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The colour in this Doppler ultrasound image (longitudinal section) of a rainbow trout helps researchers visualize the direction of blood flow through its heart. Photo: Kathleen Ma.

Ultrasound shows fish heart function

How do you take ultrasound images of a fish?

That question often came up when I told people about my work at the Western College of Veterinary Medicine (WCVM) last summer. To take the images, we anesthetized each fish and then placed it belly and chest up in a foam trough within a handmade holding container. Water containing an anesthetic drug was continuously pumped past the fish's gills so it remained unconscious throughout the procedure.

As a member of the research team led by WCVM associate professor Lynn Weber, I took dozens of ultrasound images of fish as we studied the effects of benzo[a]pyrene (BaP), an environmental toxicant, on the heart function of juvenile rainbow trout.

BaP, which is created through the combustion of organic materials, is readily found and formed in the environment. Smoking, breathing in car exhaust fumes and eating charbroiled or burned barbecue meats are all examples of people's possible, everyday exposure to BaP.

Unlike mammals, fish are primarily exposed to BaP in a different manner. "In the wild, fish are exposed to BaP from petroleum that is dissolved in water, present in sediments and throughout the food chain," explains PhD student Fred Leal, another member of Weber's research team.

He adds that different species of fish show varying sensitivities to BaP exposure. "In my research I'm using zebrafish (Danio rerio), which is one of the most resistant fish to BaP, and rainbow trout (Onchorrynchus mykiss), one of the most sensitive [fish species]."

BaP has many effects on body systems including the ability to potentially cause genetic mutations and cancerous tumours. Researchers have also suggested that BaP may have a role in cardiotoxicity (leading to heart damage).

Zebrafish is an ideal model for studying human cardiotoxicity because exposure to environmental contaminants such as BaP causes similar responses and deformities in both species, explains Leal. This research project will help us to better understand the role of BaP on genetic regulation of energy metabolism, as well as the effects of BaP on the offspring of exposed fish.

Linked to Leal's experiments, my project's goal was to determine if and how short-term exposure to BaP shows any evidence of heart damage in rainbow trout.

This information is important because fish with decreased hemodynamic (blood flow) function may be less capable of adapting to physiologically challenging situations such as an increase in water temperature (which causes a decrease in dissolved oxygen). Fish that are less able to adapt will also have lower fitness levels.

Using ultrasound technology, we can non-invasively image and record videos of the fish's beating heart, and we can also measure the heart's blood flow properties. By comparing results from the control group of fish to results from the group of fish that were exposed to BaP, we hope to detect any changes in blood flow function.

In previous experiments, former master's student Courtney Gerger also used high-frequency ultrasound to detect changes in heart function in zebrafish that were exposed to BaP for short periods of time.

Her results showed an increase in the atrioventricular conduction ratio (AV ratio) in zebrafish which can be indicative of AV block (impairment of conduction between the atria and ventricles of the heart) and arrhythmias.

"I also saw a general trend of … decreased cardiac output with BaP exposure," explains Gerger.

As rainbow trout are considered to be more sensitive to BaP, we expected similar if not amplified disturbances in blood flow function in our study's treatment groups. Length of time of exposure and differences in route of exposure (via ingestion or diffusion through the skin) are also factors that will play into the presence and severity of toxic effects on the fish.

While our study's results are still being analyzed, Leal says the findings will have applications for human health as well as for the maintenance of ecosystems.

The Natural Sciences and Engineering Research Council of Canada's Undergraduate Summer Research Awards program and the WCVM's Interprovincial Undergraduate Student Summer Research program provided funding for this research.

 

Kathleen Ma of Burnaby, B.C., is a second-year veterinary student who was part of the WCVM's Undergraduate Summer Research and Leadership program in 2014. Kathleen's story is part of a series of articles written by WCVM summer research students.
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