Epithelial Sodium Channel (ENaC) as a Novel Mechanism for Hypertension and Volume Expansion in Type 2 Diabetes

Renal sodium retention and extracellular fluid volume expansion are hallmarks of nephrotic
syndrome. There is abundant evidence that this occurs even in the absence of activation of
hormones that are known to activate renal Na transporters. Proteinuria not only reflects
glomerular damage, but also functions as a risk factor for cardiovascular disease, stroke,
end stage renal disease and is associated with extracellular volume expansion and high BP.

In the natural course of Type II diabetes, microalbuminuria and elevations in blood pressure
are thought to occur at around the same time. Blood pressure in microalbuminuric diabetics is
more sensitive to dietary salt intake than in normoalbuminuric patients despite both groups
having similar aldosterone and plasma renin activity levels. Proteolytic processing of ENaC
subunits might provide the primary defect in renal sodium handling in these microalbuminuric
individuals. However, proteinuria is not consistently identified as a risk factor for
incipient elevation in blood pressure and in some studies elevated blood pressure predicts
the advent of microalbuminuria.

Analyses of normotensive normoalbuminuric subjects in previous studies have found that higher
urinary albumin levels in the normal range predicted incident hypertension. A similar finding
was seen in a non-diabetic cohort. These studies suggest that these disparate results may be
related to the cut off that defined microalbuminuria. Another possible explanation is that an
ENaC activator, like plasmin, contributes to the generation of incident hypertension in some
individuals. Levels of albuminuria may not necessarily be reflective of ENaC activator levels
and may vary from individual to individual. Perhaps urinary plasmin and plasminogen provides
a more robust biomarker for those individuals who may develop hypertension.

Recent evidence suggests that in some individuals with glomerular damage, proteases not
normally found in urine enter the urinary space and aberrantly cleave ENaC. In this setting,
filtered plasminogen (inactive precursor) is converted to plasmin (active protease) by
urokinase that is expressed in tubular epithelial lumen. The proteolytic activation of ENaC
would generate a primary defect in renal sodium handling, a mechanism that may be a
particularly important factor leading to increases in extracellular fluid volume and BP that
accompany nephrotic syndrome.

While previous studies have examined the role of amiloride in low-renin hypertension, and as
an additional agent the conventional treatment of hypertension, no human trials have tested
whether ENaC inhibitors impact blood pressure and volume status in the setting of
proteinuria. Over a ten year period, millions of diabetics, 5.3% of Type II diabetics and 28%
of Type I diabetics develop macroscopic proteinuria.

- Inclusion Criteria:

1. Age 18 to 80 yrs at randomization

2. History of Type 2 Diabetes

3. Presence of systolic hypertension or pre-hypertension (average systolic blood
pressure (SBP) ≥120 mmHg and <180 mmHg.)

4. Urinary protein/creatinine ratio >300 mg/g creatinine at screening

5. Hemoglobin A1C<8%

6. Willing and able to give informed consent

- Exclusion Criteria:

1. Average SBP of ≥180 mmHg or diastolic blood pressure (DBP) of ≥110 mmHg

2. Current symptomatic heart failure, history of New York Heart Association Class
III or IV congestive heart failure, or left ventricular (LV) ejection fraction
(by any method) <25%; these patients may be harmed with withdrawal of diuretics

3. Serum potassium level <3.5 or >5.0 at screening

4. History of hyperkalemia in the last two years (serum K>5.5)

5. Contraindication to use of hydrochlorothiazide or amiloride

6. Unstable angina pectoris or acute myocardial infarction (MI) in last 3 months

7. Known secondary causes of hypertension (HTN) (screening for these conditions will
not be required)

8. Estimated glomerular filtration rate (GFR) <60 mL/min/1.73m², as determined by
validated estimating equations

9. On or expected to be on immunosuppressive therapy

10. Any history of solid organ transplantation

11. Significant dementia

12. Other factors likely to limit adherence during trial (eg. alcohol or substance
abuse, plan to move in next year, history of non-adherence to medications,
appointments and medical care, reluctance of close family members to participate
in trial, lack of support from primary healthcare provider)

13. Participation in another investigational trial within 4 weeks of the screening
visit

14. Arm Circumference too large or too small to allow accurate blood pressure
measurement

15. Pregnancy or currently trying to become pregnant (although this is unlikely
because of age limit

16. Incarceration