Effect of Transport Maximum on Excretion

by Thad Wilson, PhD

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    So I talked a lot about transporters. Now, what governs the transport? Well, a transporter usually has an inherent maximal range, and that means they can only transport X amount of solute per given amount of time. So for something like glucose, they can transport about 375 milligrams per minute, and that is done by the SGLT1 and SGLT2. However, if you have glucose levels that are above 375, such as a diabetic who is in a hyperglycemic episode. They may have a blood glucose of, let’s say, 400. In that case, you will always have spillover of glucose into the urine because its transport maximum is this given amount. Now, many other substances have transport maximums. It’s not just glucose. Amino acids do, proteins do, phosphate does, as well as some of the other transporters that we don’t talk about in as much detail, such as the OATP transporters that will transport drugs across the apical and basolateral membrane. If we want to think about excretion and reabsorption, how would be best to process this information? Well, we use this from a transport maximum point of view. You can only reabsorb the amount of substance based upon its transport maximum. So if we would deliver a freely filtered molecule that is one that is filtered through the glomerular barrier into the tubule lumen space, what is it dependent on? Well, it depends on the plasma concentration of that substance. So we have that now on our X-axis. On our Y-axis, we’re going to have the amount of concentration that is either reabsorbed, filtered, or excreted. The higher the plasma concentration of a molecule that’s freely filtered, will increase its concentration either in the urine or filtered. For reabsorption purposes, the transport maximum governs how high reabsorption can occur. So...

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    The lecture Effect of Transport Maximum on Excretion by Thad Wilson, PhD is from the course Renal Physiology.

    Author of lecture Effect of Transport Maximum on Excretion

     Thad Wilson, PhD

    Thad Wilson, PhD

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