Pressor and subpressor angiotensin II administration. Two experimental models of hypertension.

Administered dose is an important determinant of the type of hypertension produced by angiotensin II. With chronic administration of pressor doses, there is salt and water retention and expansion of extracellular fluid volume, and the pressure-natriuresis curve is shifted to higher pressures. Important compensatory mechanisms, including resetting of baroreceptors, de novo synthesis of vasodilator prostaglandins, and atrial natriuretic factor release, are triggered by the acute rise of blood pressure. Histologic evidence for vascular injury confounds the interpretation of findings. When angiotensin II is administered in initially subpressor doses, the rise of blood pressure is gradual, there are no detectable changes in salt and water balance, and compensatory mechanisms do not seem to be activated. Autopotentiation of pressor and vasoconstrictor responses by angiotensin II is the characteristic feature of the early stages of hypertension induced by small doses of angiotensin II. Trophic stimulation of vascular tissue, in particular restructuring of extracellular matrix, precedes and may, therefore, be the mechanism responsible for the hemodynamic changes. The pressor and subpressor models of angiotensin II-induced hypertension draw attention to the relative importance of renal and extrarenal mechanisms in the pathogenesis of hypertension. The long-term administration of initially subpressor doses of angiotensin II mimics the development of human hypertension to a greater extent than does the administration of pressor doses.