Effects of vasoactive intestinal peptide on thyroid blood flow and circulating thyroid hormone levels in the rat

The effects of vasoactive intestinal peptide (VIP) on thyroid blood flow and hormone levels were studied in rats. Tissue blood flow was determined from the distribution of radioactive microspheres after injection by cardiac puncture directly into the left ventricle of anesthetized rats. Initial results indicated that the systemic infusion of 6.25 micrograms VIP iv resulted in increased thyroid blood flow, but was also associated with hypotension, as measured by left ventricular pressure. In contrast, topical administration of VIP to the left of the thyroid increased blood flow to that lobe, but not to the right lobe, and produced no systemic cardiovascular effects. In a further set of experiments, graded doses of VIP were administered iv. Infusions of 6.25 and 0.625 micrograms VIP were associated with 2- to 3-fold increases in thyroid and pancreatic blood flows, but lower doses were ineffective. Blood flows to the adrenals, brain, small intestine, kidneys, and spleen were not altered by any dose of VIP. Mean left ventricular pressure was again reduced by the 6.25-micrograms dose of VIP, but was not affected by lower doses. The infusions of VIP had no effect on plasma TSH, T3, or T4 levels either 20 min or 2 h after treatment. These results suggest that thyroid blood flow is, in part, controlled by VIP and indicate that changes in thyroid blood flow can occur at doses of VIP that have no apparent effect on circulating thyroid hormone levels.