The Site of the Stimulatory Action of Vasopressin on Sodium Transport in Toad Bladder

Abstract

Vasopressin increases the net transport of sodium across the isolated urinary bladder of the toad by increasing the mobility of sodium ion within the tissue. This change is reflected in a decreased De resistance of the bladder; identification of the permeability barrier which is affected localizes the site of action of vasopressin on sodium transport. Cells of the epithelial layer were impaled from the mucosal side with glass micropipettes while current pulses were passed through the bladder. The resulting voltage deflections across the bladder and between the micropipette and mucosal reference solution were proportional to the resistance across the entire bladder and across the mucosal or apical permeability barrier, respectively. The position of the exploring micropipette was not changed and vasopressin was added to the serosal medium. In 10 successful impalements, the apical permeability barrier contributed 54 % of the initial total transbladder resistance, but 98 % of the total resistance change following vasopressin occurred at this site. This finding provides direct evidence that vasopressin acts to increase ionic mobility selectively across the apical permeability barrier of the transporting cells of the toad bladder. I N T R O D U C T I O NWhen the urinary bladder of the toad, Bufo marinus, is bathed by solutions containing Na +, the addition of vasopressin regularly causes a stimulation in the active transport of Na + from the mucosal to the serosal medium (8, 9). A variety of experimental approaches has provided evidence that the hormonal effect occurs at a mobility barrier early in the transport process, perhaps at the mucosal or apical face of the epithelial cell, and does not directly affect the step in transport at which metabolic energy production is coupled to the process (2,5,6,11 ). The hormonal effect on transport of Na + is regularly associated with a decrease in the DC resistance of the bladder (2). The present study was undertaken in order to determine directly the site of action of vasopressin on transport of Na + by localizing the region within the epithelial cell where vasopressin has its characteristic effect upon the resistance.

The Kinetics of Sodium Transport in the Toad Bladder

“…Studies of the site of action of the hormone on sodium transport, on the other hand, have not been so definitive. It has been thought that the hormone affects the mucosal barrier to sodium Frazier and Hammer, 1963;Civan and Frazier, 1968), and the present work offers direct confirmation of this. As shown in Table I, there is, on the average, a 77% increase in the influx of sodium from the mucosal chamber into the transport pool.…”

“…Frazier (1962) has shown that there is a small electronegative well within the cells in the short-circuited toad bladder. Although the electrical potential profile is not known during short circuit following the addition of vasopressin, Civan and Frazier (1968) have shown that under open-circuit conditions there is little or no change in the potential across the mucosal barrier following the addition of the hormone. We can therefore postulate that however vasopressin acts to enhance entry of sodium, it does so without initially altering the electrochemical potential gradient for this ion.…”

“…There is considerable indirect evidence that a site of action of this substance is on the mucosal-facing side of the epithelial cells (Civan et al, 1966;Civan and Frazier, 1968;Frazier et al, 1962). There is also indirect evidence (Morel and Bastide, 1965;Finn, 1968;Lipton and Edelman, 1969) that vasopressin effects an increase in the activity of the sodium pump as well.…”

The effect of antidiuretic hormone on Na movement across frog skin

“…Curran, Herrera & Flanigan (1963) later developed a kinetic technique for the evaluation of the rate coefficient for 24Na movement across the individual barriers of the Na-transporting compartment, and demonstrated that ADH does in fact increase the Na permeability of the outer barrier. This conclusion can also be reached on the basis of the study of Na movement across urinary bladder (Leaf & Dempsey, 1960;Bastide & Jard, 1968) and is supported by the measurement of electrical parameters (Civan & Frazier, 1968). Yet the conclusion that ADH increases the flux of Na across the outer barrier does not rest on the direct measurement of this flux.…”

“…As discussed below, the entrance of Na into the epithelium across the outer barrier has always been regarded as a rate limiting step in the overall process of Na transport across epithelial membranes (Civan & Frazier, 1968;Curran & Gill, 1962;Curran et al 1963;Frazier, Dempsey & Leaf, 1962). Comparison of control data in Tables 1 and 2, though, indicates that JOT is larger than the transepithelial influx under all comparable conditions tested.…”