Antihypertensive effect of MK-421 in rats. Role of the renal kallikrein-kinin system

Endothelin-1 augments pressor response to angiotensin II infusion in rats

To assess possible roles of endothelin in the regulation of blood pressure, we studied effects of a subpressor dose of endothelin-1 (3 micrograms/kg/day) on chronic blood pressure responses to infusion of angiotensin II and norepinephrine in rats. Rats were infused with angiotensin II at a subpressor dose (400 micrograms/kg/day i.p.) or with norepinephrine at a subpressor dose (360 micrograms/kg/day i.p.) for 6 days. Systolic blood pressure was significantly elevated during combined infusion of endothelin-1 and angiotensin II, whereas endothelin-1 alone or angiotensin II alone failed to induce any significant changes in systolic blood pressure compared with vehicle alone. This effect was sustained for the whole experimental period and was not associated with any significant changes in body weight, fluid intake, urine volume, or urinary electrolyte excretion. In contrast, combined infusion of endothelin-1 and norepinephrine failed to elevate systolic blood pressure, and no significant difference in systolic blood pressure was observed for the whole experimental period among the four groups of rats with endothelin-1 in combination with norepinephrine, endothelin-1 alone, norepinephrine alone, and vehicle alone. The present results indicate that angiotensin II and endothelin-1, but not norepinephrine and endothelin-1, work synergistically to raise the blood pressure and also suggest the possibility that endothelin-1 may modulate blood pressure control.

Role of the endogenous angiotensin II in the antihypertensive effect of MK 421 in rats made hypertensive by norepinephrine or vasopressin

To assess the mechanism by which inhibitors of angiotensin converting enzyme (ACE) lower blood pressure, we evaluated the role of endogenous angiotensin II in the antihypertensive effect of MK 421, a long-lasting ACE inhibitor, in rats made hypertensive by chronic infusion of norepinephrine or vasopressin. The hypertensive effect of norepinephrine (1.8 mg/kg/day, ip) or vasopressin (7.2 U/kg/day, ip) was inhibited by the simultaneous administration of MK 421 (6 mg/kg/day, ip). Additional administration of angiotensin II at a subpressor dose (36 micrograms/kg/day, ip) did not revert the antihypertensive effect of MK 421 in rats made hypertensive by chronic infusion of norepinephrine or vasopressin. The present results suggest that the hypotensive effect of ACE inhibitors may depend on a reduced sensitivity of the vasculature to vasoconstrictor substances. In addition, it is also suggested that the suppressed angiotensin II may not be essential for the antihypertensive effect of ACE inhibitors in rats made hypertensive by chronic infusion of norepinephrine or vasopressin.

No evidence on significant roles of the prostaglandin-thromboxane and kallikrein-kinin system in the antihypertensive effect of MK 421 in rats made hypertensive by norepinephrine or vasopressin

Inhibition of angiotensin converting enzyme by MK 421 (6 mg/kg/day ip) induced a significant increase in urinary kinin excretion in norepinephrine-infused rats (1.8 mg/kg/day ip), whereas it had no effect on urinary prostaglandin E2 excretion. In contrast, MK 421 did not induced any significant changes in urinary kinin and prostaglandin E2 excretion in vasopressin-infused rats (7.2 U/kg/day ip). The simultaneous administration of indomethacin (10 mg/kg/day sc), OKY 046 (12 mg/kg/day sc) or aprotinin (100,000 units/kg/day sc) did not affect the antihypertensive effect of MK 421 in rats made hypertensive by chronic infusion of norepinephrine or vasopressin. The present results suggest that the hypotensive effect of MK 421 may depend on a reduced sensitivity of the vasculature to vasoconstrictor substances. In addition, it is also suggested that neither the prostaglandin-thromboxane or kallikrein-kinin systems are essential for the antihypertensive effect of MK 421 in these models of hypertension.

Atrial natriuretic factor inhibits the hypertension induced by chronic infusion of norepinephrine in conscious rats

To assess the physiological role of atrial natriuretic factors in the regulation of blood pressure and sodium-water excretion, we studied the chronic effects of continuous infusion of a synthetic atrial natriuretic factor of 25 amino acids for up to 3 days on systolic blood pressure, urine volume, and urinary excretion of sodium, prostaglandin E2 and kallikrein in conscious rats, and also evaluated the antihypertensive effect of this substance in rats with hypertension caused by chronic infusion of norepinephrine. Continuous infusion of atrial natriuretic factor (150 micrograms/kg per day) into the jugular vein via osmotic minipumps did not induce any changes in systolic blood pressure, urine volume, and urinary excretion of sodium, prostaglandin E2, and kallikrein for up to 3 days, compared with those in vehicle-infused rats. When the same dose of atrial natriuretic factor was administered simultaneously with 1.8 mg/kg per day of norepinephrine infused intraperitoneally by osmotic minipumps, the systolic blood pressure of conscious rats rose on day 1 to only 127.3 +/- 6.3 mm Hg compared with the rise to 146.3 +/- 1.6 mm Hg when norepinephrine alone was infused (P less than 0.05). The antihypertensive effect of atrial natriuretic factor was sustained for 3 days in rats infused with norepinephrine. The administration of atrial natriuretic factor to rats made hypertensive by 3 days of infusion with norepinephrine alone returned the blood pressure to control levels, and the antihypertensive effect was sustained throughout the experimental period lasting for 3 days.(ABSTRACT TRUNCATED AT 250 WORDS)