Dose-response relationships for spironolactone in combination with a potassium-wasting diuretic

The plasma and urine electrolyte responses to repeated doses of spironolactone, 25, 50, and 100 mg, in combination with metolazone, 2.5, 5.0, and 10 mg, were examined in 18 healthy subjects (six at each dose of metolazone). During the period of pharmacologic steady state, there were log linear spironolactone dose-response relationships for plasma potassium, sodium, and bicarbonate (P less than 0.001 in each case) with no contraindication to parallelism between the metolazone groups. In the absence of mineralocorticoid challenge, spironolactone dose-urine electrolyte responses could not be demonstrated. However, after fludrocortisone, spironolactone log dose-response trends were linear with respect to natriuresis (P = 0.027), antikaliuresis (P = 0.020), and log 10 Na/K (P = 0.001), which is usually considered the best single index of renal antimineralocorticoid activity, and exhibited parallelism between the metolazone doses. These observations suggest that a convenient bioassay for aldosterone antagonists in normal men may be provided by the electrolyte responses to repeated doses of such drugs in combination with potassium-wasting diuretics. In view of the limitations of other methods, this approach may have particular relevance to the evaluation of potassium-sparing properties.