Abstract
The localization of Ca++-activated adenosine triphosphatase (Ca++-ATP-ase) activity was studied ultracytochemically in the transitional epithelium of the rabbit urinary bladder. The superficial cells contained two types of filaments, measuring 4-5nm and 6-10nm in diameter respectively. The former 4-5nm filaments are considered to correspond to actin filament, but the latter 6-10nm filaments seem to be composed of a little thinner, around 7nm, filaments and a little thicker, around 10nm, filaments. The thicker ones are located predominantly in the vicinity of the desmosomes and are inserted into them and are considered to be tonofilaments. The dense globular particles of the reaction products showing Ca++-ATPase activity were observed in the filament-rich field. These products lay superimposed on the filaments or immediately lateral to them. Tonofilaments were, however, not positive for the activity.
These results suggest that the 4-5nm and probably 7nm filaments also, but not 10nm tonofilaments, are the contractile proteins. The deposits of the reaction products may correspond to the site of the cross-bridge of the contraction system. Functionally these filaments might serve for the change of the cell shape during the contraction-distension cycles of the urinary bladder.
