Potentiated adrenocorticotropin (ACTH) and cortisol responses occur after the second of two small hemorrhages (hems) spaced 24 h apart in the dog. To test whether increased responses of other hormones might be associated with this effect, we examined plasma renin activity (PRA), angiotensin II (ANG II), and vasopressin after paired 10% hem (H1 and H2) spaced 5 h apart in chronically prepared conscious dogs. Cortisol secretion increased after each hem, and the response to H2 was larger (P less than 0.05; H1 peak at 6.8 +/- 1.3 micrograms/min vs. H2 peak at 18.3 +/- 5.3 micrograms/min). ACTH did not change after H1 but increased after H2, and the H2 response was larger (P less than 0.01). Vasopressin increased after each hem, and the H2 response was larger (P less than 0.01). The time courses of ACTH and vasopressin responses were similar after H2 (significant increases by 8 min). PRA and ANG II increased by 4 min after each hem, and although the difference was small the early PRA and ANG II responses were greater after H2. Blood volume and hem volume did not differ between hems. Hemodynamic responses to the hems were not different. We conclude that, although the PRA and ANG II respond rapidly enough after hem to influence pituitary responses, the slightly greater responses of these factors to H2 are not responsible for greatly increased pituitary-adrenal responses to H2. On the other hand, the markedly potentiated vasopressin response to H2, which parallels that of ACTH, suggests that vasopressin may mediate the increased ACTH responses to H2.
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