The Kinetics of Sodium Transport in the Toad Bladder

Abstract

A compartmental model of toad bladder sodium content has been developed, whereby it is possible to measure the four unidirectional fluxes across the opposite faces of the transport compartment, as well as the amount of sodium in the compartment. 24Na is added to the mucosal medium of a short-circuited bladder mounted between halves of a chamber in which the fluid is stirred by rotating impellers. After a steady state is reached, nonradioactive medium is flushed through both sides of the chamber, collected, and counted. The data from each chamber are fitted to sums of exponentials and interpreted in terms of conventional compartmental analysis. Three exponentials are required, with half-times of 0.2, 2.2, and 14.0 min. It is shown that the first of these represents chamber washout, the second the transport pool, and the third a tissue compartment which is not involved in active sodium transport and which does not communicate with the transport pool. The second compartment contains 10.5 #&Eq of sodium per 100 mg dry weight, an amount equal to approximately 30 % of total tissue sodium. The results also indicate, as expected from electrophysiological data, that the mucosal-facing side of the transport compartment is over 10 times as permeable to sodium as the serosal, or pump, side.The isolated urinary bladder of the toad, like other oriented multicellular membranes, carries out the net transport of sodium against an electrochemical potential gradient. It has been assumed that sodium must cross at least two barriers in order to traverse the bladder, one at the mucosal, the other at the serosal, boundary of the cells. In actual fact, satisfactory evidence for the presence of such a simple model has been difficult to obtain. Furthermore, there are at least four cell types which make up the epithelial layer of the bladder (Choi, 1963;Peachey and Rasmussen, 1961). Quantitative considerations make it likely that the most numerous of these, the granular layer, carries

The Kinetics of Sodium Transport in the Toad Bladder

“…Note that there is a significant increase in J,,, the influx from the mucosal medium into the transport pool, and ksl, the rate coefficient for the efflux from the pool to the serosal medium, the "pump rate constant." Values for the pool and the other unidirectional fluxes are not given, since calculation of these values, as shown in the accompanying paper (Finn and Rockoff, 1971), requires a steady state for total sodium. Since the short-circuit current is not stable during the wash period (as the effect of vasopressin is waning despite the continued presence of the hormone) this may not be a valid assumption.…”

“…One may therefore calculate the relative amount of isotope in the pool by comparing the computer-determined values of P 1 (0) with the amount of tracer present in the loading solution, PM(O). Definitions and symbols are as in the accompanying paper (Finn and Rockoff, 1971), and are shown in Fig. 1.…”

“…The toads were pithed, the bladder excised and mounted in the chamber as previously described (Finn and Rockoff, 1971). Following the establishment of a constant short-circuit current, Na was added to the mucosal medium, and allowed to remain there until a steady state for tracer was obtained.…”

“…Therefore, the effect of vasopressin in increasing both the pool size and the pump rate coefficient must be construed as a direct effect on the pump, and not one which is secondary to the increase in the pool size. Furthermore, it is shown that the effect of the hormone on the sodium pump is not dependent on the presence of sodium in the serosal medium.In the accompanying paper (Finn and Rockoff, 1971) it was shown that it is possible to represent toad bladder sodium content by a compartmental model. It was found that a system containing two noncommunicating tissue compartments satisfied the data obtained from the washout of radioactive sodium, and that one of these compartments was quite likely to be the transport pool.…”

“…In the accompanying paper (Finn and Rockoff, 1971) it was shown that it is possible to represent toad bladder sodium content by a compartmental model. It was found that a system containing two noncommunicating tissue compartments satisfied the data obtained from the washout of radioactive sodium, and that one of these compartments was quite likely to be the transport pool.…”