Responses of Rat Kidney and Toad Bladder to the Pentapeptide Ring Structures of Neurohypophysial Hormones 1

Summary

Six cyclic pentapeptides corresponding to the ring structures of oxytocin and vasopressin were studied for water and sodium transport activities in the rat kidney and the toad urinary bladder. All six cyclic pentapeptides had detectable antidiuretic activity in the rat, although the level of activity was extremely low compared to the natural hormones. They also had natriuretic activity. The natriuretic activity of the oxytocin rings was higher than that of the vasopressin rings. All six cyclic pentapeptides at appropriate dosage produced the typical hydro-osmotic (water permeability) and natriferic (sodium transport) responses in the toad bladder. The activities for the vasopressin rings were low. The oxytocin rings on the other hand retained very high hydro-osmotic and natriferic activities. Our data thus indicate that the intrinsic natriuretic, hydro-osmotic and natriferic activities of oxytocin reside primarily in the ring moiety.

The authors thank Professor Vincent du Vigeaud, Department of Chemistry, Cornell University, for the generous supplies of the cyclic pentapeptides used in this study. The authors also thank Mrs. Nydia Rivera for her technical assistance.