Tubular secretion transfers molecules from peritubular capillaries to renal tubular lumen. Secretory mechanisms in the tubules are not drug specific, but depend on non-selective carrier systems. Two main systems are responsible for the active secretion of drugs which are either weak acids (anions) or weak bases (cations).
Tubular secretion is an active secretion process and it requires a carrier and a supply of energy. Because of this, the carriers can transport drug molecules against a concentration and electrochemical gradient, allowing concentrations of the drug in the plasma to be reduced to zero. This is a particularly important feature for protein bound drugs. As each free drug molecule is transported, the equilibrium between bound and unbound is disturbed and then re-established as more bound drug disassociates. The newly free drug is also then transported.
The carriers can also be saturated, although this usually only occurs in the situations involving drug-drug interactions where one drug competitively inhibits the secretion of another.