Body's adaptive response to low-dose irradiation

Updated - September 15, 2011 02:51 am IST

Published - September 14, 2011 11:45 pm IST

A growing number of specialists think that low dose radiation exposure is not as harmful as was thought of so far.

Recently, Italian researchers claimed that low dose radiation exposure at levels considered safe by regulatory bodies can induce biological and cellular changes that might offset the hazards of radiation.

Probably, at low doses there may be protective mechanisms at work.

They studied interventional cardiologists — a group most exposed to ionizing radiation among health professionals.

Per capita exposure

They have a per capita exposure 2 to 3 times higher than that of radiologists; their exposure has increased steadily in the past 20 years.

The researchers led by Dr Gian Luigi Russo, Institute of Food Sciences, National Research Council, Rome, reported their study on-line, in the August 23, 2011 issue of the European Heart Journal.

The study groups included ten healthy interventional cardiologists with an average age of 38 years, with a median radiation exposure of 4.7 mSv per year and lifetime exposure ranging between 20 to 100 mSv, working in the cardiac catheterization unit and 10 matched unexposed controls recruited from among the hospital and laboratory workers who did not have radiation exposure. (The annual dose limit to radiation workers recommended by the International Commission on Radiological Protection is 20 mSv averaged over five years with the exposure not to exceed 50 mSv in any one year)

The researchers measured many markers of oxidative metabolism in plasma, and in other blood constituents such as red blood cells (erythrocytes) and white blood cells (lymphocytes) in the two groups.

In the exposed group, there was a threefold increase in hydrogen peroxide, a biochemical marker indicative of oxidative stress. Not surprising, as water is a major constituent of human body.

Antioxidant level

The researchers expected that the antioxidant level in the exposed group will be different compared with that in the unexposed controls. However, they did not observe any significant change in the level in the two groups. May be because the bodies of the exposed groups generated adaptive response by activating antioxidant defence mechanisms to counteract the increase in hydrogen peroxide levels.

The study demonstrated that in interventional cardiologists, chronic exposure to low dose radiation induces two specific types of cellular defences against oxidative stress.

Firstly, the oxidative stress was found to be counterbalanced by a 1.7 fold increase in glutathione, a measure of antioxidant response in the exposed group.

Secondly, the cardiologists exposed to radiation had significantly higher levels of caspase-3 activity in their white blood cells.

Programmed cell deaths

Capase-3 is a biochemical which is implicated in programmed cell deaths. Capase-3 helps to eliminate damaged cells.

Both mechanisms may compensate for the unbalanced reactive oxygen species and contribute to maintain relatively stable equilibrium in the cell.

Though the researchers saw adaptive responses at low dose radiation, they did not go overboard.

“The unprecedented radiation exposure of the interventional cardiologists and the staff in the cardiac catheterization laboratory represents a challenge and an opportunity for the cardiology community”, the researchers clarified. They urged the physicians to minimize radiation injury hazard to their patients, to their professional staff and to themselves.

“It is also an opportunity, since highly exposed interventional cardiologists are a suitable, perhaps unique, model to understand the still elusive mechanisms to biological adaptation to chronic exposure to low dose radiation”, the researchers suggested.

The emphasis

The findings of the present study emphasize that a level of radiation exposure considered ‘safe' by regulatory standards can induce profound biochemical and cellular adaptation.

We do not know whether these are adaptive modifications or hint at other relevant pathologies.

Their clinical meaning remains uncertain. If cellular damage, if any, due to irradiation is repaired, are we sure that the repair is perfect?

This preliminary study, if extended suitably, has the potential to provide answers to many vexing questions in the field of prolonged, low level radiation exposure.

K.S. PARTHASARATHY (Raja Ramanna Fellow in the Department of Atomic Energy)

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