Brief Communication Methacholine Airway Responsiveness and 24-Hour Urine Excretion of Sodium and Potassium The Normative Aging Stud y 1 - 3 DAVID SPARROW, GEORGE T. O'CONNOR,4 BERNARD ROSNER, and SCOTT T. WEISS, With the technical assistance of Susan L. DeAmario

The potential influence of diet on asthma and other forms of obstructive lung disease has received relatively little attention. Burney (1) observed that sales of table salt in England and Waleswerecorrelated with mortality from asthma for men and for children of both sexes but not for women. The same investigator's group found increased urinary excretion of sodium to be associated with increased airway responsiveness to histamine in a population of young and middle-aged men enriched with subjects reporting wheeze (2). Burney's group (3) also demonstrated in a randomized, double-blind, crossover trial that histamine airway responsiveness was greater in male asthmatic subjects on a low-sodium diet while taking a sodium chloride supplement than when they were taking placebo. Similar changes were not seen in female asthmatic subjects. In another experimental study, Javaid and coworkers (4) measured histamine airway responsiveness before and after a twofold increase in salt intake in asthmatic and healthy control subjects. Histamine responsiveness increased significantly in female as well as male asthmatic subjects when their salt intake increased, but no change in responsiveness was observed in healthy control subjects. Dietary potassium has been less well studied than dietary sodium. Burney and colleagues (2) also investigated the relationship of urinary potassium to nonspecific airway responsiveness to histamine in a population of young and middle-aged men enriched with subjects reporting wheeze.They found no significant relationship of urinary potassium to histamine airway responsiveness. No other clinical studies have investigated the influence of dietary potassium on airway responsiveness. We examined the potential relation of dietary intake of sodium and potassium, as reflected by the 24-h excretion of sodium and potassium, to methacholine airway responsiveness among a cohort of middle-aged and older men participating in the Veterans Administration's Normative Aging Study. The Normative Aging Study is a longitudinal study of aging established by the Veterans Administration in 1961 (5). Volunteers were screened at entry according to specific health criteria (5) and were free of known 722

SUMMARY Prior studies have suggested a direct relationship between dietary sodium intake and nonspecific airway responsiveness. The relationship of dietary sodium and potassium intake to methacholine airway responsiveness was examined among 273 male participants of the Normative Aging Study (age range 44 to 82 yr) using 24-h urinary excretion of these cations as a surrogate for intake. Methacholine airway responsiveness was analyzed as dose-response slope, a continuous measure of responsiveness that represents the slope of a line connecting the origin to the last point of the dose-response plot. Greater airway responsiveness to methacholine was associated with greater potassium excretion. A significant relationship between methacholine dose-response slope and potassium excretion (p = 0.014)was observed in multivariate analysis that took into account other covariates, including age, percentage of predicted FEVll cigarette smoking, and skin test reactivity. In contrast, methacholine airway responsiveness did not appear related to urinary sodium excretion. These data suggest that dietary potassium may have an influence on airway reAM REV RESPIR DIS 1991; 144:722-725 sponsiveness of middle-aged and older men.

chronic medical conditions, including asthma, chronic bronchitis, and chronic sinusitis. After entry, the men have reported for-examinations every 3 to 5 yr. Subjects are instructed to refrain from eating or drinking after midnight and to refrain from smoking after 8 p.m. of the night before each examination. Standard questionnaires based on the ATSDLD-78 (6) questionnaire were used to obtain information on respiratory symptoms and illnesses as well as smoking habits. Current smokers were defined as those men who were smoking at least one cigarette a day for at least the past year and were still smoking at least 1 month before the examination. Former smokers were defined as those who previously smoked at least one cigarette a day for at least 1yr but who had ceased smoking more than 1 month before the examination. Never smokers were defined as those who had never smoked cigarettes or smoked less than a total of 20 packs during their lifetime. Asthma was defined as a positive response to the question Have you ever had asthma? and was subclassified according to whether the subject stated this condition was still present. Spirometry and methacholine challenge were performed as previously described (7, 8). Briefly, incremental doses of methacholine wereinhaled from a DeVilbiss646nebulizer (DeVilbiss, Somerset, PA) at 5-min intervals according to the following schedule: five inhalations of mg/ml (phenol-buffered saline alone), one inhalation of 1 mg/ml, one inhalation of 5 mg/ml, four inhalations of

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5 mg/ml, one inhalation of 25 mg/ml, and four inhalations of 25 mg/ml. All inhalations were 6-s inspiratory maneuvers from residual volume to total lung capacity followed by 2 s of breath holding. Determination of nebulizer output by weight (7) indicated that the methacholine inhalation schedule corresponded to the following cumulative doses of methacholine in micromoles: 0,0.330,1.98, 8.58, 16.8, and 49.8. After completion of the methacholine challenge test, skin testing was performed, as previously described (7), by the prick method of Pepys (9). Subjects were test(Received in originaljorm August 27, 1990 and in revised form April 29, 1991) 1 From the Normative Aging Study, Department of Veterans Affairs Outpatient Clinic; the Channing Laboratory; Department of Medicine, Brigham and Women's Hospital, Harvard Medical School; and the Divisions of Pulmonary Medicine, Brigham and Women's Hospital and Beth Israel Hospital, Harvard Medical School, Boston, Massachusetts. 2 Supported by the Medical Research Service of the Department of Veterans Affairs and by Grant No. HL-34645 from the Division of Lung Diseases and Grant No. HL-39871 from the Division of Heart Diseases, National Heart, Lung, and Blood Institute. 3 Correspondence and requests for reprints should be addressed to Dr. David Sparrow, Normative Aging Study, Department of Veterans Affairs Outpatient Clinic, 251 Causeway Street, Boston, MA 02114. 4 Recipient of the American Lung Association Edward Livingston Trudeau Scholar Award.

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ed in double-blind fashion with four common aero allergen preparations preserved in glycerin (ragweed, 1:20;mixed trees, 1:20; mixed grasses, 1:20;and housedust, 1:10) along with a glycerin control. For the current analysis, a positive skin test was defined as a mean wheal diameter (half the largest diameter plus perpendicular diameter) greater than or equal to 5 mm after subtraction of the diameter of any wheal reaction to the glycerin control. The 24-h urine samples were collected at home by participants and brought in at the time of their examination. In addition, a questionnaire eliciting information on urine collection times and medication use was completed by the subject. Urinary electrolyte concentrations were measured on a Beckman Astra'" analyzer (Beckman Instruments, Inc., Brea, CA) with sodium and potassium ionselective electrodes; creatinine was measured with the same instrument by a Jaffe rate reaction. The subjects of this report are 273 men with complete data for methacholine challenge testing, allergy skin testing, and 24-h urine specimen analysis examined between July 1, 1987 and April 31, 1989. An additional 388 men examined during this period were not examined further for the following reasons: ineligible to perform methacholine challenge because of heart disease or prechallenge FEV 1 below 600,10 of the predicted value based on the regression model of Morris and coworkers (10), 81; unwilling to participate in methacholine challenge, 213; unable to perform methacholine challenge correctly, 22; failed to complete methacholine challenge for miscellaneous other reasons, 13; unwilling to participate in skin testing or 24-h urine collection, 59. Methacholine responsiveness was analyzed as a continuous variable by employing an estimate of the overall slope of the doseresponse relationship as previously described (8). Dose-response slope was defined as the decline in FEV 1 from the postsaline value (expressed as a percentage of the postsaline value) after the final dose of methacholine inhaled divided by the final cumulative dose inhaled by the subject. Dose-response slope as so defined is expressed as percentage decline in FEV 1 per micromole methacholine and represents the slope of a line connecting the origin to the last point of the doseresponse plot. The greater the responsiveness to methacholine, the larger (more positive) is the dose-response slope. As previously reported (8), among asthmatic individuals the dose-response slope is nearly perfectly correlated with the dose causing a 200,10 decline in FEV 1 (PD 2o FEV 1)' A dose-response slope of a 10,10 decline in FEV 1 per micromole methacholine corresponds to a PD 2 0 FEV 1 of 20 umol, This analytic approach allows treatment of responsiveness as a continuous variable that can be calculated for all subjects regardless of whether a 200,10 decline in FEV 1 occurred during the test.

TABLE 1 CHARACTERISTICS OF SUBJECTS WITH COMPLETE AND INCOMPLETE METHACHOLINE CHALLENGE AND 24-H URINE DATA

Characteristic Age yr, mean (range) Smoking status, n (%) Current Former Never Asthma, n (%) Current In past Never Missing data Hay fever, n (%) Yes No Missing data ;;l: 1 Positive skin test, n (%) Yes No Missing data FVC, % of predicted", mean (SEM) FEV 1 , % of predicted, mean (SEM) FEF 25 - 75 , % of predicted, mean (SEM)

Complete Data (n = 273)

Incomplete Data

62 (44-82)

62 (43-85)

(n = 388)

22 (8.1) 152 (56) 99 (36)

56 (14)* 210 (54) 122 (31)

11 (4.0) 4 (1.5) 257 (94) 1

6 (1.6) 11 (2.9) 367 (96) 4

55 (20) 218 (80) 0

81 (21) 303 (79) 4

60 213 0 97 95 89

46 140 202 94 90 82

(22) (78) (0.8) (0.8) (1.8)

(25) (75) (0.8)* (1.0)* (1.7)*

* Significantly different from subjects with complete data, p < 0.05. Predicted values are from regression equations based on asymptomatic nonsmokers in the present sample.

t

TABLE 2 24-H URINARY EXCRETION DATA IN 273 MEN

Measurement

Range of Values

Geometric Mean

Mean 10glo

SEM 10glo

Volume, ml Sodium excretion, mmol Potassium excretion, mmol Creatinine excretion, mmol

555-4,075 33-424 22-202 5.3-43.0

1,482 156 72 14

3.17 2.19 1.85 1.15

0.0106 0.0106 0.0086 0.0063

TABLE 3 DOSE-RESPONSE SLOPE ACCORDING TO TERTILES OF POTASSIUM EXCRETION AND SODIUM EXCRETION IN 273 MEN Dose-response Slope Tertile of Excretion (mmol/24 h)

Geometric Mean

Mean 10glo

SEM 10glo

Potassium 1(22-62) II (63-81) III (82-202)

0.587 0.646 0.853

-0.231 -0.190 -0.069

0.041 0.041 0.041

0.005

Sodium 1(33-129) II (130-187) III (188-424)

0.681 0.662 0.718

-0.167 -0.179 -0.144

0.042 0.042 0.042

0.706

*

p Value*

Differences between tertiles were tested for linear trend by analysis of variance.

Because of their skeweddistributions, 24-h excretion of sodium, potassium, and creatinine and dose-response slope were logarithmically transformed (log-s) for all analyses. To permit analysis in the logarithmic scale, a small constant (0.3) was added to each value of the dose-response slope to eliminate zero and slightly negative values of dose-response

slope that were observed in 21 (80,10) of the subjects. Relationships were 'explored by independent t tests, analysis of variance and covariance, multiple linear regression, and multiple logistic regression. All analyses were performed with the SAS statistical software package (SAS Institute Inc., Cary, NC). The 273 subjects with complete methacho-

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TABLE 4 MULTIPLE LINEAR REGRESSION COEFFICIENTS FOR VARIABLES IN A MODEL PREDICTING LOGARITHMIC DOSE-RESPONSE SLOPE Independent Variable Age, yr FEV1 , % of predicted Smoking status Current' Former t Skin test reactivityt Log potassium/24 h Log sodium/24 h Log creatinine/24 h Constant

Regression Coefficient

SEM

p Value

0.0075 -1.194

0.0028 0.166

< 0.001

0.134 0.017 0.086 0.444 -0.084 0.490 -0.735

0.087 0.048 0.052 0.179 0.145 0.263

• Effects are relative to never smokers (1 = current smoker; 0 = otherwise). Effects are relative to never smokers (1 = former smoker; 0 = otherwise). Effects are relative to skin test-negative subjects (1 = ~ 1 positive skin test; 0

t

*

line challenge and 24-h urine data and the 388 subjects with incomplete data were compared on severalcharacteristics (table 1).Subjects with incomplete data were more likely to be current smokers and have lower values of spirometric indicesthan subjects with complete data, reflecting exclusion from the methacholine challenge protocol of subjects with a prechallenge FEV 1 below 60070 of the predicted value. The ranges and mean values for urinary volume and 24-h excretion of sodium, potassium, and creatinine are presented in table 2. Dividing subjects into tertiles on the basis of potassium excretion revealed that methacholine dose-response slope increased (increased response to methacholine) with potassium excretion (table 3). A similar analysis revealed no significant relationship between sodium excretion and methacholine doseresponse slope. Multiple linear regression analysis (table 4) indicated that log potassium excretion was a statistically significant (p = 0.014) predictor of log dose-response slope after adjustment for the specified independent variables. Age and percentage of predicted FEY l were also independent predictors, whereas log sodium excretion appeared unrelated to log doseresponse slope.Height, weight, and body mass index werenot related to dose-response slope, and the inclusion of these covariates in the multiple regression model did not alter the relationship of dose-response slope to sodium or potassium excretion. Inclusion of interaction terms indicated that the relationships of sodium and potassium excretion to log dose-response slope were not modified by age or allergy skin test reaction. We repeated the linear regression model predicting log dose-response slope (table 4) after inclusion of a dummy variable in the model indicating use of diuretics or potassium supplements (n = 25). The relation of urinary potassium and sodium to methacholine dose-response slope was not altered. This cross-sectional study suggests that

0.008

0.123 0.722 0.099 0.014 0.560 0.063

= otherwise).

nonspecific airway responsiveness may have an important relation to potassium intake. Methacholine responsiveness was higher with increasing potassium excretion (table 3). The relation between methacholine dose-response slope and potassium excretion was observed in a multivariate analysis that took account of other covariates (table 4). In contrast, methacholine airway responsiveness did not appear related to sodium excretion. Our assumption is that our subjects were studied in a steady state in which sodium and potassium excretion are determined by sodium and potassium intake. The results of the present study differ from those of other observational and experimental studies. Schwartz and Weiss examined sodium and potassium intake based on a 24-h diet recall and after adjustment for other nutrients found no relationship of intake to wheezing in adult subjects in the Second National Health and Nutrition Examination Survey (11). Increased sodium excretion (2) or intake (3, 4) has been associated with increased airway responsiveness to histamine. Burney's group (2) also examined potassium excretion. Histamine PC 2 0 FEY 1 was directly related to urinary potassium excretion (less histamine responsiveness was associated with greater potassium excretion), but this relationship was not statistically significant. These three studies (2-4), however,involvedsamples with a younger age distribution (generally 18 to 64 yr) who were selected, at least in part, on the basis of reported asthma or wheeze. In contrast, the Normative Aging Study sample was older (44 to 82 yr) and had a lower prevalence of current asthma (4%). Thus, our study reflects more directly a general population sample not enriched for asthmatic subjects. The distributions of potassium and sodium excretion in our sample differed slightly from those reported by Burney'sgroup (2), our subjects excretingslightlymore potassium (72versus 63 mmol, table 2) and slightly less sodium (156 versus 171 mmol). How the differences in age, asthma prevalence, and sodium

and potassium intake may have lead to findings in our sample that differ from those of Burney's group is unclear. The physiologic mechanisms underlying the observed association betweenincreased potassium excretion and increased airway responsiveness are not known. Extreme elevations in the extracellular K+ concentration have been shown to augment airway responsiveness in an animal model (12), but data on the possible influence of extracellular K+ concentration within a more physiologic range on airway responsiveness are unavailable. Cellular K+ and membrane K+ channels appear to play a role in inflammatory mediator release (13, 14). K+ channels also appear to be very important in the process of smooth muscle contraction (15);however,the relevanceof dietary variation in K+ intake, as reflected in urinary K+ output, to any of these mechanisms is uncertain at best. Our study has certain limitations. First, our study design was cross-sectional; thus, it does not clearly establish that changes in dietary potassium antedate the changes in airway responsiveness. Second, 24-h excretion of potassium and sodium was used as a surrogate for long-term dietary intake. Repeated measures of potassium and sodium excretion over time may improve the precision of this analysis. Third, an additional nutrient may be associated with potassium or sodium intake; cation excretionmay be only a surrogate for this third unknown nutritional variable. Fourth, as already noted, the health-screened nature of the Normative Aging Study cohort and the older age of the subjects may limit the generalizability of our results. Finally, the clinical significance of our results is currently unknown. In summary, we have observed a relation between 24-h potassium excretion and airway responsivenessthat remains unexplained. Further investigation is necessary to verify this apparent association of potassium intake with airway responsiveness and to elucidate the mechanisms that underlie it.

Acknowledgment The writers acknowledge the expert assistance with programming and statistical computing of Deborah DeMolles, the assistance of Christina Rosch with the processing of the urine determinations, and the enthusiastic cooperation of the men of the Normative Aging Study, without whom this study would not be possible. References 1. Burney PGJ. A diet rich in sodium may poten-

tiate asthma: epidemiological evidence for a new hypothesis. Chest 1987; 91(suppl):143-8S. 2. Burney PGJ, Britton JR, Chinn S, et al. Response to inhaled histamine and 24 hour sodium excretion. Br Med J 1986; 292:1483-6. 3. Burney PGJ, Neild JE, Twort CHC, et al. Effect of changing dietary sodium on the airway re-

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sponse to histamine. Thorax 1989; 44:36-41. 4. Javaid A, Cushley MJ, Bone ME Effect of dietary salt on bronchial reactivity to histamine in asthma. Br Med J 1988; 297:454. 5. Bell B, Rose CL, Damon H. The normative aging study: an interdisciplinary and longitudinal study of health and aging. Aging Hum Dev 1972; 3:5-17. 6. Ferris BG Jr. Epidemiology standardization project. Am Rev Respir Dis 1978; 118(6, Part 2:1-88). 7. Sparrow D, O'Connor G, Colton T, Barry CL, Weiss ST. The relationship of nonspecific bronchial responsiveness to the occurrence of respiratory symptoms and decreased levels of pulmonary function. The Normative Aging Study. Am Rev Respir

Dis 1987; 135:1255-60. 8. O'Connor G, Sparrow D, Taylor D, Segal M, Weiss S. Analysis of dose-response curves to methacholine. Am Rev Respir Dis 1987; 136:1412-7. 9. Pepys J. Skin tests in diagnosis. In: Gell PGH, Coombs RRA, Lachman PJ, eds. Clinical aspects of immunology. Oxford: Blackwell, 1975: 55. 10. Morris JF, Koski A, Johnson LC. Spirometric standards for healthy nonsmoking adults. Am Rev Respir Dis 1971; 103:57-67. 11. Schwartz J, Weiss ST. Dietary factors and their relation to respiratory symptoms. Am J Epidemiol 1990; 132:67-76. 12. Murlas C, Ehring G, Suszkiw J, Sperelakis N. Kt-induced alterations in airway muscle responsive-

ness to electrical field stimulation. J Appl Physiol 1973; 34:677-82. 13. Uvnas B, Aborg CH, Lyssarides L, Danielsson LG. Intracellular ion exchange between cytoplasmic potassium and granule histamine, an integrated link in the histamine release machinery of mast cells. Acta Physiol Scand 1989; 136:309-20. 14. Kakuta Y, Okayama H, Aikawa T, et al. K channels of human alveolar macrophages. J Allergy Clin Immunol 1988; 81:460-8. 15. Black JL, Barnes PJ. Potassium channels and airway function: new therapeutic prospects. Thorax 1990; 45:213-8.