Influence of vasopressin and angiotensin on baroreflexes in the dog

Compensatory mechanisms to maintain blood pressure in paraplegic rats: implication of central tachykinin NK-1 and NK-3 receptors?

People with high level spinal cord injury (SCI) suffer from both hypotension and spontaneous hypertension due to loss of supraspinal control of spinal sympathetic outflow. Few reports have addressed whether any changes occur in central regulation of blood pressure (BP) and heart rat (HR) at the supraspinal level. Central tachykinin NK-1 and NK-3 receptors are located in many cardiovascular areas in the brain and are known to modulate BP and HR. This study examined the intracerebroventricular (i.c.v.) effects of the selective NK-1 receptor agonist [Sar(9), Met(O(2))(11)]SP (65pmol, n=6) and NK-3 receptor agonist senktide (650pmol, n=6) on mean arterial pressure (MAP) and HR before and after complete spinal cord transection at thoracic level 4 (T4). [Sar(9), Met(O(2))(11)]SP evoked increases in MAP and HR which were still present 4days after the T4 SCI. Further analysis using the beta(1)-adrenoceptor antagonist atenolol (10mgkg(-1)) revealed an increased contribution of HR in the MAP increase after SCI. For senktide, 2 and 5weeks after T4 SCI, the rise in MAP induced by senktide was significantly increased in magnitude and was similar to a normal response at 8weeks. These effects were accompanied by a bradycardia, which was still present and amplified at 8weeks. Our results reveal a transient potentiation of the senktide-mediated MAP effect and a greater contribution of the HR in MAP increase by [Sar(9), Met(O(2))(11)]SP in T4 transected rats. Although the significance of these changes remains to be established. This suggest a reorganization of supraspinal mechanisms regulating BP and HR after a high level SCI. Central NK-1 and NK-3 receptors might therefore contribute to the maintenance of MAP following high thoracic SCI.

Acute hypertension reveals depressor and vasodilator effects of cannabinoids in conscious rats

BACKGROUND AND PURPOSE

The cardiovascular effects of cannabinoids can be influenced by anaesthesia and can differ in chronic hypertension, but the extent to which they are influenced by acute hypertension in conscious animals has not been determined.

EXPERIMENTAL APPROACH

We examined cardiovascular responses to intravenous administration of anandamide and the synthetic cannabinoid, (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN55212-2), in conscious male Wistar rats made acutely hypertensive by infusion of angiotensin II (AII) and arginine vasopressin (AVP). Rats were chronically instrumented for measurement of arterial blood pressure and vascular conductances in the renal, mesenteric and hindquarters beds.

KEY RESULTS

Anandamide dose-dependently decreased the mean arterial blood pressure of rats made hypertensive by AII-AVP infusion, but not normotensive rats. Interestingly, acute hypertension also revealed a hypotensive response to WIN55212-2, which caused hypertension in normotensive animals. The enhanced depressor effects of the cannabinoids in acute hypertension were associated with increased vasodilatation in hindquarters, renal and mesenteric vascular beds. Treatment with URB597, which inhibits anandamide degradation by fatty acid amide hydrolase, potentiated the depressor and mesenteric vasodilator responses to anandamide. Furthermore, haemodynamic responses to WIN55212-2, but not to anandamide, were attenuated by the CB(1) receptor antagonist, AM251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophen yl)-4-methyl-1H-pyrazole-3-carboxamide].

CONCLUSIONS AND IMPLICATIONS

These results broadly support the literature showing that the cardiovascular effects of cannabinoids can be exaggerated in hypertension, but highlight the involvement of non-CB(1) receptor-mediated mechanisms in the actions of anandamide.

Vasopressin V1A receptor enhances baroreflex via the central component of the reflex arc

The neurohypophyseal peptide [Arg(8)]-vasopressin (AVP) exerts its physiological actions via 3 distinct receptor isoforms designated V1A, V1B, and V2. We recently showed that V1A receptor was involved in the baroreflex control of heart rate using V1A receptor knockout mice. The present study was undertaken to further clarify this finding. In conscious mice, resting blood pressure of the knockout group was lower than that of the wild-type group (wild-type, 108+/-2.0 mm Hg; knockout, 98+/-3.8 mm Hg; n=6-7) without notable change in heart rate. Although phenylephrine and nitroprusside-induced changes in blood pressure did not differ in these strains, the subsequent bradycardia and tachycardia were markedly blunted in the knockout mice (mean slopes for baroreflex curve after phenylephrine treatment; wild-type, -5.65+/-0.30 bpm/mm Hg; knockout, -3.97+/-0.52 bpm/mm Hg; those after nitroprusside treatment; wild-type, -0.51+/-0.10 bpm/mm Hg; knockout, -0.18+/-0.05 bpm/mm Hg; n=6-7). Under urethane anesthesia (1.0-1.2 g/kg, i.p.), electrical stimulation of the vagal afferent nerve evoked frequency-dependent hypotension and bradycardia in the wild-type mice. In contrast, in the knockout mice such stimulation induced a pressor, not a depressor, response and diminished bradycardia. Moreover, electrical stimulation-induced hemodynamic changes through the vagal afferent nerve in the wild-type mice were significantly attenuated by pretreatment with intravenously administered V1A receptor antagonist d(CH(2))(5)Tyr(Me)AVP. Electrical stimulation of the vagal efferent nerve-induced hemodynamic changes (depressor and bradycardia) and chronotropic responses to adrenergic and cholinergic stimuli were not different between the 2 strains. These results suggest that the V1A receptor in the central nervous system is involved in the regulation of the heart rate via the baroreflex arc.

Vasopressin receptor antagonists: pharmacological tools and potential therapeutic agents

The present survey deals with the development and applications of non-peptidergic vasopressin receptor antagonists. The existence of at least three vasopressin receptors (V(1), V(2) and V(3) respectively) is firmly established. V(1)-receptors play a relevant role in the regulation of vascular tone, whereas V(2)-receptors are known to mediate the antidiuretic activity of vasopressin at the level of the renal collecting ducts. The V(3)-receptor appears to be involved in the release of the adreno-corticotropic hormone. Vasopressin receptor antagonists which are peptides have been known for several decades, more recently, both V(1)- and V(2)-receptor blockers which are non-peptidergic have been introduced, as well as agents with affinity for both V(1)- and V(2)-receptor subtypes. A survey of these non-peptidergic antagonists is presented here. Such compounds are useful as pharmacological tools, and they can also be thought of as therapeutic agents as therapeutic agents in cardiovascular and renal diseases. Selective V(1)- and V(2)-receptor antagonists were used to study the interaction between vasopressin receptors and sympathetic neurones. Depending on the experimental model used this interaction can occur at either the pre- or postsynaptic sites. In both cases predominantly V(1)-receptors are involved. A brief survey is given of the potential use of V-receptor antagonists in the drug therapy of syndrome of inappropriate antidiuretic hormone secretion and other water retaining disorders, congestive heart failure and certain forms of hypertension (in particular in the Negroid hypertensive patients).

Intraventricular injections of tachykinin NK3 receptor agonist reduce the gain of the baroreflex in unrestrained rats

The tachykinin neuropeptides acting at NK3 receptors affect mean arterial pressure (MAP) through both neuroendocrine and neural mechanisms. NK3 receptors are found in brainstem nuclei that mediate the baroreflex, but the effects of NK3 receptor stimulation on baroreflex function are unknown. The present study tests the effects of intraventricular injections of senktide, a selective NK3 receptor agonist, on the sensitivity of the baroreflex in three stains of rats: Charles River Laboratory, Long-Evans, and Brattleboro rats, which lack the ability to synthesize vasopressin. Rats with lateral ventricle cannulas were administered injections of isotonic saline, 100 ng, or 200 ng senktide, and 5 min later arterial baroreceptor-heart rate (HR) function was examined by constructing full-range blood pressure-HR curves using alternating doses (5-20 microg kg min) of phenylephrine and nitroprusside to raise and decrease blood pressure approximately 50 mm Hg over a period of 1 min, respectively. Intraventricular injections of 200 ng senktide had no significant effect on baseline MAP, but significantly decreased the gain of the baroreflex in all three rat strains whereas the 100 ng dose had no effect on the baroreflex. These results show that NK3 receptor stimulation modulates the baroreflex that is independent of any action of vasopressin.

Mechanisms of postspaceflight orthostatic hypotension: low alpha1-adrenergic receptor responses before flight and central autonomic dysregulation postflight

Although all astronauts experience symptoms of orthostatic intolerance after short-duration spaceflight, only approximately 20% actually experience presyncope during upright posture on landing day. The presyncopal group is characterized by low vascular resistance before and after flight and low norepinephrine release during orthostatic stress on landing day. Our purpose was to determine the mechanisms of the differences between presyncopal and nonpresyncopal groups. We studied 23 astronauts 10 days before launch, on landing day, and 3 days after landing. We measured pressor responses to phenylephrine injections; norepinephrine release with tyramine injections; plasma volumes; resting plasma levels of chromogranin A (a marker of sympathetic nerve terminal release), endothelin, dihydroxyphenylglycol (DHPG, an intracellular metabolite of norepinephrine); and lymphocyte beta(2)-adrenergic receptors. We then measured hemodynamic and neurohumoral responses to upright tilt. Astronauts were separated into two groups according to their ability to complete 10 min of upright tilt on landing day. Compared with astronauts who were not presyncopal on landing day, presyncopal astronauts had 1). significantly smaller pressor responses to phenylephrine both before and after flight; 2). significantly smaller baseline norepinephrine, but significantly greater DHPG levels, on landing day; 3). significantly greater norepinephrine release with tyramine on landing day; and 4). significantly smaller norepinephrine release, but significantly greater epinephrine and arginine vasopressin release, with upright tilt on landing day. These data suggest that the etiology of orthostatic hypotension and presyncope after spaceflight includes low alpha(1)-adrenergic receptor responsiveness before flight and a remodeling of the central nervous system during spaceflight such that sympathetic responses to baroreceptor input become impaired.

Vasopressin V(1) receptor-mediated activation of central sympatho-adrenomedullary outflow in rats

The present study was designed to characterize the vasopressin receptor subtype involved in the vasopressin-induced activation of the central sympatho-adrenomedullary outflow using urethane-anesthetized rats. Intracerebroventricularly (i.c.v.) administered vasopressin (0.1, 0.2 and 0.5 nmol/animal) dose-dependently elevated plasma levels of adrenaline and noradrenaline (adrenaline>noradrenaline). The vasopressin (0.2 nmol/animal)-induced elevation of both catecholamines was significantly attenuated by [d(CH(2))(5)(1),Tyr(Me)(2),Arg(8)]-vasopressin, a selective vasopressin V(1) receptor antagonist, in a dose-dependent manner (0.1 and 0.2 nmol/animal, i.c.v.). The same doses (0.1 and 0.2 nmol/animal, i.c.v.) of [1-adamantaneacetyl(1),D-Tyr(Et)(2),Val(4),Abu(6), Arg(8,9)]-vasopressin, a potent vasopressin V(2) receptor antagonist, had no effect; however, a large dose of this antagonist (1.6 nmol/animal, i.c.v.) effectively reduced the vasopressin-induced elevation of catecholamines. On the other hand, [5-dimethylamino-1-[4-(2-methylbenzoylamino)benzoyl]-2,3,4,5-tetrahydro-1H-benzazepine], a selective vasopressin V(2) receptor antagonist (5 and 10 nmol/animal, i.c.v.), had no effect on the vasopressin-induced elevation of catecholamines. The vasopressin-induced elevation of catecholamines was abolished by indomethacin, an inhibitor of cyclooxygenase (1.2 micromol/animal, i.c.v.). These results suggest that the vasopressin activates the central sympatho-adrenomedullary outflow by brain vasopressin V(1) receptor- and cyclooxygenase-dependent mechanisms in rats.

Alteration of humoral and peripheral vascular responses during graded exercise in heart failure

We hypothesized that performance of exercise during heart failure (HF) would lead to hypoperfusion of active skeletal muscles, causing sympathoactivation at lower workloads and alteration of the normal hemodynamic and hormonal responses. We measured cardiac output, mean aortic and right atrial pressures, hindlimb and renal blood flow (RBF), arterial plasma norepinephrine (NE), plasma renin activity (PRA), and plasma arginine vasopressin (AVP) in seven dogs during graded treadmill exercises and at rest. In control experiments, sympathetic activation at the higher workloads resulted in increased cardiac performance that matched the increased muscle vascular conductance. There were also increases in NE, PRA, and AVP. Renal vascular conductance decreased during exercise, such that RBF remained at resting levels. After control experiments, HF was induced by rapid ventricular pacing, and the exercise protocols were repeated. At rest in HF, cardiac performance was significantly depressed and caused lower mean arterial pressure, despite increased HR. Neurohumoral activation was evidenced by renal and hindlimb vasoconstriction and by elevated NE, PRA, and AVP levels, but it did not increase at the mildest workload. Beyond mild exercise, sympathoactivation increased, accompanied by progressive renal vasoconstriction, a fall in RBF, and very large increases of NE, PRA, and AVP. As exercise intensity increased, peripheral vasoconstriction increased, causing arterial pressure to rise to near normal levels, despite depressed cardiac output. However, combined with redirection of RBF, this did not correct the perfusion deficit to the hindlimbs. We conclude that, in dogs with HF, the elevated sympathetic activity observed at rest is not exacerbated by mild exercise. However, with heavier workloads, sympathoactivation begins at lower workloads and becomes progressively exaggerated at higher workloads, thus altering distribution of blood flow.

Vasopressin V2 receptor enhances gain of baroreflex in conscious spontaneously hypertensive rats

The aim of the present study was to determine the receptor subtype involved in arginine vasopressin (AVP)-induced modulation of baroreflex function in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats using novel nonpeptide AVP V1- and V2-receptor antagonists. Baroreceptor heart rate (HR) reflex was investigated in both SHR and WKY rats which were intravenously administered the selective V1- and V2-receptor antagonists OPC-21268 and OPC-31260, respectively. Baroreflex function was assessed by obtaining alternate pressor and depressor responses to phenylephrine and sodium nitroprusside, respectively, to construct baroreflex curves. In both SHR and WKY rats baroreflex activity was tested before and after intravenous administration of vehicle (20% DMSO), OPC-21268 (10 mg/kg), and OPC-31260 (1 and 10 mg/kg). Vehicle did not significantly alter basal mean arterial pressure (MAP) and HR values or baroreflex function in SHR or WKY rats. The V1-receptor antagonist had no significant effect on resting MAP or HR values or on baroreflex parameters in both groups of rats, although this dose was shown to significantly inhibit the pressor response to AVP (5 ng iv; ANOVA, P < 0.05). In SHR but not WKY rats the V2-receptor antagonist significantly attenuated the gain (or slope) of the baroreflex curve (to 73 +/- 3 and 79 +/- 7% of control for 1 and 10 mg/kg, respectively), although AVP-induced pressor responses were also attenuated with the higher dose of the V2-receptor antagonist. These findings suggest that AVP tonically enhances baroreflex function through a V2 receptor in the SHR.

Discovery of novel selective hypotensive vasopressin peptides that exhibit little or no functional interactions with known oxytocin/vasopressin receptors

1. Arginine-vasopressin (VP) has both vasoconstricting and vasodilating action. We report here the discovery of four novel selective hypotensive VP analogues: d(CH2)5[D-Tyr(Et)2,Arg3,Val4]AVP; d(CH2)5[D-Tyr(Et)2,Lys3,Val4]AVP and their iodinatable Tyr-NH2(9) analogues. 2. Bioassays in rats for activities characteristic of neurohypophysial peptides showed that the four VP peptides possessed little or no V1a, V2 or oxytocin (OT) receptor agonistic or antagonistic activities. 3. In anaesthetized rats, these peptides (0.05-0.10 mg kg(-1) i.v.) elicited a marked fall in arterial blood pressure. 4. Blockade of cholinoceptors, adrenoceptors and bradykinin B2 receptors, and inhibition of prostaglandin synthesis had little effect on their vasodepressor action. 5. Classical V1a, V2 and OT receptor antagonists did not block the vasodepressor response. 6. L-NAME, 0.2 mg kg(-1) min(-1), markedly suppressed the hypotensive response to ACh but not the vasodepressor response to the hypotensive VP peptides. However, the duration of the vasodepressor response was shortened. Very high doses of L-NAME attenuated both the vasodepressor response and the duration of action. 7. These findings indicate that the vasodepressor action of these VP peptides is independent of the peripheral autonomic, bradykinin and PG systems and is not mediated by the known classical OT/VP receptors. NO does not appear to have an important role in their vasodepressor action. 8. The discovery of these novel VP peptides could lead to the development of new tools for the investigation of the complex cardiovascular actions of VP and the introduction of a new class of hypotensive agents. The two iodinatable hypotensive VP peptides could be radiolabelled as potential markers for the localization of the receptor system involved.

Peptidergic modulation of the sympathetic contraction in the rabbit ear artery: effects of temperature

1. The effects of neuropeptide Y, endothelin-1, arginine-vasopressin and angiotensin II on the vascular contraction to sympathetic nerve stimulation were studied in isolated segments, 2 mm long, from the rabbit central ear artery, a cutaneous vessel, during changes in temperature (24 degrees -41 degrees C). 2. Transmural electrical stimulation (1-8 Hz, at supramaximal voltage) produced frequency-dependent contraction, and this response, partially blocked by tetrodotoxin (1 microM) and phentolamine (1 microM), was reduced by cooling (30 degrees C -24 degrees C) and was not modified by warming (41 degrees C), as compared to that recorded at 37 degrees C. 3. Pretreatment with neuropeptide Y (10, 30 and 100 nM) increased in a concentration-dependent manner the vascular contraction to sympathetic stimulation at every temperature studied, but this potentiation was greater during cooling (34 degrees C -24 degrees C) than at 37 degrees C or warming (41 degrees C). 4. Pretreatment with endothelin-1 (3 and 10 nM) or vasopressin (0.1, 0.3 and 1 nM) increased in a concentration-dependent manner the vascular contraction to sympathetic stimulation during cooling (34 degrees C -24 degrees C), but not at 37 degrees C or warming (41 degrees C). 5. Pretreatment with angiotensin II (0.1, 0.3 and 1 microM) did not modify the contraction to sympathetic stimulation at any temperature studied. 6. These results suggest that neuropeptide Y, endothelin-1 and vasopressin, but not angiotensin II, modulate the cutaneous vasoconstriction to sympathetic nerve stimulation by potentiating this vasoconstriction during cooling.

Vascular effects of [Arg8]vasopressin in the isolated perfused rat kidney

The renal vascular effects of [Arg8]vasopressin (vasopressin) were investigated in the isolated perfused rat kidney. Vasopressin (0.01-3 nM) elicited a dose-dependent vasoconstriction in kidneys from Sprague Dawley rats, with a EC50 value of 0.206 +/- 0.044 nM. Inhibition of nitric oxide synthase by N omega-nitro-L-arginine (100 microM) shifted the vasopressin-induced vasoconstrictor response curve to the left. Inhibition of cyclooxygenase by indomethacin (10 or 30 microM) blunted the constriction induced by low concentrations of the peptide. Vasopressin, like angiotensin II but not noradrenaline, induced tachyphylaxis, SR 49059 ((2S)1-[(2R,3S)-5-chloro-3-(2-chlorophenyl)-1-(3,4-dimethoxybenzene- sulfonyl)-3-hydroxy-2,3-dihydro-1H-indole-2-carbonyl]-pyrrolidine-2- carboxamide) (1-30 nM), a new potent and selective non-peptide vasopressin V1A receptor antagonist, shifted the concentration-response curve for vasopressin to the right without decreasing the maximum contraction. Antagonism became competitive with a pA2 value (+/- S.D.) of 9.72 +/- 0.20 during inhibition of nitric oxide release. [Mpa1,D-Arg8]Vasopressin (desmopressin; 0.1-100 nM), or vasopressin (0.01-1 nM) after blockade of the vasopressin V1A receptor by SR 49059, induced no vasopressin V2 receptor-related renal relaxation in kidneys with vascular tone previously restored by noradrenaline or prostaglandin F2 alpha. These findings indicate that in the isolated perfused rat kidney vasopressin is a potent renal vasoconstrictor. The constriction depends on activation of smooth muscle vasopressin V1A receptors and is modulated by endothelial nitric oxide but not by prostacyclin or vasopressin V2 receptor-related vasodilation.

Baroreflex function in streptozotocin (STZ) induced diabetic rats

To determine whether the renal sympatho-inhibition and bradycardia in responses to acute increases in arterial pressure are altered in the diabetic state, the renal nerve discharge and heart rate were measured in streptozotocin (STZ) induced diabetic (DIA) rats. Integrated renal sympathetic nerve activity and heart rate were measured before and during an acute increase in blood pressure in anesthetized (Inactin 0.1 g/kg, i.p.) control (vehicle) and DIA rats (Sprague Dawley rats injected with STZ 65 mg/kg i.p.). Blood glucose levels were significantly elevated in the DIA group compared with the control group. Baroreflex changes in renal nerve activity and heart rate were not significantly different in the DIA rats compared with control rats at a time when the renal sympatho-inhibition in response to acute volume expansion was blunted in the diabetic rats. In addition, blocking the effect of elevated angiotensin II in diabetic rats with the converting enzyme inhibitor enalapril did not change the baroreflex function in DIA rats compared with control rats. However, administration of vasopressin failed to potentiate the baroreflex in diabetic rats as it did in normal control rats. This study demonstrates that (1) the baroreflex function is normal in STZ induced diabetic rats unlike the volume reflex during the early phase of the disease, (2) blockade of the AII system does not alter baroreflex function in diabetic rats and (3) vasopressin fails to potentiate the baroreflex in diabetic rats as it does in the euglycemic normal rats.

Effects of vasopressin on the response to carotid occlusion in conscious rabbits

To investigate the interaction between arginine vasopressin and the carotid sinus baroreflex, hemodynamic responses to bilateral carotid occlusion and to controlled reductions in carotid sinus pressure were examined. In the control state and during vasopressin infusion in conscious rabbits, mean arterial pressure, heart rate, mean aortic flow and total peripheral resistance were measured. Vasopressin infusion at 5 or 10 ng/kg/min did not raise arterial pressure, but increased resistance, and decreased heart rate and aortic flow in a dose-dependent manner. The pressure and resistance responses to carotid occlusion or changes in carotid pressure were not altered by vasopressin. The heart rate response was augmented significantly from 23 +/- 5 (mean +/- S.E.) to 40 +/- 8 and 43 +/- 8 beats/min with infusion of 5 and 10 ng/kg/min of vasopressin. Vasopressin did not augment the gain of carotid sinus reflex control of arterial pressure (3.7 +/- 0.5 in control and 3.5 +/- 0.5 during 5 ng/kg/min of vasopressin). With vasopressin infusion at 5 ng/kg/min, following vagal blockade with methylatropine both the arterial pressure and the heart rate responses to carotid pressure changes decreased to 73% and 32% of the response before blockade. The data indicate that vasopressin has little effect on control of arterial pressure by the carotid sinus baroreflex in conscious rabbits when vagal responses are activated.

The role of arginine vasopressin on peripheral cardiac parasympathetic nerve function in the rat

In rats, arginine vasopressin augments bradycardia associated with baroreflex activation. We investigated whether modulation of peripheral cardiac parasympathetic nerve function by AVP may play a role in this effect. To accomplish this we utilized an in vivo model with which we previously demonstrated both adrenergic and peptidergic modulation of cardiac parasympathetic nerve function. Urethane-anesthetized rats (250-350 g) were prepared with arterial and venous catheters and ECG leads. The cervical vagi were sectioned, and propranolol (1 mg/kg, i.v.) was administered to eliminate reflex changes in heart rate. To investigate potential preganglionic modulation by AVP, the right vagus nerve was electrically stimulated (0.5 mA; 0.5 msec; 1-10 Hz). To observe postganglionic effects through nicotinic activation, carbachol (a mixed nicotinic and muscarinic agonist) was injected (0.5 to 4.0 micrograms/kg, i.v.). To observe direct cholinergic effects at the SA node, methacholine (a pure muscarinic agonist) was injected (0.5 to 4.0 micrograms/kg). All three trials were performed before (control) and during AVP infusion (20 micrograms.kg.min). No consistent, significant differences in vagal-, carbachol- or methacholine-induced bradycardia were observed between control and AVP groups. Since endogenous plasma levels of AVP in the control situation may have saturated any vasopressinergic effect prior to AVP infusion, the experiments were repeated in Brattleboro rats, genetically deficient in AVP. Again, no consistent differences in heart rate responses to parasympathetic activation were noted between control and AVP-infused groups. These results suggest that in rats, vasopressinergic augmentation of baroreflex-induced bradycardia is not mediated by an effect on the peripheral cardiac parasympathetic innervation. However, it remains to be investigated whether AVP-mediated sympathetic withdrawal disinhibits cardiac parasympathetic nerve function.

Effects of arginine-vasopressin on regional blood volume distribution in supine humans

In healthy humans, the increase in arterial blood pressure seen in patients with autonomic dysfunction in response to exogenous vasopressin (AVP) is abolished. We tested the hypothesis that redistribution of blood from the intra- to the extrathoracic vascular compartment might contribute to this buffer response. Regional distribution of 99mTc labeled autologous red cells was assessed in healthy supine volunteers (n = 7) during arginine-vasopressin administration (1 ng.kg-1 bolus i.v. followed by a 14-min infusion of 3 ng.kg-1 x min-1), along with arterial and central venous pressures, and heart rate. Exogenous vasopressin increased plasma vasopressin concentration from 4.0 +/- 1.4 SEM to 91 pg.ml-1 +/- 12. Thoracic counts increased slightly but significantly by 2.2% +/- 0.9, while global abdominal counts remained unchanged. Most surprisingly, counts in the liver markedly increased (+8.1% +/- 1.8, p = 0.02), but significantly decreased in the spleen (-3.1% +/- 1.4). Intestinal (-2.5% +/- 2.4) and limb counts did not change significantly. Consistent with the increase in thoracic counts central venous pressure increased from 3.6 mmHg +/- 1 to 4.7 +/- 1 (p = 0.02), while arterial pressure and heart rate did not change. All changes reversed towards baseline when vasopressin administration ceased. Thus, in humans with an intact autonomic system, vasopressin, at concentrations observed during hypotension, increases liver and, albeit to a small extent, also thoracic blood volume, but decreases splenic blood content. These results: 1) are incompatible with the hypothesis that AVP induces a shift of blood from intra- to extrathoracic capacitance vessels, and 2) show that AVP increases rather than decreases central blood volume.

The effects of synthetic alpha-human atrial natriuretic peptide (alpha-hANP) on the overall baroreflex system in anesthetized mongrel dogs

The effects of alpha-hANP (0.3 micrograms/kg body weight/min) on the baroreflex system were studied in anesthetized dogs according to three different protocols. In the first protocol, the effects of alpha-hANP on an open-loop gain (G(AP)) of the overall baroreflex system and a gain (GHR) of the baroreflex heart rate control system during quick mild hemorrhage and blood infusion were studied. G(AP) was decreased from 7.6 +/- 0.6 to 2.8 +/- 0.5 by alpha-hANP with hemorrhage. G(AP) decreased 7.4 +/- 0.8 to 2.9 +/- 0.4 by alpha-hANP with blood infusion. G(AP) was restored nearly to the control levels one hour after alpha-hANP administration. GHR did not change by alpha-hANP administration. In the second and third protocol, a site of action of alpha-hANP was analysed. alpha-hANP inhibited the responses of systemic vascular resistance to quick mild hemorrhage and blood infusion, but did not affect the responses of cardiac output to quick mild hemorrhage and blood infusion in the second protocol. After carotid sinus denervation and vagotomy, alpha-hANP inhibited an arterial pressure responses to phenylephrine and nitroglycerin bolus injection in the third protocol. These findings suggest that alpha-hANP has mainly an effect on the peripheral vascular system to decrease its responsiveness, resulting in a decrease in G(AP).

The pathogenesis of DOCA-salt hypertension

Deoxycorticosterone acetate (DOCA) is an agent commonly used to induce hypertension in experimental animals. This form of hypertension is dependent on altered regulation of central pressor mechanisms including the brain renin-angiotensin system. Additionally, there are characteristic changes involving the cardiovascular system and baroreflex responses. This review will discuss aspects of the pathogenesis of DOCA hypertension and the effect of various antihypertensive agents on the development of this form of hypertension.

Sodium depresses arterial baroreceptor reflex function in normotensive humans

Sodium may contribute to the pathogenesis of hypertension by impairing arterial baroreceptor reflex function. The objectives of this study were to 1) determine whether a high sodium diet depresses arterial baroreceptor reflex function in normotensive humans, and 2) determine whether alterations in baroreceptor reflex function are related to changes in arterial compliance. Seventeen normotensive men, aged 30 +/- 2 years, received 10 and 200 meq sodium per day diets, each for 5 days, in a randomized crossover trial. Carotid baroreceptor reflex function was assessed by measuring the blood pressure response to sequential neck suction (0, -10, -20, and -30 mm Hg) and neck pressure (0, +10, +20, and +30 mm Hg). Forearm vascular resistance was determined by venous occlusion plethysmography. Arterial compliance was evaluated by calculating the quotient of the diastolic blood pressure decay time constant and forearm vascular resistance. Blood pressure averaged 124 +/- 3/62 +/- 2 mm Hg on the low sodium diet and 122 +/- 3/60 +/- 2 mm Hg on the high sodium diet (p = NS). Baroreceptor reflex slopes representing the systolic and diastolic blood pressure responses to changes in neck chamber pressure were steeper in the subjects when randomly assigned to low sodium diet than to high sodium diet. Diastolic blood pressure decay time and forearm arterial compliance were similar during low and high sodium intake. We conclude that short-term exposure to a high sodium diet depresses carotid baroreceptor reflex function in normotensive humans. This observation cannot be attributed to changes in the arterial compliance.

The hemodynamic effects of triglycyl-lysine-vasopressin (Glypressin) in patients with parkinsonism and orthostatic hypotension

The hemodynamic effects of triglycyl-lysine-vasopressin (TGLVP) were investigated in a single-blind study in seven patients with chronic orthostatic hypotension and parkinsonism. Blood pressure, heart rate, and stroke volume were measured in the supine position before and after bolus injection of either placebo or TGLVP (5.0, 7.5, or 10.0 micrograms/kg of body weight). After 40 min in the supine position, the patients were head-up tilted to 45 degrees for 20 min. All patients underwent four tilt studies with different medication. The TGLVP increased supine blood pressure by approximately 25% and total peripheral resistance by approximately 46%, and reduced heart rate by approximately 13%. No changes in supine stroke volume or cardiac output were seen. The TGLVP slightly reduced the relative fall in blood pressure and increased heart rate during the tilt. After TGLVP, blood pressure levels during tilt were similar to supine levels prior to medication. The TGLVP did not change the effects of tilt on stroke volume or cardiac output. Only few and mild side effects were experienced and no cardiotoxic effects were observed. In conclusion, TGLVP showed marked blood pressure effects of very small doses in this category of patients. The clinical effects of TGLVP and other vasopressor-specific analogs of vasopressin should be tested in these patients.

ACE inhibition improves vagal reactivity in patients with heart failure

The deranged autonomic control of heart rate was studied in 34 patients with heart failure (New York Heart Association [NYHA] functional class II to III) by examining the carotid sinus baroreflex. The carotid sinus baroreceptors were stimulated by graded suction. The slope of the regression line between increases in cycle length and the degree of neck suction was taken as an index of baroreflex sensitivity. The reflex response is mediated by a selective increase of vagal efferent activity. Baroreflex sensitivity therefore represents a measure of vagal reactivity. Using multiple regression analysis, baroreflex sensitivity (BS) correlated positively to stroke volume index (SVI) and inversely to plasma renin activity (PRA) and to age: BS = 0.47 SVI - 0.38 PRA - 0.23 age + constant (r = 0.74; p less than 0.0005). In addition to digitalis and diuretics, angiotensin-converting enzyme (ACE) inhibitors (captopril or enalapril) were given to 16 patients for a mean of 17 +/- 3 days. The patients with hemodynamic improvement (group A) exhibited improved baroreflex sensitivity (1.4 +/- 0.4 to 3.6 +/- 1.2 msec/mm Hg; p less than 0.01). Baroreflex sensitivity remained unchanged (3.1 +/- 0.8 to 2.4 +/- 1.0 msec/mm Hg; n.s.) in the patients without hemodynamic improvement (group B). The increase in reflex sensitivity did not correlate with hemodynamic alterations. Baroreflex sensitivity during ACE inhibition (BSD) was only related to the baseline baroreflex sensitivity (BSB): BSD = 2.8 BSB - 0.46 (r = 0.84; p less than 0.005). In patients with heart failure, reflex bradycardia decreases with age and with PRA and increases with stroke volume. Chronic therapy with ACE inhibitors enhances vagal reactivity in patients with hemodynamic improvement.

Evidence of an endogenous forebrain GABAergic system capable of inhibiting baroreceptor-mediated vasopressin release

In conscious rats, intracerebroventricular (i.c.v.) injections of gamma-aminobutyric acid (GABA), a GABA-uptake inhibitor (nipecotic acid), and artificial CSF (aCSF) were restricted to forebrain regions and their effect on baroreceptor-mediated arginine-vasopressin (AVP) release was studied. AVP release was stimulated by the hypotension resulting from combined treatment with a converting enzyme inhibitor (CEI) and chlorisondamine (CHLOR), a ganglionic blocking agent. CEI + CHLOR reduced mean arterial pressure (MAP) from 118 +/- 2 to 63 +/- 2 mm Hg, but pressure then rose to a compensated level of 78 +/- 1 mm Hg. The compensation in MAP was shown to be AVP-dependent at the end of the experiment since the vascular AVP antagonist, d(CH2)5Tyr(Me)AVP, reduced MAP from 78 +/- 1 to 63 +/- 1 mm Hg. While AVP was contributing to MAP maintenance, GABA (15, 50 and 150 micrograms) caused dose-related reductions in MAP (5 +/- 1.7 +/- 1 and 11 +/- 2 mm Hg, respectively). Nipecotic acid (3-350 micrograms) also caused dose-related reductions in MAP (from 3 +/- 1 to 15 +/- 2 mm Hg), while aCSF had no effect on MAP. Pretreatment with d(CH2)5Tyr(Me)AVP, antagonized completely the depressor effects of GABA and nipecotic acid. In other rats, blood samples were taken to measure the changes in plasma AVP concentrations (pAVP) induced by CEI + CHLOR and subsequent treatment with aCSF or nipecotic acid (175 micrograms). Hypotension induced by CEI + CHLOR caused a significant increase in pAVP. Forebrain-restricted nipecotic acid significantly suppressed pAVP (61 +/- 8% reduction; P less than 0.05 vs aCSF). These data provide evidence of an endogenous forebrain GABAergic system which, when activated, can inhibit baroreceptor-mediated AVP release.

Biphasic forearm vascular responses to intraarterial arginine vasopressin

Forearm vascular responses to arginine vasopressin (AVP) infused into a brachial artery in a wide range of infusion rates (0.05-2.0 ng/kg per min) were examined in 20 young healthy volunteers. Intraarterial AVP at lower doses (0.05 and 0.1 ng/kg per min) caused forearm vasoconstriction, whereas AVP at a dose of 0.2 ng/kg per min or higher caused forearm vasodilatation. The maximal forearm vasoconstriction was induced at the venous plasma AVP level of 76.3 +/- 8.8 pg/ml. Forearm vasodilatation was associated with the venous plasma AVP level of 369 +/- 43 pg/ml or higher. Forearm vasodilatation was the result of the direct effect of AVP since forearm blood flow and vascular resistance in the contralateral arm did not change. We attempted to explore the mechanisms involved in AVP-induced direct vasodilatation. The treatment with indomethacin, 75 mg/d for 3 d, did not alter AVP-induced forearm vasodilatation. In contrast, intraarterial infusion of isoosmolar CaCl2 totally prevented AVP-induced forearm vasodilatation. Intra-arterial CaCl2 also markedly attenuated forearm vasodilatation induced by intraarterial sodium nitroprusside, but did not alter forearm vasodilatation induced by intraarterial isoproterenol. These results indicate that the direct vascular effects of intra-arterial AVP on the forearm vessels are biphasic, causing vasoconstriction at lower doses and vasodilatation at higher doses. The direct vasodilatation induced by intraarterial AVP at higher doses is not mediated by prostaglandins but may involve cGMP-related mechanisms.

Vasopressin-mediated forearm vasodilation in normal humans. Evidence for a vascular vasopressin V2 receptor

Arginine vasopressin (AVP) is a potent vasopressor and antidiuretic neurohormone. However, when administered intravenously to humans, AVP causes forearm vasodilation. This effect has been attributed to sympathetic withdrawal, secondary to AVP-induced sensitization of baroreceptors. The possibility that AVP also causes forearm vasodilation directly has not been examined. Accordingly, the direct effect of AVP was determined by studying the forearm blood flow (FBF) response to intraarterial (IA) AVP infusion (0.01-1.0 ng/kg per min). Infusion of IA AVP increased FBF (96%) in the infused arm, but not the control arm, in a dose-dependent manner. The role of specific AVP V1 receptors in mediating this FBF response was determined before and after pretreatment with a V1 antagonist (AVP-A). AVP-A alone had no effect on FBF, but coadministration of AVP and AVP-A potentiated the vasodilatory response (223%). IA infusion of the V2 agonist, 1-desamino[8-D-arginine] vasopressin, caused a dose-dependent increase in FBF. These findings suggest that AVP causes direct, dose-dependent vasodilation in the human forearm that may be mediated by V2 vasopressinergic receptors. In contrast, AVP infusion caused digital vasoconstriction that was blocked by AVP-A, whereas dDAVP did not affect digital blood flow. Thus, AVP induces regionally selective vascular effects, with concurrent forearm vasodilation and digital vasoconstriction.

Hemodynamic effects of hypertonic saline in the conscious rat

The present study examines the role of vasopressin and the sympathetic nervous system on the hemodynamic effects of an infusion of hypertonic saline (NaCl 1.5 M) in conscious rats. The cardiovascular response to hypertonic saline was similar in both untreated and hexamethonium-pretreated rats. Mean arterial pressure increased by 15 mmHg as a consequence of the elevation of total peripheral resistance, while cardiac index was decreased. The administration of an antagonist to the pressor activity of vasopressin in rats with intact reflexes, partially decreased mean arterial pressure and total peripheral resistance and increased cardiac index toward basal values. In contrast, the hemodynamic response to hypertonic saline was totally reverted when the vasopressin antagonist was injected in the hexamethonium-pretreated rats. The results of the present study indicate that the hypertensive response induced by hypertonic saline in conscious rats is due to the vasoconstrictor effects of both vasopressin and the sympathetic nervous system.

Cardiac baroreflex sensitivities in conscious, unrestrained, Long Evans and Brattleboro rats

Cardiac baroreflex sensitivity (BRS; i.e. the slope of the line relating change in pulse interval to change in systemic arterial pressure) was assessed in response to an increase in mean blood pressure (MBP) evoked by methoxamine or a decrease evoked by sodium nitroprusside. Measurements were made in Long Evans (i.e. control) and in Brattleboro (i.e. vasopressin (AVP)-deficient) rats, following acute and chronic intravascular catheterization, and in water-replete and water-deprived states (the latter designed to reduce plasma volume and activate AVP-dependent mechanisms (in Long Evans rats]. There were no differences between the corresponding values for cardiac BRS in Long Evans and Brattleboro rats. Furthermore, water deprivation caused no significant changes in cardiac BRS in either strain. These results do not support the claim that absence of endogenous AVP in Brattleboro rats is associated with marked impairment of cardiac BRS, and indicate that chronic changes in circulating AVP do not cause systematic changes in cardiac BRS.

Hemodynamic and neurohormonal changes in the development of DOC hypertension in the dog

In this article we summarize studies of the hemodynamic and endocrine effects of desoxycorticosterone (DOC)-induced hypertension in dogs and also review new data of the action of this steroid on baroreceptors. The hemodynamic effect of subcutaneous injections of DOC to dogs, without supplementation of salt in their diet, consisted of increases in arterial pressure that were sustained for a 28-day observation period and associated with augmented cardiac output. At the early stage of the hypertensive response there was a rise in plasma Na+ concentration accompanied by increases in the plasma and cerebrospinal fluid (CSF) levels of vasopressin. The activity of the peripheral renin angiotensin system, as evaluated by the longitudinal changes in plasma renin activity and plasma immunoreactive angiotensin II (irAng-II), was markedly depressed in the hypertensive dogs. In contrast, the concentration of irAngII in the CSF did not change. Additional studies of the carotid occlusion reflex in anesthetized dogs revealed an enhanced buffering baroreceptor capacity in the early (less than day 10), but not the late (greater than day 28), stages of the hypertension. The abnormality in baroreflex function may be mediated by an effect of the steroid on an activity of brain angiotensin II that influences the inhibitory interaction between high and low pressure baroreceptors. The data acquired in these studies agree with the notion that excess mineralocorticoid production causes hypertension by mechanisms that influence the neurohormonal control of blood pressure by the central nervous system.

Role of baroreflex in the pressor response of rats with hypertension developed by renal artery stenosis

We examined the interrelationships between the pressor response to the administration of norepinephrine and arginine vasopressin and baroreflex function in rats with hypertension of two days' duration induced by heminephrectomy and a clip placed on the right renal artery (2-day clipped rats). Mean arterial pressure was higher in the 2-day clipped rats than in heminephrectomized rats without clips (sham-operated rats). The pressor response in the 2-day clipped rats to both agents increased as compared to the sham-operated rats. This hyperresponsiveness was attenuated by administering an angiotensin II antagonist, [1-Sar, 8-Ile] angiotensin II. Baroreflex sensitivity was studied by measuring changes in arterial pressure and pulse interval in response to the injection of phenylephrine. Baroreflex sensitivity was not decreased but markedly increased in the 2-day clipped rats and unaffected by infusing the angiotensin II antagonist. These results provide evidence that 1) in the 2-day clipped rats there are exaggerated pressor responses to vasoconstrictors; 2) the hyperresponsiveness is not causally related to the change of baroreflex sensitivity; and 3) angiotensin II plays a significant role in the increased pressor responses; however, the baroreflex mechanism is not involved in attenuation of the hyperresponsiveness by the angiotensin II antagonist.

Changes in vascular vasopressin receptors and responsiveness in DOCA/NaCl-treated rats

This study investigated the effects of treatment with desoxycorticosterone acetate and salt (DOCA/NaCl) on blood pressure, aortic vascular vasopressin receptors and in vitro vascular responsiveness in male rats. Four groups of animals were utilized in the study: a control group on normal tap water, a control group drinking a 1% NaCl solution; a DOCA/NaCl-treated group (1% NaCl in the drinking water); and a uninephrectomized (1K) DOCA/NaCl-treated group. DOCA/NaCl treatment for 4 weeks resulted in a significant elevation in blood pressure which was more pronounced in the uninephrectomized animal. The concentration of specific binding sites for vasopressin on the aorta were reduced in both the DOCA/NaCl-treated groups. However, the vascular responsiveness of the aorta to vasopressin was significantly reduced only in the hypertensive, uninephrectomized-DOCA/NaCl-(1K DOCA/NaCl) treated rat. Both the maximal contraction and the sensitivity was reduced in the 1K DOCA/NaCl group. The results of this study would suggest that the vascular alterations to vasopressin are probably post receptor mediated and result from the DOCA/NaCl-induced hypertension.

Vasopressin-induced bradycardia in fetal and adult sheep is not dependent on an increase in blood pressure

Other investigators have reported that intravenous infusion of synthetic arginine vasopressin into fetal lambs increases mean arterial pressure and decreases heart rate. To determine if the bradycardia produced by arginine vasopressin is a reflex response to the increase in blood pressure, we studied the effect of arginine vasopressin infusion on heart rate with and without blocking the increase in blood pressure. We performed 34 experiments in 12 chronically cannulated fetal lambs between 103 and 137 days' gestation. All animals had normal blood gas and pH values. Infusion of arginine vasopressin increased mean arterial pressure 10.1 +/- 1.1 mm Hg and decreased heart rate 50 +/- 8 bpm. Fetal heart rate decreased similarly when arginine vasopressin was infused and the hypertensive response was blocked with nitroprusside or a selective vasoconstrictor antagonist. [1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid) 2-(O-methyl)tyrosine] arginine vasopressin. For comparison we also studied five adult nonpregnant ewes. Bradycardia was observed in the adults after infusion of arginine vasopressin alone and when the hypertensive response was blocked with the vasoconstrictor antagonist. We conclude that arginine vasopressin infusion causes a fall in heart rate independent of any increase in blood pressure in both the fetal lamb and the adult sheep.

Brain corticotropin releasing factor in the spontaneously hypertensive rat

Corticotropin releasing factor and vasopressin were measured in major brain regions including the neurohypophysis in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) during development of hypertension. The highest concentration of corticotropin releasing factor was found in the hypothalamus in both strains. Corticotropin releasing factor was decreased in most major brain regions of SHR. In the hypothalamus, corticotropin releasing factor was lower in 3- and 6-week-old SHR than in age-matched WKY (p less than 0.01), but was similar at 12 and 24 weeks of age. The content of corticotropin releasing factor did not differ in the neurohypophysis in 3-week-old rats but began to decrease at 6 weeks of age (p less than 0.01) and continued to decrease during the development of hypertension (p less than 0.01). Brain vasopressin concentration did not differ between SHR and WKY except in the hypothalamus. The level of hypothalamic vasopressin was consistently lower in SHR than in WKY (p less than 0.01). These peptides are thought to be associated with autonomic nervous regulation, and our results may further strengthen the possibility that the deficit of the peptides may be involved in the development of spontaneous hypertension.

Vasoconstrictor role for vasopressin in experimental heart failure in the rabbit

Vasopressin's role as a vasoconstrictor in chronic heart failure, was examined in rabbits with adriamycin cardiomyopathic congestive heart failure. Chronic adriamycin treatment resulted in a decrease in cardiac output (829 +/- 38-610 +/- 36 ml/min, P less than 0.005) and blood pressure (83 +/- 2-76 +/- 3 mmHg, P less than 0.01), and an increase in peripheral resistance (8,377 +/- 381-10,170 +/- 657 dyn-s-cm-5, P less than 0.05). Plasma renin activity (4.7 +/- 0.6-10.9 +/- 2.8 ng angiotensin I/ml X h) and norepinephrine (0.7 +/- 0.1-1.3 +/- 0.2 pmol/ml, P less than 0.05) increased while plasma vasopressin levels did not change. Vasopressin infusion, however, produced significantly greater increases in peripheral resistance in animals with heart failure than in controls. Moreover, a specific vasopressin vascular antagonist reduced blood pressure (7 +/- 3%) and peripheral resistance (14 +/- 4%) and increased cardiac output (10 +/- 3%) in animals with heart failure but had no cardiovascular effects in normal rabbits. These results suggest that vascular sensitivity to vasopressin is increased in heart failure, and that it contributes significantly to the increased afterload in heart failure despite normal plasma levels. In this model of severe, chronic heart failure the sympathetic, renin-angiotensin, and vasopressin systems all appear to be activated.

Sensitization of aortic and cardiac baroreceptors by arginine vasopressin in mammals

Vasopressin facilitates the baroreflex control of the circulation. The peptide may act at several sites to augment the baroreflex. In this study we examined the effect of vasopressin on aortic baroreceptors in anaesthetized rabbits and on left ventricular mechanoreceptors in anaesthetized cats. Vasopressin (16 mu./kg. min) did not change resting nerve discharge in single fibres from aortic baroreceptors. Vasopressin (16 mu./kg. min) significantly enhanced the response of single aortic nerve fibre discharge to elevation in arterial pressure. The slope relating nerve activity to mean arterial pressure increased from 0.24 +/- 0.05 (mean +/- S.E. of mean) to 0.50 +/- 0.16 impulses/cardiac cycle. mmHg (P less than 0.05) in ten aortic medullated fibres and from 0.06 +/- 0.03 to 0.18 +/- 0.04 impulses/cardiac cycle. mmHg (P less than 0.05) in three non-medullated fibres. Vasopressin (16 mu./kg. min) did not change resting nerve discharge in single fibres from left ventricular mechanoreceptors. Vasopressin (16 mu./kg. min) significantly enhanced the response of single nerve fibre discharge from left ventricular mechanoreceptors in response to elevation of left ventricular end-diastolic pressure. The slope relating nerve activity to left ventricular end-diastolic pressure increased from 0.24 +/- 0.07 to 0.32 +/- 0.07 impulses/cardiac cycle. mmHg (P less than 0.05) in six medullated fibres and from 0.10 +/- 0.01 to 0.15 +/- 0.02 impulses/cardiac cycle. mmHg (P less than 0.05) in four non-medullated fibres. These data show that vasopressin sensitizes high and low pressure baroreceptors and suggest a mechanism by which vasopressin may facilitate the baroreflex control of the circulation.

Humoral control of water and electrolyte excretion during water restriction

The goals of the present study were twofold: first, to assess the renal excretory and hormonal responses to chronic water restriction in dogs whose sodium retaining mechanisms had been stimulated through dietary sodium (Na+) deprivation; second, to determine the mediator(s) of the natriuresis which was observed with water restriction in these sodium deprived dogs. Three groups of dogs maintained on a low Na+ diet (5 mEq/day) for two weeks underwent a three day period of water restriction. In normal, intact dogs Group 1 (N = 5), water restriction resulted in a significant increase in Na+ excretion with a net cumulative loss of 26.3 +/- 2.6 mEq over three days. The natriuresis was associated with a significant increase in plasma vasopressin (PAVP) (1.7 to 10.2 pg/mliter) and a significant fall in plasma aldosterone (PALDO) from the levels observed with Na+ restriction alone (24.9 to 12.4 ng/dliter). The natriuresis could not be explained by decreases in food intake as determined by control studies in four dogs. Group 2 (N = 6) dogs had a decrease in PALDO with water restriction that was prevented by means of continuous i.v. aldosterone infusion (6.0 micrograms/kg/day). Dogs in this group failed to demonstrate a natriuresis during three days of water restriction, despite the fact that PAVP rose from 3.3 +/- 0.8 to a peak level of 14.95 +/- 1.9 pg/mliter. Group 3 (N = 6) dogs underwent selective neurohypophysectomy, thus preventing the rise in PAVP during three days of water restriction. In this group, PALDO also remained unchanged from the Na+ deprived level during water restriction, and no natriuresis was observed. We conclude: 1) that the natriuresis which occurs with water restriction is a potent physiological response that occurs even in the Na+ restricted state; and 2) this natriuresis can be explained by a fall in PALDO and not the rise in PAVP.

Effects of small changes of plasma vasopressin on subcutaneous and skeletal muscle blood flow in man

The effect of vasopressin (AVP) on subcutaneous blood flow was studied by the 133Xenon wash-out method in 13 healthy subjects during three consecutive infusions of synthetic AVP, using increasing infusion rates. In seven of them, both subcutaneous and skeletal muscle blood flows were measured during the first infusion. The preinfusion, and infusion pAVP levels were 1.6 +/- 0.4, 3.4 +/- 0.4, 4.9 +/- 0.5 and 8.8 +/- 0.7 pg ml-1, respectively (mean +/- SE). The values are within the range normally found during dehydration. During the AVP infusions, the blood flow in subcutaneous tissues decreased 30-40% and the vascular resistance increased 60-80%. Neither heart rate nor blood pressure change significantly during the infusions. Plasma renin activity (PRA) decreased significantly. After cessation of the infusions, blood flow and vascular resistance rapidly returned to preinfusion values, while PRA increased very slowly. Skeletal muscle and subcutaneous tissues blood flows were found to be equally sensitive to small changes in the pAVP level. The present study has demonstrated that even minor increments of pAVP levels, as seen during dehydration, can significantly alter the regional blood flow in subcutaneous and skeletal muscle tissues in man.

Vasopressin reduces cardiac function and augments cardiopulmonary baroreflex resistance increases in man

We examined the effects of physiologic infusions of arginine vasopressin (AVP) on cardiovascular hemodynamics and on reflex responses initiated by decreasing cardiopulmonary baroreceptor stimulation (with lower body negative pressure) in 10 healthy, captopril-pretreated young men (19-27 yr). Their responses were compared with those of four volunteers given isosmotic infusion. Heart rate, stroke volume, blood pressure, and forearm blood flow were measured by electrocardiography, impedance cardiography, radial artery cannulation, and strain gauge plethysmography. Two 55-min infusions of AVP at rates of 0.15 and 0.40 ng/kg per min increased average plasma concentrations from control levels of 5 pg/ml to 18 and 36 pg/ml, respectively. These infusions resulted in progressive reductions of heart rate and cardiac output and increases of forearm and total peripheral resistance. Blood pressure increases were significant only during the larger AVP infusion rate. Lower body negative pressure provoked reflex increases of total peripheral resistance. These increases were enhanced 60% during AVP infusion compared with increases during control (pre-AVP). Baseline measurements and reflex responses were unchanged by isosmotic infusions. These results demonstrate that AVP has profound effects on cardiovascular function and augments cardiopulmonary baroreflex-mediated increases of peripheral resistance in man.

Vasopressin induced myocardial depression in neurally mediated and not due to impaired coronary blood flow

The mechanism of the cardiodepressant effect of vasopressin was studied by measuring simultaneously myocardial contractile force and coronary blood flow (with tracer microspheres) in anaesthetized open-chest rabbits. Lysine-vasopressin administered at two dose levels (10 and 100 mu kg-1 infused in 2 min with a maintenance dose of 2 mu kg-1 min-1 between these two loading doses) to a group of 6 rabbits caused dose-dependent myocardial depression and also severely decreased coronary blood flow in a dose-dependent manner. Blood pressure remained almost unchanged but heart rate, cardiac output and total peripheral conductance were also decreased dose-dependently. In another group of 6 rabbits treated in the same way with lysine-vasopressin, darodipine (PY 108-068, 30 and 100 micrograms kg-1) was infused intravenously. It reversed the vasopressin-induced coronary constriction and cardiodepression. The high dose of vasopressin brought back cardiac depression but did not reduce coronary blood flow below baseline values. Myocardial depression could therefore not be adequately explained by the changes in coronary blood flow. In a further group of rabbits which had been subjected to cervical vagotomy and beta-adrenoceptor blockade (propranolol 1 mg kg-1 i.v.) before the experiment, vasopressin still caused coronary constriction which was reversed by darodipine, but had no effect on myocardial contractile force and heart rate. The cardiodepressant effect of vasopressin can thus be explained fully by effects on the autonomic nervous system which are reversed by lowering blood pressure, whereas the severe reduction of coronary flow did not contribute to the vasopressin-induced myocardial depression.

Aortic baroreceptor characteristics in dogs with chronic high output failure

It has been shown that the arterial baroreflex is depressed in heart failure. The role of alterations in afferent discharge as a possible mechanism for this depression has not been investigated previously. Single unit aortic baroreceptor activity was recorded from six normal dogs and from nine dogs, each with a chronic aorto-caval fistula (AVF). At the time of the acute experiment, mean arterial blood pressure (MABP) was not significantly different in the two groups of dogs; however, pulse pressure was significantly higher in the AVF dogs (45.7 +/- 2.4 mm Hg vs, 24.4 +/- 2.0 mm Hg; p less than 0.001). Left ventricular end-diastolic pressure (LVEDP) was higher in the AVF dogs (31.3 +/- 2.0 vs 5.6 +/- 1.8 mm Hg; p less than 0.001). AVF dogs had elevated heart weight/body weight ratios. The relationship of systolic aortic pressure to systolic discharge was examined by changing aortic pressure with aortic and vena caval occluders. The peak gain (normalized to maximum discharge) averaged 2.19 +/- 0.27 in the normal dogs compared to 1.15 +/- 0.09 in the AVF group (p less than 0.01). Saturation pressures and maximum discharge rates were greater in the AVF dogs although the threshold pressures were not different in the two groups. This data suggests that there is an attenuated response of aortic baroreceptor discharge in dogs with chronic volume overload and this abnormality may partially be responsible for the abnormal baroreflex in heart failure.

Static and dynamic responses of carotid sinus baroreceptors in dogs with chronic volume overload

In this study we determined if there are any alterations in carotid sinus baroreceptor discharge in a model of heart failure in the dog. The left carotid sinus was isolated and perfused at 100 mmHg with a modified Krebs-Henseleit solution. Two groups of dogs were used: a normal group and a group with a chronic aorto-caval fistula (a.v.f.) of up to 20 weeks duration. The a.v.f. group had a significantly elevated heart rate and left ventricular end diastolic pressure as well as an increased arterial pulse pressure compared to normals. However, mean arterial pressure measured in the aortic arch was not significantly different. Carotid sinus pressure-discharge curves were constructed during increases in carotid sinus pressure by steps and by ramps of pressure change up to 200 mmHg/s. Carotid sinus diameter was also measured using sonomicrometer crystals. In six dogs from each group the sodium, potassium and water content of the right carotid sinus was measured. Several parameters of second order polynomial curves that were fitted to the data were compared between the two groups for step and ramp increases in pressure. These included maximum baroreceptor gain, threshold pressure and plateau pressure. Baroreceptor gain was significantly lower in the a.v.f. group for the step and lowest ramp rate pressure changes. Threshold pressure was significantly higher for the a.v.f. group for steps and the three slowest ramp rates. Plateau pressures were significantly higher for the steps and lowest ramp rate. The carotid sinus strain that was produced at any given carotid sinus pressure was not significantly different between the two groups. In addition, there was no difference in the sodium, potassium or water content of the carotid sinus between the two groups. It was concluded that there is a depression of the static discharge characteristics in this model of heart failure which is not due to an alteration in electrolyte or water content or to the compliance of the carotid sinus.

[Cardiovascular effect of the antidiuretic hormone arginine vasopressin]

The two major biological actions of vasopressin are antidiuresis and vasoconstriction. The antidiuretic action of low concentrations of vasopressin is well established and concentrations 10 to 100 times above those required for antidiuresis elevate arterial blood pressure. Antidiuresis is mediated by V2-receptors at the kidney, whereas vasopressin constricts arterioles by binding at V1-receptors. Pharmacological effects of specific antagonists of the vasoconstrictor activity of vasopressin (vascular or V1-receptor antagonists) are presented. Low concentrations of vasopressin do have significant hemodynamic effects. Physiological concentrations of vasopressin cause vasoconstriction and elevate systemic vascular resistance. In subjects with intact cardiovascular reflex activity, however, cardiac output falls concomitantly and blood pressure therefore does not change. In animals with baroreceptor deafferentation or in patients with blunted baroreceptor reflexes (autonomic insufficiency) a rise in plasma vasopressin causes vasoconstriction and an increase in blood pressure, because cardiac output does not fall under these conditions. Vasopressin contributes substantially via increase in systemic vascular resistance to maintain blood pressure during water deprivation. During hemorrhage and hypotension vasopressin has a major role to restore blood pressure. In experimental hypertension vasopressin contributes to the development and maintenance of high blood pressure in DOCA, but not in genetic hypertensive rats. The role of vasopressin in human hypertension is not yet clear. Vasopressin in extrahypothalamic areas of the brain affects circulatory regulation by interaction with central cardiovascular control centers. The exact mechanism of how vasopressin is involved in central regulation of blood pressure remains to be established. In contrast to our previous opinion vasopressin is a vasoactive hormone also at low plasma concentrations. Its cardiovascular action is more complex than previously assumed.

The mechanism of hypertension and bradycardia following lesions of the caudal ventrolateral medulla in the rabbit: the role of sympathetic nerves, circulating adrenaline, vasopressin and renin

Lesions of the ventrolateral medulla of the rabbit, coinciding with the A1 noradrenaline cell bodies (A1 lesions) produced fortyfold increases in the plasma levels of vasopressin and adrenaline, a twofold increase in plasma noradrenaline and a substantial increase in plasma renin activity. These increases accompanied the hypertension and bradycardia that follow A1 lesions. The vasoconstriction and hypertension were completely abolished by phentolamine, an alpha-adrenoceptor antagonist, when it was administered before lesions and were markedly reduced when it was given after lesions. On the other hand, administration of an antagonist to the vasoconstrictor action of vasopressin (d(CH2)5Tyr(Me)AVP) or an angiotensin converting enzyme inhibitor had little effect. Prior removal of the adrenal glands prevented any rise in plasma adrenaline levels but had no effect on the pressure response to subsequent A1 lesions. These results indicate that the vasoconstriction and hypertension were predominantly mediated by alpha-adrenoceptor stimulation, acting mainly through sympathetic vasoconstrictor nerves. The fall in heart rate following A1 lesions was approximately halved by pretreatment either with d(CH2)5Tyr(Me)AVP alone, or by blockade of the vagus and sympathetic with scopolamine and propranolol; it was completely abolished by combined pretreatment with all three agents. The experiments show that vasopressin release makes a major contribution to the bradycardia acting at least in part through mechanisms that are independent of cardiac vagal or sympathetic nerves.

Baroreflex impairment precedes hypertension during chronic cerebroventricular infusion of hypertonic sodium chloride in rats

Osmotic minipumps were implanted chronically for continuous 11-d infusion of hypertonic sodium chloride (NaCl) into the third cerebral ventricle (ICV) of awake rats to determine whether baroreflex sensitivity would be altered. Systolic and mean pressures, recorded from aortic catheters on day 11 while the rats were anesthetized with alpha-chloralose, were significantly higher in rats infused with artificial cerebrospinal fluid (CSF) containing hypertonic NaCl than in controls similarly infused with artificial CSF alone. Reflex changes in heart rate produced by subsequent intravenous infusions of either phenylephrine or sodium nitroprusside were inhibited, but reflex changes in renal nerve activity were unaltered. Magnitude of reflex bradycardia during pressor responses to phenylephrine, as well as of reflex tachycardia during depressor responses to sodium nitroprusside, was consistently smaller in NaCl-infused than in control rats. By contrast, group differences in attendant renal nerve firing were not significant. After sinoaortic denervation, drug-induced blood pressure effects persisted, but reflex responses in heart rate and renal nerve firing were abolished or markedly diminished. Peripheral effects produced by hypertonic NaCl leakage from the infusion site were considered unlikely because after 11 d of ICV infusion, sodium concentration, though appreciably elevated in CSF samples collected from the cisterna magna, was unaffected in corresponding serum samples. When cardiovascular responses to phenylephrine were recorded while chronic ICV infusions were in progress, awake rats receiving hypertonic NaCl were still normotensive on day 2 yet reflex bradycardia was already attenuated. In showing that baroreflex impairment preceded the development of hypertension, our results suggest that by depressing the anterior hypothalamus, chronic ICV infusion of hypertonic NaCl reduces sympatho-inhibition, and the ensuing baroreflex impairment then elevates blood pressure. However, other mechanisms could also be involved.