LOCALIZATION OF CA^<++>-ACTIVATED ADENOSINE TRIPHOSPHATASE ACTIVITIES IN THE TRANSITIONAL EPITHELIUM OF THE RABBIT URINARY BLADDER

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The localization of Ca^<++>-activated adenosine triphosphatase (Ca^<++>-ATPase) activity was studied ultracytochemically in the transitional epithelium of the rabbit urinary bladder. The superficial cells contained two types of filaments, measuring 4-5 nm and 6-10 nm in diameter respectively. The former 4-5 nm filaments are considered to correspond to actin filament, but the latter 6-10 nm filaments seem to be composed of a little thinner, around 7 nm, filaments and a little thicker, around 10 nm, 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 4-5 nm and probably 7 nm filaments also, but not 10 nm 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.
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