Linda J. Huffman

Sympathetic thyroidal vasoconstriction is not blocked by a neuropeptide Y antagonist or antiserum

Sympathetic nerve fibers to thyroid blood vessels contain both norepinephrine (NE) and neuropeptide Y (NPY). To assess the involvement of endogenous NPY in the sympathetic neural control of thyroid blood flow, appropriate doses of a selective NPY antagonist, alpha-trinositol, and an NPY antiserum (NPY-AS) were used during cervical sympathetic trunk stimulation in anesthetized rats. During all experiments, thyroid blood flow was continuously monitored by laser Doppler blood flowmetry. Neither alpha-trinositol nor NPY-AS blocked the thyroidal vasoconstriction evoked by either the first or second stimulation of the cervical sympathetic trunks. Our results suggest that NPY is not involved either directly or indirectly during acute sympathetic vasoconstriction in the rat thyroid gland.

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.

Neuropeptide control of thyroid blood flow and hormone secretion in the rat

The effects of peptide HI (PHI), neuropeptide Y (NPY), and substance P (SP) on thyroid blood flow and hormone levels were studied in anesthetized rats. Regional blood flows were determined using radioactive microspheres. No change in heart rate or mean left ventricular pressure occurred during these neuropeptide infusions (0.625 micrograms iv over 2 min). PHI treatment resulted in a four-fold increase in thyroid blood flow. Blood flows to the pancreas and salivary gland also increased during PHI treatment. Infusions of NPY or SP did not significantly alter thyroid blood flow. However, SP decreased blood flow to the spleen and small intestine. These neuropeptides had no effect on blood flows to the adrenal, kidney, brain, heart, and adipose tissues. Following PHI, NPY, and SP infusions, plasma triiodothyronine and thyroxine levels were not different from values in saline-treated rats. This study demonstrates that PHI, like vasoactive intestinal peptide, is a potent thyroidal vasodilator at a dose that does not affect circulating thyroid hormone secretion.

INHALATION OF COTTON DUST IS ASSOCIATED WITH INCREASES IN NITRIC OXIDE PRODUCTION BY RAT BRONCHOALVEOLAR LAVAGE CELLS

The inhalation of cotton or other organic dusts can cause alterations in pulmonary function, and these pulmonary effects appear, in part, due to endotoxin contamination of the dusts. Since endotoxin is a potent stimulus for the induction of nitric oxide (NO) synthesis, we examined whether the inhalation of cotton dust might also be associated with increases in NO production. Rats were exposed to normal air, cotton dust aerosol (40.6 3.7 mg/m3), or a nebulized aerosol of endotoxin (2.2 x 104 EU/m3) for 3 h, and responses were studied 18 h postexposure. Increases in inducible NO synthase (iNOS) production by bronchoalveolar lavage cells (BALC) from rats occurred following exposure to cotton dust or endotoxin as evidenced by increases in iNOS mRNA levels and in vitro nitrate and nitrite production. However, a contribution of NO to oxidant species generation by BALC, as indexed by luminol-dependent chemiluminescence, was observed only in endotoxin-exposed rats. These results indicated that while the inhalatio...

Vasoactive Intestinal Peptide Enhances Thyroidal Iodide Uptake During Dietary Iodine Deficiency

The presence of vasoactive intestinal peptide and neuropeptide Y in thyroid nerves and their effects on thyroid blood flow are well known. However, the effects of these two neuropeptides on the various processes involved in thyroid hormone biosynthesis and release have not been fully explored. We have now tested these two peptides for effects on an early step in thyroid hormone biosynthesis, namely iodide uptake, a process which is comprised of trapping and organification. In these experiments, we have used anesthetized adult male rats pretreated with thyroxine or fed a low iodine diet to increase thyroidal sensitivity. Vasoactive intestinal peptide significantly increased iodide uptake in rats fed an iodine deficient diet but not in those fed a normal iodine diet. This effect disappeared if animals were pretreated with propylthiouracil. Neuropeptide Y did not alter iodide uptake in rats on either the low or the high iodine diet, regardless of the presence or absence of propylthiouracil. The effect of vasoactive intestinal peptide on iodide uptake could be due to its influence on the organification of iodine, or on thyroid blood flow, or on both processes.

Expression of preproNPY and precursor VIP mRNAs in rats under hypo- or hyperthyroid conditions.

Both neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) are present in thyroid nerves and have been shown to alter thyroid activity. The present study was conducted to determine whether hypo- or hyperthyroidism is associated with changes in the expression of the mRNAs for these neuropeptides in the major ganglia which supply nerves to the thyroid or within the thyroid gland itself. Hypo- or hyperthyroid conditions were induced by the administration of propylthiouracil (PTU) or thyroxine (T(4)), respectively, for 6 days. Control rats received vehicle injections. Total RNA from superior cervical ganglia (SCG), local thyroid ganglia, thyroid gland, and selected other tissues was extracted and mRNA levels were analyzed using Northern blot procedures. No significant changes in preproNPY or precursor VIP mRNA levels were detected in the SCG or the local thyroid ganglia in response to PTU or T(4) treatment. However, PTU treatment was associated with an increase in preproNPY mRNA levels in the thyroid gland itself. These results indicate that changes within the thyroid axis in response to these hypo- and hyperthyroid conditions do not include alterations in steady-state preproNPY or precursor VIP mRNA concentrations in the major ganglia which supply nerves to the thyroid gland. However, intrathyroidal preproNPY mRNA levels are increased as a consequence of the thyroidal adaptation to a PTU challenge.

Vasopressin and Endocrine Function

In the field of endocrinology, there is an increasing number of examples of hormones that were originally identified and named on the basis of some particular effects and then subsequently found to exert other effects as well, some of which turn out to be of greater physiological importance than the original ones. Vasopressin (VP) is one of the earlier examples of this phenomenon in that it was identified and named because of its vasopressor properties; only later was it recognized that it is of even more importance as an antidiuretic hormone. Still later, this same peptide began attracting attention for its effects on the functions of a variety of endocrine organs, and it is this aspect of the physiology of VP that is addressed in this chapter. Because the earlier literature in this field was reviewed in 1968 by Doepfner, we concentrate on the advances since that time. As is often the case, many of these advances in our knowledge of the physiology involved have been possible only because of recent technological advances. For example, assays for the hormones affected by VP (as well as for VP itself) have improved dramatically, and the structures of some of these hormones (e.g., the hypophys-iotropic hormones) have only recently been elucidated. Also, the availability of synthetic agonists and antagonists for VP is proving to be as valuable to the study of the receptors involved in endocrine effects as it has been for the more classical pressor and antidiuretic effects of VP. Finally, the widespread use of the Brattle-boro rat, with its congenital lack of VP, has made a particularly strong contribution to the study of the endocrine effects of VP. The endocrinopathies of the Brat-tleboro rat are considered to some extent in this chapter, but the reader is also referred to a recent monograph (Sokol and Valtin, 1982) for further details.

Lack of effect of vasoactive intestinal peptide antagonists on blood flow in the rat thyroid

We used three putative vasoactive intestinal peptide (VIP) antagonists: 1) [4C1-D-Phe6,Leu17]VIP, 2) [N-Ac-Tyr1,D-Phe2] GRF(1-29)-NH2, and 3) VIP(10-28) to assess the involvement of endogenous VIP in the regulation of thyroid hormone secretion and thyroid blood flow (BF). We measured thyroid BF in ketamine-pentobarbital-anesthetized rats using the microsphere technique. Increases in thyroid BF induced by VIP administration (30 pmol-1.5 nmol/100 g b.wt.) were not affected by any of the three compounds tested at doses 10-100 times higher than that of VIP. These compounds (3-15 nmol/100 g b.wt.) also failed to affect basal thyroid BF or hormone secretion. Increases in pancreatic and salivary gland BFs induced by VIP (30 pmol/100 g b.wt.) were also not affected by [4C1-D-Phe6,Leu17]VIP or [N-Ac-Tyr1,D-Phe2]GRF(1-29)-NH2 (3 nmol/100 g b.wt.). These results indicate that the three compounds tested are not effective inhibitors of VIP receptors in the thyroid vasculature and, therefore, they cannot be used in the investigation of the functional significance of endogenous VIP in the regulation of thyroid BF.

Muscarinic Modulation of the Vasodilatory Effects of Vasoactive Intestinal Peptide at the Rat Thyroid Gland

In the thyroid gland, vasoactive intestinal peptide (VIP) and acetylcholine (ACh) are found in nerve fibers associated with secretory cells and blood vessels. We have, therefore, initiated studies to explore the actions of and interactions between cholinergic agents and VIP in the regulation of thyroid vascular conductance (VC). Thyroid and other organ blood flows were measured using radiolabelled (141Ce) microspheres injected directly into the left cardiac ventricle of anesthetized male rats. The mean systemic arterial pressure was monitored and used in the calculation of organ VC (blood flow/arterial pressure). Plasma TSH, T3, and T4 levels before and after infusions were measured by RIA. The acute administration of ACh (3 x 10(-8) mol/100 g BW) over 4 min increased thyroid VC, whereas nicotine (10(-7) mol/100 g BW) had no such effect. Circulating TSH and thyroid-hormone levels following ACh or nicotine were not different from those in vehicle-treated animals at 20 min or 2 h after infusion. This observation suggested that ACh acts through muscarinic receptors at the thyroid gland to increase VC. In order to extend these observations and to evaluate whether VIP might exert any of its thyroidal effects on VC via muscarinic receptors, we assessed the effects of ACh, methacholine chloride (MCC), and VIP in the presence and absence of the muscarinic receptor blocker atropine. Rats were treated intravenously with saline or atropine (3 mg/kg) 20 min before intravenous infusions of vehicle, ACh (3 x 10(-8) mol/100 g BW), MCC (5 x 10(-9) mol/100 g BW), or VIP (10(-11) mol/100 g BW).(ABSTRACT TRUNCATED AT 250 WORDS)

Reserpine‐induced increases in neuropeptide Y mRNA of guinea pig sympathetic ganglia using in situ hybridization

Neuropeptide Y (NPY) is synthesized in sympathetic ganglia by specific mRNA, to which rat probes are currently available. In the rat model, reserpine treatment increases NPY mRNA through a mechanism involving enhanced preganglionic activity. Probes for NPY mRNA have been used exclusively in rat models. In this study, we assessed whether a rat NPY cRNA probe could be used to index reserpine‐induced changes in NPY mRNA levels of sympathetic ganglia in the guinea pig.