Abrupt withdrawal of captopril has not been associated with a rapid increase in blood pressure. Studies in rats and cats indicate that captopril does not cross the blood-brain barrier to any significant extent. Hydrochlorothiazide Thiazides affect the renal tubular mechanism of electrolyte reabsorption. At maximal therapeutic dosage all thiazides are approximately equal in their diuretic potency. Thiazides increase excretion of sodium and chloride in approximately equivalent amounts.
Natriuresis causes a secondary loss of potassium and bicarbonate.
The mechanism of the antihypertensive effect of thiazides is unknown. Thiazides do not affect normal blood pressure. The mean plasma half-life of hydrochlorothiazide in fasted individuals has been reported to be approximately 2.5 hours. Onset of diuresis occurs in two hours and the peak effect at about four hours. Its action persists for approximately six to twelve hours.