Reflex regulation of hormonal responses during pregnancy.

1. During pregnancy in most species, the resting levels of plasma angiotensin II, plasma ACTH (corticotropin) are increased. The concentration of vasopressin is also increased relatively to the osmolality in rats and in humans. 2. In the pregnant state mean arterial pressure is decreased, despite an increase in blood volume. Vasopressin and ACTH responses to hypotension are altered in pregnant ewes; the relationship between mean arterial pressure and vasopressin or ACTH response is shifted to the left, consistent with a change in set-point for regulation of mean arterial pressure. The vasopressin and cortisol responses to hypotensive haemorrhage are also altered in the pregnant dog; in this case the slope of the relation between mean arterial pressure and hormone response is decreased. 3. The decrease in hormone responses to hypotension is stimulus-specific; ACTH responses to hypoglycaemia are increased in the pregnant ewe and AVP responses to hyperosmolality are not altered in the pregnant ewe. 4. The heart rate responses to hypotension are also decreased in pregnant ewes, consistent with the observation that baroreflex responses are decreased in the pregnant rat. 5. The data suggest that a change in regulation of arterial pressure alters the hormonal responses to hypotension in the pregnant state.