SAN FRANCISCO, Jan. 31 (Xinhua) -- A team of researchers has identified a substance that can cause severe cardiovascular effects in fish exposed to crude oil spills and, as it is abundant in air pollution, could pose a global threat to human health.
The oil pollutant, known as phenanthrene, is a type of polycyclic aromatic hydrocarbon (PAH) found in water, air and soil due to widespread use of petroleum.
"By carefully isolating heart cells from tunas, Olympians of the sea, and using electrophysiological and confocal microscopy techniques, we recorded ionic currents and found exactly where phenanthrene blocks the heart excitation-contraction coupling pathway, which is the link between the on-off switch, or excitation, and the contraction that powers every heart beat," said Stanford University professor Barbara Block and senior author of a paper published Tuesday in Nature Scientific Reports.
Experiments at Stanford's Hopkins Marine Station, have provided direct evidence of how phenanthrene impacts the heart, showing how it causes both irregular heartbeat (arrhythmia) and weaker contractions of heart cells in all three species tested: bluefin and yellowfin tunas and mackerel. At the cellular level, these athletic fish with remarkable aerobic poise are similar to higher vertebrates, which include mammals and birds. This suggests that the cardio-toxicants may also act upon the hearts of higher vertebrates, as all of these animals have similar methods of regulating the activation of heart cells.
"The mechanism which alters cardiac function in fish and the protein that phenanthrene targets - the ion channel responsible for potassium movement from the cell - is also present in humans," Fabien Brette, a research associate at Stanford University at the time of the study and co-lead author of the current paper, was quoted as saying in a news release from Stanford. "What we measured on fish cardiac cells can occur on human cardiac cells and this could mean risk of sudden death."
Also found in land-based stormwater runoff, contaminated soil from defunct industrial sites and air pollution, PAHs have been investigated as cancer-causing chemicals for nearly a hundred years. The environmental health risks of phenanthrene, in particular, have received secondary consideration to other PAHs more strongly implicated in the development of cancer. Urban air pollution, laden with PAHs, has been implicated in cardiac distress.
The current study points the finger at phenanthrene, which could enter the bloodstream through respiratory pathways such as breathing.
Each beat of a heart is dependent upon the exquisite timing of ion channels opening and closing to pace the cardiac rhythmicity. Once individual heart cells were isolated from tunas, the researchers used confocal microscopy and patch clamping, a technique enabling electrical recordings of the cardiac ion currents, to test how the different PAHs affected the strength and the rhythm of the cells' contractions.
They showed that phenanthrene disrupts cardiac excitation-coupling, the process linking excitability to muscle contraction, at three key levels. After exposure to phenanthrene, calcium ion movement into and out of the heart cells was disrupted and reduced. This exposure also led to irregularities in two major ionic currents, including a significant disruption of the outward flow of potassium ions. And the adverse changes in heat muscle cell function manifested within seconds, indicating that exposure to phenanthrene causes disruption immediately and is more than a chronic issue.
"In the short term, cardiac dysfunction in these fish can affect really metabolically demanding activities like swimming or reproduction or foraging. In the long term, if you have heart function compromised, it could be fatal," said Holly Shiels, associate professor of life sciences at the University of Manchester in England and co-lead author on the study.
In Addition, the researchers suggest that atmospheric phenanthrene deserves more attention for the possible impact it could have on the cardiovascular health of people. "This paper shows that phenanthrene has the properties of many drugs that cause abnormal heart rhythms as serious and potentially life-threatening side effects," said John Incardona, a developmental biologist with the U.S. National Oceanic and Atmospheric Administration (NOAA) Fisheries Ecotoxicology Program in Seattle, Washington state.