EFFECTS OF ANGIOTENSIN-II ON PLASMA ATRIAL-NATRIURETIC-FACTOR IN NONPREGNANT AND PREGNANT EWES

The release of atrial natriuretic factor (ANF) is primarily determined by atrial stretch, but may also be modulated by circulating angiotens in II (AngII). During pregnancy, the circulating concentrations of bot h ANF and AngII are increased. To further define possible effects of A ngII on ANF release, four doses of AngII (0.5, 5, 20, 40 ng kg(-1) min (-1)) were intravenously infused into five nonpregnant and five pregna nt (105 - 140 days of gestation) ewes alone and during the simultaneou s infusion of sodium nitroprusside at doses sufficient to abolish the presser effects of AngII. Mean arterial pressure (MAP) was increased f rom 80 +/- 2 to a maximum of 121 +/- 5 mmHg (1 mmHg = 133.3 Pa) in non pregnant ewes (p < 0.01) and from 79 +/- 2 to 116 +/- 4 mmHg in pregna nt ewes (p < 0.01) over the range of AngII infusion. MAP was unaltered during AngII plus nitroprusside infusion, averaging 78 +/- 3 mmHg in nonpregnant ewes and 80 +/- 2 mmHg in pregnant ewes. Basal ANF was hig her (p < 0.01) in pregnant sheep than in nonpregnant sheep. With AngII infusion alone, plasma ANF was increased from 13 +/- 2 to 42 +/- 4 fm ol/mL in nonpregnant ewes (p < 0.01) and from 23 +/- 5 to 72 +/- 16 fm ol/ml in pregnant ewes (p < 0.01). However, during AngII plus nitropru sside infusion, the increases in plasma ANF observed were completely a bolished in both nonpregnant and pregnant ewes. These results suggest that AngII does not have a direct stimulatory effect on ANF release bu t rather that the increase in plasma ANF is a consequence of the press er effects of AngII. This response does not appear to be altered durin g ovine pregnancy.