AN IN VIVO ESTIMATION OF THE POTENCIES AND HALF-LIVES OF SYNTHETIC BRADYKININ AND KALLIDIN

¹ Research Division, Parke, Davis & Company, Ann Arbor, Michigan

The relative vasodilating capacity of the synthetic kinins, bradykinin and kallidin, was compared in the dog autoperfused hindquarters preparation. Kallidin was found to be more potent than bradykinin in this preparation. A method utilizing this preparation was found suitable for measuring the half-life in blood of highly potent vasoactive substances which are rapidly destroyed by blood. This procedure has been applied to determine the half-life ( t) of the synthetic nonapeptide, bradykinin, and to compare it with its synthetic 1-lysyl homolog, kallidin. The t, of bradykinin (0.27 ± 0.013) remained substantially constant within the range of doses and duration of incubation in extracorporeal blood studied, while the t of kallidin increased from 0.32 to 0.48 minute as higher initial concentrations were incubated for more prolonged periods. This suggests that the inactivation of kallidin may not follow the kinetics of unimolecular reactions. A possible explanation for this observation was that some intermediate(s) in the inactivation of kallidin had sufficient vasodilating action to contribute measurably to the vasodilatation upon which the determination of the t_, depended.

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