Long known as a toxic gas, hydrogen sulfide (H2S) has recently been shown in mammals to serve as a neurotransmitter and local signaling molecule. Recent work in this laboratory (NSF IBN-0235223) suggested that H2S impacts cardiovascular function in all vertebrates and it also led to the novel hypothesis that H2S is the heretofore elusive oxygen "sensor" producing hypoxic relaxation in systemic vessels and hypoxic constriction in pulmonary vessels. This project will examine how H2S mediates hypoxic responses in blood vessels and how this signal is transduced into contraction or relaxation. It will also determine the role of H2S in general cardiovascular function in primitive vertebrates. Electrophysiological studies on single smooth muscle cells, biochemical and biophysical studies of isolated blood vessels, and complete cardiovascular measurements on unanesthetized animals will be performed on a variety of vertebrates, from primitive hagfish to mammals. These studies will provide the first comprehensive investigation of the novel hypothesis that H2S is the key component in oxygen "sensing" in blood vessels and they will identify the phylogenetic origin and impact of H2S in the vertebrate cardiovascular system. The results will have broad implications for all of vertebrate physiology by resolving the long-standing conundrum of vascular oxygen sensing and they will provide an evolutionary window into the integrative role of H2S in the vertebrate cardiovascular system. This work is also relevant to environmental biology because H2S is often a byproduct of industrial and agricultural activity and it is potentially debilitating to aquatic vertebrates. In addition, this project will support the education of the next generation of "broad spectrum" comparative physiologists as it will provide an integrative training program from molecular biology to whole-animal physiology.
Vertebrate cardiorespiratory homeostasis is inextricably dependent upon specialized cells that provide feedback on oxygen status in the tissues, blood, and on occasion, environment. These oxygen sensing cells include the blood vessels in most tissues (systemic blood vessels) that dilate in order to accommodate an increased need for oxygen delivery or blood vessels in the lung that constrict when local oxygen supply is decreased thereby preventing oxygen-poor blood from being delivered to the rest of the body. Oxygen sensitive chemoreceptors in the carotid body also stimulate breathing if blood oxygenation falls. The initial step that allows these cells to detect oxygen, the "oxygen sensor", has been the subject of intense research over the past 50 years with little success. My research, funded exclusively by the National Science Foundation, has led to the development of a novel and universal hypothesis of oxygen sensing that utilizes the metabolism of hydrogen sulfide (H2S). In this model, the concentration of biologically active H2S is regulated by the balance between cellular production and inactivation. When oxygen levels are normal, H2S is inactivated as fast as it is produced. However, when oxygen availability decreases, mitochondrial oxidation can no longer keep pace with production and the resultant increase in H2S initiates the appropriate physiological response (see figure). In support of this hypothesis we have shown the following: 1) H2S is enzymatically synthesized in all cells from common sulfur molecules, such as cysteine, and under low oxygen conditions H2S can be regenerated from thiosulfate (S2O32-). The enzymes cystathionine beta synthase (CBS), cystathionine gamma lyase (CSE) and 3-mercaptopyruvate sulfur transferase (3-MST) are involved and may also be oxygen sensitive. 2) In the presence of oxygen, H2S is rapidly oxidized (inactivated) by the mitochondria and this inactivation occurs at oxygen levels commonly encountered during hypoxia. 3) The effects of H2S on systemic and respiratory vessels from all vertebrate classes are identical to those produced by hypoxia. 4) Sulfur-donating molecules augment vessel responses to hypoxia and inhibitors of H2S production inhibit the hypoxia response. 5) Hypoxia and H2S share common downstream activation pathways, when one is active, the other cannot function. We have also shown that H2S does not circulate in the blood and it is therefore, and appropriately, confined to intracellular signaling systems. The implications of this research are broad-based, extending from explanations on the evolutionary origins of mitochondria to oxygen delivery at high altitude and during exercise. They also have enormous clinical potential in the areas of sleep apnea, heart disease, stroke and hypoxic tumors. This project has also supported the training of postdoctoral, graduate, undergraduate and highschool students.
