Endothelin-1 (ET-1), a peptide hormone best known for its vasoconstrictor action, also produces a potent positive inotropic effect and is suggested to contribute to myocardial contractility in a number of species, including man. However, the mechanism of this positive inotropic action remains unclear. We have previously shown in rat ventricular myocytes that ET-1 inhibits a steady-state background K+ current, an effect that may contribute to its positive inotropic action through prolongation of the action potential duration (APD) (James et al. 2001). As part of our investigations into the positive inotropic effect of the peptide hormone, we have examined heterogeneity in the response to ET-1 of rat ventricular myocytes.
Ventricular myocytes were isolated enzymatically from hearts excised under pentobarbitone anaesthesia (150 mg kg-1 I.P.) from adult male Wistar rats according to the UK legislation. Myocytes were superfused with Tyrode solution at 35 °C (pH 7.35) and field-stimulated at 0.3 Hz. Myocyte images were acquired via a CCD camera and cell length measured in real-time by edge-detection. The responses to ET-1 were analysed from the average of 20 traces in the absence and presence of the peptide hormone at steady-state. Data are expressed as means ± S.E.M. and analysed by Student’s paired and unpaired t tests. ET-1 (1 nM) increased cell shortening by 91.4 ± 11.5 % from 4.7 ± 0.3 to 8.5 ± 0.6 mm (n = 37; P < 0.001). ET-1 also increased the maximum rate of shortening by 85.6 ± 9.6 % from 115.9 ± 7.3 to 203.0 ± 13.2 mm s-1 (n = 37; P < 0.001) and increased the thalf of relaxation by 8.2 ± 2.6 % from 36.9 ± 1.5 ms to 39.3 ± 1.4 ms (n =37; P < 0.05). There was a notable degree of variability between cells in responses to ET-1, the increase in cell shortening ranging from 4.8 to 360.9 %. It is well known that the APD varies across the left ventricular free wall due to differences in K+ current density and this has been suggested to produce heterogeneity in Ca2+ influx (Volk et al. 1999). Therefore, the effects of ET-1 were investigated on myocytes isolated from the endocardial and epicardial regions. The mean increase in cell shortening was significantly larger in epicardial (3.5 ± 0.4 µm; n = 11) than endocardial cells (2.2 ± 0.5 µm; n = 12; P < 0.05), but this difference was not statistically significant.
Further studies are required to establish the mechanism of action of ET-1 and the basis to the large inter-cell variability in response to the hormone.
This work is kindly supported by the British Heart Foundation.
All procedures accord with current UK legislation.