Does nitric oxide contribute to the basal vasodilation of pregnancy in conscious rabbits?

Pregnancy produces marked systemic vasodilation, but the mechanism is unknown. Experiments were performed in conscious rabbits to test the hypotheses that increased nitric oxide (NO) production contributes to the increased vascular conductance, but that the contribution varies among vascular beds. Rabbits were instrumented with aortic and vena caval catheters and ultrasonic flow probes implanted around the ascending aorta, superior mesenteric artery, terminal aorta, and/or a femoral artery. Hemodynamic responses to intravenous injection of N(omega)-nitro-L-arginine (L-NA; 20 mg/kg or increasing doses of 2, 5, 10, 15, and 20 mg/kg) were determined in rabbits first before pregnancy (NP) and then at the end of gestation (P). L-NA produced similar increases in arterial pressure between groups, but the following responses were larger (P < 0.05) when the rabbits were pregnant: 1) decreases in total peripheral conductance [-3.7 +/- 0.3 (NP), -5.0 +/- 0.5 (P) ml x min(-1) x mmHg(-1)], 2) decreases in mesenteric conductance [-0.47 +/- 0.05 (NP), -0.63 +/- 0.07 (P) ml x min(-1) x mmHg(-1)], 3) decreases in terminal aortic conductance [-0.43 +/- 0.05 (NP), -0.95 +/- 0.19 ml x min(-1) x mmHg(-1) (P)], and 4) decreases in heart rate [-41 +/- 4 (NP), -62 +/- 5 beats/min (P)]. Nevertheless, total peripheral and terminal aortic conductances remained elevated in the pregnant rabbits (P < 0.05) after L-NA. Furthermore, decreases in cardiac output and femoral conductance were not different between the reproductive states. We conclude that the contribution of NO to vascular tone increases during pregnancy, but only in some vascular beds. Moreover, the data support a role for NO in the pregnancy-induced increase in basal heart rate. Finally, unknown factors in addition to NO must also underlie the basal vasodilation observed during pregnancy.

The interaction of angiotensin II and osmolality in the generation of sympathetic tone during changes in dietary salt intake. An hypothesis

At rest, sympathetic nerves exhibit tonic activity which contributes to arterial pressure maintenance. Significant evidence suggests that the absolute level of sympathetic tone is altered in a number of physiologic and pathophysiologic states. However, the mechanisms by which such changes in sympathetic tone occur are incompletely understood. The purpose of this review is to present evidence that humoral factors are essential in these changes and to detail specifically an hypothesis for the mechanisms that underlie the changes in sympathetic tone that are produced during increases or decreases in dietary salt intake. It is proposed that the net effect of changes in dietary salt on sympathetic activity is determined by the balance between simultaneous and parallel sympathoinhibitory and sympathoexcitatory humoral mechanisms. A key element of the sympathoinhibitory mechanism is the chronic sympathoexcitatory effects of angiotensin II (ANG II). When salt intake increases, ANG II levels fall, and the sympathoexcitatory actions of ANG II are lost. Simultaneously, a sympathoexcitatory pathway is triggered, possibly via increases in osmolality which activate osmoreceptors or sodium receptors. In normal individuals, the sympathoinhibitory effects of increased salt predominate, sympathetic activity decreases, and arterial pressure remains normal despite salt and water retention. However, in subjects with salt-sensitive hypertension, it appears that the sympathoexcitatory effects of salt predominate, possibly due to an inability to adequately suppress the levels or actions of ANG II. The net result, therefore, is an inappropriate increase in sympathetic activity during increased dietary salt which may contribute to the hypertensive process.

Role of angiotensin II in altered baroreflex function of conscious rabbits during late pregnancy

OBJECTIVE

Pregnancy alters baroreflex control of heart rate in conscious rabbits, but the mechanism for this action is unknown. This study tested the hypothesis that endogenous angiotensin II is the mediator.

STUDY DESIGN

To test this hypothesis the baroreflex relationship between arterial pressure and heart rate in conscious rabbits was determined before and after administration of the angiotensin II AT1 receptor antagonist losartan (n = 7) before pregnancy and at the end of gestation.

RESULTS

Pregnancy decreased mean arterial pressure, increased heart rate, and modified the reflex by shifting the mean arterial pressure-heart rate relationship to a lower pressure level, by increasing minimum heart rate, and by decreasing baroreflex gain (P < .05). Before pregnancy, losartan decreased baroreflex gain but had no other effect on reflex function. In contrast, during late gestation losartan further decreased mean arterial pressure, further decreased reflex gain, decreased maximum heart rate, and shifted the curve to a lower mean arterial pressure level (P < .05).

CONCLUSION

These results suggest that in conscious rabbits during pregnancy endogenous angiotensin II contributes to hypotension-induced tachycardia but does not decrease reflex gain or elevate minimum heart rate.

Methysergide delays the decompensatory responses to severe hemorrhage by activating 5-HT(1A) receptors

Central administration of the serotonin receptor ligand methysergide delays the decompensatory response to hypotensive hemorrhage. This study was performed to determine the receptor subtype that mediates this effect. Lateral ventricular (LV) injection of methysergide (40 microg) delayed the hypotensive, bradycardic, and sympathoinhibitory responses to blood withdrawal (1.26 ml/min) in conscious rats. The response was quantified, in part, as the blood volume withdrawal that produced a 40-mmHg fall in blood pressure. The delayed hypotensive response produced by methysergide (8.2 +/- 0.2 vs. 5.6 +/- 0.2 ml, P < 0.01) was reversed by the 5-hydroxytryptamine (HT)(1A) antagonist WAY-100635 (30 microg iv: 6.7 +/- 0.4 ml, P < 0. 01; 100 microg iv: 5.6 +/- 0.1 ml, P < 0.01). LV injection of the 5-HT(1A) agonist (+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) also delayed the hypotensive (10 microg: 8.6 +/- 0.3, P < 0.01; 20 microg: 9.2 +/- 0.3 ml, P < 0.01), bradycardic, and sympathoinhibitory responses to hemorrhage. WAY-100635 (10 microg iv) completely reversed the effects of 8-OH-DPAT (20 microg: 5.4 +/- 0.3 ml). Neither selective blockade of 5-HT(2) receptors nor stimulation of 5-HT(1B/1D) receptors had any effect on hemorrhage responses. These data indicate that methysergide stimulates 5-HT(1A) receptors to delay the decompensatory responses to hemorrhage.

Regional conductance changes during hemorrhage in pregnant and nonpregnant conscious rabbits

Late pregnant (P) conscious rabbits are less able to maintain arterial pressure during hemorrhage than nonpregnant (NP) animals. This study tested the hypothesis that the difference is due in part to less reflex vasoconstriction when the rabbits are P. Rabbits (n = 14) were instrumented with arterial and venous catheters as well as ultrasonic flow probes around the superior mesenteric, renal, and/or terminal aortic arteries. Pregnancy increased (P < 0.05) blood volume [235 +/- 5 (P) vs. 171 +/- 3 (NP) ml], terminal aortic conductance [1.88 +/- 0.11 (P) vs. 0.98 +/- 0.06 (NP) ml. min(-1). mmHg(-1)], mesenteric conductance [1.20 +/- 0.19 (P) vs. 0.80 +/- 0. 05 (NP) ml. min(-1). mmHg(-1)], and heart rate [191 +/- 4 (P) vs. 162 +/- 3 (NP) beats/min] and decreased arterial pressure [59 +/- 1 (P) vs. 67 +/- 2 (NP) mmHg; P < 0.05]. Renal conductance was unaltered. The rabbits were bled in both the NP and P states at 2% of the initial blood volume per minute until arterial pressure fell below 45 mmHg. Arterial pressure fell with less blood loss in P rabbits [28 +/- 2% (P) vs. 39 +/- 2% (NP) of initial blood volume; P < 0.001]. Terminal aortic conductance decreased (P < 0.001) before the pressure fall in both groups, but the response was reduced in P rabbits. Mesenteric and renal conductances did not change in either group before the blood pressure fall. During the pressure fall, terminal aortic conductance increased (P < 0.05) only in NP rabbits. Mesenteric conductance increased in both groups. In summary, rabbits in late gestation are less able to maintain arterial pressure during hemorrhage, at least in part because of reduced vasoconstriction in tissues perfused by the terminal aorta.

Osmolality: a physiological long-term regulator of lumbar sympathetic nerve activity and arterial pressure

Acute infusion of hypertonic fluid increases mean arterial pressure (MAP) in part by elevating nonrenal sympathetic activity. However, it is not known whether chronic, physiological increases in osmolality also increase sympathetic activity. To test this hypothesis, MAP, heart rate (HR), and lumbar sympathetic nerve activity (LSNA) were measured in conscious, 48-h water-deprived rats (WD) during a progressive reduction in osmolality produced by a 2-h systemic infusion (0.12 ml/min) of 5% dextrose in water (5DW). Water deprivation significantly increased osmolality (308 +/- 2 vs. 290 +/- 2 mosmol/kgH2O, P < 0.001), HR (453 +/- 7 vs. 421 +/- 10 beats/min, P < 0.05), and LSNA (63.5 +/- 1.8 vs. 51.9 +/- 3.8% baroreflex maximum, P < 0.01). Two hours of 5DW infusion reduced osmolality (-15 +/- 5 mosmol/kgH2O), LSNA (-23 +/- 3% baseline), and MAP (-10 +/- 1 mmHg). To evaluate the role of vasopressin in these changes, rats were pretreated with a V1-vasopressin receptor antagonist. The antagonist lowered MAP (-5 +/- 1 mmHg) and elevated HR (32 +/- 7 beats/min) and LSNA (11 +/- 3% baseline) in WD (P < 0. 05), but not in water-replete, rats. 5DW infusion had a similar cumulative effect on all variables in V1-blocked WD rats, but had no effect in water-replete rats. Infusion of the same volume of normal saline in WD rats did not change osmolality, LSNA or MAP. Together these data indicate that, in dehydrated rats, vasopressin supports MAP and suppresses LSNA and HR and that physiological changes in osmolality directly influence sympathetic activity and blood pressure independently of changes in vasopressin and blood volume.

Hemodynamic and hormonal responses to hemorrhage in conscious rabbits at mid- and late gestation

This study tests the hypothesis that conscious rabbits late in pregnancy (P), but not at midgestation (MP), are less able to maintain arterial pressure during hemorrhage. Blood volume (BV) was elevated (P < 0.05) by an average of 13 +/- 4 (MP) and 35 +/- 3% (P). Rabbits were bled in both the nonpregnant (NP) and P state at 2% of the initial BV per minute. The hemorrhage was stopped after arterial pressure decreased. In NP rabbits, arterial pressure was well maintained near control pressures of 70 +/- 2 mmHg until 38 +/- 2% of the initial BV was removed and then rapidly fell to reach a nadir at 35 +/- 2 mmHg. In contrast, in P rabbits, basal arterial pressure was lower (61 +/- 2 mmHg; P < 0.05) and gradually decreased to below control after <25% of the initial BV was removed. Moreover, the rapid hypotensive phase was triggered with a lower percent BV removal (33 +/- 2%; P < 0.05). Basal heart rate was higher during P (149 +/- 5 vs. 189 +/- 9 beats/min; P < 0.05), and reflex increases were delayed. The slope of the relationship between arterial pressure and vasopressin was not modified during P, although the line was shifted to a lower pressure (P < 0.05). Larger increases in plasma renin activity and ANG II concentration were produced during hemorrhage in P rabbits. In contrast, no differences in the changes in arterial pressure, heart rate, and vasopressin were found between NP and MP rabbits during hemorrhage, although increases in renin and ANG II were greater at MP (P < 0.05). In summary, although P conscious rabbits are less able to maintain blood pressure during hemorrhage, this change is not evident at MP. These data suggest that the factors that mediate the P-induced alterations in arterial pressure regulation are not operative until late in gestation.

Endogenous ANG II supports lumbar sympathetic activity in conscious sodium-deprived rats: role of area postrema

This study tests the hypothesis that the area postrema (AP) is necessary for endogenous ANG II to chronically maintain lumbar sympathetic nerve activity (LSNA) and heart rate (HR) in conscious sodium-deprived rats. The effect of the ANG II type 1-receptor antagonist, losartan, on LSNA and HR was determined in rats that were either AP lesioned (APX) or sham lesioned. The sham rats were divided into groups, with (SFR) or without (SAL) food restriction, to control for the decreased food intake of APX rats. Before losartan, basal mean arterial pressure (MAP), HR, and baroreflex control of LSNA and HR were similar between groups, with the exception of lower maximal reflex LSNA and higher maximal gain of the HR-MAP curve in APX rats. In all groups, losartan similarly shifted (P < 0.01) the LSNA-MAP curve to the left without altering maximal gain. Losartan also decreased (P < 0.05) minimal LSNA in all groups, and suppressed (P < 0.01) maximal LSNA (% of control) in SFR (240 +/- 13 to 205 +/- 15) and SAL (231 +/- 21 to 197 +/- 26) but not APX (193 +/- 10 to 185 +/- 8) rats. In general, losartan similarly shifted the HR-MAP curve to a lower MAP in all groups. The results suggest that the AP is not necessary for endogenous ANG II to chronically support LSNA and HR at basal and elevated MAP levels in sodium-deprived rats. However, the AP is required for endogenous ANG II to increase maximal reflex LSNA at low MAP levels.

Altered heart rate baroreflex during pregnancy: role of sympathetic and parasympathetic nervous systems

Two studies were performed to determine whether the attenuation of baroreflex control of heart rate during late pregnancy in conscious rabbits is due to changes in parasympathetic (Para) or sympathetic (Sym) control of the heart. In the first, baroreflex relationships between arterial pressure and heart rate were generated before and after treatment with propranolol (Pro) to block Sym or with methscopolamine (Meth) to block Para. Each rabbit was studied in both the pregnant and nonpregnant state. Pregnancy decreased maximum baroreflex gain from 14.9 +/- 4.0 to 4.8 +/- 0.9 beats.min-1.mmHg-1 (P < 0.01) and decreased heart rate range from 177 +/- 6 to 143 +/- 10 beats/min (P < 0.01), primarily by increasing minimum heart rate (114 +/- 6 to 134 +/- 8 beats/min; P < 0.01). The difference between pregnant and nonpregnant rabbits in baroreflex gain was not altered by Meth but was abolished by Pro, suggesting that it is due to decreased Sym control of the heart. The elevated minimum heart rate of pregnancy persisted after Pro, but was abolished by Meth, suggesting that it is mediated by decreased Para control of the heart. In the second study, isolated buffer-perfused hearts from pregnant and nonpregnant rabbits were treated with increasing doses of isoproterenol (0.3-300 mM) or acetylcholine (0.3-10,000 microM), and the heart rate responses were determined. Hearts from pregnant rabbits were more sensitive to isoproterenol (P < 0.05), but less responsive to acetylcholine (P < 0.05). In conclusion, pregnancy-induced decreases in cardiac reflex gain and range appear to be mediated by alterations in Sym and Para, respectively. The change in Sym occurs proximal to the heart, whereas the decreased contribution of Para may be due, at least in part, to decreased sensitivity of the heart to acetylcholine.

Water deprivation and rat adrenal mRNAs for tyrosine hydroxylase and the norepinephrine transporter

Experiments were performed in rats to test the hypothesis that adrenal mRNA levels of tyrosine hydroxylase (TH) and the norepinephrine transporter (NET) would be modified by water deprivation via activation of the sympathetic nervous system. TH and NET mRNA levels were measured using the ribonuclease protection assay. Adrenal TH mRNA was higher (P < 0.001) in water-deprived (921 +/- 39 fg/microgram total RNA) compared with the water-replete rats (657 +/- 45 fg/microgram total RNA). In contrast, water deprivation decreased (P < 0.01) adrenal NET mRNA levels (275 +/- 66 vs. 433 +/- 63 fg/microgram total RNA). The dehydration-induced increase in TH mRNA was prevented by prior splanchnicectomy, but the decrease in NET mRNA was produced even in the absence of adrenal nerves. Water deprivation also increased (P < 0.05) plasma adrenocorticotropic hormone (84 +/- 16 vs. 42 +/- 14 pg/ml) and corticosterone (358 +/- 87 vs. 44 +/- 15 ng/ml) levels. Interestingly, the corticosterone response was reduced (P < 0.05) by unilateral adrenal denervation. These results suggest that water deprivation increases both adrenal medullary and adrenocortical activity at least in part by stimulation of sympathetic nerve activity.

Alterations in the baroreflex occur late in pregnancy in conscious rabbits

Experiments were performed in conscious rabbits to investigate when during pregnancy cardiac baroreceptor reflex gain decreases. A decrease in gain was first observed after 3 weeks of gestation (-34.1 +/- 4.9 to -15.7 +/- 3.1 beats/min/mm Hg, p < 0.05) and became more pronounced after 4 weeks (-30.2 +/- 4.7 to -8.6 +/- 2.9 beats/min/mm Hg, p < 0.05). These results indicate that the attenuation of baroreflex function does not become fully manifest until late in pregnancy.

Chronic ANG II infusion and reflex control of norepinephrine and corticosterone in conscious rabbits

The hypothesis that long-term increases in angiotensin II (ANG II) produce pressure-independent resetting of baroreflex control of the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis was tested in rabbits by determining the effect of chronic ANG II infusion on reflex relationships between mean arterial pressure (MAP) and plasma concentrations of norepinephrine (NE) and corticosterone (CS). After 2 wk, ANG II increased MAP from 61 +/- 1 to 99 +/- 2 mmHg (P < 0.05) without altering heart rate or plasma NE concentration, but increased CS from 9.8 +/- 1.3 to 29.5 +/- 13.7 ng/ml (P < 0.05). Heart rate, NE, and CS baroreflex curves were all reset to a higher pressure level (P < 0.05) after 24 h, 1 wk, and 2 wk of ANG II. Forty minutes after stopping ANG II on the same days, MAP decreased, and curves were shifted back toward control (P < 0.05), indicating that ANG II was required for the resetting. Two findings suggest that the resetting action ofANG II is distinct from the pressor effect. First, although stopping ANG II reversed the hypertension as it reversed the resetting, reversal of the hypertension instead by prolonged infusion of nitroprusside along with ANG II did not have the same effect. Second, NE and heart rate baroreflex curves returned toward preinfusion positions after stopping ANG II (P < 0.05), even when the hypertension was nearly maintained by phenylephrine infusion. In conclusion, chronic increases in ANG II may have a global baroreflex resetting effect by a mechanism that is in part independent of the hypertension.

Sodium intake, angiotensin II receptor blockade, and baroreflex function in conscious rats

The hypothesis that endogenous angiotensin II (Ang II) chronically supports baroreflex control of lumbar sympathetic nerve activity (LSNA) and heart rate (HR) via AT1 but not AT2 receptors was tested in conscious, normotensive rats. Rats were fed either a sodium deficient diet (LS) to increase circulating Ang II or a high-sodium diet (HS) for 2 to 3 weeks. One to two days after surgery to implant catheters and nerve electrodes, baroreflex curves were produced before and 40 minutes after intravenous administration of the AT1 antagonist losartan (10 mg/kg) or the AT2 antagonist PD123319 (500 micrograms/kg + 50 micrograms.kg-1.min-1). Mean arterial pressure (MAP) after losartan was maintained at basal levels with methoxamine. Forty minutes after losartan in LS rats, LSNA (46 +/- 5 to 22 +/- 1% max) and HR (414 +/- 7 to 387 +/- 8 bpm) were decreased (P < .05). Losartan decreased reflex control of LSNA more in LS than in HS rats (P < .05), as indicated by reductions in maximum LSNA (98 +/- 2 to 78 +/- 3% max) and minimum LSNA (42 +/- 5 to 21 +/- 5% max). Losartan also shifted reflex control of LSNA to a lower pressure in both groups, but the effect was larger in LS rats (-21 +/- 3 [LS] versus -9 +/- 2 [HS] mm Hg at basal LSNA; P < .05). Maximum gain was unaltered in either group. Similarly, losartan reduced maximum HR (534 +/- 6 to 495 +/- 9 bpm) and shifted the HR curve leftward (114 +/- 5 to 105 +/- 4 mm Hg) in LS but not in HS rats. In general, no changes were observed in MAP or baroreflex control of LSNA and HR after PD123319 in LS rats. These results suggest that in conscious, normotensive LS rats, endogenous Ang II supports LSNA and HR over a wide MAP range via AT1 but not AT2 receptors.

Interactions between angiotensin II and the sympathetic nervous system in the the long-term control of arterial pressure

1. The role of the renin-angiotensin system in long-term control of sympathetic activity and arterial pressure is reviewed. 2. There is evidence that favours a necessary role for the sympathetic nervous system in long-term arterial pressure regulation. First, appropriate changes in sympathetic activity appear to be produced in response to chronic changes in blood volume or blood pressure. Second, prevention of the normal homeostatic decrease in sympathetic activity in response to an increase in sodium intake produces hypertension. 3. Long-term changes in sympathetic activity cannot be mediated by the baroreceptor reflex, because it adapts to sustained changes in pressure. Therefore, an hypothesis is presented that evokes a key role for angiotensin II (AngII) in determining the chronic level of sympathetic activity. The key feature of this model is that the role of AngII is non-adaptive: chronic changes in extracellular fluid volume produce sustained reciprocal changes in AngII, and long-term increases in AngII produce sustained increases in sympathetic activity. 4. Evidence is reviewed that suggests that a lack of the normal suppression in AngII and/or sympathetic activity in response to an increase in sodium intake produces salt-sensitive hypertension.

ANG II chronically supports renal and lumbar sympathetic activity in sodium-deprived, conscious rats

The hypothesis that chronic elevations in endogenous angiotensin II (ANG II) increase sympathetic outflow in conscious, normotensive rats was tested by determining if acute blockade of ANG II receptors with losartan (10 mg/kg iv) decreases renal sympathetic nerve activity (RSNA), lumbar sympathetic nerve activity (LSNA), or heart rate (HR) more in rats with higher ANG II levels due to a low sodium (LS) diet compared with a control sodium (CS) or high sodium (HS) diet. In LS rats, losartan decreased (P < 0.05) mean arterial pressure (MAP) in two phases: an immediate decrease of 23 +/- 2 mmHg and a slower fall to 35 +/- 4 mmHg below control 40 min postlosartan. Five minutes after losartan, RSNA (149 +/- 13%), LSNA (143 +/- 5%), and HR (109 +/- 2%) were increased (P < 0.05). Despite further falls in MAP, the elevation in RSNA and HR remained constant, and LSNA decreased toward control (119 +/- 4%). After restoration of MAP to basal levels with methoxamine or phenylephrine infusion, RSNA (46 +/- 8%), LSNA (49 +/- 11%), and HR (76 +/- 2%) were suppressed (P < 0.05). In CS rats, losartan also initially decreased (P < 0.05) MAP by 6 +/- 2 mmHg and increased (P < 0.05) RSNA to 129 +/- 13%. When MAP was returned to control, RSNA was decreased (70 +/- 8%; P < 0.05) but less than in LS rats. In contrast, no changes in MAP, RSNA, LSNA, or HR were observed after losartan in HS rats. In conclusion, endogenous ANG II chronically supports RSNA, LSNA, and HR in conscious, normotensive low and normal sodium intake rats.

Treatment of chronic pilonidal disease

PURPOSE

Pilonidal disease (PD) is a common chronic intermittent disorder of the sacrococcygeal region. Despite surgical therapy dating back more than one century, management remains controversial and recent reports have advocated different surgical approaches.

METHODS

A retrospective review was conducted of 129 patients who were treated for chronic PD in our institution during a five-year period, 1990 to 1994.

RESULTS

Excision with primary closure was performed in 56 patients; 47 underwent open excision without closure, and 26 had marsupialization procedure. All were performed electively, with only minor complications. Complete healing was fastest in the primary closure group, despite a 14 percent postoperative wound infection rate. Recurrence rates of 11, 13, and 4 percent were found for primary closure, wide resection, and marsupialization procedures, respectively. There was no correlation among recurrence rate, postoperative infection, or prior surgery.

CONCLUSION

Considering healing time, morbidity, and recurrence rate, we conclude that surgical treatment should be directed at either excision and primary closure or marsupialization. Wide excision with secondary healing should be performed only for grossly infected and complex cysts.

Captopril and stress-induced hypertension in the borderline hypertensive rat

OBJECTIVE

Daily exposure to air-jet stress (AJS) causes sustained elevations of blood pressure in borderline hypertensive rats (BHR). It is known that the renin-angiotensin system is activated during episodes of behavioral stress, and the purpose of this study was to assess the involvement of renin-angiotensin system in the development of stress-induced hypertension in the BHR.

DESIGN

Four groups of 8- and 9-week-old rats were studied: they received, respectively, oral captopril and AJS; oral captopril without AJS; AJS without captopril; and neither AJS nor captopril.

METHODS

After 10 days of AJS and captopril conditions, femoral and jugular catheters were implanted for the measurement of mean arterial pressure and pressor responses to norepinephrine and tyramine. Blood samples were collected for the measurement of norepinephrine and plasma renin activity.

RESULTS

Ten days of AJS caused a significant elevation of blood pressure in BHR exposed to AJS without receiving captopril but not in those animals given captopril concurrently with AJS. Circulating norepinephrine and blood pressure responses to exogenous norepinephrine and tyramine were similar across the four groups. Plasma renin activity was highest in BHR given captopril, and was significantly elevated during an acute episode of AJS.

CONCLUSIONS

The renin-angiotensin system may be important in the development of stress-induced hypertension in the BHR.

ANG II receptor blockade and arterial baroreflex regulation of renal nerve activity in cardiac failure

In cardiac failure, efferent renal sympathetic nerve activity (ERSNA) and the activity of the renin-angiotensin system are increased, and arterial baroreflex regulation of ERSNA is attenuated. We examined the effect of intravenous and intracerebroventricular angiotensin II AT receptor blockade with losartan on the arterial baroreflex regulation of ERSNA in conscious control (C) and congestive heart failure (CHF) rats. Intravenous losartan (10 mg/kg, 21.7 mumol/kg) decreased arterial pressure more in CHF than in C rats (-28 +/- 3 vs. -20 +/- 3 mmHg, P < 0.05). After restoration of arterial pressure to the prelosartan value with methoxamine infusion, ERSNA was decreased more in CHF than in C rats (-23 +/- 4 vs. -1 +/- 2%, P < 0.05). Maximal gain of arterial baroreflex control of ERSNA (Gmax) was lower in CHF compared with C rats (-1.94 +/- 0.10 vs. -3.78 +/- 0.21%/mmHg, P < 0.05). Intravenous losartan increased Gmax in CHF (to -3.01 +/- 0.14%/mmHg, P < 0.05) but not in C rats (to -3.56 +/- 0.19%/mmHg). Intracerebroventricular losartan (4.61 micrograms, 10 nmol) did not affect arterial pressure but decreased ERSNA more in CHF than in C rats (-13 +/- 2 vs. -8 +/- 3%, P < 0.05). Like intravenous losartan, intracerebroventricular losartan increased Gmax in CHF (from -2.11 +/- 0.18 to -3.21 +/- 0.30%/mmHg, P < 0.05) but not in C rats (from -3.98 +/- 0.25 to -3.84 +/- 0.22%/mmHg). These results suggest that increased activity of the renin-angiotensin system contributes to the increase in ERSNA and its abnormal arterial baroreflex regulation in cardiac failure.

Chronic infusion of angiotensin II resets baroreflex control of heart rate by an arterial pressure-independent mechanism

The purpose of this study was to test the hypothesis that chronic infusion of angiotensin II (Ang II) in rabbits resets the cardiac baroreflex to a higher arterial pressure level by a pressure-independent mechanism. This hypothesis was tested by determining whether the resetting would be reversed soon after the Ang II infusion was stopped even if the hypertension was maintained by infusion of another vasoconstrictor. Relationships between arterial pressure and heart rate were determined by infusion of increasing doses of nitroprusside to decrease pressure and increase heart rate, followed by increasing doses of phenylephrine to increase pressure and decrease heart rate. After 9 to 10 days of Ang II infusion (20 ng.kg-1.min-1) arterial pressure was increased from 62 +/- 2 to 94 +/- 3 mm Hg (P < .001), and heart rate was unchanged from control values of 126 +/- 7 beats per minute. The baroreflex relationship between arterial pressure and heart rate was shifted to a higher pressure level after 3 to 4 and 9 to 10 days of Ang II infusion. On these same days the Ang II infusion was replaced with phenylephrine (5.0 +/- 0.4 micrograms.kg-1.min-1), and 30 minutes later arterial pressure decreased slightly (P < .05); however, despite the relative hypotension, heart rate was decreased (P < .005) from 126 +/- 5 to 98 +/- 7 beats per minute (days 3 to 4) and from 132 +/- 4 to 103 +/- 7 beats per minute (days 9 to 10). Moreover, the cardiac baroreflex relationships were shifted back to a lower pressure level (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal-sympathetic interactions in long-term regulation of arterial pressure: an hypothesis

The importance of the sympathetic nervous system in short-term regulation of arterial pressure is well accepted. However, the question of whether neural systems participate in long-term control of pressure has been debated for decades and remains unresolved. The principal argument against such a control system is that arterial baroreceptors adapt to sustained changes in arterial pressure. In addition, denervation of baroreceptors has minimal to no effect on basal levels of arterial pressure chronically. This argument assumes, however, that baroreceptors provide the primary chronic feedback signal to the central nervous system. An alternate model is proposed in which circulating hormones, primarily arginine vasopressin and angiotensin II, provide a long-term afferent signal to the central nervous system via binding to specific receptors in central sites lacking a blood-brain barrier (circumventricular organs). Studies suggest that the release of the hormones and the sympathetic response to alterations in their plasma concentrations are nonadaptive but may be gated by baroreceptor input. Evidence that this "hormonal-sympathetic reflex" model may explain the long-term alterations in sympathetic activity in response to chronic salt depletion and salt loading as well as congestive heart failure is presented. Finally, the role of an impaired hormonal sympathetic reflex in hypertension, specifically salt-dependent hypertension, is discussed.

Pregnancy attenuates activity of the baroreceptor reflex

1. Pregnancy-induced changes in acute blood pressure regulation are reviewed. 2. Pregnant animals are less able to maintain arterial pressure during haemorrhage than non-pregnant animals. 3. Baroreceptor reflex-mediated increases in heart rate, renal sympathetic activity, vasopressin, ACTH and cortisol are reduced during pregnancy. Therefore, one explanation for the subnormal ability of pregnant animals to regulate arterial pressure during haemorrhage is that the baroreceptor reflex is not as effective. 4. Chronic increases in oestrogen levels in non-pregnant rabbits do not reduce the gain of baroreflex control of renal sympathetic activity. This and other findings suggest that oestrogen alone does not mediate the blunted baroreflex activity observed during pregnancy.

Changes in the baroreflex during pregnancy in conscious dogs: heart rate and hormonal responses

This study was performed to test the hypothesis that reflex increases in heart rate, PRA, and plasma concentrations of vasopressin, angiotensin-II, ACTH, and cortisol are reduced during pregnancy. The hypothesis was tested by measuring, in pregnant and nonpregnant conscious dogs, changes in arterial and atrial pressures, heart rate, and plasma hormone concentrations during 30-min infusions of three doses of nitroprusside (1, 2, and 4 micrograms/kg.min). Between-group differences were determined by comparing the relationships between arterial or atrial pressure and plasma hormone concentrations. Hypotension-induced increases in heart rate and plasma levels of vasopressin, ACTH, and cortisol were blunted when the dogs were pregnant. In contrast, reflex increases in PRA and angiotensin-II were increased. These results indicate that the activity of the baroreceptor reflex is altered during pregnancy in dogs.

Hemorrhage decreases arterial pressure sooner in pregnant compared with nonpregnant dogs: role of baroreflex

This study was performed to test the hypothesis that smaller reflex increases in vasopressin, cortisol, adrenocorticotropic hormone (ACTH), and angiotensin II (ANG II) concentrations are produced by hemorrhage in pregnant compared with nonpregnant conscious dogs. Equivalent hemorrhages (1% of the initial blood volume per minute) produced larger decreases in arterial pressure [P < 0.01; 107 +/- 6 to 73 +/- 10 mmHg (pregnant); 109 +/- 6 to 90 +/- 5 mmHg (nonpregnant)] but produced similar increases in plasma vasopressin concentration in the pregnant animals. As a result, the slope of the arterial pressure-to-vasopressin relationship was reduced (P < 0.05). During pregnancy, smaller increases in plasma cortisol concentration and heart rate were also produced for a given decrease in arterial pressure, but the relationship between pressure and ACTH was not significantly affected. In contrast, higher levels of plasma renin activity and plasma ANG II concentration were achieved in the pregnant dogs. In general, the relationships between plasma hormone levels and either left or right atrial pressure were not significantly altered. These results indicate that reflex increases in heart rate, vasopressin, and cortisol concentration are attenuated in pregnant dogs and that this attenuation may contribute to the inability of pregnant animals to achieve normal cardiovascular homeostasis during hemorrhage.

Pressure-independent baroreflex resetting produced by chronic infusion of angiotensin II in rabbits

The purpose of this study was to test the hypothesis that chronic infusion of angiotensin II (ANG II) in rabbits shifts or resets baroreflex control of heart rate to a higher pressure level via a mechanism that is independent of the hypertension that is produced. The baroreflex relationship between arterial pressure and heart rate was assessed by first infusing progressively increasing doses of nitroprusside (3, 6, 12, 24, and 48 micrograms.kg-1 x min-1) to lower pressure and then increasing doses of phenylephrine (0.5, 1, 2, 4, and 8 micrograms.kg-1 x min-1) to raise pressure. Two weeks of intravenous ANG II infusion (20 ng.kg-1 x min-1) increased plasma ANG II levels from 9 +/- 1 to 146 +/- 24 pg/ml (P < 0.05), increased arterial pressure from 62 +/- 2 to 95 +/- 2 mmHg (P < 0.05), and transiently increased heart rate. The baroreflex was shifted to a higher pressure level after 30 min and 1, 3, 7, 9, and 14 days of ANG II infusion. Thirty minutes after the ANG II infusion on days 1, 7, and 14 was stopped, arterial pressure decreased, and the baroreflex shifted back to control, indicating that ANG II was required for the resetting that was produced. However, when the ANG II infusion was continued and arterial pressure was instead reduced for 30 min by infusing nitroprusside on days 3 and 9, the baroreflex relationship between arterial pressure and heart rate remained positioned at a higher pressure level.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin and angiotensin II in reflex regulation of ACTH, glucocorticoids, and renin: effect of water deprivation

Angiotensin II (ANG II) and vasopressin participate in baroreflex regulation of adrenocorticotropic hormone (ACTH), glucocorticoid, and renin secretion. The purpose of this study was to determine whether this participation is enhanced in water-deprived dogs, with chronically elevated plasma ANG II and vasopressin levels, compared with water-replete dogs. The baroreflex was assessed by infusing increasing doses of nitroprusside (0.3, 0.6, 1.5, and 3.0 micrograms.kg-1.min-1) in both groups of animals. To quantitate the participation of ANG II and vasopressin, the dogs were untreated or pretreated with the competitive ANG II antagonist saralasin, a V1-vasopressin antagonist, or combined V1/V2-vasopressin antagonist, either alone or in combination. The findings were as follows. 1) Larger reflex increases in ANG II, vasopressin, and glucocorticoids, but not ACTH, were produced in water-deprived dogs compared with water-replete dogs. 2) ANG II blockade blunted the glucocorticoid and ACTH responses to hypotension in water-deprived dogs, but not water-replete dogs. In contrast, vasopressin blockade reduced the ACTH response only in water-replete dogs. 3) Vasopressin or combined vasopressin and ANG II blockade reduced the plasma level of glucocorticoids related either to the fall in arterial pressure or to the increase in plasma ACTH concentration in water-replete dogs, and this effect was enhanced in water-deprived dogs. 4) In both water-deprived and water-replete animals, saralasin and/or a V1-antagonist increased the renin response to hypotension, but a combined V1/V2-antagonist did not. These results reemphasize the importance of endogenous ANG II and vasopressin in the regulation of ACTH, glucocorticoid, and renin secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin and angiotensin II in reflex regulation of heart rate: effect of water deprivation

This study tested the hypothesis that endogenous angiotensin II (ANG II) and vasopressin enhance baroreflex-mediated increases in heart rate in water-replete dogs and in dogs water deprived to chronically elevate plasma ANG II and vasopressin concentrations. The baroreflex was assessed by examining the heart rate response to infusion of increasing doses of nitroprusside (0.3, 0.6, 1.5, and 3.0 micrograms.kg-1.min-1). The effect on the baroreflex of pretreating the dogs with the competitive ANG II antagonist saralasin, a V1-vasopressin antagonist, or a combined V1/V2-vasopressin antagonist, alone or in combination, was determined. Nitroprusside infusion produced dose-dependent increases in heart rate, and the heart rate response was greater in water-deprived dogs in association with higher plasma levels of ANG II and vasopressin than in water-replete dogs. ANG II blockade alone depressed reflex increases in heart rate in water-deprived but not water-replete dogs. In both water-replete and water-deprived dogs, blockade of V1-vasopressin receptors reduced the heart rate response to hypotension, but this effect could be produced only when ANG II receptors were also blocked. In addition, administration of saralasin and the V1/V2-vasopressin antagonist led to a further reduction of the reflex tachycardia. These data suggest that endogenous vasopressin, acting at both V1- and V2-receptors, can amplify the increase in heart rate produced by hypotension. In addition, the results further support a physiological role for chronic elevations in endogenous ANG II in the maintenance of normal baroreflex function.

Hypotension during vasopressin receptor blockade: role of V2 receptors and sympathetic nervous system

This study investigated possible mechanisms for the hypotension produced by nitroprusside infusion in conscious dogs pretreated with a V1 vasopressin antagonist. The hypothesis that an action of vasopressin at V2-like receptors contributes to the hypotension was tested by comparing the effects of a V1 antagonist to the effects of a combined V1/V2 antagonist. Nitroprusside infusion produced dose-dependent decreases in arterial and atrial pressures. Larger decreases in pressures were produced in animals pretreated with either antagonist; however, the decreases in V1/V2-blocked dogs were not less than the decreases in V1-blocked dogs. These data suggest that V2-like actions of vasopressin do not contribute to the hypotensive effects of V1 blockade. A second hypothesis was that the greater hypotension was due to activation of a cardiac reflex to cause withdrawal of sympathetic tone, a decrease in peripheral resistance, and adrenal activation. Measurement of cardiac output revealed that the larger decreases in arterial pressure were due to larger decreases in total peripheral resistance. The hypotension was also associated with decreases in heart rate, unchanging plasma norepinephrine concentration, and increases in epinephrine concentration. These data are consistent with the hypothesis that the fall in pressure observed in dogs pretreated with a V1 antagonist is secondary to a decrease in peripheral resistance that is due at least in part to withdrawal of sympathetic tone.

Effect of baroreceptor denervation on stimulation of drinking by angiotensin II in conscious dogs

Angiotensin II causes marked stimulation of drinking when it is injected centrally but is a relatively weak dipsogen when administered intravenously. However, it has been proposed that the dipsogenic action of systemically administered angiotensin II may be counteracted by the pressor action of the peptide. To test this hypothesis, the dipsogenic action of angiotensin II was investigated in dogs, in which low and high baroreceptor influences had been eliminated by denervation of the carotid sinus, aortic arch, and heart. In five sham-operated dogs, infusion of angiotensin II at 10 and 20 ng.kg-1.min-1 increased plasma angiotensin II concentration to 109.2 +/- 6.9 and 219.2 +/- 38.5 pg/ml and mean arterial pressure by 20 and 29 mmHg, respectively, but did not induce drinking. In four baroreceptor-denervated dogs, the angiotensin II infusions produced similar increases in plasma angiotensin II concentration and mean arterial pressure but, in contrast to the results in the sham-operated dogs, produced a dose-related stimulation of drinking. Water intake with the low and high doses of angiotensin II was 111 +/- 44 and 255 +/- 36 ml, respectively. The drinking responses to an increase in plasma osmolality produced by infusion of hypertonic sodium chloride were not different in the sham-operated and baroreceptor-denervated dogs. These results demonstrate that baroreceptor denervation increases the dipsogenic potency of intravenous angiotensin II and provides further support for the hypothesis that the dipsogenic action of intravenous angiotensin II is counteracted by the rise in blood pressure.

Characterization of vasopressin actions in isolated submucosal arterioles of the intestinal microcirculation

Submucosal arterioles are the final resistance vessels of the mesenteric circulation; they supply intestinal mucosa and smooth muscle and contribute significantly to total mesenteric resistance. Characterization of receptors present on submucosal arterioles has not been carried out, because these vessels have not been accessible to study by previous methods. We have used a novel optical method for on-line tracking of outside diameter from in vitro preparations of submucosal arterioles in the ileum and colon of guinea pigs, rabbits, and humans to characterize the vasoconstrictor responses to vasopressin as well as other vasopressor agents along the gastrointestinal tract. All ileal submucosal arterioles showed smoothly graded constrictor responses, whereas colonic arterioles from each species exhibited rhythmic vasoconstrictions. Vasopressin constricted guinea pig and human submucosal arterioles (EC50, 1 nM) by activating classical V1 receptors; dissociation equilibrium constants (Kd) for the V1 antagonist d(CH2)5 Tyr (Me) arginine vasopressin were 1-3 nM. This antagonist was 10-50-fold more potent in inhibiting vasopressin constrictions in rabbit submucosal arterioles (Kd = 0.05-0.1 nM). No evidence for the presence of V2 receptors was obtained in any arteriole, and no significant differences in the alpha 1-adrenoceptor-mediated constrictions were observed in these vessels. Results from this study suggest the presence of heterogeneity of V1 receptors in submucosal arterioles; these differences appear to be species dependent. Our results also suggest that intrinsic vasoconstrictor properties of submucosal arterioles differ along the length of the gastrointestinal tract; these differences appear to be species independent.

Effect of baroreceptor denervation on vasopressin and cortisol responses to angiotensin II infusion in conscious dogs

Recent studies suggest that the pressor response to exogenous angiotensin II infusion may, through baroreceptor-dependent mechanisms, counteract the stimulatory effect of the peptide on vasopressin and adrenocorticotropic hormone (ACTH) secretion. To test this hypothesis, the effect of combined cardiac and sinoaortic baroreceptor denervation on the increases in plasma concentrations of vasopressin and cortisol (used as an index of ACTH secretion) produced by angiotensin II infusion was studied in conscious dogs. In eight intact dogs, 30-min angiotensin II infusions at 5, 10, and 20 ng.kg-1.min-1 increased mean arterial pressure from 108 +/- 5 to 126 +/- 5 mmHg, from 101 +/- 4 to 130 +/- 4 mmHg, and from 99 +/- 3 to 138 +/- 4 mmHg, respectively (P less than 0.001). Plasma cortisol concentration increased from 19 +/- 4 to 27 +/- 4 ng/ml, from 19 +/- 4 to 43 +/- 4 ng/ml, and from 19 +/- 4 to 71 +/- 6 ng/ml (P less than 0.01), and plasma vasopressin concentration increased from 2.2 +/- 0.3 to 3.1 +/- 0.3 pg/ml, from 2.3 +/- 0.3 to 3.5 +/- 0.4 pg/ml, and from 2.2 +/- 0.4 to 5.0 +/- 0.5 pg/ml (P less than 0.01). In five to six baroreceptor-denervated dogs, angiotensin II infusion produced increases in mean arterial pressure, plasma vasopressin concentration, and plasma cortisol concentration that were not consistently different from those in the intact dogs. These results demonstrate that baroreceptor denervation does not enhance the vasopressin or cortisol responses to angiotensin II infusion in conscious dogs.

Role of the renin-angiotensin system in hypertension during reduced uteroplacental perfusion pressure

Utero-placental ischemia is known to cause systemic hypertension in various species, but the mechanisms are unknown. These studies were designed to test the hypothesis that the increased systemic arterial pressure that occurs during reduced utero-placental perfusion pressure is mediated by the renin-angiotensin system, possibly due to release of renin or angiotensin from the ischemic gravid uterus. In trained, chronically instrumented pregnant dogs (gestational age 47 +/- 2 days, term = 60 days) maintained on a normal Na+ intake (approximately 80 meq/day), uterine perfusion pressure was reduced to 60 mmHg with an inflatable aortic occluder positioned distal to the renal arteries but proximal to the uterine arteries and was servo-controlled at this level for 1 h. Systemic arterial pressure rose by 14 +/- 2 mmHg, from 96 +/- 7 to 110 +/- 8 mmHg. Plasma renin activity and angiotensin II levels did not change significantly. On another day in the same animals, the activity of the renin-angiotensin system was fixed by infusing captopril and sufficient angiotensin II to restore arterial pressure to normal (2-5 ng.kg-1.min-1 iv). Reduction of uterine artery pressure to 60 mmHg caused systemic arterial pressure to increase by 10 +/- 2 mmHg with the renin-angiotensin system fixed, a response not different from that in the control experiments. These data suggest that the increase in systemic arterial pressure during reduced uteroplacental perfusion pressure is independent of the renin-angiotensin system.

Vasopressin: a regulator of adrenal glucocorticoid production?

Vasopressin infusion has been shown to decrease plasma adrenocorticotropic hormone (ACTH) concentration and transiently increase plasma cortisol concentration in conscious dogs. In the present study, one experiment tested the hypothesis that vasopressin infusion decreases ACTH by activation of a V1 receptor mechanism, e.g., by increasing atrial pressures and stimulating the low-pressure baroreceptor reflex. Administration of a vasopressin V1 antagonist eliminated the increases in atrial pressure and decreases in heart rate with vasopressin infusion (1 ng.kg-1.min-1), as it eliminated the decrease in ACTH, which is consistent with baroreflex-mediated inhibition of ACTH by vasopressin. A second experiment evaluated the role of ACTH in the increase in glucocorticoids. Dexamethasone pretreatment, which inhibits ACTH secretion, abolished the increase in glucocorticoid concentration with vasopressin infusion, indicating that ACTH is necessary for the glucocorticoid response. A third experiment was performed to determine whether the glucocorticoid response could be restored in dexamethasone-treated dogs, when ACTH concentration was maintained near control levels by intravenous infusion of synthetic alpha-ACTH-(1-24) (0.3 ng.kg-1.min-1). In these dogs, vasopressin infusion produced a sustained increase in plasma glucocorticoid concentration from 22 +/- 3 to 49 +/- 8 ng/ml (P less than 0.001). Infusing higher levels of ACTH (0.5 ng.kg-1.min-1) enhanced basal glucocorticoid levels but did not enhance the response to vasopressin. Vasopressin infusion did not alter clearance of glucocorticoids. Collectively, these results suggest that vasopressin directly stimulates adrenal glucocorticoid production, provided that background levels of ACTH are present.

Vasopressin and ANG II in the control of ACTH secretion and arterial and atrial pressures

Hypotension stimulates the secretion of adrenocorticotropin (ACTH) and vasopressin (AVP) and increases plasma levels of angiotensin II (ANG II). Because AVP and ANG II increase ACTH secretion, the present experiments were performed to evaluate the role of these peptides in the increases in plasma ACTH and glucocorticoid concentrations produced by hypotension in conscious dogs. This was accomplished by determining whether administration of receptor antagonists to vasopressin, [1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid), 2-(O-methyl)tyrosine]Arg8-vasopressin, and ANG II (saralasin), reduced the ACTH and glucocorticoid responses to infusion of four doses of the vasodilator nitroprusside. Nitroprusside (NP) infusion produced dose-dependent decreases in mean arterial pressure. Larger decreases in arterial pressure were produced in dogs pretreated with the AVP antagonist or with both saralasin and the vasopressin antagonist. Left and right atrial pressures also fell with NP infusion, and larger decreases in atrial pressures were found in dogs pretreated with the AVP antagonist. Finally, NP infusion increased plasma glucocorticoid concentration and plasma ACTH concentration. Both the glucocorticoid and the ACTH responses to hypotension were reduced in dogs given the AVP antagonist and in dogs given both saralasin and the AVP antagonist, but there was no difference in the effect of AVP blockade alone vs. the effect of combined AVP and ANG II blockade. These data suggest that AVP, but not ANG II, is required for normal glucocorticoid and ACTH responses to hypotension. They also suggest that AVP is necessary for normal maintenance of arterial blood pressure and atrial pressures during NP infusion.

Intravenous vasopressin infusion decreases plasma ACTH concentration in conscious dogs

Vasopressin infusion increases arterial and atrial pressures, which could stimulate arterial and cardiac baroreceptors to inhibit adrenocorticotropin (ACTH) secretion. Therefore, the current experiments were performed to test the hypothesis that vasopressin infusion decreases plasma ACTH concentration in conscious dogs. Vasopressin was infused for 90 min in three doses (0.5, 1.0, and 2.0 ng.kg-1.min-1) that produced increases in plasma levels within the physiological range. Only the highest dose of vasopressin increased mean arterial pressure, but left atrial pressure increased with all doses, and right atrial pressure increased with the two highest doses. A bradycardia was produced with all doses of vasopressin. Plasma ACTH concentration decreased from 44 +/- 12 to 25 +/- 7 (P less than 0.01), from 50 +/- 11 to 26 +/- 9 (P less than 0.001), and from 70 +/- 15 to 28 +/- 4 pg/ml (P less than 0.001) with infusion of 0.5, 1.0, and 2.0 ng.kg-1.min-1 vasopressin, respectively. In contrast, plasma cortisol concentration first increased (P less than 0.05) with each vasopressin dose, but after 15-30 min it decreased back to control levels. These results demonstrate that intravenous infusion of vasopressin decreases plasma ACTH concentration. Because the inhibition is associated with increases in atrial pressure and decreases in heart rate, it may be mediated via activation of the baroreceptor reflex.

The role of adrenocorticotropin in the cortisol and aldosterone responses to angiotensin II in conscious dogs

The role of ACTH in the cortisol and aldosterone responses to iv angiotensin II (AII) infusion, (5, 10, and 20 ng kg-1 min-1) in dogs was evaluated by examining the effect of AII infusion in conscious dogs pretreated with dexamethasone to suppress endogenous ACTH secretion. AII infusion in untreated dogs produced dose-related increases in plasma cortisol and aldosterone concentrations. The plasma ACTH concentration also increased. Dexamethasone treatment lowered the basal cortisol concentration from 1.7 +/- 0.1 to 0.7 +/- 0.1 micrograms/dl (P less than 0.05) and the ACTH concentration from 52 +/- 3 to 41 +/- 4 pg/ml (P less than 0.05), and abolished the cortisol response to all doses of AII, indicating that ACTH was necessary for the response. On the other hand, the basal aldosterone concentration was not significantly affected by dexamethasone, although the aldosterone response to the highest dose of AII was reduced. Additional experiments were performed to determine if the cortisol and aldosterone responses to AII (20 ng kg-1 min-1) in dexamethasone-treated dogs are restored if the ACTH concentration is maintained near control levels by iv infusion of synthetic alpha ACTH-(1-24) (0.3 ng kg-1 min-1). AII still failed to increase the plasma cortisol concentration in this group of dogs; however, the aldosterone response was fully restored. To evaluate the effect of elevated ACTH levels on the steroidogenic effects of AII, dogs were treated with dexamethasone and a higher dose of ACTH (0.4 ng kg-1 min-1). This dose of ACTH increased the plasma cortisol concentration from 1.7 +/- 0.1 to 3.5 +/- 0.8 micrograms/dl (P less than 0.05), but did not significantly affect the plasma aldosterone concentration. In the presence of constant elevated levels of ACTH, AII (10 and 20 ng kg-1 min-1) increased the plasma cortisol concentration in dexamethasone-treated dogs, although the response to the 10 ng kg-1 min-1 dose was smaller than the response in untreated dogs. Infusion of AII at 5 ng kg-1 min-1 did not increase the plasma cortisol concentration. In contrast, the increased plasma aldosterone produced by AII infusion in dexamethasone-treated dogs was not altered in the presence of elevated ACTH levels. Finally, AII infusion did not alter the clearance of cortisol. Collectively, these results demonstrate that an increase in plasma ACTH is necessary for the cortisol response to AII infusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the renin-angiotensin system in the control of vasopressin secretion in conscious dogs

The present studies were designed to evaluate the physiological significance of angiotensin II in the control of vasopressin secretion in conscious dogs. They demonstrated that exogenous angiotensin II (10 ng/kg per min) increased vasopressin secretion more when the pressor effect of angiotensin II was abolished. The fact that endogenous angiotensin II levels are normally increased without an increase in arterial pressure suggests that angiotensin II may play a greater role in the control of vasopressin secretion than was previously thought. The present study also evaluated the role of endogenous angiotensin II in the control of vasopressin secretion during sodium depletion, a state in which angiotensin II levels are elevated. Intracarotid infusion of a low dose of the angiotensin II antagonist, saralasin, decreased plasma vasopressin concentration, suggesting that endogenous angiotensin II acts in an area of the brain perfused by the carotid arteries to stimulate vasopressin secretion in sodium-deprived dogs. Finally, the present experiments evaluated the role of angiotensin II in baroreceptor reflex control of vasopressin secretion. Baroreflex function was assessed by examining the relationship between the change in blood pressure and the log of the change in vasopressin secretion over a range of blood pressure levels. Exogenous angiotensin II (10 ng/kg per min) altered baroreflex function by causing a shift of this relationship to a higher pressure level in sodium-replete dogs. In sodium-depleted dogs, inhibition of the renin-angiotensin system with saralasin or captopril produced an opposite shift. These results suggest that endogenous angiotensin II may be necessary for the maintenance of normal baroreflex control of vasopressin secretion during sodium depletion. Collectively, these results support the hypothesis that endogenous angiotensin II plays a role in the control of vasopressin secretion.

Interaction between angiotensin II and the baroreceptor reflex in the control of adrenocorticotropic hormone secretion and heart rate in conscious dogs

The present studies were designed to examine the effect of angiotensin II on baroreflex control of adrenocorticotropic hormone secretion and heart rate in conscious dogs. Baroreflex function was assessed by examining the relationship between blood pressure and plasma corticosteroid concentration (used as an index of adrenocorticotropic hormone secretion) and between blood pressure and pulse interval (the inverse of heart rate). Blood pressure was varied by intravenous infusion of four doses (0.3, 0.6, 1.5, and 3.0 micrograms/kg per min) of the vasodilator nitroprusside. Nitroprusside infusion produced an increase in plasma corticosteroid concentration and a decrease in pulse interval, both of which were linearly related to the fall in arterial blood pressure. During infusion of angiotensin II (10 ng/kg per min), however, the lines relating blood pressure to plasma corticosteroid concentration or blood pressure to pulse interval were shifted to a higher pressure level, suggesting that angiotensin II resets the baroreceptor reflex. Angiotensin II blockade with saralasin, an angiotensin II antagonist, or with captopril, a converting enzyme inhibitor, in sodium-depleted dogs with elevated plasma angiotensin II levels produced a shift in the opposite direction in these relationships. Because baroreflex function in sodium-depleted dogs before angiotensin II blockade was similar to that in sodium-replete dogs, despite the increased plasma angiotensin II levels, and because treatment of sodium-replete dogs with saralasin did not affect baroreflex function, these results suggest that endogenous angiotensin II is necessary for the maintenance of normal baroreflex control of adrenocorticotropic hormone secretion and heart rate during sodium depletion. Previous studies designed to evaluate the physiological significance of angiotensin II in the control of adrenocorticotropic hormone secretion showed that high, perhaps supraphysiological, levels of exogenous angiotensin II are required to increase adrenocorticotropic hormone secretion. An additional finding of the present study was that exogenous angiotensin II produces a larger increase in adrenocorticotropic hormone secretion when the pressor effect of angiotensin II was eliminated with simultaneous infusion of the vasodilators nitroprusside or hydralazine. This result suggests that experiments that evaluate the physiological role of angiotensin II in the control of adrenocorticotropic hormone secretion with infusions of exogenous AII may underestimate the importance of endogenously produced angiotensin II, which is normally released without an increase in pressure.

Effects of blockade of brain angiotensin II receptors in conscious, sodium-deprived dogs

The present studies were designed to evaluate the physiological significance of the actions of angiotensin II (ANG II) on the brain. The effects of blockade of brain ANG II receptors by intracarotid or intravertebral infusions of saralasin were studied in conscious dogs with high circulating ANG II levels (142 +/- 16 pg/ml) due to a low-sodium diet. Three doses of saralasin were infused into each pair of arteries and intravenously: 0.1, 0.3, and 1.0 micrograms X kg-1 X min-1. Saralasin produced dose-related decreases in arterial pressure during infusion into the carotid or vertebral arteries, confirming that ANG II maintains arterial pressure during sodium deficiency. However, intravenous saralasin administration decreased pressure to a similar degree, suggesting that the hypotensive effect was due to recirculation of saralasin, rather than to blockade of a central action of circulating ANG II. Heart rate was not altered by infusion of saralasin by any route. Saralasin administration also caused a dose-dependent increase in plasma renin activity and plasma ANG II concentration. However, because the increases produced by intracarotid or intravertebral saralasin did not differ from the increase produced by intravenous infusion, these results do not provide evidence that renin release is modulated by a central action of ANG II during sodium deficiency. Plasma corticosteroid levels were reduced (2.4 +/- 0.5 to 1.4 +/- 0.2 micrograms/dl, P less than 0.05) by intravenous infusion of the highest dose of saralasin, but neither intracarotid nor intravertebral saralasin infusion altered plasma corticosteroid concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement and localization of angiotensin-like immunoreactivity in juxtaglomerular cells of the rat

The existence and distribution of angiotensin I (A I) and angiotensin II (A II) in rat kidney were examined in immunocytochemical studies using the peroxidase-antiperoxidase technique and in biochemical studies using rat kidney homogenates extracted with acid-ethanol and purified by Sephadex G-25 gel chromatography. Immunopositive A II-like staining was observed in the juxtaglomerular cells of the afferent arteriole, but no histochemical evidence for A I was found. On the other hand, renal homogenates were found to contain both A I and A II immunoreactivities which coeluted on gel chromatography with synthetic A I and A II. These results indicate that A I as well as A II immunoreactivities are present in the kidney and that A II immunoreactivity can be localized to the juxtaglomerular cells. The origin of the immunoreactive A II in the juxtaglomerular cells remains to be determined.

Analysis of the actions of angiotensin on the central nervous system of conscious dogs

Angiotensin II (ANG II) acts on the brain to elevate blood pressure (BP), stimulate drinking, increase the secretion of vasopressin and corticotropin (ACTH), and inhibit the secretion of renin. The present studies were designed to evaluate the possible physiological significance of these effects. The experiments were performed in conscious dogs with small catheters chronically implanted in both carotid and both vertebral arteries. ANG II was infused into both carotid or both vertebral arteries in doses of 0.1, 0.33, 1.0, and 2.5 ng.kg-1.min-1. Intravertebral ANG II produced dose-related increases in BP that were generally accompanied by increases in heart rate. Intracarotid angiotensin also increased BP but did not change heart rate. Intracarotid ANG II stimulated drinking and, at the highest dose only, increased the secretion of vasopressin, ACTH, and corticosteroids. Intravertebral and intracarotid ANG II suppressed plasma renin activity (PRA). In a parallel series of experiments, the effects of intravenous ANG II, in doses of 2, 5, 10, and 20 ng.kg-1.min-1, were studied. These infusions produced dose-related increases in BP and water intake and suppressed PRA. Only the highest dose of ANG II increased vasopressin or corticosteroid secretion. Analysis of these results in terms of calculated or measured changes in plasma ANG II concentration indicate that the central cardiovascular and dipsogenic actions of angiotensin, as well as the suppression of PRA, can be elicited by concentrations of the peptide that are within the physiological range. On the other hand high, probably supraphysiological, levels of ANG II are required to increase vasopressin or ACTH secretion.