Shrinkage of dog red blood cells (RBC) activates a Na transport pathway that is CI dependent, amiloride sensitive, and capable of conducting Na-proton counterflow. It is possible to establish transmembrane gradients for either Na or protons and to demonstrate that each cation species can drive reciprocal movements of the other. The nature of the coupling between Na and proton movements was investigated using the fluorescent probe diS-C3(5) and also by an indirect method in which K movements through valinomycin channels were used to draw inferences about the membrane potential. No evidence was found to suggest that the Na-proton pathway activated by shrinkage of dog RBC is a conductive one. By exclusion, it is presumed that the coupling between the counterflow of Na and protons is electroneutral . The volume-activated Na-proton fluxes in dog RBC have certain properties that distinguish them from similar transport pathways in other cell types.

The reabsorption of albumin from the pleural space was measured in eight dogs receiving 0.5 ml intrapleural injection of 131I-labeled albumin and a simultaneous intravenous injection of 125I-labeled albumin. Plasma curves for both tracers were obtained over 24 h. The 125I-albumin curve served as input function of albumin for interstitial spaces, including pleura, whereas the 131I-albumin curve represented the output function from pleural space. The frequency function of albumin transit times from pleural space to plasma was obtained by deconvolution of input-output plasma curves. Plasma recovery of 131I-albumin was complete by 24 h, and the mean transit time from pleura to plasma averaged 7.95 +/- 1.57 (SD) h. Albumin reabsorption occurred mainly via lymphatics as indicated by experiments in 16 additional dogs in which their right lymph ducts or thoracic ducts were ligated before intrapleural injection. A pleural lymph flow of 0.020 +/- 0.003 (SD) ml.kg-1.h-1 was estimated, which is balanced by a comparable filtration of fluid into the pleural space. This suggests that, under physiological conditions, the subpleural lymphatics represent an important control mechanism of pleural liquid pressure.

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