Ten-fifteen percent of all pregnancies are complicated by cardiovascular disorders. In half of this number (6-8%), maternal and fetal mortality and morbidity are threatened by the hypertension, proteinuria and pathological edema of pre-eclampsia. Studies are proposed that will further define mechanisms responsible for the control of atrial natriuretic factor (ANF) secretion, the actions of ANF on kidney, the actions of nitric oxide (NO) on the kidney, and both the short and long-term interaction of ANF and NO in the control of renal function, fluid and electrolyte balance, and arterial pressure. These studies will be conducted in vivo using non- pregnant and pregnant sheep, and then supplemented with a number of cellular and molecular approaches to further define mechanisms of action. We hypothesize that estrogen is involved in the enhancement of ANF secretion and that oxytocin may be a necessary intermediate factor in this response. The relationship between left atrial pressure and ANF will be observed in ovariectomized, non-pregnant sheep in the presence of estrogen, oxytocin, and oxytocin antagonism. The ability of estrogen to promote both expression and secretion of ANF will also be tested in culture. ANF exerts an enhanced natriuretic action on the kidney during pregnancy that cannot presently be ascribed to changes in either ANF receptors or metabolism. Since ANF and NO share a common second messenger, we hypothesize that increased NO may account for the enhanced natriuretic ability of ANF. This will be tested with intrarenal infusions of ANF challenged with intrarenal and intravenous infusions of the nitric oxide synthase inhibitor L-NAME to permit evaluation of purported direct and indirect, autonomically mediated, actions of NO that may be relevant to the increased sympathetic dependence of blood pressure during pregnancy. Alternatively, changes in ANF receptors or renal metabolism will be assessed in vitro using pure suspensions of glomeruli, proximal tubule, inner and outer medullary nephron to measure neutral endopeptidase activity and message for the different ANF receptor subtypes. During normal pregnancy, blood pressure is reduced possibly due to estrogen induced increases in NO. This suggests a shift in the renal function curve, but the interplay of NO, ANF and NaCl make the shape, i.e. degree of salt sensitivity, difficult to predict. Accordingly, renal function curves will be compared in non-pregnant sheep with various combinations of estrogen and L-NAME, and in pregnant sheep treated with L-NAME. Further studies will assess the long-term actions of elevated ANF in the presence of suppressed NO activity, as occurs in pre-eclampsia, to determine whether ANF is contributing to the severity of the pre-eclampsia-like condition is acting in a compensatory manner via its normal natriuretic actions. These studies should improve our understanding to the integrative responses of estrogens, ANF and NO during pregnancy.
