24-h monitoring of calcineurin phosphatase activity in healthy subjects.

The calcineurin inhibitors cyclosporine and tacrolimus are the cornerstone immunosuppressants used in solid organ transplantation. Studies investigating calcineurin (CaN) activity in renal transplanted patients have been published, but basic properties of the enzyme activity in healthy subjects remain to be described. The aim of this study was to investigate whether CaN displays circadian variation or sex difference is present in healthy subjects. Twenty subjects had blood samples drawn every 4 h for a 24-h period. CaN activity was determined in whole blood as the release of 32P from a phosphorylated peptide. Activity of the 32P was quantitated by liquid scintillation and results converted to units CaN utilizing a calibration curve. We found no circadian variation in CaN activity and no difference between the two sexes. The clinical importance of these findings is that blood samples for calcineurin activity can be drawn without taking the exact time of day into consideration, but only considering the time of drug intake.