Alpha 2-adrenergic receptor gene polymorphism and hypertension in blacks

Mechanism of epinephrine-induced platelet aggregation

We report that a genetic polymorphism of the alpha2-adrenergic receptor (A2AR) encoded by chromosome 10 is associated with hypertension and an increase in epinephrine-mediated platelet aggregation in humans. The mechanism responsible for this heritable contrast in sensitivity to epinephrine is unknown. We tested our hypothesis that epinephrine-induced platelet aggregation is mediated by activation of chloride transport. We measured epinephrine-mediated increases in optical density of gel-filtered platelets suspended in a bicarbonate-buffered physiological salt solution. Compared with platelets incubated in the control buffer (130 mmol/L NaCl), platelets incubated with either bumetanide, a Na/K/2Cl cotransport inhibitor; anthracene-9-carboxylic acid, a chloride channel blocker; or acetazolamide, an agent that blocks ATP-dependent chloride transport had significantly decreased aggregation responses to epinephrine. When measured fluorometrically, epinephrine significantly increased intraplatelet chloride concentrations. Chloride-dependent modifications of epinephrine-induced platelet aggregation were not attributable to changes in A2AR ligand binding characteristics or to the concentration of platelet cAMP. Finally, subthreshold concentrations of epinephrine also potentiated thrombin-induced platelet aggregation, and blockade of chloride transport diminished this synergistic action of epinephrine on thrombin-stimulated platelet aggregation. Heritable differences in epinephrine-mediated platelet aggregation may be attributable to genetic differences in chloride transport in platelets. Furthermore, because we observed a necessary role for chloride transport in epinephrine-mediated platelet aggregation, pharmacological agents that block chloride transport, such as diuretics, may provide salutary protection against vascular thrombosis in patients with hypertension independent of the effect of these drugs on blood pressure.

The application of molecular genetic techniques to the study of hypertensive diseases

The techniques of modern molecular genetics are shedding new light on hypertension and its sequelae. This article discusses techniques which have identified genes associated with hypertension and have pointed the way toward identifying the full cohort of genes operative in all forms of human hypertension. These techniques have expanded understanding of the pathophysiology of hypertension as well as its prevention.

Analyses of mutations in the human renal kallikrein (hKLK1) gene and their possible relevance to blood pressure regulation and risk of myocardial infarction

The kallikrein-kinin system is involved in the maintenance of blood pressure (BP), and studies have shown an inverse correlation between BP and urinary kallikrein levels. These and other effects, make the human tissue kallikrein (hKLK1) gene a candidate gene with respect to BP regulation as well as risk of myocardial infarction (MI). By analysis for single-stranded conformation polymorphisms (SSCPs), patterns consistent with four different variants of the gene were detected and further characterized by DNA sequencing. Three of the variants have not been described before. Two of the polymorphisms changed the codon for an amino acid. Methods based on the polymerase chain reaction (PCR) were developed to analyze these polymorphisms at the hKLK1 locus. We found no evidence of association between any genotype in the polymorphisms and normal BP level in two series of healthy, unrelated individuals. In a third series, diastolic BP exhibited a weak association with genotypes in three of the four polymorphisms. Since no such association was detected in the other two series, we conclude that no effect on normal BP level is exerted by variants in the hKLK1 as expressed in these polymorphisms. In two series of monozygotic (MZ) twin pairs, there were no differences between genotypes in within-pair difference in systolic BP or diastolic BP. Finally, no differences in allele frequencies or genotype frequencies in the four polymorphisms at the hKLK1 locus were found between a series of young MI survivors and a series of controls. Thus, genes in the four polymorphisms at the hKLK1 locus detected by SSCP and DNA sequencing did not exhibit associations with MI, and had neither "level gene" nor "variability gene" effects on normal blood pressure.

Renal alpha 2a/d-adrenoceptor subtype function: Wistar as compared to spontaneously hypertensive rats

1. The alpha 2a/d-adrenoceptor subtype in the rat kidney modulates solute excretion (osmolar clearance). Since the kidney plays a role in chronic regulation of blood pressure, altered renal function may be implicated in the development of hypertension. A second alteration-that of the alpha 2a/d-adrenoceptor subtype gene-has also been correlated with hypertension in rats and man. 2. We hypothesized that as a consequence of the altered alpha 2a/d-adrenoceptor subtype gene previously shown in spontaneously hypertensive (SH) rats, the increase in osmolar clearance following stimulation of the renal alpha 2a/d-subtype would be attenuated in SH rats as compared to normotensive Wistar rats. In contrast, based on the theory that such functional unresponsiveness of the alpha 2a/d-subtype would be genetically determined, we further hypothesized that in one kidney-one clip (1K-1C) rats, the response to stimulation of the renal alpha 2a/d-subtype would be intact as compared to the normotensive Wistar 1K-sham rats. 3. Male rats were unilaterally nephrectomized under ether anaesthesia. In the 1K-1C rats, a silver clip (diameter 0.254 mm) was also placed around the left renal artery. On the experimental day, rats were administered pentobarbitone (50.0 mg kg-1, i.p.). The carotid artery and jugular vein were cannulated for blood pressure monitoring and saline infusion. The ureter was catheterized for urine collection. A 31 gauge needle was advanced into the renal artery for infusion of the alpha 2a/d-selective agonist, guanfacine (vehicle, 1.0, 3.0 and 10.0 nmol kg-1 min-1 in Wistar and SH rats; vehicle and 10.0 nmol kg-1 min-1 in Wistar 1K-sham and 1K-1C rats). 4. In Wistar rats, guanfacine dose-dependently increased urine flow and sodium excretion. An increase in osmolar clearance but not free water clearance was also observed. However, in SH rats guanfacine failed to alter urine flow, sodium excretion, osmolar and free water clearance. In contrast, in both Wistar 1K-sham and 1K-1C rats, guanfacine increased urine flow rate. Again, this response was due solely to an increase in osmolar clearance. At these doses, guanfacine did not alter blood pressure or creatinine clearance during the experiment. 5. In summary, the ability of the alpha 2a/d-adrenoceptor subtype to mediate an increase in osmolar clearance was absent in a genetic model of hypertension, the SH rats. This effect was intact in an acquired model of hypertension (1K-1C rats). This suggested a defective modulation of solute excretion in SH rats which was probably due to alteration of the alpha 2a/d-subtype gene and not secondary to the elevated blood pressure. The altered alpha 2a/d-subtype gene and function may therefore play a causal role in the pathogenesis of hypertension.

Preliminary evidence of linkage of salt sensitivity in black Americans at the beta 2-adrenergic receptor locus

Salt sensitivity is a heritable trait that is a hallmark of hypertension in black Americans. Genes encoding adrenergic receptors are candidate loci for the inheritance of this hypertension-related trait because of the role of these receptors in the regulation of renal sodium excretion and vascular tone. We performed this study to determine whether these loci are responsible for some of the phenotypic variation in salt sensitivity. Hypertensive black American probands were ascertained, followed by sequential ascertainment of adult sib pairs among the first-, second- and third-degree relatives of the proband. Both hypertensive and normotensive siblings were tested for salt sensitivity by an intravenous sodium-loading, lasix volume-depletion protocol. Genotyping was performed with restriction fragment length polymorphisms in genomic DNA probed with clones containing the beta 2- and alpha 2c10-adrenergic receptor genes. A total of 109 sib pairs was evaluated. Salt sensitivity was defined as the change in blood pressure in each individual, comparing the sodium-loaded with the volume-depleted state. Systolic pressure decreased by an average of 9.0 +/- 9%, diastolic pressure by 1.5 +/- 11%, and mean arterial pressure by 5.0 +/- 9%. Neither blood pressure nor salt sensitivity was linked at the alpha 2c10-adrenergic receptor locus. No evidence suggested that systolic salt sensitivity and baseline blood pressure were linked at the beta 2-adrenergic receptor locus. Model-independent sib pair linkage analysis suggested that diastolic blood pressure response to sodium loading/volume depletion is linked at the beta 2-adrenergic receptor locus (P < .006). Evidence for linkage was significant at the .05 level after adjustment for the number of phenotypic traits examined.

Association of hypertension with beta2- and alpha2c10-adrenergic receptor genotype

The adrenergic receptors have been implicated in the pathogenesis of essential hypertension. We hypothesized that hypertension is associated with variants at the beta2-adrenergic receptor locus and at one of the alpha2-adrenergic receptor loci. In unrelated individuals, we measured untreated blood pressure and characterized each subject as hypertensive or normotensive. We then used genomic DNA to identify beta2- and alpha2c10-adrenergic receptor restriction fragment length polymorphisms. In 175 subjects (49 percent with hypertension, 55 percent black), both hypertension and race were associated with genotype at the beta2 locus (chi2 for hypertension = 11, P = .004, chi2 for race = 8.8, P = .012). The association with hypertension persisted in each race group separately (blacks only: chi2 = 9.6, P = .008; whites only; chi2 = 14.2, P = .001). This association persisted in a logistic model that controlled for race (P = .01). Genotype was also significantly associated with baseline systolic, diastolic, and mean arterial blood pressures (P = .05, .01, and .02, respectively). These data suggest that the beta2-adrenergic receptor gene is a candidate gene for hypertension in blacks and whites. We also genotyped subjects at the alpha2-adrenergic receptor coded on chromosome 10. There was no association between hypertension and genotype at the alpah2c10 locus in the total group or in blacks, but there was significant association in whites (chi2 = 6.7, P = .03). These data suggest that the beta2- and alph2c10-adrenergic receptor genes may contribute, in a race-specific manner, to the inheritance of essential hypertension. Linkage studies in related individuals are needed to confirm these findings.

Alpha 2-adrenergic agonists increase cellular lactate efflux

We reported previously that genetic polymorphisms of the alpha 2-adrenergic receptor are associated with hyperinsulinemia, diabetes mellitus, and hypertension in blacks. The evolutionary driving force for maintaining such deleterious mutations in the black population is unknown. Recognizing that vascular alpha 2-adrenergic receptors mediate cold-induced vasoconstriction and that temperature maintenance is a primary thrust of cellular metabolism, we postulated that vascular alpha 2-adrenergic receptors contribute significantly to metabolic heat generation in homeotherms such as humans. Using aerobic lactate production as an indicator of thermogenesis, we measured metabolic heat production in HT29 cells that expressed the gene encoding human vascular alpha 2-adrenergic receptors. Epinephrine, an alpha 2-adrenergic receptor agonist, increased net lactate efflux from 226 +/- 20 to 280 +/- 20 nmol/min (mean +/- SE) (P = .06). Clonidine, a more specific alpha 2-adrenergic agonist, increased lactate efflux from 110 +/- 6 to 156 +/- 8 nmol/min (P < .01). Similarly, in the presence of physiological concentrations of glucose (5.5 mmol/L), insulin increased lactate production from 123 +/- 6 to 175 +/- 10 nmol/min (P < .01). Because differences in aerobic glycolysis may also explain the heat intolerance and abnormal fuel homeostasis found in genetically hypertensive rats, we also measured lactate production in cultured vascular smooth muscle cells isolated from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive control Wistar-Kyoto rats (WKY). Vascular smooth muscle cells from SHRSP had significantly greater lactate efflux compared with cells from normotensive WKY (296 +/- 4 versus 172 +/- 2 nmol/min, P < .001). These differences were not due to abnormalities in glucose uptake, as lactate efflux was greater in SHRSP cells compared with WKY cells when dextrose was replaced with equimolar concentrations of fructose (230 +/- 6 versus 138 +/- 2 nmol/min, P < .001). alpha 2-Adrenergic agonists increase lactate efflux in HT29 cells, and abnormalities in vascular smooth muscle lactate metabolism in genetically hypertensive rats is independent of altered glucose uptake. These data provide support for our hypothesis that balanced polymorphisms of the alpha 2-adrenergic receptor could offer protection against cold stress by increasing the thermogenic response associated with aerobic lactate production.