Atrial natriuretic factor in ovine pregnancy: plasma levels, molecular forms and biological activity

Abstract

Pregnancy is associated with hypervolemia and elevated plasma ANF, but the time course over which ANF increases and the mechanisms that control plasma ANF levels are unclear. Plasma ANF was measured in 12 nonpregnant and 16 pregnant sheep at various gestational ages. ANF was elevated at 30–35 days of pregnancy ( 15.7±1.4 vs. 22.9±3.4 fmol/ml , P=0.04), but did not increase further with the advance of gestation. Tissue content of ANF was unchanged in the right atrium, left atrium, renal cortex, renal medulla, adrenals and lungs, but ovarian ANF content was increased during pregnancy ( 9.2±2.2 vs. 67.2±23.2 fmol/mg protein, P=0.003). However, the ovarian tissue ANF concentration was less than 0.2% of that in the atria during pregnancy. HPLC of plasma from both nonpregnant and pregnant ewes revealed the presence of a single peak that elutes in parallel with synthetic human ANF. HPLC of atrial and lung tissue homogenates revealed multiple peaks that may represent different molecular forms of ANF. The biological activity of ANF in the plasma of pregnant ewes was reduced to 23% of nonpregnant levels. ANF in lung tissue was also biologically active, but that activity was reduced to 13% of nonpregnant levels. These data suggest that elevated plasma ANF in pregnancy is not secondary to increased atrial, renal, adrenal, ovarian or pulmonary contribution. Since we have previously shown that the metabolic clearance of ANF is not decreased, other extra-atrial sites may contribute to the increased plasma ANF during pregnancy. In addition, ovine pregnancy is associated with similar molecular forms of ANF but these exhibit reduced biological activity and may, therefore, alter volume regulation during pregnancy.