ADH-INDUCED RECYCLING OF FLUID-PHASE MARKER FROM ENDOSOMES TO THE MUCOSAL SURFACE IN TOAD BLADDER

In the toad urinary bladder, the reversal of antidiuretic hormone (ADH ) stimulation results in the endocytosis of apical membrane water chan nels, along with any fluid-phase marker present in the mucosal bathing solution. We have loaded vesicles with horseradish peroxidase (HRP), then restimulated the bladders and measured the reappearance of endocy tosed HRP in the mucosal bath. HRP-loaded bladders that were restimula ted showed HRP release that peaked sharply within 15 min after restimu lation. Varying the interval between loading and restimulation did not vary HRP release significantly. Restimulation with forskolin gave HRP release values similar to ADH. The amount of HRP released correlated with the magnitude of water permeability induced. The demonstration th at fluid-phase markers can be recycled from endosomes to the apical su rface in a hormone-dependent fashion indicates that endocytosed membra ne, containing water channels, is able to rapidly recycle back to the surface in response to hormone restimulation. In addition, marker rele ase declined progressively with repeated restimulation, totaling < 30% of the retrieved amount. This result indicates that a relatively larg e proportion of the retrieved marker reaches a nonrecycling compartmen t.