Abstract
The effect of monensin on endocytosis, transcytosis, recycling and transport to the Golgi apparatus in filter‐grown Madin‐Darby canine kidney (MDCK) cells was investigated using ^125^I‐labeled ricin as a marker for membrane transport, and horseradish peroxidase (HRP) as a marker for fluid phase transport. Monensin (10 μM) stimulated transcytosis of both markers about 3‐fold in the basolateral to apical direction. Transcytosis of HRP in the opposite direction, apical to basolateral, was reduced to approximately 50% of the control by monensin, whereas that of ricin was slightly increased. Recycling of markers endocytosed from the apical surface was reduced in the presence of monensin and there was an increased accumulation of both ricin and HRP in the cells. Transport of ricin to the Golgi apparatus increased to the same extent as the increase in intracellular accumulation. No change in recycling or accumulation was observed with monensin when the markers were added basolaterally, but transport of ricin to the Golgi apparatus increased almost 3‐fold. Our results indicate that basolateral to apical transcytosis is increased in the absence of low endosomal pH, and they suggest that apical to basolateral transcytosis of a membrane‐bound marker (ricin) is affected by monensin differently from that of a fluid phase marker (HRP).