Nifedipine-sensitive noradrenergic vasoconstriction is enhanced in spontaneously hypertensive rats: the influence of chronic captopril treatment

The new quinazoline derivative (N2-methyl-N4-[(thiophen-2-yl)methyl]quinazoline-2,4-diamine) vasodilates isolated mesenteric arteries through endothelium-independent mechanisms and has acute hypotensive effects in Wistar rats

During the screening of new N2,N4-disubstituted quinazoline 2,4-diamines as phosphodiesterase-5 inhibitors and pulmonary artery vasodilators, one N2-methyl-N4-[(thiophen-2-yl)methyl]quinazoline-2,4-diamine (compound 8) presented a greater selectivity for systemic than pulmonary vasculature. The present study aimed to characterize its vasorelaxant and hypotensive effects in Wistar rats. Vasorelaxant effects of compound 8 and underlying mechanisms were evaluated on isolated mesenteric arteries. Acute hypotensive effect was evaluated in anesthetized rats. Additionally, cell viability and cytochrome P450 (CYP) activities were studied in rat isolated hepatocytes. Nifedipine was used as a comparator. Compound 8 induced a strong vasorelaxant effect, similar to nifedipine. This was unaffected by endothelium removal but was decreased by inhibitors of guanylate cyclase (ODQ) and KCa channel (iberiotoxin). Compound 8 enhanced sodium nitroprusside-induced relaxation, but inhibited vasoconstriction evoked by α1-adrenergic receptor activation and extracellular Ca2+ influx via receptor-operated Ca2+ channels. Acute intravenous infusion of compound 8 (0.05 and 0.1 mg/kg) produced hypotension. It showed similar potency to nifedipine for lowering diastolic and mean arterial blood pressure, but less so for the effect on systolic blood pressure. Compound 8 had no effect on hepatocyte viability and CYP activities except at high concentration (10 μM) at which a weak inhibitory effect on CYP1A and 3A was observed. In conclusion, this study identified a N2-methyl-N4-[(thiophen-2-yl)methyl]quinazoline-2,4-diamine with a potent vasodilator effect on resistance vessels, leading to an acute hypotensive effect and a low risk of liver toxicity or drug-drug interactions. These vascular effects were mediated mainly through sGC/cGMP pathway, opening of KCa channels, and inhibition of calcium entry.

How Do Young and Old Spontaneously Hypertensive Rats Respond to Antihypertensive Therapy? Comparative Studies on the Effects of Combined Captopril and Nifedipine Treatment

Numerous studies on the effects of antihypertensive treatment in young spontaneously hypertensive rats (SHRs) have shown that early-onset therapy may effectively reduce their blood pressure (BP) even to normotensive values. In contrast, only a few studies investigated the effects of treatment started at an advanced age. These studies revealed that antihypertensive effects are lower in adult or even in senescent SHRs compared with young SHRs. Even more, prevention of cardiac sequelae of hypertension such as hypertrophy and fibrosis is less effective when treatment starts late in life. Because, in patients, combination therapies with calcium antagonists are favored, we studied the efficacy of a combination therapy with captopril and nifedipine in young and old SHRs. We directly compared the treatment effects on BP as well as on cardiac hypertrophy and remodeling between these two animal cohorts. With antihypertensive treatment, significantly lower BP values were achieved in young SHRs despite a shorter treatment period compared with old SHRs. Although treatment effects on cardiac hypertrophy were greater in old than in young SHRs, cardiac fibrosis was significantly attenuated only in young but not in old SHRs. The results emphasize the value of antihypertensive therapy and particularly accentuate the importance of an early-onset therapy. With respect to problems such as late diagnosis and poor therapy adherence, these results may have great importance for the treatment of human hypertension.

How Effective Is a Late-Onset Antihypertensive Treatment? Studies with Captopril as Monotherapy and in Combination with Nifedipine in Old Spontaneously Hypertensive Rats

Background: A major problem in the treatment of human hypertension is the late diagnosis of hypertension and, hence, the delayed start of treatment. Very often, hypertension has existed for a long time and cardiac damage has already developed. Therefore, we tested whether late-onset antihypertensive treatment is effective in lowering blood pressure (BP) and in reducing or even preventing left ventricular hypertrophy and fibrosis. Methods: Twenty-one male 60-week-old spontaneously hypertensive rats (SHR) were included. Fourteen rats received oral treatment with captopril (CAP) either as monotherapy or combined with nifedipine (CAP + NIF) over 22 weeks. Seven untreated SHR served as controls. We examined the therapeutic effects on BP, heart weight and histological and biochemical markers of left ventricular remodeling and fibrosis. Results: At 82 weeks of age, BP was reduced in the CAP and CAP + NIF groups by 44 and 51 mmHg, respectively (p < 0.001), but not in untreated controls. Despite the late therapy start, cardiac hypertrophy and fibrosis were attenuated compared to controls. Both treatments reduced heart weight by 1.2 mg/g (25%, p = 0.001) and collagens I and III by 66% and 60%, respectively (p < 0.001), thus proving nearly equivalent cardioprotective efficacy. Conclusion: These data clearly emphasize the benefit of antihypertensive treatment in reducing BP and mitigating the development of cardiac damage even when treatment is started late in life.

Antihypertensive and cardioprotective effects of different monotherapies and combination therapies in young spontaneously hypertensive rats - A pilot study

Spontaneously hypertensive rats (SHR) are an established animal model for antihypertensive treatment. The aim of this pilot study was a systematic search for two lines of antihypertensive treatment - a monotherapy and a combination of two drugs - to be applied in a future study on old SHR. Originally, representatives of three drug classes recommended for antihypertensive therapy in humans should be applied, namely captopril (CAP) as an antagonist of the renin-angiotensin-aldosterone system, nifedipine (NIF) as calcium channel blocker and propranolol (PROP) as β-adrenergic blocker. As we observed that PROP had been poorly ingested, all groups with PROP therapy were excluded from the study. CAP (60 mg kg^-1 d^-1), NIF (10 mg kg^-1 d^-1) or both were administered orally to seven-week-old SHR over 3 weeks. A further group of SHR received no treatment (SHR/CTRL). Age-matched normotensive Wistar-Kyoto rats served as normotensive controls. We examined the effect of the antihypertensive therapies on systolic blood pressure, heart weight and on histological and biochemical markers of cardiac hypertrophy and fibrosis. CAP proved to be the most effective treatment reducing blood pressure and relative heart weight significantly compared to SHR/CTRL without reaching normotensive values. Beginning cardiac fibrosis observed in SHR/CTRL was completely abrogated with CAP treatment. Similar effects were achieved with a combination of CAP and NIF. CAP as monotherapy and CAP + NIF as combination therapy were chosen for the forthcoming study on old SHR.

Cooperation of augmented calcium sensitization and increased calcium entry contributes to high blood pressure in salt-sensitive Dahl rats

Salt hypertensive Dahl rats are characterized by sympathoexcitation and relative NO deficiency. We tested the hypothesis that the increased blood pressure (BP) response to fasudil in salt hypertensive Dahl rats is due to augmented calcium sensitization in the salt-sensitive strain and/or due to their decreased baroreflex efficiency. BP reduction after acute administration of nifedipine (an L-type voltage-dependent calcium channel blocker) or fasudil (a Rho kinase inhibitor) was studied in conscious intact rats and in rats subjected to acute NO synthase inhibition or combined blockade of the renin-angiotensin system (captopril), sympathetic nervous system (pentolinium), and NO synthase (L-NAME). Intact salt-sensitive (SS) Dahl rats fed a low-salt diet had greater BP responses to nifedipine (-31 ± 6 mmHg) or fasudil (-34 ± 7 mmHg) than salt-resistant (SR) Dahl rats (-16 ± 4 and -17 ± 2 mmHg, respectively), and a high-salt intake augmented the BP response only in SS rats. These BP responses were doubled after acute NO synthase inhibition, indicating that endogenous NO attenuates both calcium entry and calcium sensitization. Additional pentolinium administration, which minimized sympathetic compensation for the drug-induced BP reduction, magnified the BP responses to nifedipine or fasudil in all groups except for salt hypertensive SS rats due to their lower baroreflex efficiency. The BP response to the calcium channel blocker nifedipine can distinguish SS and SR rats even after calcium sensitization inhibition by fasudil, which was not seen when fasudil was administered to nifedipine-pretreated rats. Thus, enhanced calcium entry (potentiated by sympathoexcitation) in salt hypertensive Dahl rats is the abnormality that is essential for their BP increase, which was further augmented by increased calcium sensitization in salt-sensitive Dahl rats.

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Basal and Activated Calcium Sensitization Mediated by RhoA/Rho Kinase Pathway in Rats with Genetic and Salt Hypertension

Calcium sensitization mediated by RhoA/Rho kinase pathway can be evaluated either in the absence (basal calcium sensitization) or in the presence of endogenous vasoconstrictor systems (activated calcium sensitization). Our aim was to compare basal and activated calcium sensitization in three forms of experimental hypertension with increased sympathetic tone and enhanced calcium entry—spontaneously hypertensive rats (SHR), heterozygous Ren-2 transgenic rats (TGR), and salt hypertensive Dahl rats. Activated calcium sensitization was determined as blood pressure reduction induced by acute administration of Rho kinase inhibitor fasudil in conscious rats with intact sympathetic nervous system (SNS) and renin-angiotensin system (RAS). Basal calcium sensitization was studied as fasudil-dependent difference in blood pressure response to calcium channel opener BAY K8644 in rats subjected to RAS and SNS blockade. Calcium sensitization was also estimated from reduced development of isolated artery contraction by Rho kinase inhibitor Y-27632. Activated calcium sensitization was enhanced in all three hypertensive models (due to the hyperactivity of vasoconstrictor systems). In contrast, basal calcium sensitization was reduced in SHR and TGR relative to their controls, whereas it was augmented in salt-sensitive Dahl rats relative to their salt-resistant controls. Similar differences in calcium sensitization were seen in femoral arteries of SHR and Dahl rats.

Acute and chronic role of nitric oxide, renin-angiotensin system and sympathetic nervous system in the modulation of calcium sensitization in Wistar rats

Principal vasoactive systems - renin-angiotensin system (RAS), sympathetic nervous system (SNS), nitric oxide (NO) and prostanoids - exert their vascular effects through the changes in calcium levels and/or calcium sensitization. To estimate a possible modulation of calcium sensitization by the above vasoactive systems, we studied the influence of acute and chronic blockade of particular vasoactive systems on blood pressure (BP) changes elicited in conscious normotensive rats by acute dose-dependent administration of Rho-kinase inhibitor fasudil. Adult male chronically cannulated Wistar rats were used throughout this study. The acute inhibition of NO synthase (NOS) by L-NAME enhanced BP response to fasudil, the effect being considerably augmented in rats deprived of endogenous SNS. The acute inhibition of prostanoid synthesis by indomethacin modified BP response to fasudil less than the acute NOS inhibition. The chronic NOS inhibition caused moderate BP elevation and a more pronounced augmentation of fasudil-induced BP changes compared to the effect of acute NOS inhibition. This indicates both short-term and long-term NO-dependent attenuation of calcium sensitization. Long-term inhibition of RAS by captopril caused a significant attenuation of BP changes elicited by fasudil. In contrast, a long-term attenuation of SNS by chronic guanethidine treatment (in youth or adulthood) had no effect on BP response to fasudil, suggesting the absence of SNS does not affect calcium sensitization in vascular smooth muscle of normotensive rats. In conclusion, renin-angiotensin system contributes to the long-term increase of calcium sensitization and its effect is counterbalanced by nitric oxide which decreases calcium sensitization in Wistar rats.

Excitation-Contraction Coupling and Excitation-Transcription Coupling in Blood Vessels: Their Possible Interactions in Hypertensive Vascular Remodeling

Vascular smooth muscle cells (VSMC) display considerable phenotype plasticity which can be studied in vivo on vascular remodeling which occurs during acute or chronic vascular injury. In differentiated cells, which represent contractile phenotype, there are characteristic rapid transient changes of intracellular Ca2+ concentration ([Ca2+]i), while the resting cytosolic [Ca2+]i concentration is low. It is mainly caused by two components of the Ca2+ signaling pathways: Ca2+ entry via L-type voltage-dependent Ca2+ channels and dynamic involvement of intracellular stores. Proliferative VSMC phenotype is characterized by long-lasting [Ca2+]i oscillations accompanied by sustained elevation of basal [Ca2+]i. During the switch from contractile to proliferative phenotype there is a general transition from voltage-dependent Ca2+ entry to voltage-independent Ca2+ entry into the cell. These changes are due to the altered gene expression which is dependent on specific transcription factors activated by various stimuli. It is an open question whether abnormal VSMC phenotype reported in rats with genetic hypertension (such as spontaneously hypertensive rats) might be partially caused by a shift from contractile to proliferative VSMC phenotype.

Pathogenetic Mechanisms of Neurogenic Pulmonary Edema

Neurogenic pulmonary edema (NPE) is a life-threatening complication of central nervous system (CNS) injuries. This review summarizes current knowledge about NPE etiology and pathophysiology with an emphasis on its experimental models, including our spinal cord compression model. NPE may develop as a result of activation of specific CNS trigger zones located in the brainstem, leading to a rapid sympathetic discharge, rise in systemic blood pressure, baroreflex-induced bradycardia, and enhanced venous return resulting in pulmonary vascular congestion characterized by interstitial edema, intra-alveolar accumulation of transudate, and intra-alveolar hemorrhages. The potential etiological role of neurotransmitter changes in NPE trigger zones leading to enhanced sympathetic nerve activity is discussed. Degree of anesthesia is a crucial determinant for the extent of NPE development in experimental models because of its influence on sympathetic nervous system activity. Sympathetic hyperactivity is based on the major activation of either ascending spinal pathways by spinal cord injury or NPE trigger zones by increased intracranial pressure. Attenuation of sympathetic nerve activity or abolition of reflex bradycardia completely prevent NPE development in our experimental model. Suggestions for future research into NPE pathogenesis as well as therapeutic potential of particular drugs and interventions are discussed.

The interaction of calcium entry and calcium sensitization in the control of vascular tone and blood pressure of normotensive and hypertensive rats

Increased systemic vascular resistance is responsible for blood pressure (BP) elevation in most forms of human or experimental hypertension. The enhanced contractility of structurally remodeled resistance arterioles is mediated by enhanced calcium entry (through L type voltage-dependent calcium channels - L-VDCC) and/or augmented calcium sensitization (mediated by RhoA/Rho kinase pathway). It is rather difficult to evaluate separately the role of these two pathways in BP control because BP response to the blockade of either pathway is always dependent on the concomitant activity of the complementary pathway. Moreover, vasoconstrictor systems enhance the activity of both pathways, while vasodilators attenuate them. The basal fasudil-sensitive calcium sensitization determined in rats deprived of endogenous renin-angiotensin system (RAS) and sympathetic nervous system (SNS) in which calcium entry was dose-dependently increased by L-VDCC opener BAY K8644, is smaller in spontaneously hypertensive rats (SHR) than in normotensive Wistar-Kyoto (WKY) rats. In contrast, if endogenous RAS and SNS were present in intact rats, fasudil caused a greater BP fall in SHR than WKY rats. Our in vivo experiments indicated that the endogenous pressor systems (RAS and SNS) augment calcium sensitization mediated by RhoA/Rho kinase pathway, whereas the endogenous vasodilator systems (such as nitric oxide) attenuate this pathway. However, the modulation of calcium entry and calcium sensitization by nitric oxide is strain-dependent because NO deficiency significantly augments low calcium entry in WKY and low calcium sensitization in SHR. Further in vivo and in vitro experiments should clarify the interrelationships between endogenous vasoactive systems and the contribution of calcium entry and/or calcium sensitization to BP maintenance in various forms of experimental hypertension.

Ca2+ sensitization and Ca2+ entry in the control of blood pressure and adrenergic vasoconstriction in conscious Wistar-Kyoto and spontaneously hypertensive rats

BACKGROUND

Calcium entry through nifedipine-sensitive L-type voltage-dependent calcium channels (L-VDCC) is augmented in spontaneously hypertensive rats (SHR) characterized by enhanced sympathetic vasoconstriction. However, the changes of calcium sensitization mediated by RhoA/Rho kinase pathway are less understood.

METHODS AND RESULTS

The participation of calcium entry and calcium sensitization in the control of blood pressure (BP) and vascular contraction was studied in SHR and normotensive Wistar-Kyoto (WKY) rats. The acute administration of fasudil (Rho kinase inhibitor) caused BP decrease which lasted longer in SHR. Fasudil also attenuated adrenergic contraction in femoral or mesenteric arteries of WKY and SHR. BP reduction elicited by fasudil in WKY was more pronounced than that induced by L-VDCC blocker nifedipine (-33±2 vs. -15±3% of baseline BP, P<0.001), whereas both inhibitors were similarly effective in SHR (-36±4 vs. -41±2%). Fasudil pretreatment also attenuated BP elevation elicited by L-VDCC agonist BAY K8644 more in WKY than in SHR (-63±4 vs. -42±5%, P<0.001), indicating reduced calcium sensitization in SHR. Moreover, fasudil pretreatment shifted norepinephrine dose-response curves to the right more in WKY than in SHR. The additional nifedipine pretreatment shifted these curves further to the right but this shift was more pronounced in SHR than in WKY. Thus adrenergic vasoconstriction is more dependent on L-VDCC in SHR and on RhoA/Rho kinase pathway in WKY rats.

CONCLUSION

Ca sensitization mediated by RhoA/Rho kinase pathway is attenuated in SHR compared with normotensive WKY rats. This might be a part of the compensation for enhanced Ca entry through L-VDCC in genetic hypertension.

Age-related alterations in endothelial function of femoral artery in young SHR and WKY rats

The present study was designed to evaluate the effects of vascular aging in juvenescence on endothelial function in femoral arteries and to assess differences between normotensive and hypertensive rats. The aim of the study was to determine if age affected nitric oxide- (NO-) mediated relaxations in normotensive and hypertensive rats. Juvenile (7-week-old) and young adult (22-week-old) male Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were used in this study. Femoral artery (FA) reactivity was determined by wire myograph and NO synthase activity by conversion of [³H]-L-arginine. During juvenescence systolic blood pressure (tail-cuff) increased significantly only in SHR, while NO synthesis decreased significantly in both strains. Endothelium-dependent relaxations to acetylcholine were reduced in the FA of SHR compared to age-matched WKY at both ages, yet these parameters were unchanged in adult rats compared with juvenile animals. The NO-dependent component of vasorelaxation was markedly reduced, whereas the NO-independent component was increased in adult compared to juvenile rats in both strains. The endothelial dysfunction in SHR at both ages was associated with reduction of NO-independent mechanisms. In conclusion, aging in early periods of life was associated with reduction of vascular NO production and bioavailability in both strains investigated. This reduction was however fully compensated by accentuation of NO-independent mechanisms.

Contribution of Ca²⁺-dependent Cl⁻ channels to norepinephrine-induced contraction of femoral artery is replaced by increasing EDCF contribution during ageing

The activation of Ca²⁺-dependent Cl⁻ channels during norepinephrine-induced contraction of vascular smooth muscle was suggested to depolarize cell membrane and to increase Ca²⁺ entry. Hypertension and ageing are associated with altered Ca²⁺ handling including possible activation of Ca²⁺-dependent Cl⁻ channels. Our study was aimed to determine Ca²⁺-dependent Cl⁻ channels contribution to norepinephrine-induced contraction during hypertension and ageing. Norepinephrine-induced concentration-response curves of femoral arteries from 6- and 12-month-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were recorded using wire myograph. Pretreatment with Ca²⁺-dependent Cl- channel inhibitor indanyloxyacetic acid 94 [R(+)-IAA-94](IAA) attenuated norepinephrine-induced contraction in all groups, but relatively more in WKY than SHR arteries. The attenuation of norepinephrine-induced contraction after Ca²⁺-dependent Cl⁻ channels blockade was partially reduced in 12-month-old WKY rats, but substantially diminished in 12-month-old SHR. IAA effect was enhanced after NO synthase inhibition but decreased by ageing. In 20-month-old WKY rats norepinephrine-induced contraction was not affected by IAA but was almost abolished after cyclooxygenase inhibition by indomethacin or niflumic acid. In conclusion, contribution of Ca²⁺-dependent Cl⁻ channels to norepinephrine-induced contraction diminished with age, hypertension development, and/or NO synthesis inhibition. Ca²⁺-dependent Cl⁻ channels are important for maintenance of normal vascular tone while their inactivation/closing might be a pathological mechanism.

Nifedipine-Sensitive Blood Pressure Component in Hypertensive Models Characterized by High Activity of Either Sympathetic Nervous System or Renin-Angiotensin System

High blood pressure (BP) of spontaneously hypertensive rats (SHR) is maintained by enhanced activity of sympathetic nervous system (SNS), whereas that of Ren-2 transgenic rats (Ren-2 TGR) by increased activity of renin-angiotensin system (RAS). However, both types of hypertension are effectively attenuated by chronic blockade of L-type voltage-dependent calcium channel (L-VDCC). The aim of our study was to evaluate whether the magnitude of BP response elicited by acute nifedipine administration is proportional to the alterations of particular vasoactive systems (SNS, RAS, NO) known to modulate L-VDCC activity. We therefore studied these relationships not only in SHR, in which mean arterial pressure was modified in a wide range of 100-210 mm Hg by chronic antihypertensive treatment (captopril or hydralazine) or its withdrawal, but also in rats with augmented RAS activity such as homozygous Ren-2 TGR, pertussis toxin-treated SHR or L-NAME-treated SHR. In all studied groups the magnitude of BP response to nifedipine was proportional to actual BP level and it closely correlated with BP changes induced by acute combined blockade of RAS and SNS. BP response to nifedipine is also closely related to the degree of relative NO deficiency. This was true for both SNS- and RAS-dependent forms of genetic hypertension, suggesting common mechanisms responsible for enhanced L-VDCC opening and/or their upregulation in hypertensive animals. In conclusions, BP response to nifedipine is proportional to the vasoconstrictor activity exerted by both SNS and RAS, indicating a key importance of these two pressor systems for actual L-VDCC opening necessary for BP maintenance.

Involvement of BKCa and KV potassium channels in cAMP-induced vasodilatation: their insufficient function in genetic hypertension

Spontaneously hypertensive rats (SHR) are characterized by enhanced sympathetic vasoconstriction, whereas their vasodilator mechanisms are relatively attenuated compared to their high BP. The objective of our in vivo study was to evaluate whether the impaired function of BKCa and/or KV channels is responsible for abnormal cAMP-induced vasodilatation in genetic hypertension. Using conscious SHR and normotensive WKY rats we have shown that under the basal conditions cAMP overproduction elicited by the infusion of beta-adrenoceptor agonist (isoprenaline) caused a more pronounced decrease of baseline blood pressure (BP) in SHR compared to WKY rats. Isoprenaline infusion prevented BP rises induced by acute NO synthase blockade in both strains and it also completely abolished the fully developed BP response to NO synthase blockade. These cAMP-induced vasodilator effects were diminished by the inhibition of either BKCa or KV channels in SHR but simultaneous blockade of both K(+) channel types was necessary in WKY rats. Under basal conditions, the vasodilator action of both K(+) channels was enhanced in SHR compared to WKY rats. However, the overall contribution of K(+) channels to cAMP-induced vasodilator mechanisms is insufficient in genetic hypertension since a concurrent activation of both K(+) channels by cAMP overproduction is necessary for the prevention of BP rise elicited by acute NO/cGMP deficiency in SHR. This might be caused by less effective activation of these K(+) channels by cAMP in SHR. In conclusion, K(+) channels seem to have higher activity in SHR, but their vasodilator action cannot match sufficiently the augmented vasoconstriction in this hypertensive strain.

The impact of four different classes of anesthetics on the mechanisms of blood pressure regulation in normotensive and spontaneously hypertensive rats

Most anesthetics induce characteristic hemodynamic changes leading to blood pressure (BP) reduction but the role of renin-angiotensin system (RAS), sympathetic nervous system (SNS) and nitric oxide (NO) synthesis in this BP reduction is unknown. We therefore studied the influence of four widely used anesthetics - pentobarbital (P), isoflurane (ISO), ketamine-xylazine (KX) and chloralose-urethane (CU) - on the participation of these vasoactive systems in BP maintenance. BP effects elicited by the acute sequential blockade of RAS (captopril), SNS (pentolinium) and NO synthase (L-NAME) were compared in conscious and anesthetized Wistar or spontaneously hypertensive rats (SHR). Except for pentobarbital all studied anesthetics evidenced by diminished BP responses to pentolinium. The absolute pentolinium-induced BP changes were always greater in SHR than Wistar rats. KX anesthesia eliminated BP response to pentolinium and considerably enhanced BP response to NO synthase inhibition in SHR. In both rat strains the anesthesia with ISO or CU augmented BP response to captopril, decreased BP response to pentolinium and attenuated BP response to NO synthase inhibition. In conclusion, pentobarbital anesthesia had a modest influence on BP level and its maintenance by the above vasoactive systems. Isoflurane and chloralose-urethane anesthesia may be used in cardiovascular experiments if substantial BP decrease due to altered contribution of RAS, SNS and NO to BP regulation does not interfere with the respective research aim. Major BP reduction (namely in SHR) due to a complete SNS absence is a major drawback of ketamine-xylazine anesthesia.

Endothelial dysfunction in femoral artery of the hypertensive rats is nitric oxide independent

This study examined nitric oxide (NO) production, oxidative load and endothelium-dependent relaxation (NO-dependent and NO-independent) in adult male borderline hypertensive (BHR) and spontaneously hypertensive (SHR) rats as compared to normotensive Wistar-Kyoto (WKY) rats. Systolic blood pressure (BP) was determined by tail-cuff. NO production was determined by conversion of [(3)H]-L-arginine. Conjugated dienes (CD) and concentrations of thiobarbituric acid-reactive substances (TBARS) were measured for assessment of oxidative load. Vascular function was investigated in rings of the femoral artery (FA) using a wire myograph. BP of WKY, BHR and SHR was 106+/-2, 143+/-3 and 191+/-3 mm Hg, respectively (p<0.01 for each). Significant left ventricle (LV) hypertrophy and elevated levels of CD and TBARS in the LV were present in BHR and SHR as compared to WKY. NO production was elevated significantly in the aorta of BHR and SHR vs. WKY as well as in the LV of SHR vs. WKY. Acetylcholine (ACh)-induced relaxation of the FA was reduced significantly in both BHR and SHR vs. WKY. The NO-dependent component of ACh-induced relaxation had increasing tendency in hypertensive groups and it correlated positively with BP. The NO-independent component of vasorelaxation was reduced significantly in BHR and SHR vs. WKY and it correlated negatively with BP. In conclusion, the results showed that endothelial dysfunction in the experimental model of borderline hypertensive and hypertensive rats is NO-independent. The results suggest that borderline hypertension represents a risk of other cardiovascular disorders which is qualitatively similar to that of fully developed hypertension.

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Chronic endothelin A receptor blockade attenuates contribution of sympathetic nervous system to salt hypertension development in adult but not in young Dahl rats

AIM

Endothelin-1 (ET-1) plays an important role in the pathogenesis of salt-dependent forms of hypertension in adult rats, but its participation in salt hypertension elicited in immature rats is still unknown. Therefore, we compared ET-1 role in the development or the maintenance of salt hypertension induced in young (4-week-old) or adult (12-week-old) Dahl rats.

METHODS

The contribution of pressor ET-1 effects to the maintenance of high blood pressure (BP) was studied using acute injection of ET(A) receptor antagonist ambrisentan (BSF 208075, 1 mg kg(-1) iv) to young or adult rats with established salt hypertension. Furthermore, using chronic ambrisentan treatment (30 mg kg(-1) day(-1) in the drinking fluid during 5 weeks of high salt intake), we investigated the age-dependent involvement of ET(A) receptors in salt hypertension development in these two age groups.

RESULTS

Acute ET(A) receptor blockade lowered BP in both age groups of salt hypertensive Dahl rats more than in rats fed a low-salt diet (but without any age-dependent difference). Chronic ET(A) receptor blockade strongly attenuated the development of salt hypertension and cardiac hypertrophy in adult rats, but it had no significant effects on salt hypertension in young animals. Pronounced BP reduction induced in adult salt hypertensive rats by chronic ambrisentan treatment was attributed to attenuated sympathetic BP component, without changes in nitric oxide (NO)-dependent BP regulation. In contrast, chronic ambrisentan treatment of young animals did not modify sympathetic BP component but substantially attenuated NO-dependent vasodilatation.

CONCLUSIONS

ET(A) receptor-mediated ET-1 effects play an important role in salt hypertension of adult but not young Dahl rats.

Blood pressure reducing effects of Phalaris canariensis in normotensive and spontaneously hypertensive rats

The birdseed Phalaris canariensis (Pc) is popularly used as an antihypertensive agent. The aqueous extract of Pc (AEPc) was administered in adult normotensive Wistar rats and spontaneously hypertensive rats (SHR) and in prehypertensive young SHR (SHR(Y), 3 weeks old). Animals received AEPc (400 mg·kg(-1)·day(-1), by gavage) for 30 days, then groups were divided into 2 subgroups: one was treated for another 30 days and the other received water instead of AEPc for 30 days. AEPc reduced systolic blood pressure (SBP) in both adult groups; however, treatment interruption was followed by a gradual return of the SBP to baseline levels. SHR(Y) became hypertensive 30 days after weaning. AEPc minimized the increase in SBP in SHR(Y), but blood pressure rose to levels similar to those in the untreated group with treatment interruption. There were no changes in renal function, diuresis, or Na(+) excretion. Pc is rich in tryptophan, and the inhibition of the metabolism of tryptophan to kynurenine, a potential vasodilator factor, prevented the blood pressure reducing effect of AEPc. Moreover, AEPc significantly reduced sympathoexcitation. Data indicate that the metabolic derivative of tryptophan, kynurenine, may be a mediator of the volume-independent antihypertensive effect of Pc, which was at least in part mediated by suppression of the sympathetic tonus.

The role of sympathetic nervous system in the development of neurogenic pulmonary edema in spinal cord-injured rats

The pronounced activation of sympathetic nervous system is a necessary prerequisite for the development of neurogenic pulmonary edema (NPE) in rats with balloon compression of spinal cord. In this study we examined whether this is a consequence of rapid activation of spinal pathways leading to sympathetic venoconstriction, blood pressure rise, and reflex bradycardia. We found that NPE development can be prevented by epidural upper thoracic anesthesia or by transection of the upper spinal cord. This indicates an important role of spinal pathways activation. NPE development can also be prevented by moderate blood loss, supporting the role of blood redistribution to pulmonary circulation. In rats developing NPE the catecholamine surge following spinal cord compression involved not only a dramatic increase of circulating norepinephrine but also of epinephrine levels. The pretreatment of rats with α-1 adrenoceptor blocker prazosin, α-2 adrenoceptor blocker yohimbine, or calcium channel blocker nifedipine prevented NPE development, whereas the effect of β-adrenoceptor blockade with propranolol was less convincing. In conclusion, considerable activation of thoracic spinal pathways, followed by marked catecholamine secretion, play a major role in the development of NPE in spinal cord-injured rats. Enhanced α-adrenergic nifedipine-sensitive vasoconstriction is responsible for observed blood pressure changes, subsequent baroreflex bradycardia, and blood volume redistribution, which represent major pathogenetic mechanisms of NPE development.

Altered neural and vascular mechanisms in hypertension

Essential hypertension is a multifactorial disorder which belongs to the main risk factors responsible for renal and cardiovascular complications. This review is focused on the experimental research of neural and vascular mechanisms involved in the high blood pressure control. The attention is paid to the abnormalities in the regulation of sympathetic nervous system activity and adrenoceptor alterations as well as the changes of membrane and intracellular processes in the vascular smooth muscle cells of spontaneously hypertensive rats. These abnormalities lead to increased vascular tone arising from altered regulation of calcium influx through L-VDCC channels, which has a crucial role for excitation-contraction coupling, as well as for so-called "calcium sensitization" mediated by the RhoA/Rho-kinase pathway. Regulation of both pathways is dependent on the complex interplay of various vasodilator and vasoconstrictor stimuli. Two major antagonistic players in the regulation of blood pressure, i.e. sympathetic nervous system (by stimulation of adrenoceptors coupled to stimulatory and inhibitory G proteins) and nitric oxide (by cGMP signaling pathway), elicit their actions via the control of calcium influx through L-VDCC. However, L-type calcium current can also be regulated by the changes in membrane potential elicited by the activation of potassium channels, the impaired function of which was detected in hypertensive animals. The dominant role of enhanced calcium influx in the pathogenesis of high blood pressure of genetically hypertensive animals is confirmed not only by therapeutic efficacy of calcium antagonists but especially by the absence of hypertension in animals in which L-type calcium current was diminished by pertussis toxin-induced inactivation of inhibitory G proteins. Although there is considerable information on the complex neural and vascular alterations in rats with established hypertension, the detailed description of their appearance during the induction of hypertension is still missing.

Preventive dietary potassium supplementation in young salt-sensitive Dahl rats attenuates development of salt hypertension by decreasing sympathetic vasoconstriction

AIM

Increased potassium intake attenuates the development of salt-dependent hypertension, but the detailed mechanisms of blood pressure (BP) reduction are still unclear. The aims of our study were (i) to elucidate these mechanisms, (ii) to compare preventive potassium effects in immature and adult animals and (iii) to evaluate the therapeutic effects of dietary potassium supplementation in rats with established salt hypertension.

METHODS

  Young (4-week-old) and adult (24-week-old) female salt-sensitive Dahl rats were fed a high-salt diet (5% NaCl) or a high-salt diet supplemented with 3% KCl for 5 weeks. The participation of vasoconstrictor (renin-angiotensin and sympathetic nervous systems) and vasodilator systems [prostanoids, Ca(2+) -activated K(+) channels, nitric oxide (NO)] was evaluated using a sequential blockade of these systems.

RESULTS

Preventive potassium supplementation attenuated the development of severe salt hypertension in young rats, whereas it had no effects on BP in adult rats with moderate hypertension. Enhanced sympathetic vasoconstriction was responsible for salt hypertension in young rats and its attenuation for potassium-induced BP reduction. Conversely, neither salt hypertension nor its potassium-induced attenuation were associated with significant changes of the vasodilator systems studied. The relative deficiency of vasodilator action of NO and Ca(2+) -activated K(+) channels in salt hypertensive Dahl rats was not improved by potassium supplementation.

CONCLUSIONS

The attenuation of enhanced sympathetic vasoconstriction is the principal mechanism of antihypertensive action exerted by preventive potassium supplementation in immature Dahl rats. Dietary potassium supplementation has no preventive effects on BP in adult salt-loaded animals or no therapeutic effects on established salt hypertension in young rats.