Membrane Research: New Approach to Treatment of Gastrointestinal Illnesses

Robert A. Argenzio, professor of physiology, School of Veterinary Medicine, North Carolina State University, Raleigh.

Diarrhea and other gastrointestinal illnesses cause high livestock mortality rates and result in significant economic losses to the livestock industry. These losses were passed on to consumers in the form of higher prices.

During the past few years, however, research on the function of the epithelial membrane has begun to provide information in both basic and applied physiology. (This membrane of cellular tissue covers a surface or lining of a tube or cavity of an animal, serving to enclose and protect other parts of the body, to produce secretions and excretions, and to function in assimilation. Gut linings are an example.) Studies are leading to new approaches for the treatment of diarrhea and acute and chronic bowel injuries. Significant therapeutic advances based on this new knowledge are now anticipated. Some of these studies will now be discussed.

Membrane Function

The study of epithelial function is especially difficult because of the many different types of cell in the mucous membrane of an animal's gastrointestinal tract as well as the complexity of the limiting cell membranes. The functions of cells differ markedly. So we may no longer treat the epithelium as a "black box" if we are to study rigorously the underlying mechanisms and control of ion transport.

Several recent advances in methodology have opened the "black box". It is now possible to separate vinous (hairy) and crypt cells in vitro (outside the body) and to make membrane vesicles (pouches) of the two limiting membranes which can be separated by sucrose density gradients. Thus the individual membranes can be studied under conditions of precisely controlled electrochemical and pH gradients. In the past 2 years, these studies have provided sound evidence for the molecular transport characteristics of several membrane transport systems and have elucidated important information on their control.

A second recent development in methodology is the growth in culture media of a colon cancer cell line. These cells can be grown to form a membrane only one cell thick with one surface attached to glass or plastic or a nylon mesh. Transmission electron microscopy and freeze fracture techniques indicate that they form junctional complexes between adjacent cells. Preliminary studies with this system show that they possess many of the transport pathways of intact mucous membranes. Conventional electrophysiological methods such as microelectrode and patch clamp techniques also can be applied to these isolated cells.

The study of intracellular mechanisms in conjunction with transport pathways should be greatly facilitated with this method because the cells are completely isolated from external neuroendocrine influence. Further, such methodology is an alternative to the use of animals in some of these critical areas of research.

Membrane Phospholipids and Hormone Action

It was well established that one group of hormones uses cyclic adenosine monophosphate (cAMP) as an intracellular messenger linking stimulus to response. Recently, studies with several cell systems have shown that a large number of other hormones and neurotransmitters utilize calcium rather than cAMP for their actions. These agents induce a breakdown and resynthesis of a membrane phospholipid known as phosphatidylinositol. This phospholipid turnover may open a calcium gate in the membrane allowing calcium concentrations to increase inside the cell.

In addition, activation of an enzyme known as protein kinase C is linked to this turnover of phospholipids. In several cell types, activation of protein kinase C is a requirement and acts synergistically with calcium mobilization to elicit the physiological response. Recently, studies with mammalian intestines have shown that direct stimulation of this enzyme results in intestinal secretion.

Also involved in this turnover is the release of arachidonic acid, a component of phosphatidylinositol. Arachidonic acid is metabolized by two major pathways in intestinal mucosa to form Prostaglandins, which may control blood pressure and muscle contractions, and thromboxanes which regulate cell function. Also formed are compounds known as leukotrienes and other fatty acids. These compounds have diverse and potent actions. They may play key roles in the regulation of ion transport and mucosal defense.

For example, several of the Prostaglandins activate cAMP which, in turn, stimulates intestinal secretion. More recently, several products of the second arachidonic acid pathway were shown to cause colonic secretion by a mechanism independent of both cAMP and calcium. The possible interaction between these intracellular messengers makes an understanding of the stimulus-secretion coupling much more complex than was previously imagined. These recent findings will stimulate much needed research in this area for many years to come.

Medicinal Treatment of Diarrhea

This new knowledge in the control of ion transport has already led to advances in the medicinal treatment of diarrhea. For example, agents which inhibit protein kinase C and calmodulin, such as the tranquilizers trifluoperazine and chlorpromazine, inhibit intestinal secretion in experimental systems. These agents also are effective clinically. But the doses required for an antisecretory effect may have undesirable side effects such as sedation.

Nevertheless, it is expected that potent new drugs which are specific for the desired effect will be produced in the next few years.

Similarly, agents capable of inhibiting prostaglandin production seem to be effective in some types of diarrhea. The recent findings that lipoxygenase products of arachidonic acid also may be capable of causing intestinal secretion opens yet another avenue for pharmacologic intervention.

Basic knowledge in the control of ion transport by neurotransmitters and hormones has led to studies using agonists or antagonists of these agents. Recent animal studies indicate that these compounds may prove to be effective in the treatment of certain types of diarrhea.

Mucosal Defense

In recent years, it was found that Prostaglandins were capable of protecting gastric and upper small bowel mucosa from serious injury caused by certain damaging agents. Further studies have established that these Prostaglandins stimulate mucous and bicarbonate secretion, a mechanism which protects the mucosa from back diffusion of gastric acid and disruption of the "gastric mucosal barrier."

Careful studies of the time course of mucosal injury and its protection by Prostaglandins show that the proliferative cell zone is completely protected during an acute injury and therefore can initiate rapid repair of damaged surface epithelium. Proliferative activity itself seems to be at least partially controlled by metabolites of arachidonic acid. These findings have great potential in the therapy of gastric and duodenal ulcers.

Other potent and diverse effects of these metabolites are regulation of mucosal blood flow, epithelial ion transport, and chemotaxis. Further study in this rapidly expanding area may unravel the mechanisms by which these agents are involved in mucosal injury, defense, and repair.