Effect of captopril and hydralazine on arterial pressure-urinary output relationships in spontaneously hypertensive rats.

Angiotensin I converting enzyme inhibitors are typically classified as peripheral vasodilators. We studied the effect of captopril and a known vasodilator, hydralazine, on arterial pressure-urinary output relationships in adult spontaneously hypertensive rats to determine whether these drugs produced similar changes in this relationship. Tail-cuff pressure and 24-hour urine output and sodium excretion were measured under steady state conditions during ingestion of tap water or saline (1% NaCl) ad libitum. Sodium intake increased seven to nine times when rats drank saline, but in the absence of drug treatment, tail-cuff pressure was not altered significantly (water, 213 +/- 3 vs saline, 220 +/- 5 mm Hg). Daily administration of captopril (100 mg/kg p.o.) or hydralazine (15 mg/kg p.o.) for 2 weeks lowered tail-cuff pressure significantly (175 +/- 3 and 166 +/- 3 mm Hg, respectively; p less than 0.01) while rats drank tap water. Continued administration of hydralazine plus 2 weeks of drinking saline did not alter tail-cuff pressure (162 +/- 4 mm Hg), but with the addition of saline during captopril treatment, tail-cuff pressure was elevated significantly (210 +/- 5 mm Hg; p less than 0.01). Thus, hydralazine produced a parallel shift of the arterial pressure-urinary output relationship along the pressure axis. In contrast, captopril produced a marked change in the slope of this relationship, making arterial pressure extremely salt-sensitive. The results suggest that the two drugs have different effects on the mechanisms that contribute to the long-term control of arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)