The effects of exogenous purine supply on plasma concentration
and
urinary excretion of purine
derivatives (PD), which include allantoin, uric acid, xanthine and hypoxanthine,
were studied. Five
sheep, totally nourished by intragastric infusion of volatile fatty acids
and casein, were given an
abomasal infusion of a mixture of adenosine and guanosine at three levels
(5·0, 10·0 and
20·0 mmol/day) each in four infusion patterns (as one, two,
or
four 3-h infusion periods per day or
infused continuously, P1 to P4 respectively). Urine was collected hourly
over
24 h, and plasma
samples were collected hourly for 7 h from the start of purine infusion.
Both
the plasma concentration
of PD and its urinary excretion changed rapidly in response to the start
and termination of purine
infusion with a lag time of 2–3 h. Individual sheep differed
considerably in the relative proportions
of the different purine derivatives in the urine. The endogenous urinary
PD excretion averaged 176
(±S.E. 28) μmol/kg W0·75
per day. Daily PD excretion in urine increased with the amount of purine
infusion, but at each level the output decreased significantly in a
gradient from P1 to P4. The response
of total PD excretion (Y, mmol/day) to daily purine input
(X, mmol/day) was predictable and
followed the equation: Y=0·81
(±0·02)X+3·07(±0·81)
× (u e−0·27X/u
+1−u), where u is the
duration over which the daily input was infused expressed as a proportion
of
24 h. The recovery of
the infused purines averaged 81%. When the infusion was given continuously,
plasma PD
concentration was relatively stable and was linearly correlated with the
purine entry rate and daily
PD excretion in the urine. The estimated glomerular filtration rate
was 150 (±S.E. 16) litres/day and
the estimated tubular transport maximum was 2·6
(±S.E. 1·3) mmol/day. It is concluded that due
to
the rapidity in the response of plasma and urinary PD to
changes in the exogenous purine supply, spot
measurements of PD in urine or plasma can be of no value for the
estimation of exogenous purine
uptake unless the purine supply is relatively constant throughout the
day. Daily PD excretion in urine
related to the exogenous purine uptake in a predictable and reproduceable
manner and provides a
reliable index for the estimation of the exogenous purine uptake.