The etiology of the N-terminus of the ANF prohormone in pleural fluid is unknown, but two possibilities are suggested by the present data. One possibility for the N-terminus of ANF prohormone being present in pleural fluid is that it is due to capillary leakage of this relatively small (10,000 mw) peptide into pleural fluid from capillaries or postcapillary venules in the lung. Under normal conditions, the rate at which water molecules diffuse through capillary membranes is approximately 80 times as great as the rate at which plasma itself flows linearly along the capillary. The relative permeability of 3,000 to 5,000 molecular weight peptides such as pro ANF 1-30 or ANF is 20 percent of that of water under normal conditions while a 10,000 mw peptide like the whole N-terminus of the ANF prohormone is only 1 percent as permeable as water. Albumin with a molecular weight of 69,000 is <0.0001 as permeable as water under normal conditions. With capillary leakage, medium-sized peptides such as albumin become freely permeable through capillaries, implying that smaller-sized peptides such as the N-terminus of the ANF prohormone should also be freely permeable as well. Although the ability of the lung to extract either the whole N-terminus or pro ANF 31-67 from the pulmonary circulation has not been investigated, the lung has been shown to remove ANF from the pulmonary circulation. With a single bolus injection of ANF, 67 percent of ANF is removed in a single passage through rabbit lungs as measured by indicator-dilution methods. buy asthma inhaler
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