Question 1

Routine neonatal testing suggests that a baby lacks a transport protein responsible for the uptake of cysteine in the intestines and kidneys.However,although cysteine accumulates in the urine,its concentration in the plasma is normal when the child receives a diet containing protein.Which intestinal transporter probably provides for uptake of cysteine in this child?

A)CFTR
B)Na⁺/K⁺/2Cl^- cotransporter,isoform 1 (NKCC1)
C)Epithelial sodium channel (ENaC)
D)Peptide transporter 1 (PepT1)
E)SLC6A19 gene

Accepted Answer

Peptide transporter 1 (PepT1) is responsible for the uptake of cysteine in the intestines. This transporter is able to transport dipeptides and tripeptides, including those containing cysteine, across the intestinal lining. This would explain why the child's cysteine concentration in plasma is normal when receiving a diet containing protein, as the protein would be broken down into dipeptides and tripeptides for absorption by PepT1.

Question 2

A patient with severe Crohn's disease undergoes resection of her terminal ileum.After recovering from the surgery,she develops chronic diarrhea that is treated effectively with cholestyramine (a resin that can bind bile acids).If her Crohn's disease has not recurred,the diarrhea in the patient can probably be attributed to loss of which of the following membrane proteins?

A)Sodium-glucose linked transporter 1 (SGLT1)
B)Cystic fibrosis transmembrane conductance regulator (CFTR)
C)Electrogenic Na⁺/HCO₃^- cotransporter 1 (NBC1)
D)Na⁺,K⁺-ATPase
E)Apical sodium-dependent bile acid transporter (ASBT)

Accepted Answer

The patient's chronic diarrhea is likely caused by increased levels of bile acids in the colon due to the loss of the Apical sodium-dependent bile acid transporter (ASBT) in the terminal ileum. Cholestyramine works by binding to bile acids in the colon, thus reducing their levels and improving the diarrhea. The other membrane proteins listed, including SGLT1, CFTR, NBC1, and Na,K-ATPase, are not directly involved in the transport of bile acids and would not be expected to cause chronic diarrhea in the absence of an underlying condition.

Deck 30: The Small Intestinal Phase of the Integrated Response to a Meal