Alveolar epithelial permeability in baboons: histamine and capsaicin

1. We investigated if changes in alveolar epithelial permeability could be initiated by various doses of the irritant capsaicin through stimulation of pulmonary afferent nerves either directly, or indirectly through the release of an intermediate cell mediator, and whether mediation of permeability involved histamine H1 and/or H2 receptors. 2. Alveolar epithelial permeability was indicated by the clearance of peripherally deposited 99mTc-DTPA (technetium-99m-diethylenetriamine pentaacetate) aerosol in baboons. Aerosol challenge experiments were performed twice with histamine (32 mg/ml), once each with capsaicin (10(-6), 10(-5), 10(-4) and 10(-3) M), and once each with histamine (32 mg/ml) preceded by H1 (terfenadine) and H2 (ranitidine) antagonists alone and combined. 3. Mean half-time for 99mTc-DTPA clearance was 66 +/- 4 min. After histamine, it decreased to 41 +/- 5 min (P < 0.05), and after capsaicin it was 70 +/- 5 min (combined doses). Pretreatment with the H1 and H2 antagonists, either separately or in combination, did not consistently inhibit increases in 99mTc-DTPA permeability. 4. Capsaicin caused tachypnoea only at 10(-3) M (P < 0.05). Inhibition of histamine-induced tachypnoea required both antagonists (P < 0.05) suggesting the existence of a population of peripheral sensory neurons possessing H1 and H2 receptors. Changes in permeability were unrelated to the changes in respiratory frequency. Thus, neither the mechanisms that stimulate respiratory frequency, nor the respiratory frequency per se, were responsible for increased alveolar epithelial permeability. 5. Capsaicin did not change epithelial permeability at doses that cause capsaicin-sensitive C fibre stimulation as delineated by increases in respiratory frequency. Nor does C fibre stimulation, induced by mediators released from pulmonary cells activated by low doses of capsaicin, increase epithelial permeability. These data suggest that in the primate, histamine increases epithelial permeability to small solutes by a mechanism independent of the activation of capsaicin-sensitive C fibres and capsaicin-initiated release of cell mediators. 6. These findings do not support a role for C fibre activation in regulating epithelial permeability of small solutes.