Acyclovir (topical, oral and parentral }

SOLUTE TRANSPORT 

Transport proteins in the cell membranes of the nephron mediate the reabsorption and secretion of solutes and water transport in the kidneys. Acquired defects in transport proteins are the cause of 

many kidney diseases. In addition the transport proteins are important drug targets.

Transport Mechanisms

Simple diffusion

 • Net movement represents molecules or ions moving down their electrochemical 

gradient.

 • This doesn’t require energy.

Facilitated diffusion (facilitated transport) •

 A molecule or ion moving across a membrane down its concentration gradient attached to a specific 

membrane-bound protein. • This doesn’t require energy.

Active transport •

 A protein-mediated transport that uses ATP as a source of energy to move a molecule or ion against its electrochemical gradient.

Dynamics of Protein-Mediated 

TransportUniport

• Transporter moves a single molecule or ion as in the uptake of glucose into skeletal muscle or adipose tissue. This is an example of facilitated diffusion.

Symport (cotransport) 

• A coupled protein transport of 2 or more solutes in the same direction as in Na-glucose, Na-amino acid transporters.

Antiport (countertransport) 

• A coupled protein transport of 2 or more solutes in the opposite direction. Generally, protein carriers transport substances that cannot readily diffuse across a membrane. There are no transporters for gases and most lipid-soluble substances because those substances readily move across membranes by simple diffusion.

Characteristics common to all protein-mediated transport

Rate of transport A substance is transported more rapidly than it would be by diffusion, because the membrane is not usually permeable to any substance for which there is a transport protein.

Saturation kinetics • As the concentration of the substance initially increases on one side of the membrane, the transport rate increases. • Once the transporters become saturated, transport rate is maximal  (TM = transport maximum). Rate of transport is dependent upon: – Concentration of solute – Number of functioning transporters; the only way to increase TM is to add more protein carriers to the membrane • Once all the protein carriers are saturated, the solutes are transported across the membrane at a constant rate. This constant rate is TM. • There is no TM is simple diffusion.

Chemical specifi ity To be transported, the substance must have a certain chemical structure. Generally, only the natural isomer is transported (e.g., D-glucose but not L-glucose).

Competition for carrier Substances of similar chemical structure may compete for the same transporter. For example, glucose and galactose generally compete for the same transport protein.

Primary and secondary transport

 • In primary active transport, ATP is consumed directly by the transporting protein, (e.g., the Na/K-ATPase pump, or the calcium-dependent ATPase of the sarcoplasmic reticulum).

 • Secondary active transport depends indirectly on ATP as a source of energy, as in the cotransport of Na-glucose in the proximal tubule. This process depends on ATP utilized by the Na/K-ATPase pump. • Glucose moves up a concentration gradient via secondary active transport.