Pulmonary fate of histamine, isoproterenol, physalaemin, and substance P

Abstract

The biological inactivation of four vasodilator agents during a single passage through the pulmonary circulation was studied by means of an assay based on systemic depressor responses. Isoproterenol was tested in humans undergoing cardiac catheterization and in anesthetized normal dogs. Histamine and physalaemin were tested in anesthetized normal rats and dogs; substance P was tested in rats. The degree of inactivation of these four vasodilator agents was either negligible or small, rarely exceeding 15%. It is concluded that the lung is not an important site for the removal of these substances from the circulation. The small degree of pulmonary inactivation of histamine suggests that it could play some role as a "circulating" hormone. Unlike bradykinin, the "tachykinins" physalaemin and substance P undergo little pulmonary inactivation. The absence of lung inactivation of isoproterenol explains its entry into the systemic circulation when given by inhalation.