Kinetic measurement of the urinary production rate of cortisol in male piglets: Is the prerequisite 'collection until all label has disappeared' necessary?

Urinary cortisol production rate (CPR) was calculated by two different methods in five male piglets (about 3 kg bodyweight) injected i.v. with 40-120 kBq tritiated cortisol ([3H]F. After administration of [3H]F, urine was obtained from four consecutive collections for the following 2 days, during which 80-100% of the label was recovered. Total radioactivity in the urine was measured and used to calculate the total rate constant of 0.115 +/- 0.011 h-1 and, from this, the mean biological half-life (t1/2) of 6.0 +/- 0.6 h (S.D.; n = 4). It was found that the mass ratio of the two principal urinary cortisol metabolites tetrahydrocortisone (THE) and tetrahydrocortisol (THF) was strikingly less than 1.0 (0.4 +/- 0.1; n = 14), which is the reverse of that observed in older pigs, neonatal infants and man. To calculate CPR conventionally, the cumulative specific activities of THE and THF were calculated for the 2-day period of urine collection. The apparent mean CPR values on the basis of THE and THF were calculated as 11.5 +/- 1.6 (n = 5) and 12.8 +/- 3.3 (n = 5) mumol/day respectively, and 12.1 +/- 1.4 (n = 5) mumol/day for the average of THE and THF. The second method for calculating CPR consisted of determining the masses of THE and THF (mumol) per fraction of dose (m/fd) (fd refers to the ratio of radioactivity in the metabolite and dose) at different times after administration of [3H]F. The calculated m/fd values, which are synonymous with the dose divided by the specific activities of the metabolites, and the different times of urine collection were analysed by linear regression. The resulting slope is equal to the CPR. The CPR derived by this method for the average of THE and THF, 10.1 +/- 0.91 mumol/day was significantly (P less than 0.014) lower than that derived conventionally, 12.1 +/- 1.40 mumol/day. This second method may be used when CPR is determined in neonatal infants by means of nonradioactive, deuterated or 13C-enriched cortisol, where the extent of negative feedback by the relatively high dose of exogenous steroid on cortisol secretion must be kept as low as possible. This method also allows urine collections to be used at times when the tracer is still being excreted.