Effects of the circadian variation in serum cortisol on markers of bone turnover and calcium homeostasis in normal postmenopausal women

Bone turnover has a circadian pattern, with bone resorption and, to a lesser extent, bone formation increasing at night. Serum cortisol also has a circadian pattern and is a potential candidate for mediating the circadian changes in bone turnover. Thus, we measured bone formation and resorption markers before (study A) and after (study B) elimination of the morning peak of cortisol. We also assessed effects of the circadian cortisol. pattern on serum calcium, PTH, and urinary calcium excretion. Ten normal postmenopausal women, aged 63-75 yr (mean, 69 yr), were studied. Metyrapone was administered to block endogenous cortisol synthesis and either a variable (study A) or a constant (study B)infusion of cortisol was given to reproduce and then abolish the morning cortisol peak. Blood was sampled every 2 h for serum cortisol, ionized calcium, PTH, and bone formation markers [osteocalcin and carboxyl-terminal propeptide of type I collagen (PICP)], and timed 4-h urine samples were collected for measurement of calcium, phosphorus, sodium, potassium, and bone resorption markers (N-telopeptide of type I collagen and free deoxypyridinoline). During study A, serum osteocalcin had a circadian pattern, with a peak at 0400 h and a nadir at 1400 h. During study B, however, the afternoon nadir of serum osteocalcin was eliminated (P < 0.001 and P < 0.005 for the difference in the patterns of peak and nadir, respectively, on the 2 study days). In contrast, the circadian patterns of serum PICP and urinary N-telopeptide of type I collagen and free deoxypyridinoline were virtually identical during the two studies. Urinary calcium excretion declined after the cortisol peak, without differences between the 2 study days in phosphorus or sodium excretion or in serum PTH. We conclude that the circadian variation in serum cortisol is responsible for the circadian pattern of serum osteocalcin, but not that of PICP or bone resorption markers. The physiological variation in serum cortisol may also reduce urinary calcium excretion.