Background

Although systemic glucocorticoid treatment may have negative impact on the cardiovascular system, for example by increasing blood pressure, it has been shown that glucocorticoid signaling is essential for normal cardiac function and development. We recently demonstrated in isolated rat and human cardiomyocytes that dexamethasone exerts positive inotropic effects and improves excitation-contraction coupling. So far, however, the direct effects of glucocorticoids in human myocardium have remained unclear because most results of previous studies were obtained from animal models and cell culture experiments. 

Methods

We used myocardial tissue samples from 13 end-stage heart failure patients, obtained during heart transplantation or left ventricular (LV) assist device (LVAD) implantation. Myocardial tissue samples were cut into 300µm thick slices and kept in culture for 5 to 7 days. The slices were treated with either 100nM dexamethasone (Dex) or vehicle as control, and were electrically stimulated and beating with 0.5 Hz throughout the time period. We analyzed changes in maximum force (F_max), contraction duration (CD), time to peak force (TTP), time to relaxation (TTR) and maximum beating frequency (Freq_max). In addition to that we determined the force frequency relationship (FFR) and ß-adrenergic responsiveness in both groups. After culture we quantified expression of the cardiac ryanodine receptor (RyR) and the L-type Ca²⁺ channel subunit alpha-1C (CACNA1C) via Western blot analysis. Additionally, mRNA expression of CACNA1c was quantified via RT qPCR. 

Results

Human myocardial slices treated with dexamethasone exhibited a 2-fold increase in contractile force (F_max = 2070±663 µN) when compared with control slices (F_max = 1030±237 µN, p<0.05). When exposing the tissues to increasing pacing rates, we observed that the Dex-treated slices achieved higher beating rates than the control group (Freq_max = 2.76±0.101 Hz vs 2.02±0.254 Hz, p<0.001) and showed a more positive FFR. Furthermore, Dex-treated slices showed a significantly shorter contraction duration (CD90 = 671±52 ms vs 778±76 ms, p<0.05), which was caused mainly by faster force development (TTP = 286±24.8 vs 361±41.7 ms, p<0.05) rather than accelerated relaxation (TTR = 385±31.4 vs 417±38.6 ms, p=0.25). When stimulated with 100nM isoprenaline, both groups showed a comparable force increase (10.5±1.73 fold vs 8.04±1.06 fold, p=0.6, n=12), indicating that increased contractility in Dex did not result from a higher baseline level of beta-adrenergic stimulation. Additionally, we observed a Dex-induced 2.65±1.39 fold increase in RyR protein expression (p<0.05, n=7) and 23±8% increase in CACNA1C mRNA (p<0.05, n=8), although CACNAC1C protein expression did not increase significantly (2±9%, p=0.85, n=7)

Conclusions

In-vitro glucocorticoid treatment leads to an increase of contractile force and contraction velocity, and to an improved capability to follow high pacing frequencies in failing human myocardium. In addition to that it has a positive impact on the FFR. These effects might be caused by an increased expression of the ryanodine receptor and the L-Type Ca²⁺channel and subsequenty improved excitation-contraction coupling.