Respiratory Factors In Blood Pressure Regulation

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Abstract

This program is concerned with the role of respiratory factors in sodium-sensitive forms of human hypertension. It is based on previous research that developed a model of experimental hypertension in laboratory animals resulting from a synergistic interaction between high sodium intake and behavioral stress that inhibited respiration. Subsequent studies with humans found that chronic self-reported stress was associated with inhibited breathing at rest, and high end tidal CO2 at rest was an independent determinant of resting blood pressure. Both of these findings emerged more strongly in women than in men.[unreadable] [unreadable] The annual report last year reported results of a study with 36 normotensive women that tested the hypothesis that breathing frequency at rest was a determinant of blood pressure responsivity to high sodium intake. In this study, subjects were maintained on a low sodium diet for six days followed by a high sodium diet for six days. Breathing pattern, pCO2, and blood pressure were monitored before and after each diet. Slow breathing rate, high pCO2, and low hematocrit were all found to be independent predictors of change in systolic blood pressure in response to change in dietary sodium intake. [unreadable] [unreadable] Over the past year, additional findings in this study have focused on the effects of changes in dietary sodium intake on plasma concentrations and urinary output of two endogenous digitalis-like factors, ouabain and marinobufagenin (MBG). They are of interest because an endogenous digitalis-like factor has been shown to be elevated in clinical hypertension, and it has been hypothesized that this factor, which has both natriuretic and vasoconstrictor effects, mediates the development of sodium-sensitive forms of hypertension. It was hypothesized that high sodium intake would be associated with increases in plasma and urinary ouabain and MBG, available from 33 healthy, Caucasian, female participants, ages 40-70, at the end of the six-day low and high sodium diets. An MBG competitive fluoroimmunoassay was based on a polyclonal rabbit antibody raised against MBG-glycoside BSA. The ouabain assay was based on a similar principle using a ouabain-ovalbumin conjugate and a rabbit ouabain antibody. [unreadable] [unreadable] Mean 24-hr urinary MBG excretion on day six of high sodium intake was significantly greater than on day six of low sodium intake (2.33 ? 0.14 vs 1.77 ? 0.14 pmol/24 hr; t = 2.83; p <.01).[unreadable] 24-hr urinary EO excretion on day 12 of high sodium intake was not significantly different from that on day six of low sodium intake (0.59 ? 0.08 vs 0.57 ? 0.05 pmol/24 hr). No significant differences between low and high sodium diets in either plasma EO (0.17 ? .03 vs. 0.15 ? .03 pmol) or plasma MBG (0.37 ? .02 vs 0.40 ? .03 pmol) were observed.[unreadable] [unreadable] Urinary EO and MBG were positively correlated under low and high sodium diets (r = +0.61 and +0.55, respectively; p <.01). Multiple regressions of age, BMI, baseline systolic blood pressure, and renal sodium excretion on MBG during low and high sodium diets showe that on the low sodium diet, EO was the only significant independent determinant of MBG, accounting for 50% of the variance. Onn the high sodium diet, EO, age, and urine volume were all significant independent determinants of MBG, accounting for 57% of the variance.[unreadable] [unreadable] Urinary MBG excretion by salt-sensitive subjects was less than that by salt-insensitive subjects during both the low sodium (t = 2.27; p <.02) and high sodium (t = 2.84; p <.01) diets. MBG excretion during low (F2,30 = 4.94; p <.02; r2 = .25) and high (F1,31 = 7.91; p <.01; r2 =.20) sodium intake remained a significant inverse determinant of salt sensitivity after adjustment for age, BMI, and baseline blood pressure. The results of this study show that urinary MBG of normotensive humans is responsive to high sodium intake, and that its excretion is attenuated in those whose blood pressure is responsive to high sodium intake. Thus, deficient renal excretion of MBG evoked by high sodium intake might play a role in the development of sodium-sensitive forms of hypertension.[unreadable] [unreadable] The effects of changes in dietary sodium intake on heart rate variability (HRV) in normotensive women was also analyzed during the past year. The data were obtained in the study described above with 36 healthy women, ages 40-70, who were on a low sodium/low potassium diet for six days followed by a high sodium/low potassium diet for six days. At the end of each diet, individual salt sensitivity was determined from 24-hr monitoring of blood pressure and renal sodium excretion, and resting HRV obtained from an electrocardiogram was determined via time and frequency domain measures, which included standard deviations of R-R intervals and low and high frequency power functions.[unreadable] [unreadable] Each measure of HRV was significantly less in women while on low sodium diet than at baseline or on high sodium diet. No significant association of HRV with salt sensitivity of blood pressure was observed at baseline or on high sodium diet. On low salt diet, HRV was inversely correlated with sodium sensitivity of blood pressure, but this association was obviated when effects of respiratory rate were taken into account. In summary, this study found that HRV at rest is a function of dietary sodium intake, and it was concluded that individual differences in dietary sodium intake should be considered in future studies of HRV in normotensive persons. [unreadable] [unreadable] Finally, another clinical study has been initiated over the past year that investigates the extent to which the reversal of hypertension in patients with borderline hypertension by a guided breathing procedure is associated with specific changes in respiratory habits. To date, 10 humans with systolic blood pressure between 130 and 160 and/or diastolic pressure between 85 and 100 mmHg have been taught to practice daily with the Resperate, a commercially-available device that provides a series of alternating ascending and descending tones via earphones to encourage the patient to breathe more slowly, down to six breaths or less per min, during daily 15 min sessions for four weeks. Subjects in a control group engaged in a meditative relaxation exercise for 15 min per day for four weeks. Acute and chronic changes in breathing pattern, blood gases, and blood pressure are assessed in the clinic and during 24-hr ambulatory monitoring before and following the behavioral interventions. 24-hr urine samples are also collected before and following the behavioral intervention to assess effects on endogenous digitalis-like factors. This study remains in progress.