Evidence for cycling of aggregate‐containing tubules in toad urinary bladder

Antidiuretic hormone (ADH) promotes the fusion of cytoplasmic tubular structures with the luminal membrane of receptor tissues such as toad urinary bladder. To determine whether fusion is a continuous cyclic process, bladders were stimulated with ADH with colloidal gold in the luminal bathing medium. After as little as 15 min of stimulation, gold-filled tubules were seen in the cytoplasm, evidence that cycling was indeed taking place. Serial sections confirmed that these tubules had no connection with the luminal membrane, and had returned to the cytoplasm. Cessation of ADH stimulation, followed by a second stimulation, greatly reduced the number of gold-filled cytoplasmic tubules, suggesting that many tubules were capable of refusion. Mean fusion event diameter underwent significant changes, enlarging at 15 min, and contracting at 60 min. Thus, ADH initiates a process of continuous cycling of cytoplasmic tubules between cytoplasm and luminal membrane.