Problem
Question I: Using what you have learned about epithelial transport, draw a picture of the salivary duct cells and lumen. Arrange the following membrane channels and transporters on the apical and basolateral membranes so that the duct cell absorbs Na+ and secretes K+: ENaC, Na+-K+-ATPase, and K+ leak channel. With neural stimulation, the flow rate of saliva can increase from 0.4 mL/min to 2 mL/min. What do you think happens to the Na+ and K+ content of saliva at the higher flow rate?
Question II: Intestinal transport of the amino acid analog MIT (monoiodotyrosine) can be studied using the "everted sac" preparation. A length of the intestine is turned inside out, filled with a solution containing MIT, tied at both ends, and then placed in a bath containing nutrients, salts, and an equal concentration of MIT. Changes in the concentration of MIT are monitored in the bath (mucosal or apical side of the inverted intestine), in the intestinal cells (tissue), and within the sac (serosal or basolateral side of the intestine) over a 240-minute period. The results are displayed in the graph shown here. (Data from Nathans et al., Biochimica et Biophysica Acta 41:271-282, 1960.)
1. Based on the data shown, is the transepithelial transport of MIT a passive process or an active process?
2. Which way does MIT move: (i) apical to tissue to basolateral, or (ii) basolateral to tissue to apical? Is this movement absorption or secretion?
3. Is transport across the apical membrane active or passive? Explain your reasoning.Is transport across the basolateral membrane active or passive? Explain your reasoning.