The Single Injection Technique for Determination of Renal Clearance: I.Clearance of Iothalamate and Iodohippurate in Dogs

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Abstract
Renal clearance of iothalamate and iodohip-purate labelled with radioactive iodine was investigated in dogs, using the single injection technique. The clearance values were calculated from the amount excreted in the urine and the area under the plasma curve during the corresponding time-period. Four separate determinations from consecutive periods following the injection were performed in each experiment and compared with simultaneous continuous infusion clearance of the same substance. For iothalaniate the single injection clearanes agreed very well with the continuous infusion clearance, mean values of the former being only 3% lower than those of the latter. Individual determinations also showed a very good correlation between the two techniques (r=+0.98). After single injection of iodohippurate there was a gradual full of the clearance values from period to period. The niean value for the last period was only about 60% of that of the initial period. The continuous infusion clearance of iodohippurate also showed a decrease which, however. was less pronounced. The total clearance calculated from the plasma curve was 10 and 20% higher than the infusion clearance of iothalamate and iodohippurate. recpectively. It was concluded that the clearance of iothalamate is constant during falling plasma concentration, and that it can be determined equally well with the single injection and the continuous infusion technique. The clearance of iodohippurate, on the other hand, is not constant tinder these conditions and the two techniques do not give comparable results. Several properties of iodohippurate may contribute to these findings but the exact inechanism remains to be elucidated.