Cardiovascular effects of brain kinin receptor blockade in spontaneously hypertensive rats

Correlation between brain bradykinin receptor binding sites and cardiovascular function in young and adult spontaneously hypertensive rats

Intracerebroventricular (i.c.v.) effects of bradykinin (BK) B(1) and B(2) receptor agonists and antagonists were assessed on mean arterial blood pressure (MAP) and heart rate (HR) in awake unrestrained spontaneously hypertensive rats (SHR, aged of 8 and 16 weeks) and age-matched Wistar Kyoto rats (WKY). Quantitative in vitro autoradiographic studies were also performed on the brain of both strains with specific radioligands for B(2) receptors [(125)I]HPP-Hoe 140 and B(1) receptors [(125)I]HPP-des-Arg(10) and Hoe140. MAP increased linearly with doses of BK (81-8100 pmol) and the amplitudes were significantly greater in SHR, particularly at 16 weeks. While BK evoked a negative linear trend on HR (bradycardia) in WKY, a positive one (tachycardia) was observed in adult SHR. In both strains, BK-induced pressor response was blocked by equimolar doses of B(2) receptor antagonist, D-Arg-[Hyp(3), Thi(5), D-Tic(7), Oic(8)]-BK (Hoe 140), but not by B(1) receptor antagonist, AcLys[D-betaNal(7), Ile(8)]des-Arg(9)-BK (R-715). B(1) receptor agonists (Sar-[D-Phe(8)]-des-Arg(9)-BK, des-Arg(9)-BK, des-Arg(10)-Kallidin) and antagonist (R-715 alone or with Hoe 140) had no or marginal effect on MAP and HR at doses up to 8100 pmol in SHR and WKY. Higher densities of specific [(125)I]HPP-Hoe 140 labelling were found in discrete brain areas of SHR, especially in regions associated with cardiovascular function. Low levels of [(125)I]HPP-[des-Arg(10)]-Hoe140 binding sites were seen in WKY and SHR, yet densities were significantly greater in midbrain and cortical regions of SHR aged of 16 weeks. Contrary to SHR, ageing caused a downregulation of B(2) and B(1) receptor binding sites in specific brain nuclei in WKY. It is concluded that the hypersensitivity of the pressor response to i.c.v. BK in SHR occurs during both the early and established phases of hypertension in parallel with the enhancement of B(2) receptor binding sites in various cardiovascular brain centres. In contrast, brain B(1) receptors do not seem to participate in the central pressor effects of kinins nor in the maintenance of hypertension in SHR.

Kinin B2 receptor localization and expression in the hypothalamo-pituitary-adrenal axis of spontaneously hypertensive rats

OBJECTIVE

An enhanced hypothalamo-pituitary-adrenocortical (HPA) activity has been demonstrated during onset of high blood pressure in spontaneously hypertensive rats (SHR). Furthermore, compared to normotensive Wistar-Kyoto (WKY) rats, SHR show hypersensitivity to bradykinin (BK)-induced pressor responses which may be caused by an upregulation of B(2) receptor expression in the brain.

METHODS

We performed an immunohistochemical localization and measured gene expression of B(2) receptors in the hypothalamus, pituitary and adrenal glands of SHR at three ages corresponding to the development of hypertension, i.e. prehypertensive phase, onset of hypertension and established hypertension. Using reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot technique, B(2) receptor mRNA and protein levels, respectively, were measured.

RESULTS

A specific immunostaining for B(2) receptors was observed in the hypothalamic nuclei paraventricularis (PVN) and supraopticus (SON). In the pituitary and adrenal glands, a strong immunostaining was observed in neurohypophysis (NH) and adrenal medulla, respectively. At all ages tested, B(2) receptor mRNA and protein levels were higher in the hypothalamus and adrenal glands of SHR compared to age-matched WKY rats. Among SHR, the mRNA level was increased in neurohypophysis with age, and no difference was found in the adenohypophysis (AH) between SHR and WKY rats.

CONCLUSION

The data demonstrate a specific localization and an upregulation of B(2) receptor expression in the hypothalamus and adrenal glands of SHR, providing an anatomical and molecular basis for a possible contributory role to bradykinin-induced hypersensitivity of cardiovascular responses. The increased B(2) receptor expression in the hypothalamus and adrenal glands may also play a role in the abnormalities of the HPA axis in SHR during the development of hypertension.

Kinin B1 and B2 receptor mRNA expression in the hypothalamus of spontaneously hypertensive rats

The central hypertensive effects induced by bradykinin are known to be mediated via B2 receptors, which are present constitutively in the brain. B, receptors are rapidly upregulated during inflammation, hyperalgesia, and experimental diabetes. The hypothalamus plays an important role in the regulation of cardiovascular homeostasis, and all components of kallikrein-kinin system have been identified in this area. Therefore, we analyzed the mRNA expression of B1 and B2 receptors in the hypothalamus of spontaneously hypertensive rats (SHR) by RT-PCR. Male SHR were studied at three different ages corresponding to the three phases in the development of hypertension: (i) 3-4 (prehypertensive), (ii) 7-8 (onset of hypertension), and (iii) 12-13 weeks (established hypertension) after birth, and compared with age-matched Wistar-Kyoto (WKY) rats. At all ages tested, B2 receptor mRNA levels in the hypothalamus of SHR were higher than age-matched WKY rats (p < 0.001). However, the B1 receptor mRNA levels were higher at the established phase of hypertension only. We conclude that B1 and B2 receptor mRNA are differentially expressed in the hypothalamus of SHR and may play different roles in the pathogenesis of hypertension: upregulation of B2 receptor mRNA from early age may participate in the pathogenesis of hypertension, whereas an upregulation of B1 receptor mRNA in the established phase of hypertension may reflect an epiphenomenon in essential hypertension.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Dilated and failing cardiomyopathy in bradykinin B(2) receptor knockout mice

BACKGROUND

The activation of B(2) receptors by kinins could exert cardioprotective effects in myocardial ischemia and heart failure.

METHODS AND RESULTS

To test whether the absence of bradykinin B(2) receptors may affect cardiac structure and function, we examined the developmental changes in blood pressure (BP), heart rate, and heart morphology of bradykinin B(2) receptor gene knockout (B(2)(-/-)), heterozygous (B(2)(+/-)), and wild-type (B(2)(+/+)) mice. The BP of B(2)(-/-) mice, which was still normal at 50 days of age, gradually increased, reaching a plateau at 6 months (136+/-3 versus 109+/-1 mm Hg in B(2)(+/+), P<0.01). In B(2)(+/-) mice, BP elevation was delayed. At 40 days, the heart rate was higher (P<0.01) in B(2)(-/-) and B(2)(+/-) than in B(2)(+/+) mice, whereas the left ventricular (LV) weight and chamber volume were similar among groups. Thereafter, the LV growth rate of B(2)(-/-) and B(2)(+/-) mice was accelerated, leading at 360 days to a LV weight-to-body weight ratio that was 9% and 17% higher, respectively, than that of B(2)(+/+) mice. In B(2)(-/-) mice, hypertrophy was associated with a marked chamber dilatation (42% larger than that of B(2)(+/+) mice), an elevation in LV end-diastolic pressure (25+/-3 versus 5+/-1 mm Hg in B(2)(+/+) mice, P<0.01), and reparative fibrosis.

CONCLUSIONS

The disruption of the bradykinin B(2) receptor leads to hypertension, LV remodeling, and functional impairment, implying that kinins are essential for the functional and structural preservation of the heart.

Altered baroreflex control of heart rate in bradykinin B2-receptor knockout mice

Recently, we have shown that a knockout mouse strain lacking the bradykinin B2-receptor gene exhibits an accelerated heart rate (HR) under basal conditions, this alteration being associated with mildly elevated blood pressure (BP) levels and ultimately with the development of cardiomyopathy. The goal of the present study was to determine whether genetic disruption of the B2-receptor alters autonomic cardiovascular reflexes to acute or chronic changes in BP. The direct mean BP and HR levels of unrestrained B2 knockout mice (B2-/-) were higher than those of wild type (B2+/+) controls (131 +/- 2 vs. 105 +/- 2 mm Hg and 480 +/- 5 vs. 414 +/- 8 beats/min, P < 0.01 for both comparisons). The difference in HR observed between groups under basal conditions was nullified by the acute administration of propranolol and atropine as well as by hexamethonium; it was attenuated by long-term blockade of angiotensin AT1 receptors. In B2-/- mice, the presence of an alteration in baroreceptor regulation of HR was supported by a reduced gain in the HR responses to acute nitroprusside-induced hypotension or phenylephrine-induced hypertension (slope of the regression line: 0.82 +/- 0.07 vs. 5.58 +/- 0.08 beats/min per mmHg in B2+/+, P < 0.01), as well as by an exaggerated tachycardic response to chronic hypertension induced by clipping of the left renal artery (60 +/- 3 vs. 15 +/- 3 beats/min in B2+/+, P < 0.01). Our findings indicate that disruption of the bradykinin B2-receptor gene is associated with an impaired baroreflex control of HR. The combination of chronically elevated resting HR and impaired baroreflex control could contribute to the development of cardiomyopathy in these animals.

The bradykinin B1 receptor and the central regulation of blood pressure in spontaneously hypertensive rats

1. We evaluated if the brain bradykinin (BK) B1 receptor is involved in the regulation of blood pressure (BP) in conscious rats. 2. Basal mean BP and HR were 115 +/- 2 and 165 +/- 3 mmHg and 345 +/- 10 and 410 +/- 14 beats min in Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR), respectively. Intracerebroventricular (i.c.v.) injection of 1 nmol B1 receptor agonist Lys-desArg9-BK significantly increased the BP of WKY and SHR by 7+/-1 and 19+/-2 mmHg, respectively. One nmol Sar[D-Phe8]-desArg9-BK, a kininase-resistant B1 agonist, increased the BP of WKY and SHR by 19+/-2 and 17+/-2 mmHg, respectively and reduced HR in both strains. 3. I.c.v. injection of 0.01 nmol B1 antagonists, LysLeu8-desArg9-BK or AcLys[D-betaNal7,Ile8]-desArg9-BK (R715), significantly decreased mean BP in SHR (by 9+/-2 mmHg the former and 14+/-3 mmHg the latter compound), but not in WKY. In SHR, the BP response to R715 was associated to tachycardia. 4. I.c.v. Captopril, a kininase inhibitor, increased the BP of SHR, this response being partially prevented by i.c.v. R715 and reversed into a vasodepressor effect by R715 in combination with the B2 antagonist Icatibant. 5. I.c.v. antisense oligodeoxynucleotides (ODNs) targeted to the B1 receptor mRNA decreased BP in SHR, but not in WKY. HR was not altered in either strain. Distribution of fluorescein-conjugated ODNs was detected in brain areas surrounding cerebral ventricles. 6. Our results indicate that the brain B1 receptor participates in the regulation of BP. Activation of the B1 receptor by kinin metabolites could participate in the pathogenesis of hypertension in SHR.

Interaction of bradykinin and angiotensin-(1-7) in the central modulation of the baroreflex control of the heart rate

OBJECTIVE

Previous studies have shown that angiotensin-(1-7) potentiates the vascular actions of bradykinin. In the present study, we evaluated the interaction of bradykinin and angiotensin-(1-7) in the central modulation of baroreflex control of the heart rate.

MATERIALS AND METHODS

Blood pressure and reflex bradycardia, elicited by intravenous injection of phenylephrine, were evaluated in conscious male Wistar rats before and at the end of 1 h of an intracerebroventricular infusion of angiotensin-(1-7) at 0.5 or 1.0 microg/h combined with bradykinin at 2.5 microg/h; or angiotensin-(1-7) at 2.0 microg/h combined with bradykinin at 4.0 microg/h; or angiotensin-(1-7) alone at 2.0 or 4.0 microg/h; or bradykinin alone at 4.0 or 8.0 microg/h; or saline at 8 microl/h. In addition, baroreflex bradycardia was evaluated before and at the end of 1 and 2 h of intracerebroventricular infusion of angiotensin-(1-7) at 4 microg/h for 2 h; or saline at 8 microl/h in the first hour followed by HOE 140 at 90 ng/h in the second hour; or angiotensin-(1-7) at 4 microg/h in the first hour followed by angiotensin-(1-7) at 4 microg combined with HOE 140 at 90 ng/h in the second hour; or HOE 140 at 90 ng/h in the first hour followed by HOE 140 at 90th ng/h combined with angiotensin-(1-7) at 4 microg/h in the second hour; or saline at 8 microl/h for 2 h.

RESULTS

The intracerebroventricular infusion of angiotensin-(1-7) or bradykinin alone required a dose of 4.0 and 8.0 microg/h, respectively, to facilitate baroreflex control of the heart. However, a simultaneous infusion of these peptides at subeffective rates was able to produce a significant increase in baroreflex sensitivity. In addition, the facilitation of the baroreflex control of the heart rate induced by angiotensin-(1-7) at 4.0 microg/h was inhibited by HOE 140.

CONCLUSIONS

These results suggest that centrally, bradykinin and angiotensin-(1-7) can interact in order to modulate baroreflex control of the heart rate. In addition, our data indicate that the central modulatory effect of angiotensin-(1-7) on the baroreflex is mediated, at least in part, by the release of kinins.

Renovascular hypertension in bradykinin B2-receptor knockout mice

We evaluated whether kinins exert a protective action against the development of two-kidney, one clip (2K1C) hypertension, a model characterized by an activated renin-angiotensin system in the ischemic kidney and increased expression of the bradykinin (BK) B2 receptor in the contralateral kidney. BK B2-receptor knockout (B2-/-), wild-type (B2+/+), and heterozygous (B2+/-) mice underwent clipping of the left renal artery, with the other kidney remaining untouched. Basal systolic blood pressure (SBP, via tail-cuff plethysmography) was higher in B2-/- mice than in B2+/- or B2+/+ mice (121+/-2 versus 113+/-2 and 109+/-1 mm Hg; P<0.05 for both comparisons). SBP did not change from basal values after sham operation, but it increased in mice that underwent clipping. The increase in SBP was greater in 2K1C B2-/- mice than in B2+/- or B2+/+ mice (28+/-2 versus 14+/-2 and 14+/-2 mm Hg, respectively, at 2 weeks; P<0.05 for both comparisons). Blockade of the BK B2 receptor by Icatibant enhanced the pressure response to clipping in B2+/+ mice (29+/-2 mm Hg at 2 weeks). Intra-arterial mean blood pressure (MBP) was higher in 2K1C than in respective sham-operated mice, with the MBP difference being higher in B2-/- mice (32 and 38 mm Hg, at 2 and 4 weeks, respectively), and higher in B2+/+ mice given Icatibant (30 and 32 mm Hg) than in B2+/+ mice without Icatibant (17 and 18 mm Hg). At 4 weeks, acute injection of an angiotensin type 1 receptor antagonist normalized the MBP of 2K1C hypertensive mice. A tachycardic response was observed 1 week after clipping in B2-/- and B2+/- mice, but this effect was delayed in B2+/+ mice. However, the HR response to clipping in B2+/+ mice was enhanced by Icatibant. Within each strain, heart weight to body weight ratio was greater in 2K1C hypertensive mice than in sham-operated control animals (B2-/-: 5.7+/-0.1 versus 5.2+/-0.1; B2+/+: 5.1+/-0.1 versus 4.5+/-0.1; P<0.01 for both comparisons). The clipped kidney weight to nonclipped kidney weight ratio was consistently reduced in mice with 2K1C hypertension. Our results indicate that kinins acting on the BK B2 receptor exert a protective action against excessive blood pressure elevation during early phases of 2K1C hypertension.

Pathophysiology of the kallikrein-kinin system in mammalian nervous tissue

The nervous system and peripheral tissues in mammals contain a large number of biologically active peptides and proteases that function as neurotransmitters or neuromodulators in the nervous system, as hormones or cellular mediators in peripheral tissue, and play a role in human neurological diseases. The existence and possible functional relevance of bradykinin and kallidin (the peptides), kallikreins (the proteolytic enzymes), and kininases (the peptidases) in neurophysiology and neuropathological states are discussed in this review. Tissue kallikrein, the major cellular kinin-generating enzyme, has been localised in various areas of the mammalian brain. Functionally, it may assist also in the normal turnover of brain proteins and the processing of peptide-hormones, neurotransmitters, and some of the nerve growth factors that are essential for normal neuronal function and synaptic transmission. A specific class of kininases, peptidases responsible for the rapid degradation of kinins, is considered to be identical to enkephalinase A. Additionally, kinins are known to mediate inflammation, a cardinal feature of which is pain, and the clearest evidence for a primary neuronal role exists so far in the activation by kinins of peripherally located nociceptive receptors on C-fibre terminals that transmit and modulate pain perception. Kinins are also important in vascular homeostasis, the release of excitatory amino acid neurotransmitters, and the modulation of cerebral cellular immunity. The two kinin receptors, B2 and B1, that modulate the cellular actions of kinins have been demonstrated in animal neural tissue, neural cells in culture, and various areas of the human brain. Their localisation in glial tissue and neural centres, important in the regulation of cardiovascular homeostasis and nociception, suggests that the kinin system may play a functional role in the nervous system.

Enhanced blood pressure sensitivity to deoxycorticosterone in mice with disruption of bradykinin B2 receptor gene

The renal kallikrein-kinin system is activated under conditions of mineralocorticoid excess. To evaluate whether endogenous kinins exert a protective role against the development of mineralocorticoid-induced hypertension, we studied the cardiovascular effects induced by long-term administration of deoxycorticosterone (DOC; 0.3 micromol/g body wt s.c. once per week for 6 weeks) or vehicle in transgenic mice (Bk2r-/-) lacking the bradykinin B2 receptor gene and in wild-type controls (Bk2r+/+). Under basal conditions, Bk2r-/- mice showed higher systolic blood pressure (tail-cuff plethysmography) than wild-type Bk2r+/+ and heterozygous Bk2r+/- mice (121+/-2 versus 114+/-2 and 115+/-2 mm Hg, respectively; P<0.05 for both comparisons). Heart rate was higher in Bk2r-/- and Bk2r+/- than in Bk2r+/+ (459+/-12 and 418+/-7 versus 390+/-7 bpm; P<0.05 for both comparisons). Systolic blood pressure was increased by DOC in transgenic as well as in wild-type mice, whereas no change was induced by the vehicle. The pressor response to DOC was more rapid and pronounced in Bk2r-/- than in Bk2r+/+ and Bk2r+/- (30+/-5 versus 15+/-4 and 6+/-3 mm Hg, respectively, at 3 weeks; P<0.01 for both comparisons). The difference in systolic blood pressure was consistent with that detected by direct intra-arterial measurements of mean blood pressure. Neither DOC nor its vehicle altered heart rate or gain in body weight over time. Under basal conditions, urinary sodium excretion did not differ between strains. During DOC administration, cumulative urinary sodium excretion was lower in Bk2r-/- than in Bk2r+/+ (2.59+/-0.15 versus 3.31+/-0.22 mmol, respectively, during the first week; P<0.05). Urinary kinin excretion was increased by DOC in both Bk2r-/- (from 0.65+/-0.17 to 4.27+/-0.80 pmol/24 h; P<0.01) and Bk2r+/+ (from 0.55+/-0.09 to 6.27+/-1.48 pmol/24 h; P<0.05). The increase in urinary kinin excretion was similar between strains. These results show that integrity of the bradykinin B2 receptor is essential for regulation of blood pressure and heart rate under basal conditions. In addition, they indicate that activation of the kallikrein-kinin system represents a compensatory response against the development of hypertension induced by mineralocorticoid excess.

Bradykinin B2 receptors in nodose ganglia of rat and human

The present study has employed in vitro electrophysiology to characterise the ability of bradykinin to depolarise the rat isolated nodose ganglion preparation, containing the perikarya of vagal afferent neurons. Both bradykinin and kallidin elicited a concentration-dependent (1-100 nM) depolarisation when applied to the superfusate bathing the nodose ganglia, whereas the bradykinin B1 receptor agonist, des-Arg9-bradykinin, was only effective in the micromolar range. Furthermore, the electrophysiological response to bradykinin was antagonised by the bradykinin B2 receptor antagonist, D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-t hienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl+ ++-L-(2alpha,3beta,7abeta)-octahydro-1H-indole-2-carbonyl-L- arginine (Hoe 140), in a concentration-related manner. To determine the anatomical location of functional bradykinin B2 receptors, in vitro autoradiography with [125I]para-iodophenyl Hoe 140 was performed on sections of rat and human inferior vagal (nodose) ganglia and confirmed the presence of binding over vagal perikarya. Collectively, these data provide evidence for functionally relevant bradykinin B2 receptors on vagal afferent neurons, which are apparently also present on human vagal perikarya.

Blood pressure responses to acute or chronic captopril in mice with disruption of bradykinin B2-receptor gene

OBJECTIVE

To evaluate the role of kinins in the hypotensive response to angiotensin converting enzyme inhibition, we compared the blood pressure effects induced by acute or chronic captopril administration in a mouse strain (Bk2r-/-) with disruption of the bradykinin B2 receptor gene and in wild-type controls (J129 Sv mice). A second aim was to determine whether Icatibant, a selective bradykinin B2-receptor antagonist, prevented the blood pressure changes induced by acute captopril administration in Swiss, c57/B16, J129 Sv and Bk2r-/- mice.

METHODS AND RESULTS

Under basal conditions, tail-cuff systolic blood pressure (SBP) and intra-arterial mean blood pressure (MBP) were higher in Bk2r-/- than in J129 Sv (SBP: 132+/-2 versus 113+/-3 mmHg; MBP: 144+/-6 versus 122+/-10 mmHg, P< 0.05 for both comparisons). Acute captopril administration (1 mg/kg body weight, intra-arterially) reduced the MBP of Bk2r-/- and J129 Sv by 36+/-8 and 31+/-7 mmHg, respectively. Swiss and c57/B16 mice showed similar decreases in MBP following captopril. Pretreatment with Icatibant (10 nmol/kg body weight, intra-arterially) did not influence the MBP responses to acute captopril in all the strains. Chronic administration of captopril (approximately 120 mg/kg body weight per day for 2 weeks in drinking water) reduced SBP in either Bk2r-/- or J129 Sv. The magnitude of this response was higher in Bk2r-/- than in J129 Sv (65+/-3 versus 47+/-4 mmHg, respectively, P < 0.01).

CONCLUSIONS

Our results suggest that endogenous kinins do not participate in the hypotensive response to angiotensin converting enzyme inhibition in mice; in Bk2r-/-, the exaggerated blood pressure response to chronic captopril appears to be attributable to interference with unbalanced vasoconstrictor action of the renin-angiotensin system.

Distribution of bradykinin B2 receptors in sheep brain and spinal cord visualized by in vitro autoradiography

Bradykinin B2 receptors were localized in the sheep brain and spinal cord by quantitative in vitro autoradiography using a radiolabelled and specific bradykinin B2 receptor antagonist analogue, 3-4-hydroxyphenyl-propionyl-D-Arg0-[Hyp3,Thi5,D-Tic 7,Oic8]bradykinin, (HPP-HOE 140). This radioligand displays high affinity and specificity for bradykinin B2 receptors. The respective K(i) values of 0.32, 1.37 and 156 nM were obtained for bradykinin, HOE140 and D-Arg[Hyp3,D-Phe7,Leu8]bradykinin competing for radioligand binding to lamina II of sheep spinal cord sections. Using this radioligand, we have demonstrated the distribution of bradykinin B2 receptors in many brain regions which have not been previously reported. The highest density of bradykinin B2 receptors occur in the pleoglial periaqueductal gray, oculomotor and trochlear nuclei and the circumventricular organs. Moderate densities of receptors occur in the substantia nigra, particularly the reticular part, the posterior thalamic and subthalamic nuclei, zona incerta, the red and pontine nuclei, some of the pretectal nuclei and in discrete layers of the superior colliculus. In the hindbrain, moderate levels of bradykinin B2 receptor binding occur in the nucleus of the solitary tract, and in spinal trigeminal, inferior olivary, cuneate and vestibular nuclei. Laminae II, X and dorsal root ganglia display the most striking binding densities in the spinal cord, while the remainder of the dorsal and ventral horn display a low and diffuse density of binding. Bradykinin B2 receptors are extensively distributed throughout the sheep brain and spinal cord, not only to sensory areas but also to areas involved in motor activity.

Antisense inhibition of the brain kallikrein-kinin system

We used antisense oligodeoxynucleotide (ODN) strategy, based on interference of information flow from gene to protein, to determine the role of kininogen and bradykinin B2 receptor genes in the pathogenesis of genetic hypertension in rats. Mean blood pressure of 9-week-old spontaneously hypertensive rats (SHR) increased 4 hours after acute intracerebroventricular injection of synthetic 18-mer antisense ODNs targeting the translation initiation codon of kininogen mRNA (from 164 +/- 5 to 181 +/- 4 mm Hg, P < .01) or bradykinin B2 receptor mRNA (from 161 +/- 5 to 185 +/- 8 mm Hg, P < .01) and then returned to basal levels within 24 hours. Prolonged vasopressor effects were observed after repeated injections of antisense ODN targeting kininogen mRNA. Antisense ODNs to kininogen and B2 receptor mRNAs increased blood pressure of normotensive Wistar-Kyoto rats only slightly compared with SHR (from 116 +/- 3 to 124 +/- 1 and from 116 +/- 2 to 126 +/- 4 mm Hg, respectively; P < .05). Cardiovascular responses were confirmed by the use of antisense ODNs targeted to bind to different non-overlapping regions of kininogen or B2 receptor mRNA. Microinjection of antisense ODN to B2 receptor mRNA into the nucleus tractus solitarii increased mean blood pressure in SHR and prevented the vasodepressor effect induced by intranuclear microinjection of bradykinin. No significant change in mean blood pressure was induced in either strain by intravenous injection of antisense ODNs or by central injection of sense or scrambled ODNs. A strong fluorescent signal was detected at the level of the hippocampus, thalamus, hypothalamus periventricularis, midbrain, and cerebrum 1 hour after central injection of fluorescein isothiocyanate-conjugated antisense ODNs. Kininogen levels were significantly lower in the brain of rats given intracerebroventricular antisense kininogen ODN compared with controls. Our results indicate that the brain kallikrein-kinin system plays a role in the central regulation of blood pressure and suggest that this system may exert a protective action against further elevations of blood pressure levels in SHR.

Cardiovascular effects produced by bradykinin microinjection into the nucleus tractus solitarii of anesthetized rats

In this study, we characterized the cardiovascular effects produced by microinjection of doses in the femtomole range of bradykinin (BK) into the nucleus tractus solitarii of male Wistar rats (230-280 g, n = 120) anesthetized with urethane (1.2 g/kg, i.p.). Microinjections of BK (1, 10, 100 fmol, and 1 and 10 pmol, in 50 nl) or vehicle (NaCl, 0.9%) were made by using a triple-barreled glass micropipette into the medial nTS (0.4 mm anterior, 0.3 mm lateral to the obex and 0.3 mm deep from the dorsal surface). Microinjection of BK produced a shallow dose-dependent decrease in mean arterial pressure and heart rate reaching -18 +/- 6 mmHg and -21 +/- 5 beats/min, with the dose of 10 pmol. The peripheral mechanism of these effects, tested in animals treated with methylatropine (2 mg/kg, i.v.), or propranolol (2 mg/kg, i.v.) or prazosin (30 micrograms/kg, i.v.), was shown to be mainly dependent on an increase in vagal efferent activity for bradycardia and a decrease in sympathetic activity for hypotension. In order to investigate the receptor subtype involved in these effects, BK was microinjected into the nTS before and after the injection of the B1 receptor antagonist, Des-Arg9-Leu8-BK (DALBK) (11.5 pmol) or before and after the B2 receptor antagonist, HOE-140 (7.7 pmol). The cardiovascular effects of BK were significantly attenuated by the microinjection of HOE-140 and DALBK into the nTS. The effect of BK microinjected into the nTS on the baroreflex modulation was also investigated. While BK produced a significant facilitation of the baroreflex, HOE-140 and DALBK produced a significant attenuation of the baroreceptor control of heart rate. Taken together, the data presented in this study indicate the nTS as a site, in the central nervous system, for the modulatory effect of BK on the central cardiovascular control.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Hypertensive effect of tissue kallikrein in rostral ventrolateral medulla is mediated by brain kinins

Microinjections of kallikrein, 0.5-2.0 units, in the rostral ventrolateral medulla (RVLM) of brain increased arterial pressure in Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). This effect was significantly greater in SHR. The kinin B2 receptor antagonist icatibant (Hoe 140) blocked the hypertensive responses to kallikrein in both groups and caused greater hypotension and bradycardia in SHR. These results suggest that local kinins in the RVLM act to alter cardiovascular function and may be involved in the maintenance of blood pressure in the SHR.