A great deal of research is being carried out on the mechanism of the "sodium pump" which maintains the differences across cell membranes of a relatively high concentration of potassium ions and a very low concentration of sodium ions inside the cell and the reverse in the fluid bathing the cells. Frog skin is an example of a tissue which has such pumps on only one side of the cells. The experiments in this program demonstrate the uptake of radioactive sodium (24Na) through the skin of a frog’s hindlegs, right side out and everted; a counter determines the amount of sodium taken up. The second experiment measures the electrical potential across the skin. The third experiment shows the effect of inhibiting the sodium transport on the transepithelial potential. The drug ouabain, which is a specific inhibitor of the enzyme Na+ + K+—ATPase (which constitutes the sodium pump) is added to both the outside and the inside of the skin and readings of the potential are taken. The fourth experiment illustrates the effect of a hormone produced in the frog’s pituitary gland. (15 minutes)