AJP - Regulatory, Integrative and Comparative Physiology, Vol 257, Issue 5 973-R981, Copyright © 1989 by American Physiological Society
Turtle urinary bladder: regulation of ion transport by dynamic changes in plasma membrane area
D. L. Stetson
Department of Zoology, Ohio State University, Columbus 43210.
Turtle urinary bladder possesses four ion transport processes: Na+
absorption, H+ secretion, and HCO3- secretion-Cl- absorption. Each
transport process is performed by a specific epithelial cell type. Granular
cells absorb Na+ but they are not sensitive to antidiuretic hormone (ADH),
unlike toad bladder granular cells. alpha-Carbonic anhydrase-rich (CA)
cells secrete H+ via an apical H+-adenosinetriphosphatase (ATPase). Under
conditions of low CO2 tension, this active pump is contained in the
limiting membranes of certain cytoplasmic vesicles. The vesicles fuse with
the apical membrane, and H+ pumps are incorporated into that membrane, as
physiological conditions demand increased H+ secretion. The stimulus for
fusion of these vesicles with the apical membrane appears to be
intracellular acidification. beta-CA cells secrete HCO3- and reabsorb Cl-,
both processes driven by H+-ATPase in the basolateral membrane in series
with an apical Cl- -HCO3- exchanger. Increased intracellular adenosine
3',5'-cyclic monophosphate concentration in beta-cells stimulates net HCO3-
secretion and induces an electrogenic component of this flux by activating
an apical Cl- channel. This activation accompanies the fusion of an
intracellular tubulovesicular network with the apical membrane. The
membrane of this network may contain Cl- channels.
