Evidence for involvement of nitric oxide in the regulation of hypothalamic portal blood flow.

Vasoactive intestinal polypeptide and peptide histidine isoleucine, two peptides with a common precursor and with strong vasodilatory actions, have been suggested to be involved in control of blood flow through the hypothalamic portal blood vessels, in this way regulating the amounts of releasing and inhibitory factors reaching the anterior pituitary. Using the indirect immunofluorescence technique, we now show that this system also contains the enzyme nitric oxide synthase, as well as acetylcholinesterase. It is therefore likely that the control of blood flow through the portal vessels is mediated via relaxation of smooth muscle cells with a high myogenic tone by neuronal release of four vasodilatory compounds, acetylcholine, vasoactive intestinal polypeptide, peptidine histidine isoleucine, and nitric oxide, i.e. a classic neurotransmitter, two neuropeptides and a gas.