The majority of drugs do not enter the kidney tubule by glomerular filtration but do so by tubule secretion. This is an active process since drugs are carried against a favourable chemical gradient from the capillary network into the tubule.
Tubule secretion involves two carrier systems – basic carriers which transport basic drugs (amiloride, dopamine, histamine), and acidic carriers for acidic drugs (frusemide, penicillin, indomethacin).
The process of tubule secretion can have a big impact on the speed that a drug is eliminated from the body. For example, penicillin readily enters the nephron by tubule secretion and is rapidly excreted from the body in the urine. In a situation where the therapeutic effect needs to be prolonged, agents can be administered that block tubule secretion to slow the excretion of the drug.