The beat goes on: a vertebrate heart that beats without oxygen

Scientists have discovered that the heart of a carp keeps beating when it is starved of oxygen for five days. “This is the first time that a vertebrate heart has been shown to survive such prolonged periods without oxygen and actually keep beating at the same rate as when oxygen is available” says Jonathan Stecyk (Simon Fraser University), presenting his latest results at the annual SEB meeting in Edinburgh this week.

“If I were to take oxygen away from your heart it would die in two minutes” says Mr. Stecyk. “But the heart of a carp survives and keeps beating as normal for five days without oxygen.” Crucian carp live in shallow ponds in Scandinavia, which are sealed off by ice in winter. Animals have two problems in such low oxygen environments: they cannot produce enough energy, and have to cope with the build-up of lactic acid that is produced by anaerobic respiration.

“There are two possible coping strategies in low oxygen conditions” says Mr. Stecyk “Either the animal lowers its demand for energy, or it increases the rate of processes that deliver energy.” Carp rely on the second method, which requires a large supply of fuel. Luckily carp have the largest livers among vertebrates, with the largest energy stores relative to body size. By having a heart that goes on, the fish can move this fuel around the body. They can also shuttle lactic acid to the muscles, the only tissue that can convert lactic acid to ethanol, which is then transferred to the gills where it can be excreted. Maintaining circulation allows the fish to avoid the build-up of waste products which could eventually poison the animal. In the face of their hostile environments, carp have developed a complex survival strategy relying on the unique ability of their hearts to survive without oxygen.