Respiratory control in insects: integration from the gene to the organism

Apr 30 2007

Because of new imaging technology, researchers are getting a better understanding of a physiological paradox: how insects, which have a respiratory system built to provide quick access to a lot of oxygen, can survive for days without it.

The insect respiratory system is so efficient that resting insects stop taking in air as they release carbon dioxide, according to research by Stefan K. Hetz of Humboldt University in Berlin, Germany. This allows them to keep oxygen and carbon dioxide levels in balance. Too great a concentration of oxygen is toxic, causing oxidative damage to the insect's tissues, just as it does in humans.

Hetz is one of four speakers at the upcoming symposium “Respiratory control in insects: integration from the gene to the organism.” The symposium, sponsored by The American Physiological Society (APS) takes place 10:30 a.m., Sunday, April 29 during the APS annual meeting at Experimental Biology 2007 in Room 147A, the Washington Convention Center in Washington, D.C. Scott Kirkton of Union College, Schenectady, New York, will lead the symposium of four speakers.

Bees consume large amounts of oxygen, and so it might be tempting to think they are panting – tiny inaudible pants. They are not, because they do not breathe through noses or mouths. Instead, insects draw in oxygen through holes in their bodies known as spiracles and pump the oxygen through a system of increasingly tiny tubes (tracheae) that deliver oxygen directly to tissues and muscles. Insects typically have a pair of spiracles for each thoracic and abdominal segment.

The same tubes that transport oxygen into the insect body usher out carbon dioxide. Insects use different methods to release carbon dioxide, including opening the thoracic spiracles (the ones closest to the head) to take in oxygen while exhaling carbon dioxide through the abdominal spiracles. Insects also use different mechanisms to pump the oxygen to the tissues.

This system is much more efficient than the system that vertebrates evolved. Insects deliver much greater volumes of oxygen, in proportion to their size, than do mammals. They also deliver oxygen directly to the tissues, while vertebrates dissolve oxygen in blood, transport it to tissues, and then reconvert the oxygen to usable form.

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Because insects take in oxygen through spiracles which they open and close as needed, and because they can take in a large store of oxygen, they can live a long time without breathing by closing their spiracles and curbing their activity.

“Insects are able to survive hypoxic environments,” explained Kirkton, the symposium chairman. “They can shut down and survive for hours or days. They have a low metabolic rate and can close their spiracles. If you compare Lance Armstrong, the bee and the hummingbird, the bee is the champion of oxygen delivery,” he said. But at the same time, insects can survive low levels of oxygen for a comparatively long time.

Researchers have been interested in the insect tracheal respiratory system since 1911 when August Krogh researched moths and grasshoppers. Krogh's interest in oxygen delivery led him later to study blood perfusion in mammalian capillaries, for which he was awarded the Nobel Prize in 1920. But the advent of synchrotron x-rays, an advanced form of x-ray scan, has recently allowed scientists to learn much more about how insects breathe. The new imaging technology allows scientists to observe the respiration of live bugs.

This advance in technology also comes at a time when physiologists are learning more about the genes that control breathing. When physiologists gather at the symposium, they will assess these new developments and consider a roadmap for future research, said Kirkton.

The symposium, which is also sponsored by the London-based Journal of Physiology , will feature the following speakers:

Gabriel Haddad, of the University of California, San Diego, will speak on the “Genetic basis for hypoxia tolerance in Drosophila melanogaster.” Haddad is interested in the ability of drosophila (fruit fly), to survive periods of hypoxia, that is, periods of insufficient oxygen supply. He is examining the role the fruit fly's genes play in the ability of its nerve cells to remain healthy even under hypoxic conditions. The research aims to lead to better ways to protect humans who suffer periods of hypoxia due to medical emergency or accidents.

Mark Krasnow, of the Stanford University School of Medicine, Palo Alto, California, will speak on “Developmental responses to hypoxia in the insect tracheal system.” Krasnow has studied the development of the tracheal system in the embryo fruit fly, noting how cells form into trachea, and how trachea branch to smaller trachea and eventually connect to form the tracheal network. His laboratory, which has identified more than 50 genes controlling various aspects of airway development, has also studied how oxygen-starved cells behave in developing airways. Krasnow's laboratory is also investigating the development of the mammalian lung using mice. The work is aimed at learning more about human lung diseases and developing ways to reactivate lung development to restore diseased tissue.

Stefan Hetz, Humboldt University, Berlin, Germany, will talk on “Spiracular control of tracheal gases in insects.” Hetz and his colleagues, noting that some insects close their spiracles during rest, have concluded that the insects' respiratory system is designed to deliver large amounts of oxygen while they are active. The drawback is that, when they are at rest, this level of oxygen is toxic, he has theorized. Insects close their spiracles during rest to avoid an overdose of oxygen, which can result in the production of free radicals which cause tissue damage, he has said. His work could have applications to insect control in agriculture.

Jake Socha, Argonne National Laboratory, Chicago, and the University of Chicago will discuss “Control of internal convection in beetles using active tracheal compression.” Socha, a biophysicist, is best known to the public for his work on “flying” snakes. He has used synchrotron x-rays to view working tracheal tubes and air sacs in living insects that give further clues to how the system works. Insects employ autoventilation (using wings or legs to pump air through the body), abdominal pumping (using the muscles in the abdomen to pump air), and tracheal compression (getting oxygen to the head and thorax).

Physiology is the study of how molecules, cells, tissues and organs function to create health or disease. The American Physiological Society has been an integral part of this scientific discovery process since it was established in 1887.