REDUCTION OF RYANODINE BINDING AND CYTOSOLIC CA2+ LEVELS IN LIVER BY THE IMMUNOSUPPRESSANT FK506

The mechanism of action of the immunosuppressant FK506 in the liver was studied. The hypothesis was tested that FK506 exerts its effect in the liver by interacting with the ryanodine-binding Ca2+ release channel. Two types of experiments were carried out: (1) [H-3]-ryanodine binding studies with isolated microsomal fractions, and (2) cytosolic-free Ca2+ ([Ca2+](i)) measurements with the intracellular Ca2+-indicator fura-2. The inclusion of FK506 in the incubation medium significantly decreased the binding of [H-3]-ryanodine to liver microsomes. The B-max of binding in control experiments was 405 fmol/mg protein; the presence of FK506 decreased the B-max to 157 fmol/mg protein. Measurements of [Ca2+](i) in the presence and absence of FK506 showed a decrease in [Ca2+](i) in the presence of FK506. The data support the notion that FK506 interacts with the ryanodine binding Ca2+ channel in the liver and suggest a critical role for the ryanodine-binding Ca2+ channel in the hepatic responses to FK506. The interaction between FKBP-12 (FK506 binding protein) and the ryanodine-binding Ca2+ channel may be an essential link in the chain of events by which FK506 alters Ca2+-dependent cellular processes.