaCl diet. Conscious, free-moving SHR-S, SHR-R, WKY, and Sprague-Dawley rats were volume expanded with whole blood to 15% of blood volume within 6 minutes, and mean arterial pressure, heart rate, and lumbar sympathetic nerve activity were recorded. Heart rate and lumbar sympathetic nerve activity decreased significantly in SHR-R, WKY, and Sprague-Dawley rats after volume expansion. In contrast, in SHR-S neither heart rate after volume expansion nor lumbar sympathetic nerve activity was significantly different from levels before volume expansion. The blunted reflex response of heart rate and lumbar sympathetic nerve activity to volume expansion suggests impaired cardiopulmonary volume receptor function in SHR-S. This likely contributes to NaCl-induced hypertension in SHR-S on a high NaCl diet.

Adenylate cyclase of platelets from spontaneously hypertensive rats: reduction of the inhibition by GTP

We have compared the effects of Gpp[NH]p on adenylate cyclase activity of platelet membranes in SHR and WKY rats. In the presence of 50 microM forskolin, low concentrations of Gpp[NH]p (0.01 to 0.3 microM) inhibited the enzyme activity in both strains, but the maximal level of inhibition was significantly lower in SHR (- 20%). In the absence of forskolin, 0.1 microM Gpp[NH]p was inhibitory only in WKY and the adenylate cyclase activity was greater in hypertensive rats at this nucleotide concentration. Increasing Gpp[NH]p from 0.1 to 3 microM induced the same increase of enzyme activity in both strains. In SHR, GTP itself induced a lower inhibition of the enzyme stimulated by 50 microM forskolin or 0.1 microM prostaglandin E1. These results suggest that the modulatory effect of the guanine nucleotide inhibitory protein on adenylate cyclase may be reduced in platelets from SHR.

Effects of chronic alcohol consumption and alcohol withdrawal on blood pressure in stroke-prone spontaneously hypertensive rats

The development of blood pressure was monitored by the tail-cuff method in normotensive (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) receiving ethanol (alcohol) in drinking water from weaning (approximately 1 month of age). Alcohol administration over a 3-month period attenuated the development of hypertension in SHRSP and also caused a small reduction of the initial blood pressure rise in WKY. This was accompanied by a reduction of fluid intake and an increase of circulating antidiuretic hormone (arginine vasopressin; AVP). Circulatory volume remained constant. Direct measurement of arterial blood pressure in conscious rats before and after autonomic blockade confirmed the antihypertensive effect of alcohol in SHRSP and indicated that it is at least partly dependent on altered activity of neural mechanisms. Sudden withdrawal of alcohol caused an immediate increase of fluid intake followed by a rise of blood pressure lasting several days in both WKY and SHRSP. This withdrawal hypertension could not be attributed to changes in plasma catecholamines or AVP.

[Na+/H+ metabolic activity in the thrombocytes of spontaneously hypertensive rats]

The sodium-proton exchange was determined in platelets of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The platelets were suspended in sodium propionate; the cytoplasmic acidification activated the exchanger and intracellular pH (the increasing) and volume of the platelets (the swelling) were registered. The activity of Na+/H+ exchange was inhibited by isopropyl amiloride. The platelets' volume and the exchange rate constant of SHR were increased on 30-40% as compared with those of WKY.

Endogenous digoxin-like material in normotensive and hypertensive strains of rats

We measured the concentration of endogenous digoxin-like materials (EDLM) in the serum of spontaneously hypertensive rats (SHR) and three normotensive rat strains at four stages during growth using a sensitive RIA. In the SHR, there was a significant peak in the EDLM level between 0.057-0.087 ngE/mL at 6 to 8 weeks of age, shortly after the onset of hypertension. The EDLM concentration returned to normal levels by 20 weeks of age. Sprague-Dawley and Wistar-Kyoto rats had EDLM levels below 0.050 ngE/mL at all time points studied. In contrast, Fischer 344 rats displayed persistently elevated serum EDLM concentrations that exceeded 0.124 ngE/mL from 3 to 20 weeks of life. We conclude that (1) there are significant interstrain differences in serum EDLM levels in rats; and (2) the SHR has a unique peak in serum EDLM levels at 6 to 8 weeks of age, indicating a possible role for the substance in the inception of hypertension.

Effects of spontaneously hypertensive rat plasma on blood pressure and tail artery calcium uptake in normotensive rats

Previous studies have described the presence of humoral hypertensive factors in spontaneously hypertensive rats (SHR). Other studies have described factors that increase calcium uptake in vascular tissue. In this study, we attempted to confirm, and thereby correlate, the presence of both types of factors in SHR plasma. Intravenous infusion or bolus administration of dialyzed SHR plasma consistently induced an increase in blood pressure in normotensive rats. This hypertensive response was somewhat delayed, with peak blood pressures occurring 45 minutes after bolus administration and 90 minutes after infusion of SHR plasma. Spontaneously hypertensive rat plasma also increased 45Ca uptake in isolated normotensive rat tail arteries in a dose-dependent manner, with a time course similar to that for the hypertensive response to bolus administration. These findings suggest, therefore, that a substance exists in SHR plasma that can increase intracellular calcium in vascular tissues and thereby increase blood pressure.

Influence of parathyroidectomy on blood pressure and vascular reactivity in spontaneously hypertensive rats

We investigated the influence of parathyroidectomy (PTX) on blood pressure (BP) and hindlimb vascular reactivity to noradrenaline (NA) and vasopressin (AVP) in male spontaneously hypertensive (SHR) and normotensive Wistar Kyoto rats (WKR). Three groups of SHR and WKR, respectively, were investigated: Sham-operated (SO) rats on a normal calcium intake (0.95%), SO rats on moderately elevated calcium intake (1.6% calcium diet) and PTX rats on the 1.6% calcium diet. At the end of the experiment (3 months), directly or indirectly measured BP was significantly lower in the PTX-SHR group on the 1.6% calcium diet than in SO-SHR on the same diet. In WKR groups, no changes of BP were recorded. Hindlimb perfusion with oxygenated Tyrode's solution for cumulative dose response curves with NA (0.1-1000 x 10(-6) M) and AVP (0.5-500 x 10(-9) M) showed no differences between PTX and SO groups. Maximal pressures and ED50 for agents used were significantly higher in SHR than WKR groups (p less than 0.05). The results support the hypothesis that the parathyroid glands contribute to high blood pressure in SHR. However, the antihypertensive action of PTX was not mediated by a change in hindlimb vascular reactivity.

Glucose tolerance and insulin secretion in conscious and unrestrained normotensive and spontaneously hypertensive rats

We compared the glucose tolerance and insulin responses to intravenous (IV) glucose administration of a dose of 1 g/kg body weight in a conscious and unrestrained state of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) with catheters chronically indwelled into artery and vein. Both plasma glucose levels at two minutes and ten minutes following IV glucose load as well as the incremental and total areas of plasma glucose were slightly but significantly lower in SHR than in WKY. Glucose disappearance rate (K value) was 7.7 +/- 0.3%/min in SHR, being slightly but significantly higher than that of 6.8 +/- 0.3%/min in WKY. On the other hand, insulin responses to the glucose load at ten minutes and 30 minutes as well as incremental and total insulin areas were significantly lower in SHR than in WKY. There was no significant difference in insulinogenic index between SHR and WKY. Our observations suggest that in a conscious and unrestrained state, SHR have the greater glucose tolerance associated with reduced insulin secretion than do WKY.

Erythrocytosis in spontaneously hypertensive rats

Spontaneously hypertensive rats (SH) with an increased number of red blood cells (RBC), microcytosis, and normal hemoglobin (Hb) concentration were used to study the effect of different manipulations of the erythron on erythropoietin production and on erythroid progenitor proliferation by bone marrow cells in order to gain insight regarding the regulation of erythropoiesis. The serum erythropoietin (Ep) level was increased in untreated SH rats. After stimulation by either bleeding, hemolysis, or acute hypoxia, both the erythropoietin level and erythroid colony-forming unit (CFU-E) proliferation by bone marrow cells increased in SH rats to levels that were similar to those of normotensive Wistar (W) rats. Exposure to chronic hypoxia induced an increase in Hb concentration in SH rats concomitantly with the increase in RBC. The results obtained in SH rats raise the possibility of a defect in nonEp stimulators of erythropoiesis that may alter Hb synthesis.

Calcium transport and vitamin D in three breeds of spontaneously hypertensive rats

Conflicting results have been published by different laboratories comparing the rate of intestinal calcium transport and concentration of circulating 1,25-dihydroxyvitamin D (1,25-[OH]2D) in the spontaneously hypertensive rat (SHR) and control Wistar-Kyoto rat (WKY): They have been reported to be greater, the same, or lower in the SHR than in the WKY. We tested the possibility that the conflicting results might be breeder-related by measuring 1) the rate of intestinal mucosal calcium transport, 2) the concentration of circulating 25-hydroxyvitamin D (25-OH-D) and 1,25-(OH)2D, and 3) the concentration of intestinal mucosal receptor for 1,25-(OH)2D in the two strains of animals from three different breeders. Sodium and water transport were also measured because of their relevance to hypertension. Blood pressure was always higher and calcium, as well as mean sodium and water transport, was always lower in the SHR than in the WKY. The concentration of 1,25-(OH)2D was the same, higher, or lower in the SHR than in the WKY and was age- and breeder-dependent. Mean mucosal 1,25-(OH)2D receptor concentration was higher in the SHR and was variable, depending on breeder. We conclude that 1) the rate of calcium transport is lower in the SHR than in the WKY and independent of breeder and concentration of 1,25-(OH)2D in serum, 2) the variability in 1,25-(OH)2D concentration among investigators may be breeder-dependent, and 3) the higher receptor concentration in the intestinal mucosa of the SHR could be a compensatory response to the decreased rate of calcium transport. These differences in calcium and sodium transport may be an expression in the enterocyte of factors etiological for hypertension.