Role of nitric oxide in the development of corticotropin-induced hypertension in sheep.

1. The possibility that altered synthesis of vascular nitric oxide (NO) plays a role in the development of corticotropin-induced hypertension in sheep was examined by determining the effect of concomitant infusion of L-arginine, a precursor of NO, on the development of the hypertension. 2. Corticotropin (5 microg/kg per h) infused over 2 days increased mean arterial pressure (MAP) from 83+/-4 to 99+/-4 mmHg in five conscious sheep. Concomitant infusion of L-arginine (60 mg/kg per h) did not alter this response; infusion of L-arginine alone had no effect on blood pressure. 3. The dose of L-arginine (60 mg/kg per h) used blocked the rise in MAP (+16 mmHg) in response to a 5 h infusion of N-nitro-L-arginine (1 mg/kg per h). 4. These findings suggest that disruption of NO synthesis does not play a role in the development of corticotropin hypertension in sheep.

Evidence against a central pressor mechanism for adrenocortical steroid hypertension in sheep.

The possibility that corticotropin (ACTH)-induced hypertension results from a direct central effect of the adrenocortical steroids released by ACTH was investigated in sheep. Using two approaches, steroid levels were increased in the brain while peripheral levels remained sub-pressor. The blood pressure response to intravenous infusion of a combination of 7 steroids (aldosterone, cortisol, deoxycorticosterone, corticosterone, 11-deoxycortisol, 17 alpha hydroxyprogesterone and 17 alpha 20 alpha dihydroxyprogesterone), which causes a similar pressor effect to ACTH, was compared with that caused by intracarotid infusion of the steroids at rates calculated to give concentrations in the brain equivalent to those achieved after intravenous infusion. We also examined the effects of infusing the combination of steroids directly into the central nervous system via the lateral cerebral ventricles. Intravenous infusion of the steroids increased mean arterial pressure (MAP) from a control average of 84.0 +/- 1.1mmHg to 98.2 +/- 2.2mmHg (p < 0.001) on day 5. There was no increase in MAP during intracarotid infusion, nor during intracerebroventricular infusion. These findings suggest that the adrenocortical steroids released by ACTH do not act directly on central steroid receptors to increase blood pressure.

Role of nitric oxide in the attenuated pressor responses of pregnant or sodium-deplete sheep.

Reduced pressor responsiveness to angiotensin II (Ang II) during pregnancy and sodium depletion is a well-known but little understood phenomenon; whether the same mechanisms are involved in both situations is unclear. In pregnant humans, altered vascular reactivity to norepinephrine (NE) has also been demonstrated. Nitric oxide (NO) has been implicated in the modulation of blood pressure (BP) and the maintenance of vascular tone and may be involved in these attenuated responses. We examined the role of NO in the pressor responses to (a) Ang II (5, 10, 25, 50 micrograms/h) and NE (0.32, 0.65, 1.62, 3.24 mg/h) in pregnant and postpartum sheep, and (b) to Ang II (5, 7, 5, 10, 25, 50 micrograms/h) in sodium-replete sheep and sheep made sodium deplete by 24 h of parotid salivary drainage. Vascular NO production was inhibited by pretreatment with N omega-nitro-L-arginine (NOLA 10 mg/kg), a NO-synthase inhibitor. Pregnancy significantly reduced (p < 0.001) pressor responses to Ang II, which ranged from 5.1 +/- 0.2-30.6 +/- 1.2 mm Hg as compared with postpartum increases of 10.3 +/- 0.5-52.2 +/- 3.4 mm Hg. Pretreatment with NOLA partially restored Ang II responses to postpartum levels. Pregnancy did not alter pressor responses to NE. Sodium depletion also significantly reduced responses to Ang II by the same amount as in pregnancy, and these responses returned to normal with pretreatment with NOLA. NO thus has a role in modulating the attenuated pressor responses to Ang II in pregnant and sodium-deplete sheep.

Prolonged regional vasoconstriction produced by NG-nitro-L-arginine in conscious sheep.

Nitric oxide (NO) is a potent endothelium-derived vasodilator whose synthesis can be blocked both in vitro and in vivo by structural analogues of its precursor, L-arginine (L-ARG). We examined the dose-response profile of one such analogue, NG-nitro-L-arginine (NOLA) in conscious sheep (n = 4) and used continuous monitoring techniques to study long-term changes in mean arterial pressure (MAP), heart rate (HR), and cardiac output (CO) and the relative responsiveness of the coronary, mesenteric, renal, and hindlimb vascular beds to NOLA [10 mg/kg, intravenous (i.v.) bolus] in 5 sheep. NOLA (3 and 10 mg/kg) increased MAP at 1 h from 73 +/- 4 to 86 +/- 3 mm Hg (p < 0.05) and 73 +/- 1 to 106 +/- 8 mm Hg (p < 0.05), respectively. CO and HR decreased significantly after 10 mg/kg NOLA. Plasma endothelin (ET) level was unchanged after all doses of NOLA. Continuous monitoring of MAP, CO, and blood flow for 24 h before and after NOLA injection showed that MAP increased rapidly owing to a decrease in total peripheral conductance (TPC), with short-term reflex decreases in HR and prolonged decreases in CO and stroke volume (SV). Coronary and iliac conductances changed comparatively little. Renal conductance decreased by 43% at 80 min, but was not different from control after 6 h. The greatest and most sustained decrease in conductance, by a maximum of 55% of control levels at 110 min, occurred in the mesenteric bed.(ABSTRACT TRUNCATED AT 250 WORDS)

Haemodynamic and hormonal effects of N-nitro-L-arginine, an inhibitor of nitric oxide biosynthesis, in sheep.

1. The haemodynamic and hormonal responses to N-nitro-L-arginine (NOLA), a potent inhibitor of nitric oxide biosynthesis in endothelial cells, were investigated in conscious sheep. 2. Mean arterial blood pressure (MAP), heart rate (HR) and cardiac output by thermodilution (CO) were measured in four oophrectomized ewes. Two other ewes were surgically implanted with aortic electromagnetic flow probes and an indwelling carotid arterial line for monitoring CO and MAP over 40 h. 3. After a control period, NOLA (10 mg/kg) was injected intravenously and MAP, HR and CO monitored and blood samples taken at intervals over the following 24 h. 4. NOLA increased blood pressure within minutes, from 76 +/- 4 to a maximum of 99 +/- 4 mmHg (P less than 0.001) at 6 h after injection. It remained elevated 24 h after injection. CO and HR fell but these falls were not sustained longer than 6 h. Calculated total peripheral resistance increased to a maximum of 2 h, but had returned to control levels 24 h after injection. There were no significant changes in plasma concentrations of renin, atrial natriuretic factor, vasopressin, noradrenaline or endothelin during the first hour. 5. NOLA may be a useful tool in understanding the role of the endothelium and nitric oxide in the control of blood pressure.

Lack of pressor response to intracerebroventricular infusion of aldosterone in sheep.

1. Studies in the rat and the dog have shown that infusion of aldosterone for several weeks into the cerebral ventricles (ICV) can produce hypertension at doses that do not have an effect when infused systemically. We have previously shown that a high physiological dose of aldosterone infused intravenously at 10 micrograms/h in sheep produces an increase in blood pressure of 7 mmHg within 2 days. 2. In this paper we report the effects of ICV infusion of aldosterone at 2 micrograms/h for 6 days in conscious sheep. 3. Neither blood pressure nor heart rate were altered, and there were no consistent changes in any of the metabolic parameters measured. 4. These results do not support a role for central effects of aldosterone in the hypertension produced by systemic infusion of the steroid in sheep.

Cyclosporine-induced hypertension in sheep. The role of thromboxanes.

Thromboxanes have been implicated in the CsA-induced hemodynamic changes and impairment in renal function in humans and in rats. We have previously shown that administration of intravenous CsA to sheep for 5 days at 12 mg/kg/day produces a hypertension that is resistance mediated and independent of nephrotoxicity. In this study we used a thromboxane synthetase inhibitor, U63,557A, to examine the role of thromboxanes in the CsA-induced hypertension in the sheep. The thromboxane synthetase inhibitor had no effect on blood pressure in normotensive sheep. Serum thromboxane levels were not elevated with CsA, and the inhibitor had a minimal effect on blood pressure during CsA treatment, suggesting that thromboxanes are not a major contributor to the rise in blood pressure seen in the sheep. A study of the dose-response relationship for CsA at 3, 6, and 24 mg/kg/day for 5 days indicated that maximal blood pressure responses were attained with 6 mg/kg/day.

Central effect of the enkephalin analogue FK-33824 on vasopressin secretion in conscious sheep.

The role of opioids in the regulation of arginine vasopressin release from the posterior pituitary is a subject of controversy. In the present study, we examined the effects of central administration of met-enkephalin, leu-enkephalin, the enkephalin analogue FK-33824, and the opiate antagonist naloxone, and the effects of systemic administration of met-enkephalin and FK-33824 on AVP secretion in conscious normal sheep. Intracerebroventricular infusion of FK-33824 significantly increased the plasma concentration of immunoreactive AVP in a dose-dependent manner, but met-enkephalin, leu-enkephalin and naloxone failed to change plasma concentration of AVP. Intravenous infusion of met-enkephalin and FK-33824 also failed to change plasma concentration of AVP. The opiate antagonist naloxone given both centrally and systemically attenuated the increase in plasma concentration of AVP induced by FK-33824. We conclude that basal AVP release is stimulated by central administration of FK-33824.

Effects of enkephalins and the analogue FK-33824 on mean arterial pressure and heart rate in conscious sheep.

Experiments were designed to evaluate the central and systemic effects by enkephalins and the enkephalin analogue FK-33824 on mean arterial pressure (MAP) and heart rate (HR) in conscious sheep. Intracerebroventricular infusion of FK-33824 increased both MAP and HR in a dose-dependent manner in normal sheep. The increases in MAP and HR were attenuated by naloxone administered centrally, but not systemically. Intracerebroventricular infusion of met-enkephalin, leu-enkephalin and naloxone failed to change both MAP and HR significantly. However, intravenous infusion of met-enkephalin, leu-enkephalin and FK-33824 resulted in bradycardia. Haemorrhage alone decreased both MAP and HR. Intracerebroventricular infusion of FK-33824 blunted the reduction in MAP in response to haemorrhage. The increases in MAP and HR following FK-33824 were also accompanied by elevated levels of plasma renin concentration. It is suggested that the tachycardia and pressor effect produced by the intracerebroventricular administration of FK-33824 in normal conscious sheep may result from a combined action of both neural and chemical pathways which are involved in cardiovascular control, and are mediated via the mu-opioid receptors. Opioids may have opposite effects on cardiovascular control depending on the route of administration.

Met-enkephalin and the enkephalin analogue FK-33824 centrally inhibit adrenocorticotrophic hormone secretion in sheep.

1. The effects of central administration of met-enkephalin, leu-enkephalin, the enkephalin analogue FK-33824 and the opiate antagonist naloxone on plasma concentration of adrenocorticotrophic hormone (ACTH) were examined in conscious sheep. 2. Intracerebroventricular infusion of met-enkephalin and FK-33824 significantly decreased the basal plasma concentration of ACTH. 3. Intracerebroventricular infusion of FK-33824 inhibited the haemorrhage-induced increase in plasma concentration of ACTH. 4. Intracerebroventricular infusion of naloxone attenuated the central inhibition of plasma concentration of ACTH induced by FK-33824, but intravenous infusion of naloxone had no effect on the reduction in plasma concentration of ACTH induced by FK-33824. 5. These studies suggest that in sheep met-enkephalin may play a central inhibitory role in the control of ACTH secretion.

Cyclosporin A and pressor responsiveness in sheep.

Pressor response to graded infusion of angiotensin II, noradrenaline, arginine-vasopressin, and serotonin and blood pressure change following indomethacin, an inhibitor of cyclooxygenase, were examined in conscious sheep, before and during the development of cyclosporin A-induced hypertension. Cyclosporin caused an increase in mean blood pressure from 68 +/- 2 to 82 +/- 3 mm Hg (p less than 0.001) and in heart rate from 67 +/- 4 to 91 +/- 4 beats/min (p less than 0.001). Pressor and heart rate responses to all substances tested were not changed by cyclosporin treatment suggesting that changes in pressor responsiveness are unlikely to be involved in the development of cyclosporin hypertension in sheep.

Dihydrocyclosporin D in sheep: haemodynamic and renal effects.

1. This study was designed to test the haemodynamic and renal effects in sheep of dihydrocyclosporin D (dCyD), an immunosuppressant agent derived from the fungus Tolypocladium inflatum Gams. 2. dCyD was infused for 5 days at 12 mg/kg per day. Mean arterial pressure (MAP) was elevated after 24 h, but thereafter returned to control levels. Heart rate was significantly elevated throughout the infusion and was still high 24 h postinfusion. Cardiac output rose after 5 days, but total peripheral resistance was unchanged during the infusion. 3. Glomerular filtration rate, renal blood flow and effective renal plasma flow remained unchanged, although urine sodium excretion rose for the first 48 h. 4. Infusion of the castor oil-based vehicle for cyclosporin, Cremaphore EL, for 5 days in four sheep did not produce any sustained changes in any of the parameters measured.

The effects of calcium and vitamin D on blood pressure in conscious sheep.

The metabolic and haemodynamic effects of elevating plasma calcium levels were examined in both normal and ACTH-hypertensive sheep. Six weeks of dietary Ca++ supplementation did not alter plasma calcium levels, blood pressure or heart rate. Five days of CaCl2 infusion (2 mmol/h) or intravenous vitamin D injections elevated plasma ionised and total Ca++ levels and heart rate but mean arterial pressure was unchanged. As in other species, elevation of plasma Ca++ levels over 4 hours by infusion of CaCl2 at 2, 5, and 10 mmol/h increased mean arterial pressure and decreased heart rate. The course of ACTH-induced hypertension was not altered in animals supplemented with CaCl2 in their drinking water for 6 weeks nor by intravenous injection of vitamin D for 5 days. This study does not support a major role for altered plasma ionised or total Ca++ levels in the genesis of ACTH-dependent hypertension in the sheep.

Somatostatin centrally inhibits vasopressin secretion during haemorrhage.

Somatostatin is a hypothalamic inhibiting factor which has been reported to be involved in the regulation of the secretion of several anterior pituitary hormones. To determine if somatostatin plays a role in the control of vasopressin secretion from the posterior pituitary, we examined the effects of intracerebroventricular infusion of somatostatin-14 and a super-active analogue, cyclo-(N-Me-Ala-Tyr-D-Trp-Lys-Val-Phe), on the concentration of plasma vasopressin in response to haemorrhage in conscious sheep. Haemorrhage (15 ml/kg over 15 min) elevated plasma vasopressin. Treatment with either somatostatin-14 or the analogue inhibited the elevation of plasma vasopressin induced by haemorrhage. The inhibition may result from an effect of somatostatin on neurotransmitter afferent inputs to the hypothalamus which trigger vasopressin release during haemorrhage. Our study demonstrates for the first time that somatostatin administered centrally inhibits vasopressin secretion during haemorrhage in the conscious animal.

Haemodynamic and renal effects of a novel prostaglandin analogue (L644,122(+isomer)) in sheep.

1. Merino-cross ewes were given an intravenous injection of a prostaglandin analogue, (+)-4-(3-[3-[2-(1-hydroxycyclohexyl)ethyl]-4-oxo-2-thiazolidinyl]- propyl) benzoic acid, at doses of 0.01, 0.05 and 0.10 mg/kg, on separate days, to determine renal and haemodynamic responses. 2. Peripheral vasodilatory effects, indicated by increases in heart rate and cardiac output, and falls in total peripheral resistance, peaked at 20 min at the two highest doses. By 60 min most values had returned to pre-injection levels. There were no changes in mean arterial pressure. 3. At the highest dose of 0.10 mg/kg there was a fall in glomerular filtration rate, renal blood flow and effective renal plasma flow within 20 min. Urinary sodium and potassium excretion also fell with all three doses tested. 4. Plasma renin concentration increased at the 0.05 and 0.10 mg/kg doses and was still elevated at 60 min. 5. The results of this study in the sheep contrast with others in the dog, where renal blood flow is increased and the rat, where blood pressure is increased, and indicate a species specificity in regard to the analogue's actions.

Ritanserin and serotonergic mechanisms in blood pressure and fluid regulation in sheep.

1. In previous studies, exogenous serotonin (5-HT), administered intravenously, caused dose-related increases in mean arterial pressure and heart rate in conscious sheep. The 5-HT2 antagonist ketanserin (0.1 mg/kg per h, i.v.) was shown to lower blood pressure in the conscious sheep primarily through antagonism of alpha-adrenoceptors. 2. A newer 5-HT2 antagonist, ritanserin, is a more selective antagonist in vivo, as it attenuated or abolished pressor responses to exogenous 5-HT, but not to phenylephrine. 3. When infused alone, ritanserin (0.1 mg/kg per h, i.v.) failed to produce a decrease in blood pressure, suggesting that 5-HT antagonistic properties are not sufficient by themselves to lower blood pressure. 4. Ritanserin displayed a different metabolic profile to ketanserin, with a markedly decreased water intake. The mechanism of this effect is unresolved, but may imply a permissive role for 5-HT in the modulation of drinking responses in the sheep. 5. Ritanserin did not modify ACTH-induced hypertension in sheep.

The haemodynamic effects of cyclosporin A in sheep.

1. Cyclosporin A (CyA; 12 mg/kg/day) was infused into six conscious sheep over 5 days to examine the haemodynamic effects of the drug in normal animals. 2. Mean arterial pressure was increased from 73(1) mmHg to 90(4) mmHg (P less than 0.001). There was no change in cardiac output but calculated total peripheral resistance was elevated from 16(1) to 21(2) mmHg min/1 (P less than 0.001) on day 4. 3. There was no change in plasma [Na], but a fall in plasma [K]. Urinary Na excretion decreased. Glomerular filtration rate, filtration fraction, renal blood flow, renal vascular resistance, body weight, plasma renin and blood aldosterone concentration were unchanged. 4. CyA produces an increase in blood pressure in sheep associated with an increase in total peripheral resistance on days 1, 3, and 4, in the absence of changes in renal function. This suggests that CyA hypertension is not simply a consequence of nephrotoxicity.

Intracerebroventricular infusion of a cyclic hexapeptide analogue of somatostatin inhibits hemorrhage-induced ACTH release.

This study examines the effect of intracerebroventricular infusion of the 'superactive' somatostatin (SRIF) analogue cycl--(N-Me-Ala-Tyr-D-Trp-Lys-Val-Phe) on the plasma adrenocorticotrophic hormone (ACTH) response to hemorrhage in conscious sheep. Hemorrhage (15 ml/kg over 15 min) increased plasma ACTH from 55 +/- 20 to 815 +/- 148 pg/ml at 30 min (p less than 0.001). Infusion of the SRIF analogue intracerebroventricularly at 0.08 microgram/min for 10 min prior to and for 60 min after commencement of hemorrhage resulted in a partial inhibition of the plasma ACTH response. Infusion of the analogue at 0.8 microgram/min blocked the increase in plasma ACTH at 30 min. Plasma ACTH was 21 +/- 3 pg/ml as control and 66 +/- 47 pg/ml at 30 min. The SRIF analogue had no effect on plasma ACTH in the same animals in control experiments with no hemorrhage. These studies suggest that SRIF may act as a central inhibitor of ACTH release. The mechanism by which the SRIF analogue inhibits ACTH secretion is unknown but could involve inhibition of corticotropin-releasing factor release and/or an inhibition of ACTH release from the pituitary gland. The route of administration of the SRIF analogue and the finding that the SRIF analogue did not prevent systemic corticotropin-releasing factor stimulation of ACTH suggest an effect of the SRIF analogue in the hypothalamus.

Onset and dose relationships of ACTH effects on blood pressure in sheep.

The threshold and dose-response relationships for the blood pressure and metabolic effects of adrenocorticotropic hormone (corticotropin, ACTH) were examined in conscious sheep. Corticotropin was infused at five rates (0.5 micrograms/kg/day, n = 4; 1 micrograms/kg/day, n = 4;2 micrograms/kg/day, n = 6; 5 micrograms/kg/day, n = 5; and 10 micrograms/kg/day, n = 5) for 3 days, and the time of onset of the rise in blood pressure was assessed with a computer-based system. The effects of equimolar infusion of beta-endorphin and ACTH at 5 micrograms/kg/hour also were examined. Corticotropin infusion at 0.5 microgram/kg/day had no effect on mean arterial pressure. An ACTH infusion of 1.0 microgram/kg/day significantly increased mean arterial pressure (p less than 0.001), but the rise was less than that at the three higher doses, all of which produced similar effects. Changes in heart rate were significant at the 10 micrograms/kg/day level only (p less than 0.01). Initial urinary sodium retention was present at the three higher but not the two lower rates of infusion. Corticotropin infusion had no effect on urinary potassium excretion at any rate but produced hypokalemia at rates of 1.0 microgram/kg/day and above, which appeared to be dose related. Plasma sodium concentration was increased significantly only at the three higher rates (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of intracerebroventricular infusions of CRF, sauvagine, ACTH (1-24) and ACTH (4-10) on blood pressure in sheep.

This study reviews the effects of intracerebroventricular (i.c.v.) infusion of corticotrophin releasing factor (CRF) and adrenocorticotrophic hormone (ACTH) (1-24) on blood pressure in conscious sheep. The effects of sauvagine, a peptide with 50% homology with CRF and ACTH (4-10) and other analogues of ACTH were studied. Intracerebroventricular infusion of CRF for 24 h at 10 and 100 micrograms/h increased blood pressure and heart rate. Sodium (Na) excretion also increased. Sauvagine at 10 micrograms/h also increased blood pressure. Both peptides raised body temperature and produced 'arousal' behaviour. ACTH (1-24) at 0.8 microgram/h for 48 h raised blood pressure and body temperature but had no significant effect on Na excretion or behavior. ACTH (4-10) and other related analogs (alpha-MSH, alpha gamma 1-MSH) at up to 10 micrograms/h for 48 h had no effects. These studies show that neuropeptides involved in the physiological response to 'stress' may have central effects on blood pressure.