Renal Excretion of Exogenous Creatinine in the Aglomerular Toadfish,Opsanus tau

We have presented elsewhere a quantitative analysis of the relationship between plasma concentration and urine flow, and the tubular excretion of phenol red by the aglomerular fishes, Lophius piscatorius and Opsanus tau. 1 It was shown, in each of these species, that above 6.0 mg. % plasma phenol red there was no significant increase in the rate of excretion and that this maximal rate was not influenced by the extent or direction of the diffusion gradient. The present study represents an attempt at a similar analysis of the tubular excretion of exogenous creatinine by Opsanus. Marshall and Grafflin 2 have shown in this species that the fraction of injected creatinine which is excreted in an 18-hour period decreases progressively with increasing amounts of injected creatinine; furthermore, the fraction of a small injection excreted in a given period of time increases with increasing urine flow.

The present experiments may be divided into 2 groups. In the first group one urine collection period, 10 to 12 hours in duration and beginning 8 to 12 hours after the injection of creatinine, was made after single doses of 50 to 1500 mg. per kg. Under these conditions the rate at which the plasma concentration falls is so slow that a single blood sample drawn at the end of the urine collection period suffices for the calculation of the clearance. Observations of this nature on 81 fish are given in Fig. 1 and summarized in Table I. In the second group of experiments, an effort was made to obtain in the same fish 2 urine collection periods at high plasma creatinine levels but at different rates of urine flow. An initial period was taken as described above, after which a second injection of creatinine (400 to 1000 mg. per kg.) was usually given and the second urine collection period started abut 10 hours later.