Hypothalamic-Pituitary Vasculature: Evidence for Retrograde Blood Flow in the Pituitary Stalk

Retrograde transport of pituitary hormones in the pituitary stalk vasculature was investigated in anesthetized male rats in which the pituitary gland was intact and in animals in which the anterior pituitary, posterior pituitary, or entire pituitary gland had been removed 30 to 60 min before use. Blood was collected for 1.5 to 2 h by free flow from a single long portal vessel through a microcannula, the tip of which was pointed toward the hypothalamus. An arterial blood sample was obtained at the end of each collection of portal blood. The concentrations (ng\ml) of LH, TSH, prolactin, ACTH, α-MSH, and vasopressin, determined by radioimmunoassay, in portal plasma from rats with intact pituitary glands were as follows: LH, 2,320 ± 874 (mean and SE); TSH, 10,180 ± 1,471; prolactin, 4,858 ± 884; ACTH, 82 ± 17.0; a-MSH, 103 ± 17.8; vasopressin, 2.4 ± 1.0. The concentrations of these hormones in arterial plasma of these rats were as follows: LH, <20; TSH, 149 ± 22; prolactin, 25 ± 5.0; ACTH, 0.36 ± 0.05; α-MSH, 0.25 ± 0.05; vasopressin, <0.02. Compared to levels in animals with intact pituitaries, LH, TSH, prolactin and ACTH levels in portal blood from anterior lobectomized rats were greatly depressed; α-MSH concentrations were decreased moderately, and vasopressin levels were unaffected. After removal of the posterior lobe, the concentrations of LH and TSH in portal plasma did not change, but those of prolactin, ACTH, α-MSH, and vasopressin decreased markedly. After hypophysectomy, the concentrations of all pituitary hormones in portal blood were greatly reduced. These findings are consistent with the hypothesis a) that pituitary hormones are transported retrograde in certain vascular channels of the pituitary stalk toward the hypothalamus, and b) that pituitary hormones in the retrograde vasculature can reach blood in the long portal vessels going to the anterior pituitary, possibly by diffusion involving a countercurrent mechanism.