Agonist specific L-type Ca2+-current stimulation in ventricular myocytes by a novel steroid-like compound

In cardiac muscle the amplitude of Ca2+ transients can be increased by enhancing Ca2+ influx. Among the processes leading to increased Ca2+ influx, agonists of the L-type Ca2+-channel can play an important role. Known pharmacological Ca2+-channel agonists act on different binding sites on the channel protein, which may lead not only to enhanced peak currents, but also to distinct changes in other biophysical characteristics of the current. In this study, membrane currents were recorded with the patch–clamp technique in the whole-cell configuration in guinea pig isolated ventricular myocytes in combination with confocal fluorescence Ca2+ imaging techniques and a variety of pharmacological tools. Testing a new positive inotropic steroid-like compound, we found that it increased the L-type Ca2+-current by 2.5-fold by shifting the voltage-dependence of activation by 20.2 mV towards negative potentials. The dose–response relationship revealed two vastly different affinities (EC50(high-affinity) = 4.5 ± 1.7 nM, EC50(low-affinity) = 8.0 ± 1.1 μM) exhibiting differential pharmacological interactions with three classes of Ca2+-current antagonists, suggesting more than one binding site on the channel protein. Therefore, we identified and characterized a novel positive inotropic compound (F90927) as a member of a new class of Ca2+-channel agonists exhibiting unique features, which set it apart from other presently known L-type Ca2+-channel agonists.