Over the past three years, the physiology team at the TRCC has primarily focused on the cardiac physiology of tunas and other scombrid fishes. The tuna heart is larger, beats faster, and pumps blood faster than that of most other bony fishes. The TRCC team has applied a multidisciplinary approach integrating biochemistry, electrophysiology, microscopy, and tissue physiology studies to answer two main questions: 1) What cellular and physiological specializations are responsible for the exceptional cardiac performance of tunas, and 2) How does the tuna heart respond to temperature. 

Beginning at the whole-heart level, the team has studied cardiac performance and temperature sensitivity in yellowfin tuna and bluefin tuna, measuring heart rate, stroke volume and cardiac output at different temperatures. These experiments show that the hearts of both species are temperature sensitive, with heart rate and cardiac output slowing at lower temperatures. Notably, the bluefin tuna heart is less-temperature sensitive than that of the yellowfin. This difference helps to explain the thermal ecology of wild tunas, as bluefin tuna routinely migrate and dive into much colder waters than those frequented by yellowfin tuna.

Cellular studies of the tuna heart are focused on how heart muscle cells move calcium to initiate contraction and relaxation. All muscle cells rely on a rise in Ca2+ concentration as the signal to contract. The Ca2+ that triggers contraction in a heart cell can enter the cell from the extracellular space through L-type Ca2+ channels or be released from intracellular organelles known as sarcoplasmic reticulum (SR). Intracellular Ca2+ release (and reuptake) is a faster process than extracellular Ca delivery. Using electrophysiological techniques, we have found that the function of the L-type Ca2+ channels in bluefin tuna is slightly enhanced relative to related fishes. Previous research at the TRCC has indicated that intracellular calcium was more important in tunas than in other bony fishes. Recently, we have measured the activity of the pump protein that mediates intracellular uptake of Ca2+ into the SR in several tuna species as well as mackerel. Our results indicate that the SR is more important in tunas than mackerel. In addition, higher SR pump activity observed in bluefin heart ventricle relative to yellowfin may help to explain the wider thermal tolerance of the bluefin tuna heart and thus, the bluefin tuna.

Ongoing experiments seek to expand upon these results, measure temperature dependence of contractions heart cells, and determine the spatial structure of the sarcoplasmic reticulum.