The B(1) type receptor of bradykinin (Bk B(1)R) is believed to be physiologically inert but highly inducible by inflammatory mediators and tissue damage. To explore the potential participation of the Bk B(1)R in blood pressure (BP) regulation, we studied mice with deleted Bk B(2)R gene with induced experimental hypertension, either salt-dependent (subtotal nephrectomy with 0.5% NaCl as drinking water) or renin/angiotensin-dependent (renovascular 2-kidney-1-clip). Compared with the wild-type controls, the B(2)R gene knockout mice had a higher baseline BP (109.7+/-1.1 versus 101.1+/-1.3 mm Hg, P:=0.002), developed salt-induced hypertension faster (in 19.3+/-2.3 versus 27.7+/-2.4 days, P:=0.024), and had a more severe end point BP (148+/-3.7 versus 133+/-3.1 mm Hg, P:<0.05). On the contrary, renovascular hypertension developed to the same extent (149.7+/-4.3 versus 148+/-3.6 mm Hg) and in the same time frame (14+/-2.2 versus 14+/-2.1 days). A bolus infusion of a selective B(1)R antagonist at baseline produced a significant hypertensive response (by 11.4+/-2 mm Hg) in the knockout mice only. Injection of graded doses of a selective B(1)R agonist produced a dose-dependent hypotensive response in the knockout mice only. Assessment of tissue expression of B(1)R and B(2)R genes by reverse transcription-polymerase chain reaction techniques revealed significantly higher B(1)R mRNA levels in the B(2)R knockout mice at all times (normotensive baseline and hypertensive end points). At the hypertensive end points, there was always an increase in B(1)R gene expression over the baseline values. This increase was significant in cardiac and renal tissues in all hypertensive wild-type mice but only in the clipped kidney of the renovascular knockout mice. The B(2)R gene expression in the wild-type mice remained unaffected by experimental manipulations. These results confirm the known vasodilatory and natriuretic function of the Bk B(2)R; they also indicate that in its absence, the B(1)R can become upregulated and assume some of the hemodynamic properties of the B(2)R. Furthermore, they indicate that experimental manipulations to produce hypertension also induce upregulation of the B(1)R, but not the B(2)R, in cardiac and renal tissues.

Abstract-Presynaptic ␣ 2 -adrenergic receptors (␣ 2 -AR) are distributed throughout the central nervous system and are highly concentrated in the brain stem, where they contribute to neural baroreflex control of blood pressure (BP). To explore the role of the ␣ 2A -AR subtype in this function, we compared BP and plasma norepinephrine and epinephrine levels in genetically engineered mice with deleted ␣ 2A -AR gene to their wild-type controls. At baseline, the ␣ 2A -AR gene knockouts (nϭ11) versus controls (nϭ10) had higher systolic BP (123Ϯ2.5 versus 115Ϯ2.5 mm Hg, PϽ0.05), heart rate (730Ϯ15 versus 600Ϯ18 b/min, PϽ0.001), and norepinephrine (1.005Ϯ0.078 versus 0.587Ϯ0.095 ng/mL, PϽ0.01), respectively. When submitted to subtotal nephrectomy and given 1% saline as drinking water, both ␣ 2A -AR gene knockouts (nϭ14) and controls (nϭ14) became hypertensive, but the former required 15.6Ϯ2.5 days versus 29.3Ϯ1.4 days for the controls (PϽ0.001). End-point systolic BP was similar for both at 155Ϯ2.1 versus 152Ϯ5.2 mm Hg, but norepinephrine and epinephrine levels were twice as high in the knockouts at 1.386Ϯ0.283 and 0.577Ϯ0.143 versus 0.712Ϯ0.110 and 0.255Ϯ0.032 ng/mL, respectively, PϽ0.05 for both. We conclude that the ␣ 2A -AR subtype exerts a sympathoinhibitory effect, and its loss leads to a hypertensive, hyperadrenergic state.(Hypertension. 1999;34:403-407.)Key Words: adrenergic receptors Ⅲ mice, knockout Ⅲ hypertension, sodium-dependent Ⅲ hyperadrenergic state T he sympathetic nervous system (SNS) is one of the major pressor systems involved in the regulation of blood pressure (BP). Aberrant function of various neuroendocrine components of the SNS contributes to abnormal BP in response to environmental stimuli, such as excessive sodium intake. Indeed, a large body of literature indicates that 1 of the mechanisms by which salt-loading raises BP is via activation of the central SNS, which leads to increased sympathetic outflow. 1,2 Experimental studies in animals with pharmacological probes in vivo 3-5 and radioligand techniques in vitro 6,7 have suggested that salt-induced hypertension is associated with alterations in the functional characteristics of the ␣ 2 -adrenergic receptors (␣ 2 -AR) in the central nervous system (CNS). None of these methods, however, can differentiate among the ␣ 2 -AR subtypes (␣ 2A , ␣ 2B , or ␣ 2 c) involved in this process.Recently, genetically engineered mice that are deficient in each one of the ␣ 2 -AR subtypes became available. 8,9 In our first series of experiments with these mice, we used animals deficient for the ␣ 2B -AR gene (ϩ/Ϫ) and ␣ 2C -AR gene knockouts (Ϫ/Ϫ) compared with their wild-type counterparts (␣ 2B ϩ/ϩ and ␣ 2C ϩ/ϩ, respectively) to study the role of each subtype in determining salt-sensitivity. We found that the ability to develop hypertension in response to salt-loading requires a full complement of the ␣ 2B -AR subtype gene. Indeed, the ␣ 2B ϩ/Ϫ mice failed to raise their BP after subtotal nephrectomy and dietary salt-loading for 5 weeks, whereas their wild-ty...

AVP seems to play a more important role as a pressor hormone in maintaining the elevation of arterial pressure in African-American hypertensives than it does in Caucasian hypertensives.

-Our data suggest that salt-depleted salt-sensitive hypertensives with blunted renin responses exhibit enhanced catecholamine-stimulated endothelin levels and may therefore respond better than unselected patients with essential hypertension to endothelin receptor blockers.

Neurohormonal activation with increased plasma renin activity and norepinephrine and vasopressin levels is characteristic of congestive heart failure and contributes to further decompensation and poor prognosis. We treated 20 such patients with the centrally acting sympathoinhibitory drug clonidine 0.15 mg BID and obtained hemodynamic measurements by cardiac catheterization and plasma neurohormone levels before and 2 to 3 hours after the first dose; in 7 patients, these measurements were taken again after 1 week of therapy. The initial dose produced significant decreases of 8% in mean arterial pressure, 23% in right atrial pressure, 21% in pulmonary capillary wedge pressure, 19% in mean pulmonary artery pressure, and 12% in heart rate, a 17% increase in stroke volume; and no significant changes in cardiac output and systemic vascular resistance. All changes remained virtually constant after 1 week. Plasma norepinephrine decreased by 28% after the initial dose and 62% after 1 week (P < 0.1), whereas plasma renin activity remained essentially unchanged. Plasma vasopressin tended to increase, its levels being inversely correlated with those of posttreatment norepinephrine (r = -.48 P < .03). Patients with baseline norepinephrine levels > 0.400 ng/mL has significantly poorer baseline hemodynamic parameters and tended to show more improvement with clonidine, although their data remained significantly worse than patients whose baseline norepinephrine was within the normal range. Sympathetic suppression with clonidine in congestive heart failure reduces preload, heart rate, and arterial pressure, all indexes of myocardial energy demand; the lack of significant reduction in systemic vascular resistance and increase in cardiac output might be attributable in part to enhanced release of vasopressin.2+ f2p4

Essential hypertension, a clinically significant elevation in blood pressure with no recognizable cause, is believed to be attributable to the collective effect of genetic predisposing factors in combination with specific environmental factors, such as diet and stress. Of the genetic causes, genes coding for proteins involved in blood pressure regulation, such as the alpha- and beta-adrenergic receptors, are obvious candidates. The alpha2-adrenergic receptor plays a key role in the sympathetic nervous system by mediating the effects of epinephrine and norepinephrine. To evaluate the potential role between the alpha2B receptor and essential hypertension, we scanned the alpha2B-receptor gene for genetic variation in 108 affected sibling pairs. The screening revealed two major forms of the receptor. They differ by the presence of either 9 or 12 glutamic acid residues in the acidic domain of the third cytoplasmic loop of the protein. Investigation of the pattern of this variation in hypertensive sibling pairs suggests that the alpha2B receptor locus does not contribute substantially to genetic susceptibility for essential hypertension.

Previously, we have reported two major alpha 2-adrenergic receptor transcripts in rat brain of 3.8 and 3.0 kb and the cloning and characterization of the rat brain complementary DNA (cDNA) (RB alpha 2C) specific for the 3.0-kb messenger RNA. In this report, we used rat brain cDNAs specific for the 3.0 and 3.8 kb transcripts, which encode the alpha 2C- and alpha 2A-adrenergic receptors, respectively, and the RNG alpha 2 cDNA, which encodes for the nonglycosylated alpha 2B-adrenergic receptor in rat, to study tissue-specific expression of the three alpha 2-adrenergic receptor genes in rat. To eliminate cross-hybridization of probes with transcripts from other alpha 2 genes, we subcloned fragments that encode for the highly divergent third cytoplasmic loop of each rat alpha 2-adrenergic receptor cDNA and used RNase protection analysis to detect specific transcripts. We show that the three rat alpha 2-adrenergic receptor genes have diverse patterns of tissue expression, and although transcripts specific for each alpha 2-adrenergic receptor gene are found in brain and kidney, the levels of expression of each subtype differ in these tissues. We speculate on the significance of tissue-specific expression of the alpha 2-adrenergic receptor genes.

SUMMARY To evaluate the partial contributions and interaction of three vasopressor systems in blood pressure maintenance, nephrectomized rats and rats with intact kidneys were submitted sequentially to catecholamine depletion, elimination of vasopressin's vasoconstrictor action, and (for those with kidneys in situ) angiotensin blockade. Catecholamine depletion decreased blood pressure and stimulated vasopressin levels in all rats, but significantly more so in the anephric ones. Subsequent injection of an antagonist to the vasopressor effect of vasopressin produced a lasting fall of blood pressure in anephric rats, but only transient fall in those with intact kidneys. Infusion of teprotide -an angiotensin converting enzyme inhibitor -in the latter animals also produced transient blood pressure fall, but if this were followed by injection of the vasopressin antagonist, the pressure remained low for several hours. Blood pressure levels were closely correlated with those of plasma catecholamines throughout these maneuvers. Catecholamine levels were inversely correlated with those of plasma vasopressin, which were far greater in anephric rats through both stimulation and accumulation. Plasma renin activity was increasingly stimulated by falling blood pressure after each maneuver in rats with intact kidneys. Thus, it appears that in the resting state the sympathetic nervous system is more involved in the maintenance of blood pressure, whereas vasopressin and renin are important backup mechanisms. The present experiments were designed to evaluate the contribution of each one of these three vasopressor systems to the maintenance of normal blood pressure. Catecholamine depletion and bilateral nephrectomy were intended to produce conditions comparable -albeit extreme -to those likely to be encountered in the therapy of human hypertension, e.g., treatment with sympatholytic agents and advanced renal failure. Under these conditions, we studied the interaction and relationships of catecholamines, vasopressin, and the renin-angiotensin system.