Nitric oxide and the plasticity of cardiac performance to temperature changes in the teleost Anguilla anguilla

The intrinsic regulation of the heart, i.e. the Frank-Starling Law, is a fundamental property of the vertebrate myocardium which allows that, when the end-diastolic volume increases, 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 Starling response. With the use of isolated working heart preparations, where cardiac loading and heart rate can be carefully controlled, the focus of the present study was to investigate the relationship between NO modulation of Starling response and temperature challenges in the eel. The results showed that while NO Synthase (NOS) inhibition by L-N5(1-iminoethyl)ornithine (L-NIO) significantly reduced the Frank-Starling response when the experiments were performed at the same acclimation temperature (i.e. 20°C or 10°C), it was without effect when the experiments were carried out under thermal shock conditions (i.e. from 20°C to 15 or 10°C; from 10°C to 15 or 20°C), suggesting that in A. anguilla NO modulation of the Starling response is abolished by an acute thermal change. Western blotting analysis revealed a decrement of peNOS and pAkt expression in samples subjected to thermal shock compared to the control (heart working at the same acclimation temperature). Moreover, an increase in Hsp90 protein levels was observed under heat thermal stress. Taken together, these data suggest that the NOS-NO-dependent modulation of the principal cardiac regulatory mechanism in fish is sensitive to thermal shocks.