The pressor responsiveness to angiotensin II and norepinephrine was examined in rats before and during blockade of converting enzyme activity with the nonapeptide SQ 20881. Responses to angiotensin II were impaired by sodium deprivation but enhanced by sodium loading or bilateral nephrectomy. During the period of converting enzyme blockade, a twofold increase in the angiotensin II pressor response was observed in the salt-restricted rats, whereas only a small change occurred in the salt-loaded rats. Infusion of the inhibitor produced a profound fall in the blood pressure of the salt-depleted rats with a relatively minor fall in the sodium-loaded rats. Norepinephrine pressor responses were slightly potentiated in the salt-restricted rats after administration of SQ 20881, but no change occurred in the salt-loaded or the nephrectomized rats. These observations support the view that the decrease in angiotensin II pressor activity during salt deprivation is the result of a prior occupancy of receptor sites by endogenous hormone. Therefore, a change in the number or the affinity of receptors consequent to changes in sodium balance need not be postulated to explain the phenomenon.
KEY WORDS pressor responsesodium depletion and loading blood pressure norepinephrine coverting enzyme blockade bilateral nephrectomy• Most studies of the renin-angiotensin system depend on measurements of the circulating levels of renin and angiotensin II. A reciprocal relationship between plasma renin activity and sodium balance has been well documented (1, 2). Thus, sodium depletion stimulates renin secretion, and sodium loading suppresses renin release. However, under circumstances of body fluid volume depletion, during which increased renin levels are found, pressor reactivity to exogenous angiotensin becomes markedly impaired (3-7). Conversely, when sodium is given in excess, especially if a mineralocorticoid is also administered, the pressor response to angiotensin infusion is enhanced (6). Thus, angiotensin pressor reactivity appears to correlate directly with sodium balance but in-