Cardiac function in an endothermic fish: cellular mechanisms for overcoming acute thermal challenges during diving.

Understanding the physiology of vertebrate thermal tolerance is critical for predicting how animals respond to climate change. Pacific bluefin tuna experience a wide range of ambient sea temperatures and occupy the largest geographical niche of all tunas. Their capacity to endure thermal challenge is due in part to enhanced expression and activity of key proteins involved in cardiac excitation-contraction coupling, which improve cardiomyocyte function and whole animal performance during temperature change. To define the cellular mechanisms that enable bluefin tuna hearts to function during acute temperature change, we investigated the performance of freshly isolated ventricular myocytes using confocal microscopy and electrophysiology. We demonstrate that acute cooling and warming (between 8 and 28°C) modulates the excitability of the cardiomyocyte by altering the action potential (AP) duration and the amplitude and kinetics of the cellular Ca(2+) transient. We then explored the interactions between temperature, adrenergic stimulation and contraction frequency, and show that when these stressors are combined in a physiologically relevant way, they alter AP characteristics to stabilize excitation-contraction coupling across an acute 20°C temperature range. This allows the tuna heart to maintain consistent contraction and relaxation cycles during acute thermal challenges. We hypothesize that this cardiac capacity plays a key role in the bluefin tunas' niche expansion across a broad thermal and geographical range.

Hypercapnic acidosis reduces contractile function in the ventricle of the armored catfish, Pterygoplichthys pardalis.

The armored catfish, Pterygoplichthys pardalis (formerly Liposarcus pardalis), is a freshwater, facultative air-breathing teleost that experiences seasonal hypercapnia in the water systems of South America. We studied the tolerance of the P. pardalis heart to hypercapnic acidosis using an isolated ventricular muscle strip preparation. Force generation and kinetic variables were examined across a range of contraction frequencies under normocapnic and hypercapnic conditions in the absence and presence of sarcoplasmic reticulum (SR) inhibitors. Pterygoplichthys pardalis ventricle exhibited robust contractile force, on par with athletic fish species such as trout and tuna and a relatively flat force-frequency relationship between 0.2 and 1.5 Hz under normocapnic conditions (1% CO2, pH 7.78 +/- 0.02). Hypercapnic acidosis (7.5% CO2, pH 7.78 +/- 0.03) did not alter the shape of the force-frequency response but reduced force by approximately 50% across all frequencies tested, with only partial recovery upon return to normocapnic conditions. A subsequent and more severe acidotic challenge (15% CO2, pH 6.77 +/- 0.05) caused an additional 20% decrease in force. Force recovered to the level at which it had stablized after the first hypercapnic insult. SR inhibition had no steady state effect on force production at 0.2 Hz but resulted in a negative force-frequency relationship, suggesting that SR Ca2+ is recruited to a greater extent at high contraction frequencies. Surprisingly, SR-inhibited muscle was more resistant to hypercapnic acidosis (force decreased by approximately 40% across all frequencies) and displayed improved recovery upon return to normocapnic conditions. The significance of this latter finding is not clear. In aggregate, our results demonstrate robust contractile force, which extends across a range of frequencies and appears to be supported by SR Ca2+ cycling. Hypercapnic acidosis reduced contractile force but may provide preconditioning-like protection from subsequent insults.

Effect of thermal acclimation on action potentials and sarcolemmal K+ channels from Pacific bluefin tuna cardiomyocytes.

To sustain cardiac muscle contractility relatively independent of temperature, some fish species are capable of temporarily altering excitation-contraction coupling processes to meet the demands of their environment. The Pacific bluefin tuna, Thunnus orientalis, is a partially endothermic fish that inhabits a wide range of thermal niches. The present study examined the effects of temperature and thermal acclimation on sarcolemmal K(+) currents and their role in action potential (AP) generation in bluefin tuna cardiomyocytes. Atrial and ventricular myocytes were enzymatically isolated from cold (14 degrees C)- and warm (24 degrees C)-acclimated bluefin tuna. APs and current-voltage relations of K(+) channels were measured using the whole cell current and voltage clamp techniques, respectively. Data were collected either at the cardiomyocytes' respective acclimation temperature of 14 or 24 degrees C or at a common test temperature of 19 degrees C (to reveal the effects of acclimation). AP duration (APD) was prolonged in cold-acclimated (CA) cardiomyocytes tested at 14 degrees C compared with 19 degrees C and in warm-acclimated (WA) cardiomyocytes tested at 19 degrees C compared with 24 degrees C. This effect was mirrored by a decrease in the density of the delayed-rectifier current (I(Kr)), whereas the density of the background inward-rectifier current (I(K1)) was unchanged. When CA and WA cardiomyocytes were tested at a common temperature of 19 degrees C, no significant effects of temperature acclimation on AP shape or duration were observed, whereas I(Kr) density was markedly increased in CA cardiomyocytes. I(K1) density was unaffected in CA ventricular myocytes but was significantly reduced in CA atrial myocytes, resulting in a depolarization of atrial resting membrane potential. Our results indicate the bluefin AP is relatively short compared with other teleosts, which may allow the bluefin heart to function at cold temperatures without the necessity for thermal compensation of APD.