Question

You are a scientist studying the metabolism and excretion of a new drug called X-79S. Patients who take one dose of X-79S daily maintain average levels of 0.01 milligram (mg) of the drug per milliliter (ml) of their plasma. Since our kidneys filter about 100 ml of our plasma every minute, 1 mg of X-79S enters each patient's nephrons each minute through filtration.

When you test the patients' urine, you find that every hour (60 minutes) they are excreting 70 mg of X-79S. How do you explain these results based on what you know about the processes that go on in our nephrons?

Answer 1

The renal clearance of the drug is mainly regulated by glomerular filtration, active tubular secretion and tubular reabsorption.

• In the given case, in an hour, patients are excreting 70 mg of the drug.
• Going by the given calculation, each minute 100 ml of the plasma with 1 mg of the drug enters the nephrons. In an hour 60 mg of the drug.
• But they are excreting 70 mg. This shows active tubular secretion.
• The proximal tubules actively secrete the drug into the filtrate.
• Tubular reabsorption may be taking place reducing the extent of secretion, but it is not enough to bring the secretion to normal levels.
• If the drug can be bound to a carrier or hapen its filtration rate can be decreased.