Control of sodium excretion in NE-ACTH hypertension: role of pressure natriuresis.

The purpose of this study was to test the hypothesis that, in the presence of high circulating catecholamines, ACTH decreases renal excretory capability and that its natriuretic effects are caused by increased renal arterial pressure (RAP). In six conscious dogs, norepinephrine (NE, 0.4 micrograms.kg-1.min-1) alone for 5 days caused a small but significant rise in arterial pressure (AP) from 102 +/- 6 to 115 +/- 8 mmHg. An infusion of ACTH (600 micrograms/day) for 7 days, superimposed upon the NE, caused a further rise in AP to a plateau of 143 +/- 9 mmHg after 5 days while cumulative sodium balance fell to -149 +/- 41 meq by the 7th day. Cumulative water balance fell to -660 +/- 232 ml by the 3rd day and then increased slightly. In contrast, when ACTH infusion was repeated during NE infusion while RAP was prevented from increasing using a servo-controlled aortic occluder, cumulative sodium balance increased to 197 +/- 35 meq and AP rose from 108 +/- 5 to 168 +/- 5 mmHg after 7 days and did not plateau. Cumulative water balance rose to 2,325 +/- 445 ml. Thus, in dogs receiving a background infusion of NE, ACTH causes moderate hypertension and natriuresis. However, when RAP is not allowed to rise, ACTH is associated with sodium retention and severe systemic hypertension, suggesting that the natriuretic effects of ACTH are caused by increased RAP and that the natriuresis blunts the chronic hypertensive effects of ACTH.