Alteration of the circadian rhythm of intact parathyroid hormone following a 96-hour fast.

Abstract

PTH(1-84) secretion in normal male subjects follows a circadian rhythm. The control of this rhythm is multifactorial with both neuroendocrine and chemical influences. The aim of this study was to assess the effect of a 96-hour fast on the circadian rhythm of PTH(1-84), serum calcium, phosphate and nephrogenous cAMP (NcAMP), an index of PTH(1-84) bioactivity. Blood samples for estimation of all analytes were obtained over a 24-hour period at 30-minute intervals. Urine samples were obtained 4 hourly during the daytime and overnight. Each subject was studied on two occasions after being randomized to either (a) normal hospital diet or (b) a 96-hour fast with water freely available. Six healthy adult males aged between 28 and 40 years, mean 32 years. PTH(1-84) was measured by an in-house immunoradiometric assay. Serum calcium, phosphate, albumin, creatinine and urinary creatinine were measured by standard automated techniques. Calcium was adjusted for albumin. Plasma cAMP was estimated by a commercial method and urine cAMP by in-house radioimmunoassay and NcAMP obtained by calculation. Rhythm parameters were analysed by cosinor techniques. There were alterations in the circadian rhythms of serum phosphate, PTH(1-84) and NcAMP following a 96-hour fast. Fasting abolished the nocturnal rise in phosphate, PTH(1-84) and NcAMP but had little effect on the pattern of adjusted calcium over a 24-hour period. The mean concentrations of serum phosphate, adjusted calcium and NcAMP decreased significantly following the fast and mean PTH(1-84) increased during day time. Fasting for 96 hours significantly alters the circadian rhythm of PTH(1-84) secretion by lowering the mean calcium concentration and attenuating the circadian rhythm of serum phosphate.