Effect of histamine on tracheobronchial afferent and caudal mediastinal efferent lymph

We prepared nine sheep with acute tracheobronchial afferent (TBN) and caudal mediastinal efferent (CMN) lymph fistulas. After a baseline period (B) in 3 sheep, we administered histamine (H) continuously for 4 h. In six sheep, we elevated left atrial pressure (PLA) and reestablished steady-state conditions prior to H administration. The afferent lymph to plasma protein concentration ratio (CA/CP) was significantly lower than the efferent ratio (CE/CP) during periods of B, H, elevated PLA, and elevated PLA with H. H administration increased lymph flow rates (QlA and QlE) and both CA/CP and CE/CP, albeit insignificantly. During elevated PLA, QlA and QlE increased, while CA/CP and CE/CP fell. QlE increased, while QlA did not change during elevated PLA with H. CE/CP increased from its PLA level. CA/CP did not increase. Afferent data suggest that histamine may increase pulmonary microvascular surface area, but does not alter the permeability of the pulmonary circulation. While we cannot exclude the possibility of an increase in pulmonary microvascular permeability from efferent results, the difference between TBN afferent and CMN efferent results likely represent the action of histamine at the CMN.