While studying the impact of the 2010 Deepwater Horizon oil spill on tuna, scientists have discovered that crude oil interferes with fish heart cells.
The toxic consequence is a slowed heart rate, reduced cardiac contractility, and irregular heartbeats that can lead to cardiac arrest and sudden cardiac death.
Crude oil is a complex mixture of chemicals, some of which are known to be toxic to marine animals. Past research has focused in particular on “polycyclic aromatic hydrocarbons” (PAHs), which can also be found in coal tar, creosote, air pollution, and stormwater runoff from land. In the aftermath of an oil spill, PAHs can persist for many years in marine habitats and cause a variety of adverse environmental effects.
The researchers report that oil interferes with cardiac cell excitability, contraction, and relaxation—vital processes for normal beat-to-beat contraction and pacing of the heart.
Their tests revealed that very low concentrations of crude oil disrupt the specialized ion channel pores—where molecules flow in and out of the heart cells—that control heart rate and contraction in the cardiac muscle cell.
This cyclical signaling pathway in cells throughout the heart is what propels blood out of the pump on every beat. The protein components of the signaling pathway are highly conserved in the hearts of most animals, including humans.
As reported in Science, the researchers found that oil blocks the potassium channels distributed in heart cell membranes, increasing the time to restart the heart on every beat. This prolongs the normal cardiac action potential, and ultimately slows the heartbeat.
The potassium ion channel impacted in the tuna is responsible for restarting the heart muscle cell contraction cycle after every beat, and is highly conserved throughout vertebrates, raising the possibility that animals as diverse as tuna, turtles, and dolphins might be affected similarly by crude oil exposure. Oil also resulted in arrhythmias in some ventricular cells.
“The ability of a heart cell to beat,” explains Barbara Block, a professor of marine sciences at Stanford University, “depends on its capacity to move essential ions like potassium and calcium into and out of the cells quickly. This dynamic process, which is common to all vertebrates, is called ‘excitation-contraction coupling.’
“We have discovered that crude oil interferes with this vital signaling process essential for our heart cells to function properly.” Block is also a senior fellow at the Stanford Woods Institute for the Environment.
“We’ve known from NOAA research over the past two decades that crude oil is toxic to the developing hearts of fish embryos and larvae, but haven’t understood precisely why,” says coauthor Nat Scholz, leader of the Ecotoxicology Program at NOAA’s Northwest Fisheries Science Center in Seattle.
“These new findings more clearly define petroleum-derived chemical threats to fish and other species in coastal and ocean habitats, with implications that extend beyond oil spills to other sources of pollution such as land-based urban stormwater runoff.”
Risk to humans
The new study also calls attention to a previously underappreciated risk to wildlife and humans, particularly from exposure to cardioactive PAHs that also exist at relatively enriched levels in air pollution.
“When we see these kinds of acute effects at the cardiac cell level,” Block says, “it is not surprising that chronic exposure to oil from spills such as the Deepwater Horizon can lead to long-term problems in fish hearts, as our NOAA colleagues have observed in studies of larval fish development.
“The protein ion channels we observe in the tuna heart cells are similar to what we would find in any vertebrate heart and provide evidence as to how petroleum products may be negatively impacting cardiac function in a wide variety of animals. This raises the possibility that exposure to environmental PAHs in many animals—including humans—could lead to cardiac arrhythmias and bradycardia, or slowing of the heart.”
Electronic tagging and fisheries catch data indicate that Atlantic bluefin spawn in the area where the Deepwater Horizon drilling rig collapsed, raising the possibility that eggs and larvae, which float near the surface waters, were exposed to oil.
The spill occurred in the major spawning ground of the western Atlantic population of bluefin tuna in the Gulf of Mexico. The most recent stock assessment, conducted in 2012, estimated the spawning population to be at only 36 percent of the 1970 baseline population. Additionally, many other pelagic fishes were also likely to have spawned in oiled habitats, including yellowfin tuna, blue marlin, and swordfish.
Living heart cells
Taking advantage of captive populations of bluefin and yellowfin tunas at the Tuna Research and Conservation Center, the research team was able to directly observe the effects of crude oil samples collected from the Gulf of Mexico on living fish heart cells.
Block and her team bathed isolated cardiac cells from the tuna in low dose crude oil concentrations similar to what fish in early life stages may have encountered in the surface waters where they were spawned after the April 2010 oil spill in the Gulf of Mexico.
They measured the heart cells’ response using a combination of electro-physiological techniques to record how ions flowed into and out of the heart cells, and to identify the specific proteins in the excitation-contraction pathway that were affected by crude oil chemical components.
“We can examine the function of healthy heart cells in vitro and actually measure in the microscope how they respond to the presence of crude oil in real time,” says Fabien Brette, a research associate in Block’s lab and lead author on the study.
“The normal sequence and synchronous contraction of the heart requires rapid activation in a coordinated way of the heart cells,” Block says. “Like detectives, we dissected this process using laboratory physiological techniques to ask where oil was impacting this vital mechanism.”
The research, published in Science, is part of the ongoing Natural Resource Damage Assessment of the April 2010 Deepwater Horizon oil spill.
The authors will present the work at the 2014 annual meeting of the American Association for the Advancement of Science in Chicago.
NOAA, Stanford, and the Monterey Bay Aquarium Foundation funded the study.
Source: Stanford University