Fatimunnisa Qadri

Chronic kinin receptor blockade attenuates the antihypertensive effect of ramipril.

The contribution of endogenous kinins to the chronic antihypertensive effect of angiotensin converting enzyme inhibitors was investigated in two-kidney, one clip hypertensive Wistar rats, using the new bradykinin B2-receptor antagonist HOE 140 (D-Arg, [Hyp3, Thi5, D-Tic7, Oic8]-bradykinin). In a first protocol, rats were pretreated orally with the angiotensin converting enzyme inhibitor ramipril (1 mg/kg per day), for 4 weeks. Acute blockade of bradykinin receptors by intravenous injections of HOE 140 at doses of 8.4 and 100 micrograms/kg, which inhibited the depressor responses to exogenous bradykinin, did not affect the antihypertensive effect of ramipril in these animals. Bradykinin receptors were then blocked chronically by subcutaneous infusion of HOE 140 (500 micrograms/kg per day) via osmotic minipumps for 6 weeks, while ramipril treatment was continued. HOE 140 partially reversed the antihypertensive effect of ramipril from 115.3 +/- 4.6 to 123.8 +/- 3.3 mm Hg (mean arterial blood pressure) after 3 weeks and to 121.3 +/- 2.9 mm Hg after 6 weeks. In contrast, in controls (ramipril plus subcutaneous vehicle infusion) mean arterial blood pressure decreased further from 112.0 +/- 6.0 to 110.3 +/- 4.9 mm Hg after 3 weeks and to 103.7 +/- 5.0 mm Hg after 6 weeks (p less than 0.05 and p less than 0.01, HOE 140 versus controls). Plasma catecholamines were not significantly different between the two groups at the end of the experiment, indicating that the partial reversal of the antihypertensive effect was not due to a bradykinin-like agonistic effect on catecholamine release.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II evokes noradrenaline release from the paraventricular nucleus in conscious rats

In vitro and in vivo experiments have provided indirect evidence that some of the central actions of angiotensin II (ANG II) involve catecholaminergic pathways in the brain. In this study in conscious rats we investigated the effect of stimulation of periventricular ANG II receptors on blood pressure and on catecholamine release (microdialysis and HPLC) from the paraventricular nucleus (PVN), a hypothalamic area thought to be instrumental in the central pressor responses to ANG II through the release of vasopressin into the blood. Intracerebroventricular (i.c.v.) injections of pressor doses of ANG II (1 ng and 100 ng) led to significant dose-dependent increases of the noradrenaline (NA) release in the PVN (1 ng: 30.95 +/- 6.01 to 47.38 +/- 6.79 pg/sample, P less than or equal to 0.01; 100 ng: 32.93 +/- 5.38 to 73.18 +/- 11.4 pg/sample, P less than or equal to 0.01). These changes coincided in extent and duration with the respective pressor responses. A subpressor dose of ANG II (100 pg) did not release catecholamines from the PVN. Dopamine (DA) and the NA and DA, metabolites 3,4-dihydroxyphenylethylglycol and 3,4-dihydroxyphenylacetic acid, were not influenced by i.c.v. injections of ANG II at any dose. Pretreatment with the novel non-peptide ANG II-AT 1 receptor antagonist DuP 753 (5 micrograms, i.c.v.) abolished the effect of 100 ng ANG II on blood pressure and on NA release. Our results show for the first time in vivo that stimulation of periventricular ANG II-AT 1 receptors induces a selective NA release in the PVN. They further support the hypothesis that ANG II engages a noradrenergic pathway in the PVN to release vasopressin.

HOE 140, a new highly potent and long-acting bradykinin antagonist in conscious rats

The inhibitory effects of the new bradykinin antagonist HOE 140 (D-Arg-Arg-Pro-Hyp-Gly-Thi-Ser-D-Tic-Oic-Arg) on depressor responses to exogenous bradykinin were investigated in conscious rats and compared with those of the bradykinin antagonist B4146 (D-Arg-Hyp-Pro-Gly-Thi-Ser-D-Pro-Thi-Arg). HOE 140 showed a 250-700-fold higher potency in vivo and a much longer biological half-life than B4146. Plasma catecholamines were not increased after application of HOE 140, indicating that this compound did not interfere with catecholamine release. HOE 140 proved to be a highly potent, specific and long-acting bradykinin B2-receptor antagonist.

Release of angiotensin in the paraventricular nucleus in response to hyperosmotic stimulation in conscious rats: a microdialysis study.

Angiotensin peptides are thought to act as neurotransmitters or neuromodulators in central osmoregulation. We tested the hypothesis that angiotensin peptides are released in the paraventricular nucleus (PVN) of the hypothalamus upon local osmotic stimulation. Brain microdialysis and radioimmunoassay (RIA) techniques were used to measure the release of immunoreactive angiotensin II (irANG II) in the PVN following direct stimulation of this area with hyperosmotic solutions. In conscious rats, perfusion of the PVN with 0.3 M and 0.6 M NaCl in artificial cerebrospinal fluid (aCSF) elicited concentration-dependent increases in irANG II release to 5.52 +/- 0.53, (P < 0.01, n = 8) and 9.01 +/- 1.03 pg/100 microliters, (P < 0.001, n = 7), respectively, from basal values of 3.04 +/- 0.46 pg/100 microliters. Local perfusion of the PVN with 1.2 M glucose in aCSF also resulted in an increased release of irANG II from 3.07 +/- 0.87 to 6.24 +/- 0.45 pg/100 microliters (P < 0.05, n = 5). Fractionization of angiotensin peptides by HPLC followed by RIA revealed that ANG II (1-8) and ANG III (2-8) were released in similar amounts in the perfusate collected during 0.6 M NaCl stimulation (4.79 +/- 0.69 and 3.45 +/- 0.76 pg/100 microliters, respectively). Our results show that both, ANG II and ANG III are released in the PVN in response to local hyperosmotic stimulation. They support the concept that angiotensin peptides in the PVN are involved as neurotransmitters in central osmotic control.

Angiotensin II-induced noradrenaline release from anterior hypothalamus in conscious rats : a brain microdialysis study

Interaction with aminergic transmitter substances has been implicated in the central actions of angiotensin II (ANG II). We used the novel technique of brain microdialysis in conscious rats to investigate whether ANG II influences the release of endogenous catecholamines (CA) from the anterior hypothalamus (AH). Intracerebroventricular (i.c.v.) administration of ANG II (1 ng and 100 ng) increased mean arterial pressure. ANG II at 1 ng had no effect on the release of noradrenaline (NA) from the AH but ANG II at 100 ng significantly increased NA release. Intracerebroventricular pretreatment with the ANG II-receptor antagonist sarilesin (Sar1, Ile8-ANG II; 3 micrograms) prevented the ANG II-induced NA release. The release of the intraneuronal NA and dopamine (DA) metabolites 3,4-dihydroxyphenylethyl glycol (DOPEG) and 3,4-dihydroxyphenylacetic acid (DOPAC) from the AH was not altered by i.c.v. ANG II. Our results provide the first in vivo evidence for NA release from the hypothalamus induced by periventricular ANG II receptor stimulation. They support the hypothesis that hypothalamic noradrenergic mechanisms are involved in the central actions of this peptide.

Osmotically induced natriuresis and blood pressure response involves angiotensin AT1 receptors in the subfornical organ.

Objective In the present study we tested the hypothesis of whether the centrally induced natriuresis and blood pressure increase after intracerebroventricular injection of hypertonic saline involves the subfornical organ, as suggested by the occurrence of osmosensitive cells as wellas a high concentration of angiotensin II receptors in this brain area. Methods All experiments were performed in conscious Wistar rats. A chronic cannula was inserted into the lateral brain ventricle for intracerebroventricular injection and a chronic indwelling intracranial guide cannula for microinjection was placed in the subfornical organ. In addition, the rats were provided with ureter catheters for urine collection. Results Intracerebroventricular injections of hypertonic saline (0.3 mol/l; n = 7) increased renal sodium excretion from 180.0 ± 30.0 to 279.0 ± 34.0 mol/l/60 min (P < 0.001) accompanied by an increase in mean arterial pressure of 8.3 ± 1.2 mmHg (P < 0.01). No change in urinary volume was observed. After injection of the specific AT1 receptor antagonist, losartan, into the subfornical organ (5 μg/200 nl; n = 8) the natriuresis and blood pressure response to intracerebroventricular hypertonic saline was completely abolished. Control injections of losartan into areas adjacent to the subfornical organ had no effect on the responses to hypertonic saline. Conclusion: Our results suggest that the centrally induced natriuresis and blood pressure responses to hypertonic saline are mediated by an angiotensinergic mechanism involving the subfornical organ.

Substance P injection into the dorsal raphe increases blood pressure and serotonin release in hippocampus of conscious rats

Microinjections of substance P (SP, 100 pmol) into the dorsal raphe nucleus (DRN) in conscious rats increased blood pressure and heart rate for 30-40 min. Concomitantly, the extracellular levels of 5-hydroxytryptamine (5-HT) in the ventral hippocampus, monitored by microdialysis, increased by 30% for 20 min compared with the vehicle control. Pretreatment with the 5-HT2 receptor antagonist, ritanserin (1 mg/kg i.v.), prevented the pressor response to SP but not the increase in heart rate. Pretreatment with the partial 5-HT1A receptor agonist, 8-methoxy-2-(N-2-chloroethyl-N-n-propyl)amino tetralin (8-MeO-CLEPAT, 10 micrograms/kg i.v.) prevented the increase in both blood pressure and heart rate. It is suggested that microinjections of SP into the DRN increase blood pressure through activation of serotonergic DRN neurons and that the postsynaptic receptor responsible for the pressor response is of the 5-HT2 type.

In vivo Alterations in the Gonadotropin-Releasing Hormone Pulse Generator and the Secretion and Clearance of Luteinizing Hormone in the Uremic Castrate Rat

To investigate the mechanisms subserving the reported alterations in the pulsatile release of LH in uremia, we simultaneously studied endogenous accumulation of GnRH in the pituitary gland and the secretion and clearance of LH in vivo in experimentally uremic, orchidectomized rats. The temporal pattern of GnRH secretion was assessed by intrapituitary microdialysis, the dynamics of plasma LH by continuous exchange transfusion. Studies were performed in rats rendered uremic by subtotal nephrectomy (n = 8) and control rats which were either fed ad libitum (n = 8) or pair-fed with the uremic animals (n = 8). Blood samples were obtained at 5-min and microdialysate samples at 10-min intervals over a period of 270 min. The pulsatile secretory characteristics of GnRH and LH and the half-life of plasma LH were estimated by multiple-parameter deconvolution analysis. The temporal relationship between the hormone concentrations and between the secretory events of GnRH and LH was assessed by cross-correlation analysis and hypergeometric coincidence analysis. We observed that: (1) the estimated half-life of plasma LH was prolonged in uremic (59 +/- 10 min) rats compared to ad libitum-fed (17 +/- 3 min, p = 0.014) and pair-fed controls (19 +/- 3 min, p = 0.025); (2) the LH production rate was decreased in uremic animals (18 +/- 5 ng/ml.270 min) compared to ad libitum-fed (37 +/- 4 ng/ml.270 min, p = 0.002) and pair-fed controls (48 +/- 9 ng/ml.270 min, p = 0.0006); (3) the reduction of LH secretion rate in the uremic animals was accounted for by a decrease in detectable LH pulse frequency (2.1 +/- 0.2 peaks/h) compared to ad libitum-fed (3.1 +/- 0.1 peaks/h, p = 0.01) and pair-fed controls (2.8 +/- 0.2 peaks/h, p = 0.06) and a diminished mass of hormone released per burst (uremic 1.8 +/- 0.2 ng/ml, ad libitum-fed 2.6 +/- 0.3 ng/ml, p = 0.05, pair-fed 3.8 +/- 0.8 ng/ml, p = 0.025); (4) the secretion rate of GnRH was reduced to a similar degree in uremic rats (180 +/- 15 pg/tube.270 min, p = 0.04) and pair-fed controls (170 +/- 26 pg/tube.270 min, p = 0.04) compared to ad libitum-fed controls (270 +/- 36 pg/tube.270 min). In contrast to the reduced number of detectable LH secretory events, the frequency of GnRH secretory peaks in uremic rats was not different from ad libitum-fed and pair-fed controls.(ABSTRACT TRUNCATED AT 400 WORDS)

Central noradrenergic pathways are not involved in the pressor response to intracerebroventricular substance P

Intracerebroventricular (i.c.v.) pretreatment of Wistar rats with the catecholaminergic neurotoxin, 6-hydroxydopamine, significantly reduced noradrenaline in the pons/medulla (by 73%) and almost abolished the noradrenaline levels in the spinal cord (by 89%). The pressor response of these animals to i.c.v. angiotensin II (ANGII, 10 ng) was significantly attenuated. In contrast, there was no significant effect on the pressor response to i.c.v. substance P (SP, 100 ng). The results provide evidence that different central pathways mediate the pressor response to ANGII and SP.

Interaction of Angiotensin II with Catecholamines in the Brain

Almost 30 years ago, the circulating effector hormone of the reninangiotensin system (RAS), the octapeptide angiotensin II (ANG II), was first reported to act not only on peripheral vascular structures but also on the central nervous system (CNS) (Bickerton and Buckley, 1961). In the following years, evidence has accumulated that the brain, in addition to possessing ANG II-sensitive sites, contains the precursor molecule as well as the enzymatic apparatus to generate its own angiotensin peptides, suggesting a local synthesis of ANG II in the brain (Brosnihan et al., 1988; Ganten et al., 1971; Unger et al., 1988). In addition, the protein genes of the RAS are expressed in the CNS and mRNAs have been localized in specific brain areas (Hellmann et al., 1988; Jin et al., 1988), although not all components of the RAS were demonstrated within the same neuron. ANG II receptors were found in the rat brain mainly in the thalamus, hypothalamus, midbrain, septum, and medulla (Bennett and Snyder, 1976; Mendelsohn et al., 1984; Sirett et al., 1977), with higher concentrations in the lateral septum, superior colliculus, and area postrema and very high concentrations in the subfornical organ (SFO), median preoptic area (MnPO), organum vasculosum of the lamina terminalis (OVLT), medial preoptic area (MPO), paraventricular nucleus (PVN), supraoptic nucleus (SON), and nucleus tractus solitarii (NTS).