Physiological cortisol levels block the inhibition of vascular reactivity produced by prolactin.

Cortisol in concentrations similar to the unbound levels of the hormone in human plasma can reverse the inhibition of vascular reactivity produced by prolactin. In the rat mesenteric vascular bed, cortisol alone in similar concentrations had no significant effect on the pressor responses to norepinephrine: the action of cortisol was seen only when prolactin was present. The relationships between the effects of different concentrations of prolactin and cortisol suggest that at some point there is a competitive interplay between the effects of the two hormones. There is indirect evidence that this interplay is at the level of prostaglandin synthesis or release. We suggest that cortisol has no effect on basal prostaglandin production but blocks the synthesis or release occurring in response to polypeptide hormone stimulation.