Spectral analysis of unequally spaced sampled time series

Abstract

The diurnal excretion patterns of several electrolytes and urea were studied in 5 freely moving rats on an elemental diet (Vivonex) administered either orally or as a continuous intragastric feeding; the lights were on from 07.00–19.00 h and off from 19.00–07.00 h. During continuous feeding the diurnal rhythms persisted, although the amplitude was lower than during oral feeding. The maximum excretion rates of potassium, phosphate and urea all coincided under both feeding regimens. The time of the maximum calcium excretion rate coincided with that of sodium during oral feeding but not during continuous feeding. During continuous feeding the maximum calcium excretion rate shifted from 04.00 to 10.00 h. The diurnal excretion of magnesium during continuous feeding was abolished, possibly as a result of the low calcium intake. From this study it can be concluded that duringcontinuous intragastric feeding all of the studied excretion patterns persist, with the exception of magnesium, which indicates that these rhythms are not merely the result of the intermittent intake of food and minerals.