Our broad area of interest is the integrative regulation of the respiratory and cardiovascular systems in humans. This includes an interest in the physiology of exercise, as exercise provides the greatest single stress on these systems in most normal circumstances. It also includes a particular interest in the roles of carbon dioxide and oxygen in these processes, because they are the principal gases exchanged by the lungs and because they play such a central role in the regulation of both the respiratory and cardiovascular systems.

Central to the work of our group has been the technique of dynamic end-tidal forcing. This technique enables the levels of carbon dioxide and oxygen in the arterial blood to be set irrespective of any changes in metabolism or ventilation of the lungs. We have used this technique extensively to provide quantitative descriptions, through the use of mathematical models, of the dynamic responses of the lungs and the blood vessels to variations in the levels of carbon dioxide and oxygen in the blood.

As an extension of the technique of dynamic end-tidal forcing, our group developed a chamber in which the levels of carbon dioxide and oxygen in the arterial blood could be maintained constant for many hours/days. This enabled us to study, under very controlled conditions, the slower responses to low oxygen (the so called acclimatization responses to hypoxia) that occur when individuals travel to high altitude. Using this approach, we were able to show that respiratory (or ventilatory) acclimatization to hypoxia did not arise through mechanisms involving changes in the acidity of the body fluids as had been previously thought, but rather as a direct consequence of the low oxygen itself.

Over the past decade or so, a very important advance has been the discovery of the family of transcription factors known as hypoxia-inducible factors (HIF) that regulate gene expression in response to cellular hypoxia. Much of our current research is now focussed on trying to gain an understanding of the role of HIF in the integrative responses of the cardiovascular and respiratory systems to sustained hypoxia in humans. Towards this aim, we have complemented our integrative studies of human function with experimental work on genetically-modified mice with specific alterations within the HIF system; with molecular studies of human cellular responses to hypoxia – which we aim to relate through to integrative function; and with studies of the genetic adaptation of the world’s high-altitude populations to the low oxygen of their environment.

Much of the work within our group is undertaken by graduate students working towards a doctorate. In many cases, these students are jointly supervised with other colleagues in the Department, which adds breadth to their research training. The laboratory has an international feel with students coming from many different countries – currently the UK, Cyprus, Italy and Peru. Recent students have been funded through Rhodes Scholarships, Marshall Scholarships, Felix Scholarships, Clarendon Bursaries, ORS awards, as well as through funding arising from their native country.

Current Research Programme

Some specific questions that are currently being addressed in our laboratory are as follows:

Are the responses of the respiratory and cardiovascular systems to sustained hypoxia related to one another?

From previous work, we have shown that the time course of the response to sustained hypoxia is similar for pulmonary ventilation, heart rate, pulmonary vascular resistance and systemic vascular resistance. There is an immediate effect of hypoxia, which is generally complete within the first five minutes of exposure, a “plateau” phase, and then a further response to the hypoxia which begins between 30-60 min after the onset of hypoxia and continues over a period of hours. Within this common framework however, there are significant differences in the overall magnitude of the responses observed between individuals. We are now seeking to determine whether these differences are consistent across the various systems within an individual. If so, then it becomes possible to define individuals, at an integrative level, as phenotypically high, medium or low responders to hypoxia.

Are there alterations in autonomic function induced by sustained hypoxia that mirror those occurring within the respiratory and cardiovascular systems?

The transcription factor, hypoxia-inducible factor (HIF), is known to regulate a number of genes that are of key importance to the autonomic nervous system. This raises the possibility that there are also changes in the function of the autonomic nervous system in response to sustained hypoxia that mirror those we observe in the respiratory and cardiovascular systems. To investigate this, we are recording sympathetic nervous activity from the peroneal nerve to determine what effect sustained hypoxia has on sympathetic activity.

What phenotypes are associated with individuals who have rare genetic diseases of the hypoxia-inducible factor (HIF) signalling pathway?

One rare disease of the HIF signalling pathway is Chuvash Polycythemia. We have shown that, apart from the increased red cell mass, these individuals also have pulmonary hypertension, hypocapnia – as a result of excessive lung ventilation – and extremely powerful pulmonary vascular and ventilatory responses to acute hypoxia. These findings underlie the importance of the HIF pathway to homeostasis at the level of the whole person. As the HIF pathway has also been shown to be important for metabolic regulation at the cellular level, we are now exploring to what degree the metabolic responses of these individuals are altered in relation to the stress of both digestion and exercise. Since the discovery of the molecular mechanism underlying Chuvash Polycythemia, other rare diseases of the HIF-signalling pathway have been described. Such patients provide a potential route to understand the effects on phenotype of alterations of different aspects of the HIF-signalling pathway, and we are currently investigating their physiology.

Can the properties of the cardiovascular and respiratory systems be modified by modulation of known physiological co-factors of the hypoxia-sensing step of the hypoxia-inducible factor (HIF) signalling pathway?

The central oxygen-sensing step of the HIF pathway involves the hydroxylation of specific proline residues within the alpha subunit of HIF. This reaction, catalysed by prolyl hydroxylases, is dependent on both iron and ascorbate as co-factors. We have been exploring whether modulation of these co-factors can affect any of the cardiovascular and respiratory responses to sustained hypoxia. To date we have shown that iron chelation induces EPO production and increases pulmonary artery pressure in humans, and iron infusion inhibits the response of the pulmonary vasculature to sustained hypoxia. We are now exploring whether manipulation of the body’s iron stores can influence pulmonary artery pressure in more clinical settings, and whether such manipulation can influence the acclimatization process to altitude, including both susceptibility to acute mountain sickness and the ability to undertake muscular exercise at high altitude.

Can we identify differences between normal individuals with respect to the hypoxia-inducible factor (HIF) signalling system?

In order to address this question, we took white blood cells from individuals and incubated them in primary cell culture at different levels of hypoxia. We found differences between individuals in the induction by hypoxia of a number of different HIF-regulated genes (using qRT-PCR). We are now trying to extend this result in an integrative direction to determine whether our biochemical phenotype is related at all to our cardio-respiratory responses to hypoxia. We also feel it should be possible to extend this result in a more reductive direction by looking for genotypic variations that significantly influence the biochemical phenotype.

Are the human populations that live at high altitude genetically adapted to their low oxygen environment?

The hypoxia-inducible factor (HIF) signalling pathway allows cells to mount a co-ordinated response to alterations in oxygen availability. The very existence of such a system suggests that there are advantages to having a different physiological “makeup” at different oxygen tensions. As such, we should not be surprised if exposure to the hypoxia of high altitude provides a selective pressure for genetic adaptation. In collaboration with others, we are trying to identify signatures of natural selection to life at high altitude within the genomes of the world’s high altitude populations.

How does ventilatory sensitivity to hypoxia vary over long-term exposure to hypoxia?

Exposure to hypoxia over days to months causes an increase in the ventilatory sensitivity to hypoxia. However, exposure to hypoxia over years blunts this sensitivity. We are trying to determine the time course of blunting and recovery from blunting by studying individuals in Peru who are either moving from sea level to live at high altitude, or who are moving from high altitude to live at sea level.

Is there a role for learning in calibrating the cardiovascular response to exercise in humans?

Many motor acts are learnt through practise. For learning to occur, repeated trials are required which generate sensory feedback from which performance in each trial may be judged. Recently, we have shown in humans that the respiratory response to exercise may be altered by repeated trials of exercise in which the feedback from the chemoreceptors is artificially altered during the trials. We are now exploring whether a similar mechanism may calibrate the autonomic regulation of the vasculature during exercise. Our protocol again employs repeated trials of exercise in which feedback knowledge of arterial pressure from the baroreceptors is artificially altered. If such a mechanism were present, then we would expect the blood pressure during exercise to be altered following such a period of training.

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Peter Robbins