Diurnal rhythm of prostanoid secretion from bone/marrow organ in the rat

The secretion of prostanoids from an adult diaphysial rat bone organ was assessed throughout a 24 h period at 4 h intervals as well as the 24 h activities of bone alkaline phosphatase isoenzyme and serum corticosterone levels. Femurs were removed from 16 rats at each interval and incubated in the absence or presence of indomethacin (100 μg). The levels of prostaglandin E2 (PGE2) prostaglandin F2α (PGF2α), and the stable metabolites of thromboxane and prostacyclin, thromboxane B2 (TxB2), and 6-keto prostaglandin F1α (6-keto PGF1α) were measured by radioimmunoassay (RIA). The patterns for each of the variables was subjected to cosinor analysis for predetermined various periods. The chronograms obtained indicated that PGF2α, PGE2, and TxB2 showed 24 h rhythms with computed peak hour acrophases at 1700, 1800, and 2200 h, respectively, and prostacyclin demonstrated a 19 h rhythm with a peak secretion at 1100 h. Corticosterone levels and bone alkaline phosphatase isoenzyme activity in the serum were at peak at 1630 and 2200 h, and at nadir at 0500 and 1000 h, respectively, both exhibiting 24 h rhythms similar to those of PGF2α, PGE2, and TxB2. Bone alkaline phosphatase isoenzyme activity in the femursʹ incubation media showed a 12 h diurnal rhythm with peaks at 1230 and 2330 h and nadirs at 0600 and 2330 h (p< 0.01). In summary, this study demonstrated for the first time a 24 h rhythm of prostanoid secretion from diaphysial bone. Except for prostacyclin, elevated secretion was noted at evening and night hours and preceded the rhythm of diaphysial bone metabolism in the rat as reflected by the rhythm of serum bone alkaline phosphatase isoenzyme. The diurnal changes in PGs secretion from the bone/marrow organ could be of clinical importance.