michigan hypertension core curriculum - State of Michigan

intake. 4
Figure 1;
Ion transport in collecting tubule cell
Schematic representation of sodium and potassium transport in the sodium reabsorbing cells in the
collecting tubules. The entry of filtered Na into the cells is mediated by selective sodium channels in
the apical (luminal) membrane; the energy for this process is provided by the favorable electrochemical
gradient for Na (cell interior electronegative and low cell Na concentration). Reabsorbed Na is pumped
out of the cell by the Na-K-ATPase pump in the basolateral (peritubular) membrane. The reabsorption
of cationic Na makes the lumen electronegative, thereby creating a favorable gradient for the secretion
of K into the lumen via K channels in the apical membrane. Aldosterone, after combining with the
cytosolic mineralocorticoid receptor (Aldo-R), leads to enhanced Na reabsorption and potassium
secretion by increasing both the number of open Na channels and the number of Na-K-ATPase pumps.
Atrial natriuretic peptide, on the other hand, acts primarily in the inner medullary collecting duct by
combining with its basolateral membrane receptor (ANP-R) and activating guanylate cyclase. ANP
inhibits sodium reabsorption by closing the Na channels. The potassium-sparing diuretics act by closing
Na channels, amiloride and triamterene directly and spironolactone by competing with aldosterone.
Used with permission from Dr. Raymond Townsend @ Uptodate
Diagnosis and Evaluation
Recent guidelines published by the Endocrine Society in 2008 detail the evaluation of PA.
(Figure 2) The first test is the plasma aldosterone /renin ratio (ARR), preferably performed the morning.
This screening test has both false positives and false negatives. Although many antihypertensives can
affect levels of renin and aldo, only two render the test uninterpretable. The test must be performed
in patients who have not taken either an aldosterone antagonist or a renin inhibitor within the
past 4-6 weeks. A positive screen consists of an aldosterone level > 14 ng /dl (416 pmol/l), with a PRA
less than 1 ng /ml/hr (12.8 pm/l in SI units); nevertheless, a not inconsequential proportion of patients
with PA will not have circulating aldosterone levels > 14 ng/dl. PRA is now being performed in some
laboratories as a direct renin concentration or DRC, which requires conversion. Knowing local values
100 Hypertension Core Curriculum