Elevated osmolytes in rainbow smelt: the effects of urea, glycerol and trimethylamine oxide on muscle contractile properties

Trimethylamine-N-oxide acutely increases cardiac muscle contractility

Cardiovascular disease is a major cause of morbidity and mortality among patients with chronic kidney disease (CKD). Trimethylamine-N-oxide (TMAO), a uremic metabolite that is elevated in the setting of CKD, has been implicated as a nontraditional risk factor for cardiovascular disease. While association studies have linked elevated plasma levels of TMAO to adverse cardiovascular outcomes, its direct effect on cardiac and smooth muscle function remains to be fully elucidated. We hypothesized that pathological concentrations of TMAO would acutely increase cardiac and smooth muscle contractility. These effects may ultimately contribute to cardiac dysfunction during CKD. High levels of TMAO significantly increased paced, ex vivo human cardiac muscle biopsy contractility (P < 0.05). Similarly, TMAO augmented contractility in isolated mouse hearts (P < 0.05). Reverse perfusion of TMAO through the coronary arteries via a Langendorff apparatus also enhanced cardiac contractility (P < 0.05). In contrast, the precursor molecule, trimethylamine (TMA), did not alter contractility (P > 0.05). Multiphoton microscopy, used to capture changes in intracellular calcium in paced, adult mouse hearts ex vivo, showed that TMAO significantly increased intracellular calcium fluorescence (P < 0.05). Interestingly, acute administration of TMAO did not have a statistically significant influence on isolated aortic ring contractility (P > 0.05). We conclude that TMAO directly increases the force of cardiac contractility, which corresponds with TMAO-induced increases in intracellular calcium but does not acutely affect vascular smooth muscle or endothelial function of the aorta. It remains to be determined if this acute inotropic action on cardiac muscle is ultimately beneficial or harmful in the setting of CKD.NEW & NOTEWORTHY We demonstrate for the first time that elevated concentrations of TMAO acutely augment myocardial contractile force ex vivo in both murine and human cardiac tissue. To gain mechanistic insight into the processes that led to this potentiation in cardiac contraction, we used two-photon microscopy to evaluate intracellular calcium in ex vivo whole hearts loaded with the calcium indicator dye Fluo-4. Acute treatment with TMAO resulted in increased Fluo-4 fluorescence, indicating that augmented cytosolic calcium plays a role in the effects of TMAO on force production. Lastly, TMAO did not show an effect on aortic smooth muscle contraction or relaxation properties. Our results demonstrate novel, acute, and direct actions of TMAO on cardiac function and help lay the groundwork for future translational studies investigating the complex multiorgan interplay involved in cardiovascular pathogenesis during CKD.

Red muscle function and thermal acclimation to cold in rainbow smelt, Osmerus mordax, and rainbow trout, Oncorhynchus mykiss

Climate change affects the thermal environment of aquatic organisms. Changes in the thermal environment may affect muscle function in the eurythermal rainbow smelt, Osmerus mordax, and relatively more stenothermal rainbow trout, Oncorhynchus mykiss. Literature suggests that the trout will be more sensitive to changes in environmental temperature, as they experience a more limited range of environmental temperatures. To examine the effects of thermal environment on red muscle function, both the smelt and trout were thermally acclimated to either a warm (12-15°C) or cold (4-5°C) temperature, after which studies of swimming performance and muscle mechanics were performed. The data on swimming performance and maximum muscle shortening velocity in rainbow smelt were previously published. In both species, cold-acclimated (CA) fish swam with a significantly faster maximum aerobic swimming speed than warm-acclimated fish, when tested at a common temperature of 10°C. Similarly, CA smelt and trout had faster red muscle contraction kinetics. However, smelt displayed a greater shift in contractile properties, such as having a significant shift in maximum muscle shortening velocity that was not observed in trout. The smelt red muscle outperformed trout, with twitch and tetanic times of relaxation being significantly faster for CA smelt compared with CA trout, especially when contraction kinetics were tested at 2°C. The smelt shows a greater thermal acclimation response compared with trout, with more robust increases in maximum swimming speed and faster muscle contractile properties. These differences in acclimation response may contribute to understanding how smelt and trout cope with climate change.