Effects of furosemide and acute salt loading on vasopressin and renin secretion in the fetal lamb

Circulating arginine vasopressin (AVP) and plasma renin activity responses to furosemide (2 mg/kg) and acute hypertonic saline (10 mEq/kg) were studied in the fetal lamb from 100 days gestation to term. The baseline to peak plasma AVP response (delta 3.7 +/- 1.2 uU/ml) and area under the response curve (209 +/- 57 uU/ml/65 min) in the fetal lambs > 123 days were greater than in those < 106 days gestation (delta 1.8 +/0 1.1 and (171 +/- 61, respectively), P < 0.02. The plasma renin activity/AVP ratio after furosemide was similar in the two gestational groups. The log plasma AVP responses corrected for rise in plasma osmolality (0.090 +/- .01 uU/ml) 30 min after infusion, and the area under the response curve (253 +/- 49 uU/ml/30 min) was greater (P < 0.02) in the fetal lambs > 120 days than in those under 115 days gestation (.035 +/0 0.01 and 88 +/0 29, respectively), P < 0.02. These results confirm that the fetal lamb responds to an osmotic stimulus with increased plasma AVP levels and documents that this response significantly matures during the last trimester of gestation. The fetal lamb also manifests a hypothalamus-posterior pituitary AVP response to furosemide that is proportional to the maturing renal renin response.