Diurnal and nutritional adjustments of intracellular Ca(2+) release channels and Ca(2+) ATPases associated with restricted feeding schedules in the rat liver

BACKGROUND: Intracellular calcium is a biochemical messenger that regulates part of the metabolic adaptations in the daily fed-fast cycle. The aim of this study was to characterize the 24-h variations of the liver ryanodine and IP(3) receptors (RyR and IP(3)R) as well as of the endoplasmic-reticulum and plasma-membrane Ca(2+)-ATPases (SERCA and PMCA) in daytime restricted feeding protocol. METHODS: A biochemical and immunohistochemical approach was implemented in this study: specific ligand-binding for RyR and IP(3)R, enzymatic activity (SERCA and PMCA), and protein levels and zonational hepatic-distribution were determined by immunoblot and immunohistochemistry respectively under conditions of fasting, feeding, and temporal food-restriction. RESULTS: Binding assays and immunoblots for IP(3)R1 and 2 showed a peak at the light/dark transition in the ad-libitum (AL) group, whereas in the restricted-feeding (RF) group the peak shifted towards the food-access time. In the case of RyR binding experiments, both AL and RF groups showed a modest elevation during the dark period, with the RF rats exhibiting increased binding in response to feeding. The AL group showed 24-h rhythmicity in SERCA level; in contrast, RF group showed a pronounced amplitude elevation and a peak phase-shift during the light-period in SERCA level and activity. The activity of PMCA was constant along day in both groups; PMCA1 levels showed a 24-h rhythmicity in the RF rats (with a peak in the light period), meanwhile PMCA4 protein levels showed rhythmicity in both groups. The fasted condition promoted an increase in IP(3)R binding and protein level; re-feeding increased the amount of RyR; neither the activity nor expression of SERCA and PMCA protein was affected by fasting–re-feeding conditions. Histochemical experiments showed that the distribution of the Ca(2+)-handling proteins, between periportal and pericentral zones of the liver, varied with the time of day and the feeding protocol. CONCLUSIONS: Our findings show that RF influences mainly the phase and amplitude of hepatic IP(3)R and SERCA rhythms as well as discrete zonational distribution for RyR, IP(3)Rs, SERCA, and PMCA within the liver acinus, suggesting that intracellular calcium dynamics could be part of the rheostatic adaptation of the liver due to diurnal meal entrainment/food entrained oscillator expression.