Nitric oxide improves the hemodynamic performance of the hypoxic goldfish (Carassius auratus) heart.

Goldfish tolerate prolonged and severe hypoxia, thus representing a well-suited model to study the maintenance of cardiac function when O(2) availability represents a limiting factor. Using a working heart preparation, we explored the role of the intracardiac nitric oxide synthase (NOS)-derived nitric oxide (NO) under normoxic and hypoxic conditions. Cardiac performance was examined both under basal (constant preload and afterload) and loading conditions, i.e. preload-induced increases in stroke volume (SV) and hence cardiac output at constant afterload (the Frank-Starling response). Hypoxic hearts showed an increased basal mechanical performance compared to the normoxic counterpart. Under basal conditions, in both normoxic and hypoxic hearts, NOS and soluble guanylyl cyclase (sGC) inhibition increased SV, while exogenous NO supply decreased it. The normoxic heart was very sensitive to filling pressure increases; the maximum SV = 1.08 ± 0.09 mL/kg body mass was obtained at 0.4 kPa. Acute hypoxia increased this sensitivity, SV reaching the maximum value (1.45 ± 0.12 mL/kg body mass) at 0.25 kPa. NOS inhibition by L-NMMA reduced the Frank-Starling response under normoxia, but was ineffective under acute hypoxia, where NO may come from nitrite reduction. In both conditions, sGC inhibition induced a reduction of the cardiac response to preload. Moreover, under acute hypoxia, NO scavenging significantly reduced the Frank-Starling response. The hypoxia-induced hemodynamic patterns were complemented by Western blotting analysis which revealed increased expressions of NOS and hypoxia inducible factor α(HIF-1α). In conclusion, we demonstrated that intracardiac NO/NOS enhances goldfish heart performance, remarkably expanding its hypoxic tolerance.

Effects of temperature on the nitric oxide-dependent modulation of the Frank-Starling mechanism: the fish heart as a case study.

The Frank-Starling law is a fundamental property of the vertebrate myocardium which allows, when the end-diastolic volume increases, that the consequent stretch of the myocardial fibers generates a more forceful contraction. It has been shown that in the eel (Anguilla anguilla) heart, nitric oxide (NO) exerts a direct myocardial relaxant effect, increasing the sensitivity of the Frank-Starling response (Garofalo et al., 2009). With the use of isolated working heart preparations, this study investigated the relationship between NO modulation of Frank-Starling response and temperature challenges in the eel. The results showed that while, in long-term acclimated fish (spring animals perfused at 20 °C and winter animals perfused at 10 °C) the inhibition of NO production by L-N5 (1-iminoethyl)ornithine (L-NIO) significantly reduced the Frank-Starling response, under thermal shock conditions (spring animals perfused at 10 or 15 °C and winter animals perfused at 15 or 20 °C) L-NIO treatment resulted without effect. Western blotting analysis revealed a decrease of peNOS and pAkt expressions in samples subjected to thermal shock. Moreover, an increase in Hsp90 protein levels was observed under heat thermal stress. Together, these data suggest that the NO synthase/NO-dependent modulation of the Frank-Starling mechanism in fish is sensitive to thermal stress.