Circadian oscillations of calcium, sodium and potassium in chick serum, ultrafiltered serum and pineal gland

Previous studies showed that calcium in rat serum exhibited increases when rats were acclimated to a constant light cycle. The authors suggested that melatonin could exert a feedback regulation on serum cation levels. This study examines the circadian changes in pineal and plasma cations and melatonin of chicks acclimated to three different light cycles (12L : 12D, 24L : L0D and 0L : 24D). Serum was used to measure total cation concentrations (Ca, Na and K) and serum was ultrafiltered to assess free cation concentrations; similar measurements were done on homogenized pineal glands of these birds. Melatonin levels were also quantitated by a radioimmunoassay in all samples. The results show that all studied cations exhibited a circadian cycle, with the characteristic parameters being determined by cosinor analysis. When animals were acclimated to a 12L : 12D cycle, calcium exhibited high concentrations during the entire light phase, while sodium and potassium levels increased only during the last hour of the light phase. Primary oscillations in cation levels were seen with the free cation fractions; free cations are biologically active. The cations showed no variations over the light:dark cycle when cation-bound-protein fractions were measured. In ultrafiltered fractions, calcium exhibited variations in their profile when the light:dark cycle changed. Their values increased when animals were acclimated to 24L : 0D and decreased in a 0L : 24D cycle. However, no variations were detected in either sodium or potassium levels when the light cycle changed. We conclude that melatonin may regulate serum cation levels, and the free cation level could be related to pineal activation-deactivation mechanisms.