Ion and Water Transport by the Teleost Urinary Bladder

During the past year our efforts to understand the mechanism of salt and water transport by the urinary bladder of the sea water and fresh water acclimated starry flounder, Platichthys stellatus, have continued. These investigations have been extended to the urinary bladder of the fresh water rainbow trout, Salmo gairdneri, as well. Isotopic tracer techniques in conjunction with electrophysiological methods have been used to study active transport and passive permeability of the bladders to ions (Na , K and Cl-) and nonelectrolytes (non-metal). These have been compared to the fluid transport rates and passive water permeabilities assessed using gravimetric techniques. The results indicate that bladders from both sea water and fresh water acclimated flounder, as well as the trout, actively transport sodium and chloride ions which results in fluid transport. The coupling of fluid transport to ion transport is seen in many other epithelial membranes. Fresh water flounder urinary bladders transport sodium chloride at about twice the rate observed in sea water bladders while they transport fluid at only about one-half the rate observed in sea water bladders. Curiously the urinary bladders of both of the species of fish which we have examined exhibit a negative correlation between their passive permeabilities to ions and to water. The opposite is usually seen in epithelial membranes. Since the epithelial membranes which form the lining of the urinary bladders are composed of a single layer of cells joined together at their sides, there are two possible routes for the movement of ions and water from one side of the bladder to the other, i.e., through the cells or between the cells. A major thrust of the efforts of this study continues to be a determination of the relative importance of these two pathways in determining the transport properties of the epithelium. Our results indicate that in spite of their relatively large passive permeabilities to ions, which in other epithelia indicate that ions move between cells, most of the ion permeation across the fresh water trout and flounder urinary bladders is through the cells. We have also been examining the relationship between the transport of sodium and potassium in order to determine the role of a specific transport enzyme (Na-K activated ATPase) in the active sodium transport across the urinary bladder. A variety of techniques have been employed including the use of pore forming antibiotics and intracellular microelectrodes. Our results indicate that, unlike other transport epithelia which have been examined, the active transport