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Vasopressin regulation of blood pressure and volume: findings from V1a receptor–deficient mice
mini review
http://www.kidney-international.org & 2009 International Society of Nephrology
Vasopressin regulation of blood pressure and volume: findings from V1a receptor–deficient mice Toshinori Aoyagi1, Taka-aki Koshimizu2 and Akito Tanoue1 1
Department of Pharmacology, National Research Institute for Child Health and Development, Tokyo, Japan and 2Department of Pharmacology, Division of Molecular Pharmacology, Jichi Medical University, Tochigi, Japan
[Arg8]-vasopressin (AVP) has several functions via its three distinct receptors, V1a, V1b, and V2. The V1a vasopressin receptor (V1aR) is expressed in blood vessels and involved in vascular contraction. Recently, we generated V1a receptor–deficient (V1aR/) mice and found that they were hypotensive. In addition, V1aR/ mice exhibited (1) blunted AVP-induced vasopressor response, (2) impaired arterial baroreceptor reflex, (3) decreased sympathetic nerve activity, and (4) decreased blood volume, all of which could contribute to the observed hypotension. In relation to their decreased blood volume, V1aR/ mice had decreased plasma aldosterone levels, which could result not only from decreased activity of the renin–angiotensin system (RAS), but also from impaired AVP-stimulated aldosterone release in the adrenal glands. V1aR was found to specifically co-express at the macula densa cells with cyclooxygenase (COX)-2 and with neuronal nitric oxide synthase, which produces potent stimulators of renin, PGE2, and NO. The expression levels of renin, COX-2, and nNOS were significantly decreased in V1aR/ mice, which led to the suppression of RAS activity and consequent decreases in aldosterone and blood volume. Furthermore, V1aR is also expressed in collecting duct cells and involved in regulating water reabsorption by affecting V2/aquaporin 2 function. Thus, AVP regulates blood pressure and volume via V1aR by exerting diverse functions in vivo. Kidney International (2009) 76, 1035–1039; doi:10.1038/ki.2009.319; published online 19 August 2009 KEYWORDS: [Arg8]-vasopressin; renin–angiotensin system; V1a receptor
Correspondence: Akito Tanoue, Department of Pharmacology, National Research Institute for Child Health and Development, 2-10-1, Okura, Setagaya-ku, Tokyo 157-8535, Japan. E-mail: [email protected] Received 26 April 2009; revised 16 June 2009; accepted 23 June 2009; published online 19 August 2009 Kidney International (2009) 76, 1035–1039
AVP RECEPTORS AND KNOCKOUT MICE
The neurohypophyseal peptide [Arg8]-vasopressin (AVP) is involved in diverse functions including regulation of body fluid homeostasis, vasoconstriction, and adrenocorticotropic hormone release. These physiological effects are mediated by three subtypes of AVP receptors, designated V1a, V1b, and V2.1 The V1a vasopressin receptor (V1aR) is widely expressed, whereas the V1b receptor (V1bR) is specifically expressed in the anterior pituitary glands and pancreatic islets; the V2 receptor (V2R) is predominantly expressed in the kidneys.1,2 V1aR mediates vascular contraction, cellular proliferation, platelet aggregation, glycogenolysis, lipid metabolism, protein catabolism, and glucose tolerance.1,2 V1bR mediates adrenocorticotropic hormone and insulin release.2 Both V1aR and V1bR couple to the Gq protein and act through phosphatidylinositol hydrolysis to mobilize intercellular Ca2 þ .1 V2R is expressed in the thick ascending limbs (TAL) of Henle’s loop and the collecting ducts in the kidney. Through V2R, AVP stimulates Gs protein and adenylate cyclase to increase cellular cAMP, which stimulates the translocation of aquaporin (AQP)-2 in the collecting duct, and thereby increases water resorption.1 As V1aR and V2R are both detected in the juxtaglomerular apparatus, the TAL, and the collecting duct, both could be involved in regulating body fluid homeostasis.1,3 Recently, we generated mice deficient in either V1a (V1aR/) or V1b (V1bR/). As AVP exerts its various actions through V1aR and V1bR, as well as through V2R, mutant mice deficient in V1aR or V1bR exhibit a variety of phenotypes. Although V1bR/ mice exhibited altered hypothalamic–pituitary–adrenal axis activity,2 enhanced insulin sensitivity,2 and altered psychological behavior,4 V1aR/ mice also exhibited alterations in endocrine activity, metabolism, and feeding and social behavior.2,5,6 In addition, V1aR/ mice showed hypotension accompanied with decreased plasma volume.5,7 In this review, we suggest possible mechanism(s) by which AVP/V1aR may regulate blood pressure based on our studies with V1aR/ mice. BLOOD PRESSURE IN V1aR/ MICE
Blood pressure is regulated by many vasoactive factors such as epinephrine.8 Among them, AVP is one of the most potent 1035
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vasoconstrictors and is known to affect blood pressure by regulating vascular tonus and body fluid through V1aR and/or V2R.1 Yet the role of AVP in blood pressure maintenance is ill-defined, as the cardiovascular effects of AVP are complex. We studied the role of V1aR by observing V1aR/ mice, and found that the mutant mice are hypotensive.7 Although vascular V1aR is well recognized, the hypotensive phenotype of V1aR/ mice was unexpected, because previous pharmacological studies with selective V1aR antagonists showed that blocking V1aR did not induce any significant change in basal blood pressure level.9 Therefore, participation of vascular V1aR in maintaining peripheral vascular tonus is unexpectedly small or nonexistent in normal animals and humans at resting state. To elucidate the possible mechanism(s) by which AVP/V1aR regulates blood pressure, we analyzed V1aR/ mice and found several changes that could affect blood pressure homeostasis. Blunted AVP-induced vasopressor response
V1aR/ mice exhibited remarkable hypotension without a significant change in heart rate during rest.7 An echocardiogram study revealed no difference in cardiac function between V1aR/ and wild-type (WT) mice.7 As AVP is a potent vasoconstrictor, pressor responses to AVP were assessed in perfused mesenteric arterial beds. Whereas isolated perfused mesenteric arterial beds from WT and V1aR/ mice exhibited comparable responses to KCl, the pressor response to AVP seen in WT mice was absent in V1aR/ mice, indicating that the AVP-induced vascular contraction was caused primarily through V1aR.7 In addition to this in vitro finding, in vivo AVP-induced pressor response was abolished in non-anesthetized V1aR/ mice, which seemed at first to imply that a blunted blood pressure response to AVP could cause the hypotension. However, vasodilatation and depressor responses were induced in V1aR/ arterial beds and non-anesthetized animals, respectively, by high concentrations of AVP far exceeding the physiological range.1,7,10 Considering our finding and other reports on V1aR antagonists, it seems that other cause(s), rather than the blunted AVP-induced pressor response, could contribute to the decreased blood pressure observed in V1aR/ mice. Impaired arterial baroreceptor reflex
Functional relationships between blood pressure and reflex changes in heart rate were examined in non-anesthetized mice. We found that baroreceptor reflexes in V1aR/ mice were significantly impaired. When blood pressure was increased by phenylephrine, an a1 adrenergic receptor agonist, or decreased by nitrates, markedly attenuated heart rate changes were evident in V1aR/ mice and in WT mice treated with a selective V1aR blocker, although blood pressure changed by similar amounts in the mutants and the control mice.7 Our observation was in line with the attenuated baroreflex sensitivity seen in AVP-deficient rats, and indicates involvement of V1aR in that reflex.11 In 1036
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addition, the bradycardia induced by electrical stimulation of the hemilateral vagus nerve, which was exposed and cut at the cervical level, was markedly attenuated in V1aR/ mice, suggesting that their central baroreflex arc is impaired.12 When V1aR expression in the nucleus of the solitary tract where the vagal afferents terminate was examined through in situ hybridization analysis, V1aR was highly expressed in the nucleus of the solitary tract and area postrema in WT mice, but not in V1aR/. Thus, V1aR in the central nervous system could have a crucial role in maintaining the baroreflex control of heart rate and blood pressure homeostasis. Decreased sympathetic nerve activity in central nervous system
Although previous pharmacological studies with selective V1aR antagonists did not induce any significant change in basal blood pressure level, such drugs can exert a hypotensive effect in hypertensive animals.9,13 This suggests that the AVP/V1aR signal could be involved in developing or maintaining hypertension. To clarify the underlying mechanisms, we generated a salt-induced hypertension model. In WT mice, blood pressure was significantly increased during salt loading after subtotal nephrectomy. In V1aR/ mice, in contrast, blood pressure increased during salt loading, but the final blood pressure after salt loading was significantly lower, indicating that V1aR deficiency leads to resistance to salt-induced hypertension (Koshimizu T et al., manuscript in preparation). Plasma norepinephrine levels were increased by the salt loading in both WT and V1aR/ mice, but the increase in norepinephrine was smaller in V1aR/ mice, suggesting that sympathetic nerve activity was suppressed in V1aR/ mice. Furthermore, V1aR antagonists markedly lowered the blood pressure in salt-loaded WT mice, but not in control WT mice or in salt-loaded or control V1aR/ mice. Related to the salt-loaded hypertension model are several previous studies involving crosses of genetically hypertensive rat strains with vasopressin-deficient Brattleboro rats, intended to evaluate the importance of AVP in the development of hypertension. These studies revealed that genetic background contributes to salt- and water-retention ability.14,15 SHRs (stroke-prone spontaneously hypertensive rats) have normal extracellular fluid volumes and become hypertensive on salt loading,16,17 whereas New Zealand genetically hypertensive (NZGH) rats have reduced extracellular fluid volumes and are insensitive to salt loading.18,19 When crossed with Brattleboro rats, vasopressin-deficient SHR developed full hypertension,14 whereas the blood pressures of vasopressin-deficient NZGH rats were lower than those of the original NZGH strain.15 It seems that the blood pressure phenotype of V1aR/ mice is more similar to that of NZGH than to that of SHR, because AVP action is indispensable for the basal blood pressure seen in V1aR/ mice, as it is for the hypertensive response seen in NZGH rats. We also examined the role of central V1aR in blood pressure control by administering AVP into the cerebral Kidney International (2009) 76, 1035–1039
T Aoyagi et al.: New insights into the role of V1a receptor
ventricle and found that, whereas pressor and tachycardiac responses were induced in WT mice in a sympathetic activitydependent manner, these events were completely abolished in V1aR/ mice. In addition, pressor and tachycardiac responses induced by hypertonic saline in the cerebral ventricle were greatly diminished in V1aR/ mice compared with those in WT mice. These findings imply that centrally acting AVP has a crucial role in developing or maintaining hypertension by stimulating sympathetic nerve activity through V1aR. Decreased plasma volume in V1aR/ mice
In investigating the mechanism(s) responsible for the observed hypotension in V1aR/ mice, we found that their circulating blood volume was significantly reduced by 9%.7 This reduction could be a major factor, if not the only factor, contributing to the lowered resting blood pressure observed in V1aR/ mice. Lower plasma volume due to the decreased plasma aldosterone level in V1aR/ mice. To investigate the mechanism under-
lying the reduction in blood volume, we further examined hematocrit, blood urea nitrogen, plasma AVP, atrial natriuretic peptide, and aldosterone in V1aR/ mice. V1aR/ mice had higher hematocrit, blood urea nitrogen, and plasma AVP values,5 and lower atrial natriuretic peptide, and aldosterone levels, compared with WT mice.5 In addition, urine volume of V1aR/ mice was higher than that of WT mice under both basal and water-loading conditions.20 These findings suggest that the decrease in aldosterone level could be a cause of the impaired regulation of body fluid retention seen in V1aR/ mice, which led to their decreased circulating blood volume. Before this hypothesis is accepted, however, it needs to be proven experimentally by restoring aldosterone levels in V1aR/ mice to normal, and measuring their blood pressure and blood volume.5,7 In the study with the mineralocorticoid receptor-deficient mice, mentioned above, the mutant mice exhibited dehydration accompanied by higher hematocrit levels than those seen in WT mice;21 this finding supports our observation that V1aR/ mice also suffer from dehydration. Decreased plasma aldosterone level due to the decreased RAS activity in V1aR/ mice. As we found that the blood
pressure, plasma volume, and aldosterone level were decreased in V1aR/ mice,5,7 we evaluated the activity of the renin–angiotensin system (RAS). We found that plasma renin activity and angiotensin II levels were decreased in V1aR/ mice compared with those in WT mice under both basal and water-restricted conditions,20 implicating that RAS activity was suppressed in V1aR/ mice. To further confirm the decreased renin activity, we investigated renin expression in the kidney. Renin in the granule cells was detected through immunohistochemical study as previously described.22 In a control kidney, renin expression was detected not only in the granule cells but also in the renal tubules.20,23 In the V1aR/ kidney, renin was expressed in the same two locations, but at significantly lower levels, indicating that decreased expression Kidney International (2009) 76, 1035–1039
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of renin underlies the decreased renin activity in V1aR/ mice.20 In good agreement with this finding, renin expressions at the mRNA and protein levels were also decreased in V1aR/ mice compared with those in WT mice under basal and water-restricted conditions.20 As neuronal nitric oxide synthase (nNOS), which is expressed in macula densa cells, and cyclooxygenase (COX)-2, expressed in macula densa cells and the TAL, also have important roles in tubuloglomerular feedback by producing NO and prostaglandin E2, and stimulating renin production in granule cells,22 we next compared the expressions of nNOS and COX-2 between WT and V1aR/ mice. In both WT and V1aR/ kidneys, an immunohistochemical study revealed that nNOS was specifically expressed in macula densa cells and COX-2 was present in macula densa cells and TAL, but the expressions of nNOS and COX-2 were weaker in V1aR/ kidney.20 In agreement with the decreased COX-2 expression, urinary and plasma prostaglandin E2 levels were also decreased in V1aR/ mice.20 In addition to this local stimulation of renin production, sympathetic nerve activity also has a crucial role in renin secretion.24 These findings and those of other previous reports suggest that a blunted ability to regulate renin production in granule cells and the TAL due to impairment of both local and central stimulation processes caused the decrease in renin production in V1aR/ mice, which contributed to their low plasma volume and hypotension. AVP is well known to regulate body fluid homeostasis through the V2R–AQP2 system in the kidney. In contrast, the functional role of V1aR in the kidney has not been fully elucidated even though it has been localized in several regions of the kidney, including TAL, collecting ducts, and renal vascular cells.3 As the expression levels of nNOS and COX-2 were decreased in V1aR/ mouse kidneys, we performed in situ hybridization for V1aR in WT mouse kidney to determine whether V1aR was coexpressed with nNOS or COX-2. V1aR was detected in the macula densa cells, where it was co-localized with nNOS and COX-2, and in the TAL where it was co-localized with COX-2.20 This finding suggests that AVP regulates nNOS and COX-2 through V1aR at coexpressed cells and that the blockade of V1aR leads to the suppression of nNOS and COX-2, resulting in reduced renin activity. Decreased plasma aldosterone level due to impaired AVPstimulated aldosterone release from adrenal glands in V1aR/ mice. Aldosterone secretion is known to be regulated by
factors such as AVP, angiotensin II, and adrenocorticotropic hormone.25 As aldosterone levels were decreased in V1aR/ mice, aldosterone release from adrenal gland cells in response to AVP stimulation was examined. As in previous reports on human adrenal gland,26 AVP stimulated aldosterone release from the dispersed adrenal gland cells of WT mice, and this response was inhibited by the V1aR antagonist, implying that AVP mediates aldosterone release from the adrenal glands in mice.27 Supporting this finding, AVP-induced aldosterone release from the adrenal gland cells of V1aR/ mice was impaired, whereas there was no difference in 1037
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adrenocorticotropic hormone-induced aldosterone release between V1aR/ and WT mice.27 This finding, observed both in V1aR/ mice and in mice treated with the selective antagonist, shows that blockade of the AVP/V1aR pathway could lead to suppression of the signaling pathways of aldosterone production and/or secretion in the adrenal cortex, resulting in a decrease in the plasma aldosterone level. Thus, the decreased plasma aldosterone levels observed in V1aR/ mice could be caused by the suppression of RAS activity and the impairment of AVP-stimulated aldosterone secretion, although, in vivo, the RAS has a more major role in regulating plasma aldosterone in the adrenal gland than AVP-stimulated aldosterone secretion does. Lower plasma volume due to the increased urine volume caused by altered V2R–AQP2 function in V1aR/ mice. As the
RAS and tubuloglomerular feedback were altered due to the lack of V1aR expression in V1aR/ mice, we further assessed their renal function. Urine volume was greater and urine osmolarity was lower in V1aR/ mice than in WT mice, particularly when water was loaded. The GFR, urinary NaCl excretion, AVP-dependent cAMP generation, and V2R and AQP2 expression in the kidney were lower in the mutant mice, indicating that the diminished GFR and V2R–AQP2 system led to impaired urinary concentration in V1aR/ mice.20 In addition, increased renal vascular resistance and decreased NO production in V1aR/ mouse kidneys under
the basal condition could indicate impairment of tubuloglomerular feedback in mutant mice.20 Angiotensin II facilitates the AVP-stimulated V2R–AQP2 system.28 This observation, taken together with the data from this study, indicates that AVP can regulate body fluid homeostasis through V1aR in macula densa cells by activating the RAS and subsequently the V2R–AQP2 system, and that AVP/V1aR signaling may have a part in tubuloglomerular feedback (Aoyagi T et al.,20 Figure 1). In general, a decrease in RAS activity could be expected to lead to higher levels of Na excretion and polyuria, but the disruption of V1aR resulted in decreased urinary Na excretion and polyuria. To understand this discrepancy, we will have to measure the fractional excretion of Na in future studies. These data may explain whether alterations in sodium excretion have their basis in tubular re-absorption or in filtered load. Comparison between V1aR/ mice and Brattleboro rats
AVP-deficient Brattleboro rats are well known as a model of central diabetes insipidus; they exhibit normal blood pressure despite impaired baroreflex and polyuria, whereas V1aR/ mice exhibit hypotension.29 Hypersecretion of renin due to hyperactivity of the sympathetic nerve may help maintain normotensive status in Brattleboro rats.30 Conversely, our AVP
AVP V1aR
Macula densa V1aR cell
Collecting duct cell
nNOS/NO COX2/PGE2
V2R Sympathetic nerve activity
AQP2
nNOS COX2
Angiotensinogen
Granule cell
ACE Angiotensin II
Renin angiotensin system
Heart rate
Body fluid retention
Figure 1 | Schematic representation of the regulation of body fluid retention by vasopressin and vasopressin receptor type V1a and V2. Hyperosmolarity and decreased plasma volume under the water-deprived condition lead to secretion of vasopressin (AVP) from the posterior pituitary, which can bind to V1a receptor (V1aR) in macula densa cells and to V2 receptor (V2R) in collecting duct cells. AVP/V1aR stimulates the expression of neuronal nitric oxide synthase (nNOS) and cyclooxygenase (COX)-2, leading to the production of NO and prostaglandin E2 (PGE2) by macula densa cells, respectively. Both NO and PGE2 stimulate renin production from granule cells, and subsequent increases in angiotensin II and aldosterone levels promote water reabsorption. AVP/V2R enhances translocation of aquaporin (AQP)-2 to the apical membrane from the cytoplasm. Water reabsorption is further accelerated by the AVP/V1aR-stimulated renin–angiotensin system (RAS). The AVP/V2R system and the RAS enhanced by AVP/V1a critically regulate body fluid retention.
Aldosterone
Vascular contraction Body fluid retention Regulation of cardiovascular function Blood pressure
1038
AQP2
Renin Angiotensin I
Baroreflex sensitivity
Renin
V2R
Regulation of body fluid
Figure 2 | Contribution of vasopressin and vasopressin receptor type V1a and V2 in the regulation of blood pressure homeostasis. Blood pressure could be controlled by changes in body fluid, cardiac output, and vascular contraction. Activation of the AVP/V2R system induces the translocation of AQP2 and water reabsorption in the kidney. The AVP/V1aR system, on the other hand, stimulates RAS activity and aldosterone release. AVP also directly stimulates aldosterone secretion from the adrenal grand. In addition, AVP stimulates vascular contraction through V1aR. V1aR also enhances baroreflex sensitivity, which controls the heart rate. Central AVP/V1aR activates sympathetic nerve outflow and regulates vascular as well as cardiac functions. These V1aR-mediated cardiovascular and body-fluid-homeostasis effects could have crucial roles in blood pressure regulation in health and disease. Broad arrows indicate a physiological pathway directly regulated by V1aR. ACE, angiotensin-converting enzyme; AQP2, aquaporin-2; AVP, vasopressin; COX2, cyclooxygenase-2; nNOS, neuronal nitric oxide synthase; RAS, renin–angiotensin system; V1aR, V1a receptor; V2R, V2 receptor. Kidney International (2009) 76, 1035–1039
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T Aoyagi et al.: New insights into the role of V1a receptor
observations of V1aR/ mice indicated that their decreased sympathetic nerve activity may reduce the secretion of renin in addition to impairing the tubuloglomerular feedback system. Hence, dysfunction of RAS due to blockade of V1aR led to hypotension in V1aR/ mice.
8. 9.
10. 11.
CONCLUSION
In summary, AVP has a crucial role in regulating blood pressure through V1aR by stimulating vascular contractions, arterial baroreceptor reflex, sympathetic nerve activity, and water re-absorption, and blockade of the AVP/V1aR signal results in decreased blood pressure (Figure 2). Studies using the knockout model have several limitations, including the difficulty of determining cause and effect, the difficulty of determining direct versus indirect effects, and the possibility of problems occurring due to the lifetime deletion of the receptor causing functional compensations. In addition, the importance of the observed knockout phenotype needs to be further verified through studies involving other pharmacological or genetic interventions. Nevertheless, knockout studies such as ours will provide valuable information and clues contributing to the investigation of the mechanisms of AVP/V1aR signaling in humans. DISCLOSURE
All the authors declared no competing interests. REFERENCES 1. Jackson EK. Vasopressin and other agents affecting the renal conversion of water. In: Hardman JG, Limbird LE, Gilman AG (eds). Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 9th ed. McGraw-Hill: New York, 1996, pp 715–731. 2. Tanoue A. New topics in vasopressin receptors and approach to novel drugs: effects of vasopressin receptor on regulations of hormone secretion and metabolisms of glucose, fat, and protein. J Pharmacol Sci 2009; 109: 50–52. 3. Terada Y, Tomita K, Nonoguchi H et al. Different localization and regulation of two types of vasopressin receptor messenger RNA in microdissected rat nephron segments using reverse transcription polymerase chain reaction. J Clin Invest 1993; 92: 2339–2345. 4. Stewart LQ, Roper JA, Young WS 3rd et al. Pituitary-adrenal response to acute and repeated mild restraint, forced swim and change in environment stress in arginine vasopressin receptor 1b knockout mice. J Neuroendocrinol 2008; 20: 597–605. 5. Aoyagi T, Birumachi J, Hiroyama M et al. Alteration of glucose homeostasis in V1a vasopressin receptor-deficient mice. Endocrinology 2007; 148: 2075–2084. 6. Egashira N, Tanoue A, Matsuda T et al. Impaired social interaction and reduced anxiety-related behavior in vasopressin V1a receptor knockout mice. Behav Brain Res 2007; 178: 123–127. 7. Koshimizu T, Nasa Y, Tanoue A et al. V1a vasopressin receptors maintain normal blood pressure by regulating circulating blood volume and baroreflex sensitivity. Proc Natl Acad Sci USA 2006; 103: 7807–7812.
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http://www.kidney-international.org & 2009 International Society of Nephrology
Vasopressin regulation of blood pressure and volume: findings from V1a receptor–deficient mice Toshinori Aoyagi1, Taka-aki Koshimizu2 and Akito Tanoue1 1
Department of Pharmacology, National Research Institute for Child Health and Development, Tokyo, Japan and 2Department of Pharmacology, Division of Molecular Pharmacology, Jichi Medical University, Tochigi, Japan
[Arg8]-vasopressin (AVP) has several functions via its three distinct receptors, V1a, V1b, and V2. The V1a vasopressin receptor (V1aR) is expressed in blood vessels and involved in vascular contraction. Recently, we generated V1a receptor–deficient (V1aR/) mice and found that they were hypotensive. In addition, V1aR/ mice exhibited (1) blunted AVP-induced vasopressor response, (2) impaired arterial baroreceptor reflex, (3) decreased sympathetic nerve activity, and (4) decreased blood volume, all of which could contribute to the observed hypotension. In relation to their decreased blood volume, V1aR/ mice had decreased plasma aldosterone levels, which could result not only from decreased activity of the renin–angiotensin system (RAS), but also from impaired AVP-stimulated aldosterone release in the adrenal glands. V1aR was found to specifically co-express at the macula densa cells with cyclooxygenase (COX)-2 and with neuronal nitric oxide synthase, which produces potent stimulators of renin, PGE2, and NO. The expression levels of renin, COX-2, and nNOS were significantly decreased in V1aR/ mice, which led to the suppression of RAS activity and consequent decreases in aldosterone and blood volume. Furthermore, V1aR is also expressed in collecting duct cells and involved in regulating water reabsorption by affecting V2/aquaporin 2 function. Thus, AVP regulates blood pressure and volume via V1aR by exerting diverse functions in vivo. Kidney International (2009) 76, 1035–1039; doi:10.1038/ki.2009.319; published online 19 August 2009 KEYWORDS: [Arg8]-vasopressin; renin–angiotensin system; V1a receptor
Correspondence: Akito Tanoue, Department of Pharmacology, National Research Institute for Child Health and Development, 2-10-1, Okura, Setagaya-ku, Tokyo 157-8535, Japan. E-mail: [email protected] Received 26 April 2009; revised 16 June 2009; accepted 23 June 2009; published online 19 August 2009 Kidney International (2009) 76, 1035–1039
AVP RECEPTORS AND KNOCKOUT MICE
The neurohypophyseal peptide [Arg8]-vasopressin (AVP) is involved in diverse functions including regulation of body fluid homeostasis, vasoconstriction, and adrenocorticotropic hormone release. These physiological effects are mediated by three subtypes of AVP receptors, designated V1a, V1b, and V2.1 The V1a vasopressin receptor (V1aR) is widely expressed, whereas the V1b receptor (V1bR) is specifically expressed in the anterior pituitary glands and pancreatic islets; the V2 receptor (V2R) is predominantly expressed in the kidneys.1,2 V1aR mediates vascular contraction, cellular proliferation, platelet aggregation, glycogenolysis, lipid metabolism, protein catabolism, and glucose tolerance.1,2 V1bR mediates adrenocorticotropic hormone and insulin release.2 Both V1aR and V1bR couple to the Gq protein and act through phosphatidylinositol hydrolysis to mobilize intercellular Ca2 þ .1 V2R is expressed in the thick ascending limbs (TAL) of Henle’s loop and the collecting ducts in the kidney. Through V2R, AVP stimulates Gs protein and adenylate cyclase to increase cellular cAMP, which stimulates the translocation of aquaporin (AQP)-2 in the collecting duct, and thereby increases water resorption.1 As V1aR and V2R are both detected in the juxtaglomerular apparatus, the TAL, and the collecting duct, both could be involved in regulating body fluid homeostasis.1,3 Recently, we generated mice deficient in either V1a (V1aR/) or V1b (V1bR/). As AVP exerts its various actions through V1aR and V1bR, as well as through V2R, mutant mice deficient in V1aR or V1bR exhibit a variety of phenotypes. Although V1bR/ mice exhibited altered hypothalamic–pituitary–adrenal axis activity,2 enhanced insulin sensitivity,2 and altered psychological behavior,4 V1aR/ mice also exhibited alterations in endocrine activity, metabolism, and feeding and social behavior.2,5,6 In addition, V1aR/ mice showed hypotension accompanied with decreased plasma volume.5,7 In this review, we suggest possible mechanism(s) by which AVP/V1aR may regulate blood pressure based on our studies with V1aR/ mice. BLOOD PRESSURE IN V1aR/ MICE
Blood pressure is regulated by many vasoactive factors such as epinephrine.8 Among them, AVP is one of the most potent 1035
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vasoconstrictors and is known to affect blood pressure by regulating vascular tonus and body fluid through V1aR and/or V2R.1 Yet the role of AVP in blood pressure maintenance is ill-defined, as the cardiovascular effects of AVP are complex. We studied the role of V1aR by observing V1aR/ mice, and found that the mutant mice are hypotensive.7 Although vascular V1aR is well recognized, the hypotensive phenotype of V1aR/ mice was unexpected, because previous pharmacological studies with selective V1aR antagonists showed that blocking V1aR did not induce any significant change in basal blood pressure level.9 Therefore, participation of vascular V1aR in maintaining peripheral vascular tonus is unexpectedly small or nonexistent in normal animals and humans at resting state. To elucidate the possible mechanism(s) by which AVP/V1aR regulates blood pressure, we analyzed V1aR/ mice and found several changes that could affect blood pressure homeostasis. Blunted AVP-induced vasopressor response
V1aR/ mice exhibited remarkable hypotension without a significant change in heart rate during rest.7 An echocardiogram study revealed no difference in cardiac function between V1aR/ and wild-type (WT) mice.7 As AVP is a potent vasoconstrictor, pressor responses to AVP were assessed in perfused mesenteric arterial beds. Whereas isolated perfused mesenteric arterial beds from WT and V1aR/ mice exhibited comparable responses to KCl, the pressor response to AVP seen in WT mice was absent in V1aR/ mice, indicating that the AVP-induced vascular contraction was caused primarily through V1aR.7 In addition to this in vitro finding, in vivo AVP-induced pressor response was abolished in non-anesthetized V1aR/ mice, which seemed at first to imply that a blunted blood pressure response to AVP could cause the hypotension. However, vasodilatation and depressor responses were induced in V1aR/ arterial beds and non-anesthetized animals, respectively, by high concentrations of AVP far exceeding the physiological range.1,7,10 Considering our finding and other reports on V1aR antagonists, it seems that other cause(s), rather than the blunted AVP-induced pressor response, could contribute to the decreased blood pressure observed in V1aR/ mice. Impaired arterial baroreceptor reflex
Functional relationships between blood pressure and reflex changes in heart rate were examined in non-anesthetized mice. We found that baroreceptor reflexes in V1aR/ mice were significantly impaired. When blood pressure was increased by phenylephrine, an a1 adrenergic receptor agonist, or decreased by nitrates, markedly attenuated heart rate changes were evident in V1aR/ mice and in WT mice treated with a selective V1aR blocker, although blood pressure changed by similar amounts in the mutants and the control mice.7 Our observation was in line with the attenuated baroreflex sensitivity seen in AVP-deficient rats, and indicates involvement of V1aR in that reflex.11 In 1036
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addition, the bradycardia induced by electrical stimulation of the hemilateral vagus nerve, which was exposed and cut at the cervical level, was markedly attenuated in V1aR/ mice, suggesting that their central baroreflex arc is impaired.12 When V1aR expression in the nucleus of the solitary tract where the vagal afferents terminate was examined through in situ hybridization analysis, V1aR was highly expressed in the nucleus of the solitary tract and area postrema in WT mice, but not in V1aR/. Thus, V1aR in the central nervous system could have a crucial role in maintaining the baroreflex control of heart rate and blood pressure homeostasis. Decreased sympathetic nerve activity in central nervous system
Although previous pharmacological studies with selective V1aR antagonists did not induce any significant change in basal blood pressure level, such drugs can exert a hypotensive effect in hypertensive animals.9,13 This suggests that the AVP/V1aR signal could be involved in developing or maintaining hypertension. To clarify the underlying mechanisms, we generated a salt-induced hypertension model. In WT mice, blood pressure was significantly increased during salt loading after subtotal nephrectomy. In V1aR/ mice, in contrast, blood pressure increased during salt loading, but the final blood pressure after salt loading was significantly lower, indicating that V1aR deficiency leads to resistance to salt-induced hypertension (Koshimizu T et al., manuscript in preparation). Plasma norepinephrine levels were increased by the salt loading in both WT and V1aR/ mice, but the increase in norepinephrine was smaller in V1aR/ mice, suggesting that sympathetic nerve activity was suppressed in V1aR/ mice. Furthermore, V1aR antagonists markedly lowered the blood pressure in salt-loaded WT mice, but not in control WT mice or in salt-loaded or control V1aR/ mice. Related to the salt-loaded hypertension model are several previous studies involving crosses of genetically hypertensive rat strains with vasopressin-deficient Brattleboro rats, intended to evaluate the importance of AVP in the development of hypertension. These studies revealed that genetic background contributes to salt- and water-retention ability.14,15 SHRs (stroke-prone spontaneously hypertensive rats) have normal extracellular fluid volumes and become hypertensive on salt loading,16,17 whereas New Zealand genetically hypertensive (NZGH) rats have reduced extracellular fluid volumes and are insensitive to salt loading.18,19 When crossed with Brattleboro rats, vasopressin-deficient SHR developed full hypertension,14 whereas the blood pressures of vasopressin-deficient NZGH rats were lower than those of the original NZGH strain.15 It seems that the blood pressure phenotype of V1aR/ mice is more similar to that of NZGH than to that of SHR, because AVP action is indispensable for the basal blood pressure seen in V1aR/ mice, as it is for the hypertensive response seen in NZGH rats. We also examined the role of central V1aR in blood pressure control by administering AVP into the cerebral Kidney International (2009) 76, 1035–1039
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ventricle and found that, whereas pressor and tachycardiac responses were induced in WT mice in a sympathetic activitydependent manner, these events were completely abolished in V1aR/ mice. In addition, pressor and tachycardiac responses induced by hypertonic saline in the cerebral ventricle were greatly diminished in V1aR/ mice compared with those in WT mice. These findings imply that centrally acting AVP has a crucial role in developing or maintaining hypertension by stimulating sympathetic nerve activity through V1aR. Decreased plasma volume in V1aR/ mice
In investigating the mechanism(s) responsible for the observed hypotension in V1aR/ mice, we found that their circulating blood volume was significantly reduced by 9%.7 This reduction could be a major factor, if not the only factor, contributing to the lowered resting blood pressure observed in V1aR/ mice. Lower plasma volume due to the decreased plasma aldosterone level in V1aR/ mice. To investigate the mechanism under-
lying the reduction in blood volume, we further examined hematocrit, blood urea nitrogen, plasma AVP, atrial natriuretic peptide, and aldosterone in V1aR/ mice. V1aR/ mice had higher hematocrit, blood urea nitrogen, and plasma AVP values,5 and lower atrial natriuretic peptide, and aldosterone levels, compared with WT mice.5 In addition, urine volume of V1aR/ mice was higher than that of WT mice under both basal and water-loading conditions.20 These findings suggest that the decrease in aldosterone level could be a cause of the impaired regulation of body fluid retention seen in V1aR/ mice, which led to their decreased circulating blood volume. Before this hypothesis is accepted, however, it needs to be proven experimentally by restoring aldosterone levels in V1aR/ mice to normal, and measuring their blood pressure and blood volume.5,7 In the study with the mineralocorticoid receptor-deficient mice, mentioned above, the mutant mice exhibited dehydration accompanied by higher hematocrit levels than those seen in WT mice;21 this finding supports our observation that V1aR/ mice also suffer from dehydration. Decreased plasma aldosterone level due to the decreased RAS activity in V1aR/ mice. As we found that the blood
pressure, plasma volume, and aldosterone level were decreased in V1aR/ mice,5,7 we evaluated the activity of the renin–angiotensin system (RAS). We found that plasma renin activity and angiotensin II levels were decreased in V1aR/ mice compared with those in WT mice under both basal and water-restricted conditions,20 implicating that RAS activity was suppressed in V1aR/ mice. To further confirm the decreased renin activity, we investigated renin expression in the kidney. Renin in the granule cells was detected through immunohistochemical study as previously described.22 In a control kidney, renin expression was detected not only in the granule cells but also in the renal tubules.20,23 In the V1aR/ kidney, renin was expressed in the same two locations, but at significantly lower levels, indicating that decreased expression Kidney International (2009) 76, 1035–1039
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of renin underlies the decreased renin activity in V1aR/ mice.20 In good agreement with this finding, renin expressions at the mRNA and protein levels were also decreased in V1aR/ mice compared with those in WT mice under basal and water-restricted conditions.20 As neuronal nitric oxide synthase (nNOS), which is expressed in macula densa cells, and cyclooxygenase (COX)-2, expressed in macula densa cells and the TAL, also have important roles in tubuloglomerular feedback by producing NO and prostaglandin E2, and stimulating renin production in granule cells,22 we next compared the expressions of nNOS and COX-2 between WT and V1aR/ mice. In both WT and V1aR/ kidneys, an immunohistochemical study revealed that nNOS was specifically expressed in macula densa cells and COX-2 was present in macula densa cells and TAL, but the expressions of nNOS and COX-2 were weaker in V1aR/ kidney.20 In agreement with the decreased COX-2 expression, urinary and plasma prostaglandin E2 levels were also decreased in V1aR/ mice.20 In addition to this local stimulation of renin production, sympathetic nerve activity also has a crucial role in renin secretion.24 These findings and those of other previous reports suggest that a blunted ability to regulate renin production in granule cells and the TAL due to impairment of both local and central stimulation processes caused the decrease in renin production in V1aR/ mice, which contributed to their low plasma volume and hypotension. AVP is well known to regulate body fluid homeostasis through the V2R–AQP2 system in the kidney. In contrast, the functional role of V1aR in the kidney has not been fully elucidated even though it has been localized in several regions of the kidney, including TAL, collecting ducts, and renal vascular cells.3 As the expression levels of nNOS and COX-2 were decreased in V1aR/ mouse kidneys, we performed in situ hybridization for V1aR in WT mouse kidney to determine whether V1aR was coexpressed with nNOS or COX-2. V1aR was detected in the macula densa cells, where it was co-localized with nNOS and COX-2, and in the TAL where it was co-localized with COX-2.20 This finding suggests that AVP regulates nNOS and COX-2 through V1aR at coexpressed cells and that the blockade of V1aR leads to the suppression of nNOS and COX-2, resulting in reduced renin activity. Decreased plasma aldosterone level due to impaired AVPstimulated aldosterone release from adrenal glands in V1aR/ mice. Aldosterone secretion is known to be regulated by
factors such as AVP, angiotensin II, and adrenocorticotropic hormone.25 As aldosterone levels were decreased in V1aR/ mice, aldosterone release from adrenal gland cells in response to AVP stimulation was examined. As in previous reports on human adrenal gland,26 AVP stimulated aldosterone release from the dispersed adrenal gland cells of WT mice, and this response was inhibited by the V1aR antagonist, implying that AVP mediates aldosterone release from the adrenal glands in mice.27 Supporting this finding, AVP-induced aldosterone release from the adrenal gland cells of V1aR/ mice was impaired, whereas there was no difference in 1037
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adrenocorticotropic hormone-induced aldosterone release between V1aR/ and WT mice.27 This finding, observed both in V1aR/ mice and in mice treated with the selective antagonist, shows that blockade of the AVP/V1aR pathway could lead to suppression of the signaling pathways of aldosterone production and/or secretion in the adrenal cortex, resulting in a decrease in the plasma aldosterone level. Thus, the decreased plasma aldosterone levels observed in V1aR/ mice could be caused by the suppression of RAS activity and the impairment of AVP-stimulated aldosterone secretion, although, in vivo, the RAS has a more major role in regulating plasma aldosterone in the adrenal gland than AVP-stimulated aldosterone secretion does. Lower plasma volume due to the increased urine volume caused by altered V2R–AQP2 function in V1aR/ mice. As the
RAS and tubuloglomerular feedback were altered due to the lack of V1aR expression in V1aR/ mice, we further assessed their renal function. Urine volume was greater and urine osmolarity was lower in V1aR/ mice than in WT mice, particularly when water was loaded. The GFR, urinary NaCl excretion, AVP-dependent cAMP generation, and V2R and AQP2 expression in the kidney were lower in the mutant mice, indicating that the diminished GFR and V2R–AQP2 system led to impaired urinary concentration in V1aR/ mice.20 In addition, increased renal vascular resistance and decreased NO production in V1aR/ mouse kidneys under
the basal condition could indicate impairment of tubuloglomerular feedback in mutant mice.20 Angiotensin II facilitates the AVP-stimulated V2R–AQP2 system.28 This observation, taken together with the data from this study, indicates that AVP can regulate body fluid homeostasis through V1aR in macula densa cells by activating the RAS and subsequently the V2R–AQP2 system, and that AVP/V1aR signaling may have a part in tubuloglomerular feedback (Aoyagi T et al.,20 Figure 1). In general, a decrease in RAS activity could be expected to lead to higher levels of Na excretion and polyuria, but the disruption of V1aR resulted in decreased urinary Na excretion and polyuria. To understand this discrepancy, we will have to measure the fractional excretion of Na in future studies. These data may explain whether alterations in sodium excretion have their basis in tubular re-absorption or in filtered load. Comparison between V1aR/ mice and Brattleboro rats
AVP-deficient Brattleboro rats are well known as a model of central diabetes insipidus; they exhibit normal blood pressure despite impaired baroreflex and polyuria, whereas V1aR/ mice exhibit hypotension.29 Hypersecretion of renin due to hyperactivity of the sympathetic nerve may help maintain normotensive status in Brattleboro rats.30 Conversely, our AVP
AVP V1aR
Macula densa V1aR cell
Collecting duct cell
nNOS/NO COX2/PGE2
V2R Sympathetic nerve activity
AQP2
nNOS COX2
Angiotensinogen
Granule cell
ACE Angiotensin II
Renin angiotensin system
Heart rate
Body fluid retention
Figure 1 | Schematic representation of the regulation of body fluid retention by vasopressin and vasopressin receptor type V1a and V2. Hyperosmolarity and decreased plasma volume under the water-deprived condition lead to secretion of vasopressin (AVP) from the posterior pituitary, which can bind to V1a receptor (V1aR) in macula densa cells and to V2 receptor (V2R) in collecting duct cells. AVP/V1aR stimulates the expression of neuronal nitric oxide synthase (nNOS) and cyclooxygenase (COX)-2, leading to the production of NO and prostaglandin E2 (PGE2) by macula densa cells, respectively. Both NO and PGE2 stimulate renin production from granule cells, and subsequent increases in angiotensin II and aldosterone levels promote water reabsorption. AVP/V2R enhances translocation of aquaporin (AQP)-2 to the apical membrane from the cytoplasm. Water reabsorption is further accelerated by the AVP/V1aR-stimulated renin–angiotensin system (RAS). The AVP/V2R system and the RAS enhanced by AVP/V1a critically regulate body fluid retention.
Aldosterone
Vascular contraction Body fluid retention Regulation of cardiovascular function Blood pressure
1038
AQP2
Renin Angiotensin I
Baroreflex sensitivity
Renin
V2R
Regulation of body fluid
Figure 2 | Contribution of vasopressin and vasopressin receptor type V1a and V2 in the regulation of blood pressure homeostasis. Blood pressure could be controlled by changes in body fluid, cardiac output, and vascular contraction. Activation of the AVP/V2R system induces the translocation of AQP2 and water reabsorption in the kidney. The AVP/V1aR system, on the other hand, stimulates RAS activity and aldosterone release. AVP also directly stimulates aldosterone secretion from the adrenal grand. In addition, AVP stimulates vascular contraction through V1aR. V1aR also enhances baroreflex sensitivity, which controls the heart rate. Central AVP/V1aR activates sympathetic nerve outflow and regulates vascular as well as cardiac functions. These V1aR-mediated cardiovascular and body-fluid-homeostasis effects could have crucial roles in blood pressure regulation in health and disease. Broad arrows indicate a physiological pathway directly regulated by V1aR. ACE, angiotensin-converting enzyme; AQP2, aquaporin-2; AVP, vasopressin; COX2, cyclooxygenase-2; nNOS, neuronal nitric oxide synthase; RAS, renin–angiotensin system; V1aR, V1a receptor; V2R, V2 receptor. Kidney International (2009) 76, 1035–1039
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observations of V1aR/ mice indicated that their decreased sympathetic nerve activity may reduce the secretion of renin in addition to impairing the tubuloglomerular feedback system. Hence, dysfunction of RAS due to blockade of V1aR led to hypotension in V1aR/ mice.
8. 9.
10. 11.
CONCLUSION
In summary, AVP has a crucial role in regulating blood pressure through V1aR by stimulating vascular contractions, arterial baroreceptor reflex, sympathetic nerve activity, and water re-absorption, and blockade of the AVP/V1aR signal results in decreased blood pressure (Figure 2). Studies using the knockout model have several limitations, including the difficulty of determining cause and effect, the difficulty of determining direct versus indirect effects, and the possibility of problems occurring due to the lifetime deletion of the receptor causing functional compensations. In addition, the importance of the observed knockout phenotype needs to be further verified through studies involving other pharmacological or genetic interventions. Nevertheless, knockout studies such as ours will provide valuable information and clues contributing to the investigation of the mechanisms of AVP/V1aR signaling in humans. DISCLOSURE
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