Effects of brain mineralocorticoid receptor blockade on blood pressure and renal functions in DOCA-salt hypertension

In normotensive rats, we have previously demonstrated a role of brain mineralocorticoid receptors in blood pressure and renal function control. In the present study, the coordinate cardiovascular and renal effects of brain mineralocorticoid receptor blockade were examined by intracerebroventricular (i.c.v.) administration of a selective mineralocorticoid receptor antagonist (RU28318; 3,3-oxo-7 propyl-17-hydroxy-androstan-4-en-17yl-propionic acid lactone) in rats with hypertension induced by deoxycorticosterone acetate (DOCA) and salt. DOCA pellets were implanted s.c. in male Wistar rats given 0.9% NaCl as drinking solution 3 or 5 weeks before assessment of the effects of i.c.v. injection of RU28318 on cardiovascular and renal functions. Changes in expression of brain angiotensinogen, atrial natriuretic peptide (ANP) and mineralocorticoid receptor mRNA in specific brain areas in 3-week DOCA-salt rats were evaluated by in situ hybridization. The rise in systolic blood pressure induced by DOCA-salt treatment was most marked during the first 3 weeks. At 3 and 5 weeks after implantation of the DOCA-pellets a single i.c.v. injection of 10 ng of RU28318 significantly decreased systolic blood pressure during 24 h as assessed at 2, 8 and 24 h, while heart rate was not altered. Increased urinary excretion of water and electrolytes was observed in 3- and 5-week DOCA-salt rats during the period 0-8 h after i.c.v. injection of RU28318 while the suppressed plasma renin activity was not affected. The expression of brain angiotensinogen, ANP and mineralocorticoid receptor mRNA was not altered by 3-week DOCA-salt treatment, but 3 h after i.c.v. injection of RU28318, mineralocorticoid receptor mRNA expression in hippocampal cell fields responded with an increase of about 40%. In conclusion, these results demonstrate that in rats with hypertension induced by DOCA-salt, brain mineralocorticoid receptor blockade affects renal function and blood pressure regulation.     

Intracerebroventricular choline reverses hypotension induced by acute chemical sympathectomy

1 The effect of centrally administered choline on blood pressure was investigated in rats made hypotensive by chemical sympathectomy. Chemical sympathectomy was produced by intravenous (i.v.) injection of 50 mg kg^?1 of 6-hydroxydopamine (6--OHDA). Intracerebroventricular (i.c.v.) administration of choline (50–150 μg) 2 h after 6-OHDA treatment increased blood pressure and reversed the hypotension in a dose-dependent manner without affecting heart rate. The pressor response was associated with an increase in plasma vasopressin levels. 2 Pretreatment of rats with the nicotinic receptor antagonist, mecamylamine (50 μg, i.c.v.), but not the muscarinic receptor antagonist atropine (10 μg, i.c.v.), blocked both the pressor and vasopressin responses to choline (150 μg). Pretreatment of rats with hemicholinium-3 (HC-3), a high affinity choline uptake inhibitor, greatly attenuated the pressor response to i.c.v. choline (150 μg). 3 The vasopressin V1 receptor antagonist, (β-mercapto-β,β-cyclopentamethylenepropionyl-O-Me-Try,Arg)-vasopressin (10 μg kg^?1; i.v.), given 5 min after i.c.v. choline, decreased the blood pressure but failed to return it to the pre-choline levels. Prazosine (0.5 mg kg^?1; i.p.), an antagonist of α-adrenoceptors, also decreased blood pressure. Administration of both antagonists together eliminated the pressor response to choline, and the blood pressure was reduced further to below the pre-choline levels. 4 It is concluded that i.c.v. choline can increase blood pressure in rats made hypotensive by acute chemical sympathectomy through the activation of central nicotinic receptors by presynaptic mechanisms. An elevation in plasma levels of both vasopressin and catecholamines (possibly released from the adrenal medulla) is involved in the pressor response to choline.     

Central effects of endothelin and its antagonists on sympathetic and cardiovascular regulation in SHR-SP.

Intracerebroventricular injections of endothelin-1 (ET-1) are reported to cause dose-related increases in sympathetic nerve activity and blood pressure in anesthetized normotensive rats. These studies were performed to determine the following: which endothelin receptor, A or B, is involved in mediating sympathetic and cardiovascular effects of ET-1 injected centrally; whether central endothelin tonically participates in blood pressure regulation in normotensive rats; and whether the altered endothelin system in the central nervous system contributes to blood pressure elevation in hypertensive rats. ET-1, ET-A antagonist (BQ-123), or ET-B antagonist (RES-701-1) was injected into the lateral cerebral ventricle (i.c.v.) of urethane-anesthetized normotensive Wistar and Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHRs), and stroke-prone SHRs (SHR-SPs). In Wistar rats, i.c.v. injections of ET-1 (1, 5, 10 pmol) consistently increased sympathetic nerve activity, thereby elevating blood pressure in a dose-related manner. The pressor responses induced by i.c.v. ET-1 were abolished after intravenous pretreatment with phentolamine. Neither ET-A nor ET-B antagonist, when injected centrally, altered basal levels of sympathetic nerve activity, heart rate, or blood pressure in Wistar rats. However, sympathetic activation and pressor responses induced by i.c.v. injection of endothelin were completely abolished after i.c.v. pretreatment with ET-A antagonist but were unaffected after pretreatment with ET-B antagonist. Although i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure in WKY rats, SHRs, and SHR-SPs, the magnitudes of these responses did not differ among these three groups. In contrast, i.c.v. injections of ET-A antagonist decreased sympathetic nerve activity, blood pressure, and heart rate only in SHR-SPs, but not in WKY rats and SHRs. In addition, the depressor effects of i.c.v. ET-A antagonist in SHR-SPs were ascertained while these rats were awake. In summary, i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure via ET-A receptors but not via ET-B receptors. Central ET might tonically activate sympathetic nerve activity to thereby contribute to blood pressure elevation in SHR-SPs, but not in WKY rats and SHRs.     

Reversal of Haemorrhagic Shock in Rats by Tetrahydroaminoacridine

The cardiovascular effects of tetrahydroaminoacridine (tacrine; THA) were investigated in haemorrhaged rats. Intracerebroventricular (i.c.v.) injection of THA (10, 25 and 50 microg) restored blood pressure in a dose- and time-dependent manner. Atropine (10 microg, i.c.v.), a muscarinic receptor antagonist, attenuated the pressor response to THA (25 microg, i.c.v.), while mecamylamine (50 microg, i.c.v.), a nicotinic receptor antagonist, caused only a slight blockade in the pressor effect of THA. Simultaneous pretreatment with atropine and mecamylamine almost abolished the blood pressure effect of i.c.v. THA (25 microg). Haemorrhage increased plasma levels of adrenaline, noradrenaline, vasopressin and plasma renin activity. THA (25 microg, i.c.v.) administration caused additional increases in vasopressin and adrenaline levels but not of renin activity and noradrenaline levels. The reversal of hypotension by THA was greatly attenuated by administration of either prazosin, an alpha(1)-adrenoceptor antagonist (0.5 mg/kg, i.v.) or by the vasopressin V(1) receptor antagonist [beta-mercapto-beta,beta-cyclopenta-methylenepropionyl(1), O-Me-Tyr(2)-Arg(8)]-vasopressin (10 microg/kg, i.v.). Pretreatment of rats with both prazosin and the vasopressin antagonist simultaneously completely inhibited the pressor response. Intravenous administration of THA (1, 1.5 and 3 mg/kg) also reversed hypotension in rats. Atropine (10 microg, i.c.v.) greatly attenuated the pressor response to THA (1.5 mg/kg, i.v.), while mecamylamine (50 microg, i.c.v.) failed to change the pressor effect of THA. In anaesthetised haemorrhaged rats, THA (1.5 mg/kg, i.v.) increased blood pressure and survival time of the animals. These results show that centrally and peripherally injected THA reverses haemorrhagic hypotension and increases survival time in rats. Activation of central muscarinic and nicotinic receptors is involved in the pressor response to i.c.v. THA. The pressor effect of i.v. THA is solely mediated by central muscarinic receptors. Moreover, the increase in plasma adrenaline and vasopressin levels appears to be involved in the pressor effect of THA.     

Restoration of blood pressure by choline treatment in rats made hypotensive by haemorrhage.

1. Intracerebroventricular (i.c.v.) injection of choline (25-150 micrograms) increased blood pressure in rats made acutely hypotensive by haemorrhage. Intraperitoneal administration of choline (60 mg kg-1) also increased blood pressure, but to a lesser extent. Following i.c.v. injection of 25 micrograms or 50 micrograms of choline, heart rate did not change, while 100 micrograms or 150 micrograms i.c.v. choline produced a slight and short lasting bradycardia. Choline (150 micrograms) failed to alter the circulating residual volume of blood in haemorrhaged rats. 2. The pressor response to i.c.v. choline (50 micrograms) in haemorrhaged rats was abolished by pretreatment with mecamylamine (50 micrograms, i.c.v.) but not atropine (10 micrograms, i.c.v.). The pressor response to choline was blocked by pretreatment with hemicholinium-3 (20 micrograms, i.c.v.). 3. The pressor response to i.c.v. choline (150 micrograms) was associated with a several fold increase in plasma levels of vasopressin and adrenaline but not of noradrenaline and plasma renin. 4. The pressor response to i.c.v. choline (150 micrograms) was not altered by bilateral adrenalectomy, but was attenuated by systemic administration of either phentolamine (10 mg kg-1) or the vasopressin antagonist [beta-mercapto-beta,beta-cyclopenta-methylenepropionyl1, O-Me-Tyr2,Arg8]-vasopressin (10 micrograms kg-1). 5. It is concluded that the precursor of acetylcholine, choline, can increase and restore blood pressure in acutely haemorrhaged rats by increasing central cholinergic neurotransmission. Nicotinic receptor activation and an increase in plasma vasopressin and adrenaline level appear to be involved in this effect of choline.     

Cardiovascular effects of intracerebroventricularly injected CDP-choline in normotensive and hypotensive animals: the involvement of cholinergic system

Intracerebroventricular (i.c.v.) administration of CDP-choline (0.25, 0.5, 1 and 2 micromol) induced prompt, dose- and time-dependent increase in blood pressure in normotensive rats. Equimolar dose of CDP-choline (1 micromol; i.c.v.) and choline (1 micromol; i.c.v.) caused similar increases in blood pressure while cytidine (1 micromol; i.c.v.) failed to produce any pressor effect. In haemorrhagic shock, CDP-choline (0.1, 0.25, 0.5 and 1 micromol; i.c.v.) increased blood pressure dose- and time-dependently. The complete reversal of hypotension was observed with the i.c.v. injection of CDP-choline (1 micromol) and choline (1 micromol). Cytidine (1 micromol; i.c.v.) produced small, but significant ( P<0.05) increase in blood pressure in haemorrhaged rats. Dose-related bradycardia was observed with the injection of CDP-choline in normotensive rats, but the changes in heart rate were not significantly different ( P>0.05) in hypotensive conditions. Choline levels in lateral cerebral ventricle and hypothalamus increased about nine- and fivefold, respectively, after CDP-choline (1 micromol) administration in normotensive rats. In haemorrhagic shock extracellular choline levels in hypothalamus increased sevenfold after an i.c.v. administration of CDP-choline (1 micromol). Hemicholinium-3 (20 microg; i.c.v.), a neuronal high affinity choline uptake blocker, and mecamylamine (50 microg; i.c.v.), nicotinic receptor antagonist, pretreatment abolished the pressor effect of CDP-choline in normal rats. The increase in blood pressure was also attenuated by atropine (10 microg; i.c.v.) pretreatment. Atropine blocked the bradycardic response observed after CDP-choline. In haemorrhaged rats, the pressor effect of CDP-choline was attenuated by hemicholinium-3 and mecamylamine while atropine failed to alter the pressor response to CDP-choline. I.c.v. CDP-choline increased plasma adrenaline and vasopressin levels in normal rats. Haemorrhage, itself, increased plasma catecholamines and vasopressin levels. CDP-choline (1 micromol) produced additional increases in the elevated plasma levels of these hormones. An alpha(1)-adrenoceptor blocker, prazosin (0.5 mg/kg; i.v.), or vasopressin V(1) receptor antagonist, [beta-mercapto, beta,beta-cyclopenta-methylenepropionyl(1), O-Me-Tyr(2)-Arg(8)]-vasopressin (10 micro/kg; i.v.), pretreatments partially blocked the pressor response to CDP-choline (1 micromol; i.c.v.). Simultaneous administration of these two antagonists completely blocked the pressor effect of CDP-choline in haemorrhagic shock. These results show that the exogenous administration of CDP-choline increases blood pressure and reverses hypotension in haemorrhagic shock. In normotensive conditions, increase in blood pressure appears to be due to the activation of both nicotinic and muscarinic central cholinergic receptors through the activation of presynaptic cholinergic mechanisms. In hypotensive rats, activation of nicotinic cholinergic receptors is solely involved in the pressor effect. Increase in plasma vasopressin and adrenaline mediates the pressor response of CDP-choline in both normotensive and hypotensive conditions.     

THE CARDIOVASCULAR EFFECTS OF CENTRALLY ADMINISTERED 5-HYDROXYTRYPTAMINE IN THE CONSCIOUS NORMOTENSIVE AND HYPERTENSIVE RAT

• 1 The cardiovascular effects of centrally administered 5-hydroxytryptamine (5-HT) have been analysed in conscious normotensive and hypertensive rats.
• 2 In conscious normotensive rats, 5-HT, (1–30 μg) administered intracerebroventricularly (i.c.v.) produced profound and immediate dose-related decreases in heart rate and small increases in blood pressure. The initial pressor responses were followed by secondary secondary depressor responses at high doses of 5-HT.
• 3 Similar effects were produced by 5-HT i.c.v. in conscious DOCA-salt and spontaneously hypertensive rats, although the magnitude of the pressor responses was substantially greater in hypertensive than normotensive rats.
• 4 Pretreatment with either N-methylatropine or atenolol intra-arterially reduced the 5-HT-induced bradycardia in normotensive rats; the reduction was enhanced when both antagonists were given in combination.
• 5 The 5-HT₂ antagonist, cyproheptadine (10 μg i.c.v.) increased basal blood pressure and heart rate in normotensive rats. Subsequent administration of 5-HT i.c.v. produced biphasic effects on heart rate consisting of an initial tachycardia followed by a marked bradycardia.
• 6 Methysergide (10 μg i.c.v.) pretreatment did not alter resting heart rate, but attenuated the 5-HT induced bradycardia. A higher dose of methysergide, (30 μg i.c.v.), decreased resting blood pressure and heart rate.
• 7 This study has demonstrated, therefore, that the 5-HT induced bradycardia is produced by not only a centrally mediated decrease in sympathetic tone, but also an increase in vagal drive to the heart. The bradycardia is antagonised by centrally administered methysergide, but not by cyproheptadine, which suggests that it is probably mediated through a ‘5-HT₁-like’ receptor mechanism.

     

Choline potentiates the pressor response evoked by glycyl-glutamine or naloxone in haemorrhaged rats

1. Severe blood loss initially lowers arterial pressure through a central mechanism that is thought to involve opioid and cholinergic neurons. The present study tested the hypothesis that simultaneous administration of a cholinergic agonist and an opioid receptor antagonist would produce a synergistic effect in the treatment of haemorrhage. Specifically, we tested whether choline, a precursor of acetylcholine, potentiates the pressor effect of the beta-endorphin derived peptide glycyl-glutamine (Gly-Gln) or the opioid receptor antagonist naloxone following acute haemorrhage. 2. Conscious rats were treated intracerebroventricularly (i.c.v.) with choline chloride (180 nmol) alone or combined with Gly-Gln (10 nmol) or naloxone (10 nmol) 2 min after blood withdrawal (2.5 mL/100 g bodyweight over 20 min) was completed; mean arterial pressure and heart rate were monitored for 30 min. 3. Combined treatment with choline and Gly-Gln elevated mean arterial pressure but did not affect heart rate significantly. Choline and Gly-Gln had no effect on cardiovascular function when administered alone to haemorrhaged rats or when given together to normotensive animals. Choline also potentiated the pressor and tachycardic effect of naloxone in haemorrhaged rats. 4. These data show that choline potentiates the pressor effect of Gly-Gln and naloxone in haemorrhaged rats.     

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.     

Paraventricular nucleus of the hypothalamus glutamate neurotransmission modulates autonomic,neuroendocrine and behavioral responses to acute restraint stress in rats

In the present study, the involvement of paraventricular nucleus of the hypothalamus (PVN) glutamate receptors in the modulation of autonomic (arterial blood pressure, heart rate and tail skin temperature) and neuroendocrine (plasma corticosterone) responses and behavioral consequences evoked by the acute restraint stress in rats was investigated. The bilateral microinjection of the selective non-NMDA glutamate receptor antagonist NBQX (2 nmol/ 100 nL) into the PVN reduced the arterial pressure increase as well as the fall in the tail cutaneous temperature induced by the restraint stress, without affecting the stress-induced tachycardiac response. On the other hand, the pretreatment of the PVN with the selective NMDA glutamate receptor antagonist LY235959 (2 nmol/100 nL) was able to increase the stress-evoked pressor and tachycardiac response, without affecting the fall in the cutaneous tail temperature. The treatment of the PVN with LY235959 also reduced the increase in plasma corticosterone levels during stress and inhibited the anxiogenic-like effect observed in the elevated plus-maze 24 h after the restraint session. The present results show that NMDA and non-NMDA receptors in the PVN differently modulate responses associated to stress. The PVN glutamate neurotransmission, via non-NMDA receptors, has a facilitatory influence on stress-evoked autonomic responses. On the other hand, the present data point to an inhibitory role of PVN NMDA receptors on the cardiovascular responses to stress. Moreover, our findings also indicate an involvement of PVN NMDA glutamate receptors in the mediation of the plasma corticosterone response as well as in the delayed emotional consequences induced by the restraint stress.     

Role of cholinergic neurons in the cardiovascular responses evoked by central injection of bradykinin or angiotensin II in conscious rats

Intracerebroventricular (i.c.v.) injection of bradykinin (0.1–10 μg) or angiotensin II (0.01–10 μg) in conscious, freely moving rats evoked dose-related increases in arterial pressure. The pressor response to bradykinin (BK) was accompanied by an increase in heart rate while angiotensin II (ANG II) decreased heart rate. Pretreatment with hemicholinium-3 to deplete brain acetylcholine levels produced a choline-reversible blockade of the cardiovascular response to BK. In contrast, the pressor response to ANG II was only weakly inhibited by hemicholinium-3 and the bradycardia was unaffected. Central pretreatment with the nicotinic antagonist, hexamethonium (50 μg) was more effective than the muscarinic antagonist atropine (20 μg) at blocking the cardiovascular responses to i.c.v. injection of BK. Both blocking agents produced a weaker inhibitory effect on the pressor response to ANG II although no anticholinergic pretreatment significantly inhibited the fall in heart rate. These results are consistent with the possibility of a peptidergic-cholinergic interaction in the central cardiovascular actions of BK and perhaps for a component of the pressor response to ANG II.     

NORMOTENSIVE WISTAR RATS DIFFER FROM SPONTANEOUSLY HYPERTENSIVE AND RENAL HYPERTENSIVE RATS IN THEIR CARDIOVASCULAR RESPONSES TO OPIOID AGONISTS

1. The effects of three opioid receptor agonists on the blood pressure and heart rate of anaesthetized normotensive, spontaneously hypertensive and renal hypertensive rats were measured. 2. Mu agonist morphiceptin i.c.v. induced a pressor response and increase in heart rate in hypertensive rats, but hypotension in normotensive rats. After intravenous (i.v.) injection, morphiceptin produced a hypotensive response in all three groups of rats. 3. In contrast, the delta agonist DTLET i.c.v. decreased blood pressure and heart rate in hypertensive rats, but increased both pressure and beat rate in normotensive rats. After i.v. injections DTLET produced a hypertensive response and increase in heart rate in all groups of rats. 4. Kappa agonist U-50, 488H given i.c.v. induced effects similar to morphiceptin: an increase in blood pressure and heart rate in hypertensive and a decrease in normotensive rats. After i.v. injections U-50, 488H produced decreases in blood pressure and heart rate in all treated groups of rats. 5. Pretreatment with naloxone antagonized the activity of morphiceptin but prevented only the stimulating effect of DTLET in normotensive rats. Cardiovascular actions of U-50, 488H were not blocked by naloxone. 6. The results suggest that opioid agonists exert similar changes in cardiovascular function at central and peripheral sites in both models of experimental hypertension and these effects are different in normotensive rats.     

Vasopressin: mechanism of central cardiovascular action in conscious rats

Arginine vasopressin (AVP) and AVP binding sites have been localized in brain areas involved in cardiovascular control. To elucidate the mechanisms by which brain AVP may participate in central blood pressure regulation, we investigated the effects of central AVP receptor stimulation on mean arterial pressure (MAP), heart rate (HR), efferent splanchnic nerve (SpNA), and renal nerve activity (RNA) in conscious chronically instrumented rats. Intracerebroventricular (i.c.v.) injections of AVP (1-100 ng) produced dose-dependent increases in MAP together with marked increases in HR, SpNA, and RNA. The pressor responses were inhibited by peripheral alpha-adrenoceptor blockade with i.v. phentolamine. In contrast, the pressor responses to either i.v. or intracarotid injections of AVP were accompanied by baroreceptor reflex (BRR)-mediated decreases in HR and SpNA. Central (i.c.v.) pretreatment with d(CH2)5AVP, a V1-AVP receptor antagonist, completely abolished the responses to i.c.v. but not to i.v. AVP. The same antagonist had no effect on the responses to i.c.v. angiotensin II. Our results demonstrate that: (a) central AVP overrides the BRR by sympathetic stimulation, whereas blood-borne AVP activates the BRR; (b) specific AVP receptors in the brain probably of the V1-subtype are involved in the central pressor responses to AVP; (c) the central pressor actions of AVP are transmitted to the periphery by stimulation of the sympathetic nervous system. We conclude that brain AVP may contribute to central cardiovascular control by modulating the sympathetic outflow to the periphery.     

Sympathetic inhibition and vasopressin mediation during centrally induced responses to serotonin in rats

To study mechanisms underlying the cardiovascular effects of centrally administered serotonin, we recorded responses to intracerebroventricular (i.c.v.) injections of serotonin in urethane-anesthetized rats. Dose-related increases in blood pressure accompanied by reductions in heart rate and sympathetic nerve firing were elicited consistently. The diminution in sympathetic nerve activity implies that while sympathetic inhibition could contribute to the bradycardia, sympathetic activation alone cannot account for the pressor response. Pressor and bradycardic responses must have been caused by activation of specific serotonergic receptors in the brain because both responses were inhibited following serotonin blockade produced by i.c.v. injection of methysergide. By contrast, intravenous injection of a vasopressin antagonist inhibited the pressor response selectively, thereby suggesting that peripheral mediation of the pressor (but not the bradycardic) response involves release of endogenous vasopressin. Collectively, our results are compatible with the interpretation that i.c.v.-injected serotonin acts on serotonergic brain receptors to elevate blood pressure by releasing endogenous vasopressin, and slow the heart through sympathetic inhibition.     

Intravenously injected CDP-choline increases blood pressure and reverses hypotension in haemorrhagic shock: effect is mediated by central cholinergic activation

Intravenous (i.v.) administration of cytidine-5′-diphosphate choline (CDP-choline) (100, 250 and 500 mg/kg) increased blood pressure in normal rats and reversed hypotension in haemorrhagic shock. Choline (54 mg/kg; i.v.), at the dose equimolar to 250 mg/kg CDP-choline decreased blood pressure of rats in both conditions and caused the death of all hypotensive animals within 2-5 min. Equimolar dose of cytidine (124 mg/kg; i.v.) did not change cardiovascular parameters. Choline levels in plasma, lateral cerebral ventricle and hypothalamus increased after CDP-choline administration. Intracerebroventricular (i.c.v.) hemicholinium-3 pretreatment (20 μg), greatly attenuated the pressor effect of CDP-choline in both conditions. Atropine pretreatment (10 μg; i.c.v.) did not change the pressor effect of CDP-choline while mecamylamine (50 μg; i.c.v.) abolished the pressor response to drug. Besides, acetylcholine (1 μmol; i.c.v.) produced similar increases in blood pressure in normal and hypotensive conditions to that observed in CDP-choline given rats. CDP-choline (250 mg/kg; i.v.) increased plasma catecholamines and vasopressin levels but not plasma renin activity. Pretreatment of rats with either prazosin (0.5 mg/kg; i.v.) or vasopressin V₁ receptor antagonist, [β-mercapto,β,β-cyclopentamethylenepropionyl¹,O-Me-Tyr²-Arg⁸]vasopressin (10 μg/kg; i.v.), attenuated the pressor response to CDP-choline while simultaneous administration of these antagonists before CDP-choline injection completely blocked the pressor effect. Results show that i.v. CDP-choline increases blood pressure and reverses hypotension in haemorrhagic shock. Activation of central nicotinic cholinergic mechanisms by the increases in plasma and brain choline concentrations appears to be involved in the pressor effect of this drug. Moreover, the increases in plasma catecholamines and vasopressin levels mediate these effects.     

Exaggerated pressor response to centrally administered renin in freely moving, spontaneously hypertensive rats

The cardiovascular responses to intracerebroventricular (i.c.v.) injection of renin were compared between freely moving normotensive Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The i.c.v. injection of renin (0.05-1.0 mU) produced a dose-dependent and a long-lasting rise in mean blood pressure associated with variable changes in heart rate (HR) in both WKY and SHR. However, the blood pressure and HR were not affected by intravenously injected renin (0.1 mU). The pressor response to i.c.v. injected renin was greater in SHR than in WKY, the dose-response curve for renin in SHR being to the left of that in WKY. Central (i.c.v.) pretreatment with [Sar1, Ile8]angiotensin II (50 micrograms) largely abolished the pressor response to i.c.v. injected renin in both WKY and SHR. The i.c.v. injection of angiotensin II (ANG II) (10-100 ng) induced a dose-dependent pressor response which was antagonized by central pretreatment with [Sar1, Ile8]ANG II (50 micrograms). The pressor response to ANG II was also greater in SHR than in WKY. These results suggest that the pressor response to centrally administered renin as well as to ANG II, which is mediated via ANG II receptors located in the brain, is enhanced in SHR.     

CARDIOVASCULAR EFFECTS OF CENTRALLY ADMINISTERED ARACHIDONIC ACID IN HAEMORRHAGE-INDUCED HYPOTENSIVE RATS: INVESTIGATION OF A PERIPHERAL MECHANISM

• 1 The aims of the present study were to determine the cardiovascular effects of arachidonic acid (AA) and to investigate the peripheral mechanisms mediating these effects in haemorrhage‐induced hypotensive rats.
• 2 Acute haemorrhage was induced by withdrawing a total volume of 2.2 mL blood/100 g bodyweight over a period of 10 min. Rats were then injected with 75–300 µg, i.c.v., AA and cardiovascular changes were monitored over the next 60 min. Plasma catecholamine and vasopressin levels, as well as plasma renin activity (PRA), were measured 10 min after injection of 150 µg AA in haemorrhage‐induced hypotensive awake rats. In addition, rats were pretreated with saline (1 mL/kg, i.v.), the vasopressin V₁ receptor antagonist [β‐mercapto‐β,β‐cyclopentamethylenepropionyl¹,O‐Me‐Tyr²,Arg⁸]‐vasopressin (10 µg/kg, i.v.), the α₁‐adrenoceptor antagonist prazosin (500 µg/kg, i.v.), the non‐specific angiotensin II receptor antagonist saralasin (250 µg/kg, i.v.) or a combination of these three antagonists 5 min before injection of AA (150 µg, i.c.v.). The effects of these antagonists on responses to AA were determined.
• 3 Arachidonic acid caused dose‐ and time‐dependent increases in mean arterial pressure and heart rate and reversed hypotension in haemorrhaged rats. Haemorrhage itself produced an increase in plasma catecholamine and vasopressin levels, as well as PRA; injection of AA produced further increases in these parameters, ranging from 39–123%, under hypotensive conditions. Under hypotensive conditions, pretreatment of rats with all three receptor antagonists produced similar partial blockade of the pressor response to AA, but not the increase in heart rate. Moreover, combined administration of all three receptor antagonists prior to the i.c.v. injection of 150 µg AA completely abolished the pressor response to AA in haemorrhage‐induced hypotensive rats.
• 4 These results indicate that centrally administered AA reverses hypotension by increasing blood pressure and heart rate in the hypotensive setting. The observed increases in plasma catecholamine and vasopressin levels, as well as PRA, mediate the pressor response to AA in haemorrhage‐induced hypotensive rats.

     

Spontaneously hypertensive rats cholinergic hyper-responsiveness: central and peripheral pharmacological mechanisms.

1. The mechanisms and the subtypes of muscarinic receptors implicated in the cardiovascular effects of physostigmine were investigated in conscious normotensive and spontaneously hypertensive rats. 2. Intravenous (i.v.) physostigmine (50 microg kg-1) induced in both strains a long pressor response, accompanied by a bradycardia. This pressor response was larger in spontaneously hypertensive (+41+/-6 mmHg) than in Wistar-Kyoto (+25+/-2 mmHg) rats (P<0.05). 3. Pretreatment with atropine sulphate (0.4 mg kg-1 i.v.), completely abolished the physostigmine-induced pressor response in both normotensive and hypertensive rats. In both strains, the physostigmine pressor response was significantly reduced by the systemic administration of either an alpha1-adrenoceptor antagonist (prazosin, 1 mg kg-1) or a V1A-vasopressin receptor antagonist (AVPX, 20 microg kg-1). This physostigmine pressor effect was completely abolished in both strains when both antagonists were administered concomitantly. 4. In WKY rats, the pressor response to physostigmine (50 microg kg-1 i.v.) was inhibited in a dose-dependent manner by i. c.v. administration of atropine (ID50=3.70 nmoles), the M1 receptor antagonist pirenzepine (ID50=10.71 nmoles), the M2 receptor antagonist methoctramine (ID50=4.31 nmoles), the M3 receptor antagonist p-F-HHSiD (ID50=60.52 nmoles) and the M4 receptor antagonist tropicamide (ID50=214.20 nmoles). In the hypertensive strain, the ID50 were found to be significantly higher for atropine (7.34 nmoles), pirenzepine (21.60 nmoles) and p-F-HHSiD (139.50 nmoles) (P<0.05). 5. The present results indicate that physostigmine acts in normotensive and spontaneously hypertensive rats, through stimulation of both central M2 and M1 cholinoceptors to induce a rise in blood pressure mediated by an increase in plasma vasopressin and sympathetic outflow. Moreover, our results suggest that some modifications of the M1 receptor subtypes in terms of expression or affinity could be responsible for the hyper-responsiveness of the hypertensive strain to cholinomimetic agents.     

IMPORTANCE OF ENDOGENOUS VASOPRESSIN IN THE HYPERTENSIVE AND BRADYCARDIC RESPONSE TO CENTRAL α1-ADRENERGIC STIMULATION

To determine the interaction between alpha 1-adrenergic and vasopressinergic mechanisms in the central regulation of cardiovascular functions, the effects of intracerebroventricular (i.c.v.) administration of the alpha 1-agonists, methoxamine and phenylephrine, were examined in conscious Long-Evans (LE) rats and Brattleboro rats with hereditary hypothalamic diabetes insipidus (DI). In LE rats, i.c.v. methoxamine and phenylephrine (3-30 micrograms/kg) increased blood pressure and decreased heart rate in a dose-related manner, while they had no detectable cardiovascular effects in DI rats. Neither i.c.v. (0.5 ng/kg per min, 1 h) nor intravenous (i.v., 2 ng/kg per min, 2 h) infusion of vasopressin (AVP) restored the cardiovascular response to i.c.v. phenylephrine in DI rats. In LE rats, however, i.v. pretreatment with the specific antagonist to the pressor effect of AVP, d(CH2)5Tyr(Me)AVP (10 micrograms/kg), attenuated the hypertensive and bradycardic effects of i.c.v. phenylephrine, while i.c.v. pretreatment with AVP antagonist (300 ng/kg) did not alter the cardiovascular response to i.c.v. alpha 1-agonist. The cardiovascular response to i.c.v. phenylephrine was blocked by i.c.v. pretreatment with the alpha 1-antagonist, prazosin (2 micrograms/kg). Intracerebroventricular phenylephrine increased plasma AVP levels 14-fold without affecting plasma angiotensin II levels. The present study clearly demonstrated that endogenous AVP plays a significant role in the cardiovascular response to i.c.v. alpha 1-agonist.     

Effects of intracerebroventricular injected choline on cardiovascular functions and sympathoadrenal activity.

Intracerebroventricular (i.c.v.) injection of choline (50-150 micrograms) increased blood pressure (SP) and decreased heart rate (HR) in freely moving rats. Intracerebroventricular pretreatment of rats with mecamylamine (50 micrograms) blocked the reduction in HR and reduced the increase in SP induced by i.c.v. choline (150 micrograms). Central muscarinic blockade with atropine (10 micrograms, i.c.v.) reduced the pressor response to i.c.v. choline (150 micrograms) by about 70%, without influencing the decrease in HR. The decrease in HR induced by i.c.v. choline was prevented by intraarterial (i.a.) treatment of atropine methylnitrate (2 mg/kg). Intracerebroventricular choline (150 micrograms) produced a fivefold increase in catecholamine concentrations in adrenal venous plasma. Bilateral adrenalectomy reduced, but did not block, choline's effect on SP. Intracerebroventricular choline (150 micrograms) showed an ability to increase and restore SP in rats subjected to spinal cord transection or pretreatment with hexamethonium (15 mg/kg, i.a.) or with phentolamine (10 mg/kg, i.a.). Intracerebroventricular choline (150 micrograms) increased plasma vasopressin (VP) levels from 2.2 +/- 0.4 to 25.6 +/- 2.5 pg/ml. Pretreatment of rats with a VP antagonist reduced the pressor response to i.c.v. choline. It is concluded that (a) the reduction in HR results from a central nicotinic receptor-mediated increase in vagal tone, (b) the increase in SP appears to be due to activation of both nicotinic and muscarinic central cholinergic receptors, and that (c) the central activation of the adrenal medulla and the increase in plasma levels of VP are involved in the pressor response to i.c.v. choline.