The influence of sodium intake on the pressor response to angiotensin II in the unanaesthetized rat.

1. Dose-response curves for the pressor activity of angiotensin II have been determined in unanaesthetized rats receiving diets containing 2-5% (w/w) or 0-007% (w/w) sodium; the different diets were administered in various sequences. 2. In comparison with those from rats receiving a low sodium diet, the dose-response curves were displaced to the left on the high sodium diet, indicating a greater response to angiotensin, and this displacement persisted for a period of approximately 7 days after the diet was changed from high to low sodium. The dose-response curve subsequently shifted to the right when the low sodium diet was maintained for longer. 3. There was a negative correlation between the slope of the dose-response curve and the basal blood pressure in all groups; the correlation was significant in three out of the five different treatment groups. 4. Basal blood pressures were significantly raised in rats on the high sodium diet for 7 days. 5. A number of possible mechanisms have been considered to explain both the parallel shift of the dose-response curve and alteration in its slope. It is concluded that the observed findings are compatible with an action of sodium-loading on the sensitivity of the smooth muscle cell to angiotensin, on the resting of the renin-angiotensin system, on the rate of inactivation of angiotensin and on a change in initial length of the muscle fibre.