Helodermin, but not cholecystokinin, somatostatin, or thyrotropin releasing hormone, acutely increases thyroid blood flow in the rat

In the present study, we investigated whether peptides located within the thyroid gland, but not directly found in nerve fibers associated with blood vessels, might influence thyroid blood flow. Specifically, we evaluated the effects of helodermin, cholecystokinin (CCK), somatostatin (SRIF) and thyrotropin releasing hormone (TRH) given systemically on thyroid blood flow and circulating thyroid hormone levels. Blood flows in the thyroid and six other organs were measured in male rats using 141Ce-labeled microspheres. Circulating thyrotropin (TSH) and thyroid hormone levels were monitored by RIA. Helodermin (10(-10) mol/100 g BW, i.v. over 4 min) markedly elevated thyroid blood flow (52 +/- 6 vs. 10 +/- 2 ml/min.g in vehicle-infused rats; n = 5). Blood flows to the salivary gland, pancreas, lacrimal gland and stomach (but not adrenal and kidney) were also increased during helodermin infusions. CCK, SRIF, and TRH were without effect on blood flows to the thyroid and other organs even though these peptides were tested at higher molar doses than helodermin. Helodermin, CCK, or SRIF did not affect thyroid hormone or plasma calcium levels. As expected however, plasma TSH and T3 levels were increased at 20 min and 2 h, respectively, following TRH infusions. Since helodermin shares sequence homology with VIP, we next compared the relative effects of these two peptides on thyroid and other organ blood flows. VIP (10(-11) mol/100 g BW, i.v.) was more potent in increasing blood flows to the thyroid, salivary gland, and pancreas than an equimolar dose of helodermin. This study shows that while helodermin, like VIP, has the ability to increase thyroid and other organ blood flows, it appears to be a less potent vasodilator.