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(Hypertension. 1996;27:914-918.)
© 1996 American Heart Association, Inc.
Articles |
From the Division of Nephrology, Department of Medicine, National Cardiovascular Center, Osaka, Japan.
| Abstract |
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Key Words: hypertension, essential sodium, dietary pressure-natriuresis relationship antihypertensive therapy hypertension, sodium sensitive diuretics
| Introduction |
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| Methods |
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Study Protocol
After the initial hospitalization lasting more than 1 week, the
patients were studied during five stages, each lasting 7 days: a high
sodium diet containing 15 to 18 g NaCl per day (stages I and V), a low
sodium diet containing 1 to 3 g NaCl per day (stage II), and a medium
sodium diet containing 5 to 7 g NaCl per day (stages III and IV).
During stages I through III, patients were given no antihypertensive
drugs. During stages IV and V, they were given 25 mg mefruside once a
day. Mefruside, one of the most commonly used antihypertensive
diuretics in Japan, is a thiazide-type diuretic
that has a sulfonamide group and a structural similarity with loop
diuretics.5 Urinary sodium excretion rate
(UNaV) and creatinine excretion rate were
measured on the last 3 days of each stage. Serum sodium, potassium,
uric acid, and creatinine concentrations were measured on
the last day of each stage. Creatinine clearance was
calculated from serum creatinine and the average values of
the urinary creatinine excretion rate. BP was measured with
patients in the supine position with an automatic sphygmomanometer
(model BP-103, Nippon Kohrin Co) every hour from 6 AM to 9
PM on the last day of each stage, and the average reading
of 16 measurements was adopted. Mean arterial pressure
(MAP) was calculated as one third of the pulse pressure plus the
diastolic pressure.
Determination of the Pressure-Natriuresis Curve
A pressure-natriuresis curve was drawn by linking data
points in each patient obtained in a steady state of sodium balance
under three different amounts of sodium intake before mefruside
administration and under two different amounts of sodium intake after
mefruside administration. MAP (in millimeters of mercury) and
UNaV (in millimoles per day) were plotted on the
x and y axes, respectively.6 7 8 To
compare the major characteristics of the curve, that is, the shift of
the curve along the arterial pressure axis and steepness of
the curve, before and after mefruside administration, we calculated the
extrapolated x intercept, A (millimeters of mercury), and
the slope, B (millimoles per day per millimeter of mercury), as
follows4 9 10 :
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where H and L are the data obtained in a steady state of sodium balance under relatively high and relatively low sodium diets, respectively. The reciprocal of the slope of the curve, 1/B (millimeters of mercury per millimole), corresponds to the salt sensitivity index.11 12 13
Statistical Analysis
Results are expressed as mean±SD, and the significance of the
effects of salt restriction and mefruside was tested by a two-way
classification ANOVA and ANCOVA with repeated measures. The comparison
of the extrapolated x intercept and slope of the
pressure-natriuresis curve between before and after mefruside
administration was done by Student's t test for paired
samples. The correlation coefficient was obtained by the least squares
method. The reciprocal of the slope (1/B, millimeters of mercury per
millimole) was used for analysis of the slope (B, millimoles
per day per millimeter of mercury), because dietary sodium intake was
altered primarily and a consequent secondary change in MAP was
observed.4 9
| Results |
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Pressure-Natriuresis Relationship Before Mefruside
Administration
The pressure-natriuresis curve (arterial
pressureurinary sodium output relationship) before mefruside
administration was obtained by plotting UNaV in stages I,
II, and III on the ordinate as a function of MAP on the abscissa (Fig 1
). The extrapolated x intercept, A, and
slope, B, of the pressure-natriuresis curve were compared between
two phases: a decreasing phase from stage I to II (A: 104±6 mm Hg; B:
20.8±10.5 [mmol/d]/mm Hg) and an increasing phase from stage II to
III (A: 104±7 mm Hg; B: 23.3±13.0 [mmol/d]/mm Hg). The
x intercept and slope were not significantly different
between these two phases. In addition, there was a significant positive
overall linear relationship between MAP and the corresponding
UNaV during stages I through III all together
(r=.617, P<.01, n=24). These results indicate
that the pressure-natriuresis curve was virtually linear.
Therefore, the pressure-natriuresis curve from stage I to II was
adopted for the curve before mefruside administration.
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Effect of Mefruside on the Pressure-Natriuresis Curve
The pressure-natriuresis curve after mefruside administration
was obtained by linking two data points of stages IV and V. The
extrapolated x intercept, A, was not different before and
after mefruside administration (104±6 versus 101±9 mm Hg,
P=NS), whereas the slope, B, was increased after mefruside
administration (20.8±10.5 versus 143±85 [mmol/d]/mm Hg,
P<.005). These results indicated that the hypotensive
action of mefruside was based on the increased slope of the
pressure-natriuresis curve but was not based on the leftward shift
of the curve along the BP axis. In addition, there was a significant
negative relationship between the increase in the slope by mefruside
and the steepness of the slope before mefruside administration
(r=-.81, P<.02), showing that the
hypotensive effect of mefruside by steepening the slope of the curve
was greater in patients with a depressed slope and therefore higher
sodium sensitivity.
Difference in Hypotensive Mechanisms of Mefruside Under Different
Amounts of Sodium Intake
The hypotensive effect of mefruside during two different amounts
of sodium intake was analyzed in relation to the hypotensive
effect of sodium restriction from stage I to II. The hypotensive effect
of mefruside during high sodium from stage I to V was positively
correlated with that of sodium restriction (r=.84,
P<.01, Fig 2
), whereas that during medium
sodium from stage III to IV was not (r=.50,
P=NS). Therefore, the hypotensive effect of mefruside during
the two different amounts of sodium intake was analyzed in
relation to the changes by mefruside in both the extrapolated
x intercept, A, and slope, B, of the
pressure-natriuresis curve. The hypotensive effect during high
sodium from stage I to V was significantly correlated with the increase
in the slope (r=.83, P<.02, Fig 3
) but was not correlated with the change in the
x intercept (r=-.06, P=NS). On
the other hand, the effect during medium sodium from stage III to IV
had a tendency to correlate with the decrease in the x
intercept (r=.67, P<.1) but not with the change
in the slope (r=.20, P=NS). These results suggest
that the hypotensive mechanisms of mefruside differ between a high and
medium sodium intake. Mefruside may lower BP by increasing the slope of
the pressure-natriuresis curve during high sodium and by shifting
the curve leftward toward a lower BP level along the BP axis during
relatively low sodium.
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Serum Chemistry and Creatinine Clearance
Serum sodium and creatinine concentrations did not
change significantly among the five stages (Table 2
).
Serum potassium concentration was elevated by sodium restriction and
was lowered by mefruside. Serum uric acid concentration was
significantly elevated by both sodium restriction and mefruside. On the
other hand, creatinine clearance was reduced only in stage
IV compared with the other stages by the combined effect.
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| Discussion |
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Recently, we proposed that hypertension can be ascribed to one of three
major renal mechanisms12 13 : (1) increased
preglomerular vascular resistance from heart to
glomeruli, mainly due to afferent arteriolar resistance; (2) decreased
whole-kidney ultrafiltration coefficient, due to decreased
filtration surface area per glomerulus, decreased hydraulic
permeability of the glomerular filtration barrier, and/or
decreased number of glomeruli; and (3) increased rate of tubular sodium
reabsorption. The first mechanism produces
nonsodium-sensitive hypertension, and the latter two produce
sodium-sensitive hypertension. The hypotensive mechanism of
diuretics may be the reverse of the third mechanism above. When
renal tubular sodium reabsorption is inhibited by diuretics,
sodium balance becomes negative if glomerular filtration
rate and tubular sodium load remain normal, resulting in a fall in body
fluid volume and BP. As systemic BP is lowered, glomerular
capillary hydraulic pressure and glomerular filtration rate
are also reduced. The resultant decrease in tubular sodium load makes
it possible for sodium balance to be maintained despite inhibited
tubular sodium reabsorption. In this way, when tubular sodium
reabsorption is inhibited, sodium balance can be maintained only with
the reduction in tubular sodium load because of the fall in systemic
and glomerular capillary pressures. In other words, by
inhibiting tubular sodium reabsorption, diuretics multiply
urinary sodium output created at any level of glomerular
filtration rate and tubular sodium load as well as at any level of
systemic and glomerular capillary pressures.6
The above relationship between sodium balance and BP can be illustrated
clearly by the effect of a thiazide-type diuretic,
mefruside, on the pressure-natriuresis curve.6
Mefruside steepened the slope of the pressure-natriuresis curve,
especially in patients whose slope was depressed before mefruside
administration (Fig 1
). Since the reciprocal of the slope of the
pressure-natriuresis curve reflects the sodium sensitivity of
BP,11 12 13 mefruside lowers BP especially in patients with
high sodium sensitivity by reducing their sodium sensitivity. Mefruside
made it possible for sodium balance to be maintained under lowered
systemic BP based on steepening the slope of the
pressure-natriuresis curve by inhibiting tubular sodium
reabsorption.
The hypotensive effect of mefruside was greater during high sodium than low sodium, and BP became relatively insensitive to the amount of sodium after mefruside treatment. It is reported that serum potassium is further reduced by high sodium intake with diuretics, since tubular sodium load and sodium-potassium exchange at distal tubules are enhanced by increased sodium intake.19 This finding and the fact that the hypotensive effect of diuretics is diminished by an excessive amount of sodium intake20 usually encourage clinicians to restrict sodium intake during diuretic administration. However, the present study has indicated that this effect was almost negligible. Instead, uric acid was significantly elevated by mefruside in combination with sodium restriction. Taken together, these data suggest that strict sodium restriction may be unnecessary during diuretic administration.
Our data also suggest that the hypotensive mechanisms of mefruside may
be different under different amounts of sodium intake. It is certain
that the hypotensive effect of mefruside during high sodium was based
on its diuretic action, because the effect was correlated with
both the hypotensive effect of sodium restriction (Fig 2
) and the
increase in the slope of the pressure-natriuresis curve (Fig 3
).
Since the slope is believed to be determined by glomerulotubular
balance of sodium between the glomerular ultrafiltration
coefficient and tubular sodium reabsorption,12 13 the
steepened slope by mefruside can be attributed to the inhibition of
tubular sodium reabsorption, as discussed above. On the other hand, the
hypotensive effect of mefruside during relatively low sodium was
correlated with neither the hypotensive effect of sodium restriction
nor the increase in the slope. The hypotensive effect had a tendency to
correlate with the shift of the pressure-natriuresis curve toward a
lower BP level along the BP axis. Since the x intercept of
the curve seemed determined mainly by the preglomerular
vascular resistance from heart to glomeruli,12 13 the
effect during low sodium may be based on the reduction in
preglomerular resistance, probably because of the
vasodilating action of diuretics. Diuretics initially
lower cardiac output and increase total peripheral vascular
resistance. However, by 1 week, "reverse whole body
autoregulation" is occurring, and peripheral resistance
decreases while cardiac output returns to normal.6 8 21
Thus, after 7 days, diuretics are acting as vasodilators. The
very high sodium diet may prevent or retard this reverse autoregulatory
process. Mefruside, which is structurally similar to loop
diuretics, may also have a weak direct vasodilating action in
addition to the above indirect action.
We have previously proposed classifying antihypertensive drugs into three groups based on their effects on the pressure-natriuresis relationship.4 22 The first group, consisting of vasodilators such as calcium antagonists, shifts the pressure-natriuresis curve to the left without affecting slope. The hypotensive effect of this group can be obtained independently of the amount of salt intake, and the hypotensive effects of this group and sodium restriction are thought to be additive. The second group, consisting of ß-blockers and angiotensin-converting enzyme inhibitors, shifts the pressure-natriuresis curve leftward, with a decrease in slope. The hypotensive effect is augmented under sodium restriction and weakened under sodium overload and is thought to be synergistic with sodium restriction. The third group of antihypertensive drugs, consisting of diuretics, can increase the slope of the pressure-natriuresis curve, as described in this study. Since the hypotensive effect of diuretics is greater during sodium overload than during sodium restriction, the hypotensive effects of diuretics and sodium restriction are thought to be antagonistic. In addition, diuretics will potentiate the hypotensive effects of the ß-blocker propranolol23 24 and the angiotensin-converting enzyme inhibitor captopril25 26 by increasing the slope that has been depressed by the two drugs. This classification of antihypertensive drugs may be useful in clinical practice for selecting the most effective drug or combination of drugs in the treatment of hypertensive patients with various pressure-natriuresis curves and sodium sensitivities.
It is already well known that the pressure-natriuresis curve is shifted to the right and the slope is depressed with increased severity of hypertension.1 2 3 4 Parfrey and colleagues2 3 reported that children born to hypertensive parents showed a pressure-natriuresis curve shifted to the right even before the onset of hypertension compared with children born to normotensive parents. Thus it is clear that as essential hypertension progresses from borderline to mild and severe forms, the pressure-natriuresis curve will be shifted farther to the right and the slope depressed. Therefore, our data suggest that the hypotensive effect of diuretics is less effective in patients with borderline and mild essential hypertension, who have a steep slope and low sodium sensitivity, than in those with severe hypertension, who have a depressed slope and high sodium sensitivity.
| Acknowledgments |
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| Footnotes |
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Received October 24, 1995; first decision November 9, 1995; accepted January 15, 1996.
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