Inotropic response to endothelin-1, isoprenaline and calcium in cardiomyocytes isolated from endotoxin treated rats: Effects of ethyl-isothiourea and dexamethasone

Abstract

1. The contractile effects of endothelin-1, isoprenaline and extracellular calcium were assessed on ventricular cardiomyocytes isolated from lipopolysaccharide-treated rats. The involvement of nitric oxide was investigated using dexamethasone (in vivo) and ethyl isothiourea (in vitro). 2. Male Wistar rats (n = 70) were injected with either saline (1 ml kg-1) or lipopolysaccharide (LPS; 5 mg kg-1) alone, or following pre-treatment with dexamethasone (DEX + LPS; 5 mg kg-1). Ventricular cell shortening was recorded using a video edge detection system, and concentration-response relationships were established for endothelin-1, isoprenaline and calcium, in the absence or presence of ethyl isothiourea (ETU; 10 μM). iNOS expression was assessed using reverse transcription-polymerase chain reaction. 3. iNOS mRNA expression was greater (P < 0.001) in the LPS (iNOS/GAPDH ratio: 0.90 ± 0.09) treated group compared to saline (iNOS/GAPDH ratio: 0.36 ± 0.02). Baseline contractile amplitude was reduced (P < 0.05) in the LPS (7.3 ± 0.2 μm) and DEX + LPS groups (6.7 ± 0.3 μm) compared to saline (8.0 ± 0.2 μm). 4. The concentration-dependent contractile response to endothelin-1 was attenuated (P < 0.05) in the LPS group compared to saline (maximum change: 0.45 ± 0.2 vs 1.8 ± 0.2 μm). Neither ETU nor dexamethasone improved contractile function in the LPS-treated animals. 5. The concentration-dependent increase in the contractile response to isoprenaline was attenuated in the LPS-treated group compared to saline (P < 0.05; maximum change: 1.7 ± 0.4 vs 3.1 ± 0.4 μm). This effect was reversed by ETU (maximum change: 3.7 ± 0.6 μm). Pre-treatment with dexamethasone prevented a significant fall in contraction amplitude (maximum change: 2.4 ± 0.4 μm). 6. The contractile response to calcium was reduced (P < 0.05) in the LPS group compared to saline (maximum change: 8.7 ± 0.6 vs 10.7 ± 0.8 μm). Neither ETU nor dexamethasone restored contractile function in the LPS-treated group. 7. In conclusion, a nitric oxide-mediated inhibitory pathway is not responsible for the diminished contractile response to either endothelin-1 or extracellular calcium, but contributes to the hyporesponsiveness to isoprenaline in lipopolysaccharide treated rats.