Indiana University       Research & Creative Activity       September 2000 • Volume XXIII Number 2

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Digesting Modern Physics

Anyone suffering from a stomach ulcer knows the feeling: intense pain as the stomach’s naturally-produced hydrochloric acid eats away at the stomach lining. In healthy individuals, stomach acid acts as a catalyst, activating enzymes that digest our food, and suppresses bacteria. In either role, stomach acid is strongly caustic.

If it is remarkable that the stomach produces such a vicious reagent, it is even more remarkable that a healthy stomach is unharmed by the acid it secretes. Understanding the mechanisms of this and other digestive tract mechanisms at the cellular level is the research goal of Marshall H. “Chip” Montrose, professor of physiology and biophysics at the IU School of Medicine.

“Why doesn’t the stomach digest itself? For a few centuries now, that has been an active question,” Montrose says. “Our work tries to answer a very old question about how the stomach defends itself from the acid it secretes.”

Montrose has made a career of studying the structure and functions of epithelial cells in the gastrointestinal tract, first at Johns Hopkins University and, since 1998, at IU. His research on stomach acid secretion is causing a rethinking of the way the stomach defends against acid.

“We’re learning new facts about the basic process of digestion,” Montrose says. “Did you know your stomach can convert to an alkali-secreting organ, as opposed to an acid-secreting one, when you fast?”

His team has discovered, through microscope studies of cells in vivo, that there is an unusually reactive pH microdomain on the surface of cells lining the stomach. This microdomain responds to its environment, changing the pH from acidic to alkaline and back again, according to conditions of feasting or fasting. The presence of this dynamically responsive microdomain was previously unknown. Now, any scientific account of stomach function must take this newly observed function into account.

Montrose researches the cellular and subcellular mechanisms of colonic absorption as well. The stomach sends partially digested food to the small intestine, where nutrients are absorbed into the body. Undigested food is sent to the colon, the body’s last chance to scavenge nutrients before the food is removed as waste.

“The colon absorbs about 20 percent of our daily salt and water needs,” Montrose explains. “If it fails to claim enough nutrients, we go into a negative electrolyte balance.”

Bacteria in the colon produce compounds that not only stimulate sodium absorption in the colon, but are themselves then absorbed and metabolized. Montrose’s laboratory team has discovered that these compounds stimulate sodium absorption by regulating the pH outside the absorptive cells, another case of a surprising microdomain that is altering ideas about gastrointestinal function.

“It’s really a symbiotic relationship, mutually beneficial, between colon bacteria and ourselves,” Montrose says.

Montrose’s research on cell-level mechanisms in the gastrointestinal tract has potential implications for treatment of stomach and colon cancer, inflammatory bowel disease, and gastric and duodenal ulcers. In another new and highly experimental research area, Montrose is seeking to understand the endogenous fluorophores of cancerous cells. Natural fluorescence of cells (the mechanism of absorbing light energy and re-emitting light of a different color) can change during carcinogenesis. It may be possible, Montrose theorizes, to detect cellular aberration, even before tumors become visible to endoscopic imaging, by comparing wavelengths of light.

To do this, Montrose and his laboratory team are developing a spectroscopy approach, aimed at analyzing the spectral fingerprint of subcellular organelles in human colon biopsies. This will give researchers better information about what and where the fluorophores (molecules emitting fluorescence) are and which wavelengths are best for detection and possible diagnosis.

All of the research conducted by Montrose and his lab partners relies on access to imaging techniques and the quantitative interpretation of pictures of living cells and tissues. In part, it is this application that sets biophysics apart from other fields.

“Biophysics defines areas of quantitative biology and physiology that use the methods derived from modern physics to answer questions,” Montrose explains. “Our domain has been the use of high-resolution imaging and optical methods applied to complex living structures.”

Marshall H. "Chip" Montrose, professor of physiology and biophysics at the IU School of Medicine, relies on this recently installed two-photon microscope in his research. Photo Tyagan Miller.

Confocal microscopy is a mainstay of Montrose’s laboratory work. This special microscope device mitigates a problem encountered when a sample has fluorescent molecules present throughout: light from nontarget areas interferes with or obscures light from the target area.

“This device records fluorescent images after discarding the out-of-focus light that is always present,” Montrose explains.

Techniques such as confocal microscopy minimize out-of-focus light, but a new imaging technique called two-photon microscopy may completely eliminate out-of-focus light interference. With the two-photon system, fluorescence is generated only at the precise target focus, using two low-energy photons instead of one high-energy photon to excite fluorophores. Because neither of the two photons has sufficient power alone to excite fluorescence of the target, only the exact focal spot where photons are abundant has sufficient energy (photon density) to fluoresce. All two-photon fluorescence originates from the constrained areas.

“Two-photon fluorescence gives us huge improvements inhigh-resolution imaging when focused deep into thick specimens like living organs,” Montrose says. “It should cause a revolution in in vivo imaging over the next few years.

“The two-photon system has an added advantage,” Montrose continues. “Conventional fluorescent microscopy causes bleaching of the entire target, which is often toxic to the living tissues we study. The two-photon approach limits bleaching to the focal point only, so damage is reduced.” In late April, Montrose and his laboratory installed a new multiphoton microscope, only the second such unit on the IU Medical Center campus.

Montrose currently teaches master’s, doctoral, and medical students and is graduate adviser in the IUPUI Department of Physiology and Biophysics. His research has clearly changed what he teaches about the gastrointestinal tract.

“Every textbook in the world says that the stomach protects itself from acid by trapping bicarbonate at the surface as a protective layer,” Montrose says. “Our research disagrees. I avoid teaching the party line on that. I want to change the textbooks.”

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