Researchers Hope to Devise Test for Respiratory Failure

Vol. 15 •Issue 17 • Page 13
Researchers Hope to Devise Test for Respiratory Failure

Medical science has devised a simple blood test to determine whether someone has suffered a “silent” heart attack. Why not develop a comparable test to determine whether someone has suffered a “breath attack,” i.e., respiratory failure?

Admittedly, respiratory failure is not as lethal as heart failure. The heart, after all, is a solo act, with no backup muscle waiting in the wings like an understudy to take over its blood-pumping function. If the heart muscle gives out, so does its owner. The diaphragm, on the other hand, should it malfunction or grow fatigued, has a supporting cast of intercostals, abdominals and even, superficially at least, neck muscles that can step in and facilitate inspiration/expiration.

Never mind. Steve Iscoe, PhD, and his colleagues still want to develop a simple blood test that can indicate the presence of respiratory failure similar to the test cardiologists use to detect heart attacks.

“We hope to develop a test which would allow the clinician to detect a ‘breath attack’, which may or may not involve frank respiratory fatigue or failure,” Iscoe told ADVANCE. At the very least, such a test “would indicate that some sort of dysfunction is present,” he said.

Devising a simple ELISA test for respiratory failure like that used by cardiologists for heart failure “is still a long way off,” he conceded. “But the cardiac test does provide a useful precedent.”

REGULATORY PROTEINS

To achieve their goal, Iscoe and his colleagues, protein biochemist Jennifer Van Eyk and a PhD student, Jeremy Simpson, must determine the molecular basis for muscle weakness and/or fatigue.

They are focusing on two regulatory proteins in muscle cells known as troponins I and T. These two, along with troponin C, form a complex that regulates interactions between the thick (myosin) and thin (actin) muscle filaments.

Muscles contract thanks to the formation and release of connections called “cross-bridges” between actin and myosin in a ratchet-like manner, Iscoe said.

People who have suffered heart attacks are now regularly tested for the presence of these proteins in their serum, he added, although there is debate about what exactly is being measured and the sensitivity of the test.

“Any change in the structure of a member of the troponin complex is likely to affect the ability of the muscle to contract,” he said. In fact, a small change in the structure of troponin I in the hearts of laboratory mice results in their premature deaths.

STRESS MODIFICATIONS

Iscoe and his co-workers seek to characterize how disease and other forms of stress change or modify the members of the troponin complex. They have studied these proteins by analyzing blood taken for other purposes (and which would otherwise be discarded) from patients with respiratory disease. They are also examining rats with respiratory muscle fatigue, which they have induced by stressing the diaphragm and other inspiratory muscles via resistive-loaded breathing.

“We detect fatigue because we can measure the output of the muscles, the pressure generated at the trachea, the trans-diaphragmatic pressure and what the rat ‘wants’ to do, by measuring electrical activity in the phrenic nerve innervating the diaphragm,” he said.

When muscle output fails to increase or decrease despite an increase in phrenic nerve activity, “we know fatigue is present.”

However, a complication stands in the way of correctly categorizing modifications to troponin I and T. These proteins can undergo many modifications such as oxidation, glycosylation, phosphorylation, cleavage and covalent complex formation.

“We suspect that the nature of the modification depends on the nature of the stress,” said Iscoe, whose research is funded by the Canadian Institutes for Health Research and the Ontario Thoracic Society. “Our ability to detect these modifications depends on the antibodies we use to bind to the different forms of the proteins. Obviously, the fewer antibodies available, the fewer modifications we can detect.”

SKELETAL MUSCLES

By refining their methods for detecting and characterizing the troponin proteins, whether intact or modified, after they are released into the blood, the team hopes to perfect a simple blood test to detect and quantify respiratory muscle injury and/or fatigue.

“We hope, eventually, to take a small sample of blood, about 2 microliters, and, using the correct antibodies as part of an ELISA test, determine the functional status of the respiratory muscles,” Iscoe said. “But before we can do this, we need to understand what modifications the proteins undergo. To do this, we need the correct antibodies.”

Blood tests for detecting heart attacks became available because of the clinical and commercial importance of swift intervention when the heart muscle fails. The situation is more complicated for the respiratory muscles for two major reasons, according to Iscoe.

“First, the respiratory muscles are skeletal muscles, so we have to ensure that what we are measuring comes from them and not some other muscle,” he said. “Second, unlike the heart, which has only one isoform of troponin I, skeletal muscle has two—the fast and slow isoforms, associated with fast- and slowly-contracting fibers, respectively. What we detect and how we use that for diagnosis may well depend on which isoform is released.

So far, we have only a few good antibodies to detect skeletal as opposed to cardiac troponin I, he said.

You can reach Michael Gibbons at mgibbons@merion.com.

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