1. The protective action of the renal medulla was studied in one-kidney renal-clip hypertension in rats with unilateral hereditary hydronephrosis and almost complete atrophy of the medulla of the affected kidney.

2. Rats were unilaterally nephrectomized. The first group had a normal kidney remaining, and the animals from the second and third groups were left with a hydronephrotic kidney and received renomedullary and renocortical autotransplants respectively. Two weeks later all rats were made hypertensive by placing a silver clip (0·2 mm) on the renal artery.

3. From the fourth day after clipping until the end of the experiment blood pressure was found to be significantly (P < 0·01) lower in rats with medullary transplants than in the other groups. No differences in renal excretory function, plasma volume and plasma renin activity were found between the groups either before or during development of hypertension (5 and 21 days after clipping). Early in the course of hypertension (5 days) cardiac output was significantly (P < 0·05) lower in the rats with medullary transplants than in the other groups, although an increase in plasma volume was noted in all three groups. At that time no difference in total peripheral resistance was found between the groups.

4. The results are consistent with the hypothesis that the renomedullary antihypertensive substance(s) mitigates hypertension by preventing a hypertensive haemodynamic response to sodium/volume overload.

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