Racial Differences in Epinephrine and β2-Adrenergic Receptors
Abstract This study examined the effects of ethnicity and hypertension on β2-adrenergic receptors and on plasma catecholamines in a group of 77 unmedicated mildly hypertensive and normotensive men. Black hypertensive subjects had the most sensitive and white hypertensive subjects the least sensitive β-receptors (as assessed by isoproterenol-stimulated cyclic AMP in lymphocytes [P=.02]). In contrast, postreceptor adenylate cyclase activation (as assessed by forskolin stimulation) was similar among groups. As with β-receptor sensitivity, black hypertensive subjects had the highest β-receptor density and white hypertensive subjects the lowest (P=.03). Blacks demonstrated lower plasma epinephrine values compared with whites (P=.03). Across all subjects, plasma epinephrine was negatively correlated with β-receptor density (r=−.26, P<.05) and sensitivity (r=−.25, P<.05). There were no group differences in binding affinity to the β-antagonist iodopindolol. The findings support the notion of increased β-adrenergic receptors in hypertension in blacks.
Numerous studies document racial differences in the efficacy of β-blocking antihypertensive medication. For example, there is a gradient of response to the antihypertensive propranolol, with Chinese men being most sensitive and black men least sensitive and white men showing an intermediate sensitivity.1 2 3
Such findings are often attributed to possible racial differences in drug pharmacokinetics.1 4 Black men, for example, show a lower plasma concentration of propranolol and its metabolite 4-hydroxypropranolol compared with white men.4 Such racial differences in pharmacokinetics do not entirely account, however, for the noted differences in antihypertensives.1
A parallel line of research suggests that racial differences in β-blocking antihypertensive efficacy could be due to differences in end-organ receptor sensitivity.1 2 5 6 7 8 Unfortunately, the data often conflict, with reports of both increases and no difference in β-adrenergic receptors between blacks and whites.2 5 6 7 8 Most studies, however, have examined racial differences in normotensive, not hypertensive, individuals and have used rather small samples of approximately 20 individuals.5 6 7 8 In addition, studies have paid little attention to possible confounding factors such as catecholamines, which like sodium exert a regulating effect on adrenergic receptors.9 10 Epinephrine, for example, is the most potent endogenous agonist of the β2-adrenergic receptor.
This study examined the effect of ethnicity on lymphocyte β2-adrenergic receptors and on plasma catecholamines in a group of 77 unmedicated hypertensive and normotensive black and white men. All were studied in the controlled environment of a clinical research center where sodium intake was constant.
Volunteers were located through community blood pressure screening or word-of-mouth referral. Twenty-five untreated mildly hypertensive (blood pressure >140 mm Hg systolic or >90 mm Hg diastolic at screening) and 52 normotensive black and white males (mean age, 39 years) were studied. Two of the hypertensive patients had previously been taking antihypertensive medication but not for at least 3 months before the study; the remaining hypertensive subjects had never taken antihypertensive medication. Blood pressure diagnoses were made on the basis of three blood pressure measurements taken on each of two screening occasions approximately 1 week apart. Only those who remained in the same diagnostic category on both screening occasions participated. The protocol was approved by the University of California at San Diego (UCSD) Institutional Review Board, and all subjects gave written consent.
Subjects were evaluated at the UCSD Medical Center Clinical Research Center after a 2-day stay, during which an isocaloric diet provided 200 mmol sodium and 100 mmol potassium per day. On the morning of the third day, before the subject was ambulatory, blood was sampled from a forearm vein heparin lock that had been inserted the previous night.
Lymphocytes were isolated and assayed according to previously published methods.11 β2-Adrenergic receptor sensitivity was determined in whole cells by quantifying cyclic AMP (cAMP) accumulation after a 2-minute incubation with 10 μmol/L isoproterenol. Briefly, lymphocytes were suspended in cold Dulbecco’s modified Eagle medium (DMEM). Incubations in triplicate were begun by adding 0.1 mL of approximately 2×105 cold cells to 0.9 mL of 37°C DMEM containing 100 μmol/L isobutylmethylxanthine (IBMX) and 100 μmol/L Ro20-1724 to inhibit cyclic nucleotide phosphodiesterase activity. Half of the tubes also contained 10−5 mol/L isoproterenol. Isoproterenol, acting via the β-adrenergic receptor, typically causes an approximate threefold to fivefold increase (over basal level) in intracellular cAMP levels in lymphocytes. After 2 minutes, the reactions were terminated by centrifuging the tubes at 11 000 rpm, aspirating the supernatant, resuspending the pellet in 0.4 mL of 50 mmol/L sodium acetate containing 0.2 mmol/L IBMX (pH 4.0), and placing the tubes in a boiling water bath for approximately 5 minutes. The tubes were then frozen and later assayed for cAMP (Amersham). To evaluate postreceptor adenylate cyclase activation, cells were also stimulated with forskolin (20 μmol/L).12 Basal nonstimulated cAMP was also determined.
For the determination of β2-adrenergic receptor density, lymphocyte membranes were suspended in 50 mmol/L Tris, 8 mmol/L MgCl2, and 0.5 mmol/L EDTA at pH 7.5. Saturation radioligand binding isotherms were performed using [125I]iodopindolol at six concentrations from 10 to 320 pmol/L for 1 hour at 37°C. Specific binding was determined by using 10−6 mol/L propranolol. Incubations were terminated by filtration with 20 mL Tris wash buffer. β-Adrenergic receptor density and the dissociation constant (Kd) were calculated using a nonlinear regression receptor binding software program (graphpad).
The lymphocyte serves as a general model for human β2-receptors because lymphocyte β2-receptor density and sensitivity are similar to β2-receptors on other tissue such as heart and lung.13 14 15 Lymphocytes, however, represent a heterogeneous collection of cell subsets possessing different β-adrenergic receptor characteristics. For example, suppressor/cytotoxic T and natural killer cells have a greater density and more sensitive β2-adrenergic receptors than other cell types.16 17 To control for a possible racial and/or diagnosis effect on cell populations, and thus on the lymphocyte β-receptor model, we also quantified suppressor/cytotoxic T and natural killer cell populations. Flow cytometry was used to quantify the following cell types (also listed are the respective antibodies used for identification): CD8, anti-Leu 2a; CD16, anti-Leu 11b; CD56, anti-Leu 19; and CD57, anti-Leu 7.18
Blood samples for the plasma catecholamines were collected on ice and separated in a refrigerated centrifuge. The plasma was stored at −80°C until assay. Plasma catecholamines were analyzed by radioenzymatic assay.19
Data were analyzed by two-way ANOVA and correlation analysis (BMDP Statistical Software).
Twenty-four-hour urinary sodium excretion was higher in hypertensive compared with normotensive subjects (F=4.57, P=.036). Whites were older than blacks (F=7.68, P=.007); weight20 was similar among subjects (Table 1⇓).
There were two significant race-by-diagnosis interactions indicating that black hypertensive subjects had the most sensitive and highest-density β-receptors and white hypertensive subjects the least sensitive and lowest-density β-receptors (isoproterenol-stimulated cAMP, F=5.72, P=.020 and β-adrenergic receptor density, F=4.9, P=.030), respectively (Table 2⇓; Figure⇓). There were no differences in the Kd or in forskolin-stimulated cAMP production. Although plasma norepinephrine levels were similar, blacks demonstrated lower plasma epinephrine values when compared with whites (F=4.6, P=.035). There were no significant group differences in white cell subtypes (Table 2⇓).
Across all subjects, plasma epinephrine was negatively correlated with β-receptor density (r=−.26, P<.05) and sensitivity (r=−.25, P<.05). Age correlated with epinephrine (r=.29, P<.05) but not norepinephrine (r=.21, P=NS) and negatively correlated with β-receptor density (r=−.47, P<.001) but not with sensitivity (r=−.11, P=NS). The racial differences in β-receptor sensitivity and density were not significantly changed when covarying for group differences in sodium excretion, epinephrine, or age.
This study examined the effect of ethnicity on β-adrenergic receptors and agonists in a group of hypertensive and normotensive men. The findings suggest that, independent of age and sodium excretion, black hypertensive men show an increased β-receptor sensitivity and density compared with white hypertensive men. The between-group similarities in forskolin-stimulated cAMP suggest that the racial differences in β-receptor sensitivity lie in the β-receptor itself (or its coupling to the Gs nucleotide) and not with postreceptor adenylate cyclase activation.12 In contrast to hypertensive subjects, the white and black normotensive subjects had similar β-receptors.
These data may help clarify an inconsistency in prior studies examining ethnicity and lymphocyte β-receptor studies in normotensive men.5 6 7 8 In the present study, the greatest receptor contrast was between the black and white hypertensive subjects. That is, the significance emerged from the interaction of race and diagnosis of hypertension. When these same data were examined without considering the diagnosis of hypertension, ie, grouping only according to ethnicity, there were no racial differences in β-receptor characteristics. Therefore, it may be that previous inconsistencies in studies resulted from not attending to the interaction of race and diagnosis of blood pressure status.
This study attempted to control for possible confounding factors, such as sodium and antihypertensive medication, both of which can affect adrenergic receptor physiology.9 11 21 22 Twenty-four-hour sodium excretion was similar among the groups, and those hypertensive subjects previously on antihypertensive medication were tapered off medication for at least 3 weeks before study.
A better design may have involved placing the subjects on the sodium diet for 1 week. It is possible that the 2 days on the Clinical Research Center sodium diet was not enough time for the receptors to adjust if there were sizable differences in sodium intake before admission. However, there were no significant racial differences in 24-hour sodium excretion nor was there a significant relation of sodium excretion to either β-receptor sensitivity or density. Typically, studies demonstrating such an effect of dietary sodium examine subjects across an extreme range of dietary sodium, such as a low of 10 mmol/d to a high of 200 or 400 mmol/d.9 21 23
These in vitro findings of an increased β-receptor sensitivity in black compared with white hypertensive subjects replicate an earlier in vivo observation from our laboratory on different subjects using the “chronotropic 25 dose” (CD25) of isoproterenol (ie, the dose of isoproterenol needed to raise heart rate 25 beats per minute; a low dose indicates high β-receptor sensitivity and vice versa). Black hypertensive subjects had the lowest CD25 (most sensitive β-receptors) and white hypertensive subjects the highest CD25.21 We have reported similar findings for α-adrenergic receptor sensitivity as assessed by the pressor response to infused norepinephrine.23
We used lymphocyte β-receptors as a model for human β-receptors.12 13 14 In an effort to control for possible racial effects on white-cell subpopulations, and thus on the model, we measured suppressor/cytotoxic T and natural killer subsets, since these cells express higher β-receptors compared with other white cells.15 16 There were no differences in leukocyte-subset populations across the four groups. Thus, it is unlikely that the group differences in β-receptor characteristics reflect differences in leukocyte subsets.
Across all subjects we found an inverse relationship between plasma epinephrine and β-receptors. This finding is not novel and is supportive of the notion of adrenergic agonist regulation of adrenergic receptors.10
A novel finding was that blacks had lower epinephrine compared with whites. Although studies report increased epinephrine levels among hypertensive subjects,24 we are not aware of reports of lower epinephrine among blacks. This may be due to technical difficulties with the assay of epinephrine. Plasma epinephrine levels in resting subjects are at the lower limit of sensitivity for most assays. We used an assay that is 10 times as sensitive as prior methods19 and perhaps as a result were able to detect significantly decreased epinephrine levels among blacks. Current studies in this laboratory are aimed at determining possible mechanisms responsible for the differential β-receptor–mediated cAMP production in black versus white hypertensive subjects.
In sum, the findings from this study suggest that ethnicity and hypertension interact to affect the β-adrenergic receptor; postreceptor adenylate cyclase appears, in contrast, unaffected. The findings help clarify previous inconsistencies in this body of literature where blood pressure status was not considered5 6 7 8 and support the notion that racial differences in β-adrenergic receptor function might contribute to the well-documented racial differences in the efficacy of β-blockade.
This work was supported by grants MO1RR-00827, HL-36005, HL-40102, and HL-47074 from the National Institutes of Health. The authors are grateful to Ann Rearden, MD, and the staff of the HLA and Immunogenetics Laboratory at the University of California at San Diego for determination of the lymphocyte subsets.
Reprint requests to Paul J. Mills, University of California at San Diego Medical Center, 200 W Arbor Dr, San Diego, CA 92103-0804.
- Received August 16, 1994.
- Revision received September 8, 1994.
- Accepted October 3, 1994.
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