Cardiac electrophysiological effects of U-50,488H on guinea-pig papillary muscle

The opioid tramadol blocks the cardiac sodium channel Nav1.5 in HEK293 cells

AIMS

Opioids are associated with increased risk of sudden cardiac death. This may be due to their effects on the cardiac sodium channel (Nav1.5) current. In the present study, we aim to establish whether tramadol, fentanyl, or codeine affects Nav1.5 current.

METHODS AND RESULTS

Using whole-cell patch-clamp methodology, we studied the effects of tramadol, fentanyl, and codeine on currents of human Nav1.5 channels stably expressed in HEK293 cells and on action potential (AP) properties of freshly isolated rabbit ventricular cardiomyocytes. In fully available Nav1.5 channels (holding potential -120 mV), tramadol exhibited inhibitory effects on Nav1.5 current in a concentration-dependent manner with an IC50 of 378.5 ± 33.2 µm. In addition, tramadol caused a hyperpolarizing shift of voltage-gated (in)activation and a delay in recovery from inactivation. These blocking effects occurred at lower concentrations in partially inactivated Nav1.5 channels: during partial fast inactivation (close-to-physiological holding potential -90 mV), IC50 of Nav1.5 block was 4.5 ± 1.1 μm, while it was 16 ± 4.8 μm during partial slow inactivation. The tramadol-induced changes on Nav1.5 properties were reflected by a reduction in AP upstroke velocity in a frequency-dependent manner. Fentanyl and codeine had no effect on Nav1.5 current, even when tested at lethal concentrations.

CONCLUSION

Tramadol reduces Nav1.5 currents, in particular, at close-to-physiological membrane potentials. Fentanyl and codeine have no effects on Nav1.5 current.

The diverse molecular mechanisms responsible for the actions of opioids on the cardiovascular system

The actions of opioid agonist and antagonist drugs have not been well characterized in the heart and cardiovascular system. This stems from the limited role opioid receptors have been perceived to have in the regulation of the cardiovascular system. Instead, the focus of opioid receptor research, for many years, relates to the characterization of the actions of opioid drugs in analgesia associated with receptor activation in the CNS. However, recent studies suggest that opioid receptors have a role in the heart and cardiovascular system. While some of these actions may be mediated by activation of peripheral opioid receptors, others are not, and may result from direct or receptor-independent actions on cardiac tissue and the peripheral vascular system. This review will outline some of the diverse molecular mechanisms that may be responsible for the cardiovascular actions of opioids, and will characterize the role opioid receptors have in several cardiovascular pathophysiological disease states, including hypertension, heart failure, and ischaemic arrhythmogenesis. In many instances, it would appear that the effects of opioid agonists (and antagonists) in cardiovascular disease models may be mediated by opioid receptor-independent actions of these drugs.

Characterization of the opioid receptor subtypes mediating the negative inotropic effects of DAMGO, DPDPE and U-50, 488H in isolated human right atria strips

The present investigation was aimed at elucidating if mu-, delta- and kappa-opioid receptors are involved in the effects of DAMGO (selective mu-agonist), DPDPE (selective delta-agonist) and U-50,488H (selective kappa-agonist) in isolated electrically driven human right atria strips. The negative inotropic effects induced by the opioid agonists used in this study were not antagonized in presence of naloxone (preferentially mu-antagonist), naltrindole (selective delta-antagonist) and norbinaltorphimine (selective kappa-antagonist). These data suggest that the opioid receptors are not involved in the cardiac depressant effects induced by mu-, delta- and kappa-opioid agonists.

Diazoxide blocks the morphine induced lengthening of action potential duration on guinea-pig papillary muscle

1. Intracellular microelectrodes were used to evaluate the possible involvement of potassium currents in the action potential prolongation induced by morphine. To this purpose we investigated the electrophysiological effect of morphine on the isolated guinea pig right ventricular papillary muscle in the presence of the potassium channel opener and inhibitor diazoxide and glibenclamide respectively. 2. Diazoxide (1 microM), which is devoid of effect on its own, blocks the lengthening of action potential duration (APD) induced by morphine (5 mM). 3. However, in the presence of glibenclamide (1 microM), morphine (5 mM) prolonged APD in approximately the same proportion as that observed when used alone. 4. These results suggest that diazoxide but not glibenclamide sensitive potassium channels could mediate the APD prolongation induced by morphine.