93 PRESENCE OF DIURNAL VARIATION FOR IONIZED AND TOTAL Ca EXCRETION IN PREMATURE INFANTS

Abstract

Diurnal variation in urinary calcium is observed in adult subjects, and is ascribed primarily to food intake, exercise habits, a day/night routine, and lastly an intrinsic rhythm. We wished to see whether premature infants showed diurnal variation in urinary Ca, and hypothesized that this would be largely an intrinsic rhythm since the infants were exposed to 24 h light and regular feeds around the clock.We studied 5 infants (3 male, 2 female) birth weight 1.17±0.20 kg (mean±SD), gestational age 28±0.7 wks at 34±5 days of life, who were being fed every 2-3 hours. Urine was collected each time the infant voided for a period of 24 hours. Each specimen was analyzed for ionized Ca (iCa), total Ca (tCa), and pH, and excretion rates for Ca were calculated. The mean three-hourly Ca excretion rate for each infant was expressed as a percentage above or below the mean 3-hourly rate for the 24 h from midnight to midnight, and the average patterns of the five subjects graphed. Urinary iCa and tCa showed a diurnal variation (p<0.05). The values between 04.00 and 09.00 h were 17% above the 24 h mean and between 16.00 and 24.00 h were 19% below the 24 h mean. This is very similar to the intrinsic rhythm for Ca excretion described in adults, which coincides with the diurnal rhythm of cortisol secretion by the adrenal cortex, and is possibly linked to a rhythm in bone resorptive mechanisms. Thus this intrinsic rhythm of Ca excretion is present even in premature infants, and should be remembered when timed urine collections are made.