The overall objective of this proposal is to clarify physiological interactions between atrial natriuretic peptide (ANP) and the lung. Our first hypothesis is that ANP modulates pulmonary hypertensive responses, both acute and chronically. To explore this, we will measure plasma ANP levels by RIA in venous, pulmonary arterial (PA), and systemic arterial blood during acute exposures to hypoxia, angiotensin-II or phenylephrine in awake, chronically instrumented rats. We will then attempt to inhibit the action of ANP using ANP antibodies and to augment the action of ANP using inhibitors of ANP metabolism in an attempt to modify the physiologic response. We will perform similar studies in rats with chronic pulmonary hypertension induced by chronic hypoxia and monocrotaline injection, and in a strain of rat that develops spontaneous pulmonary hypertension. We will assay plasma ANP levels in venous and PA blood during the development of pulmonary hypertension, and tissue ANP levels in right and left atria and ventricles and in lung following development of pulmonary hypertension. We will also assay mRNA levels for ANP in these tissues and will determine the effects of a chronic infusion of ANP on the severity of pulmonary hypertension. Based upon preliminary observations in cell culture systems demonstrating stimulatory effects of ANP on elastin production and inhibitory effects on smooth muscle cell proliferation, we also hypothesize that ANP modulates the degree of vascular remodeling in pulmonary hypertension. We will determine dose response relationships of ANP effects on connective tissue and DNA synthesis and mRNA levels for elastic and collagen in rat PA and thoracic aortic vessel explants. We will also attempt to obtain in vivo correlations for these observations by assessing changes in mRNA levels for elastin and collagen and tissue levels for elastin and collagen in pulmonary and aortic vessels and in lung tissue of nonmoxic and chronically hypoxic rats following intravenous infusions of exogenous ANP. A third hypothesis is that the lung plays an important role in the clearance and metabolism of circulating ANP. We will determine specificity of uptake of ANP in isolated rat lungs and the proportion of binding sites that are clearance (or C) receptors using a specific C-receptor ligand. We will also examine lung effluent and homogenates following injection of ANP for metabolites using reverse phase HPLC, and will determine whether acute or chronic hypoxia affects uptake. Finally, we will determine the location of specific ANP binding sites in the lung using electron microscopy and autoradiography. By exploring the above interactions of ANP and the lung, we aim to gain insight into how this recently discovered peptide hormone participates in the physiologic regulation of pulmonary hypertensive responses. Such insights may lead to new approaches to pharmacotherapy of pulmonary hypertension.
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