Involvement of Calcium in Rhythmic Activity Induced by

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Agonist specific L-type Ca 2+-current stimulation in ventricular myocytes by a novel steroid-like compound

In cardiac muscle the amplitude of Ca 2+ transients can be increased by enhancing Ca 2+ influx. Among the processes leading to increased Ca 2+ influx, agonists of the L-type Ca 2+-channel can play an important role. Known pharmacological Ca 2+-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 Ca 2+ imaging techniques and a variety of pharmacological tools. Testing a new positive inotropic steroid-like compound, we found that it increased the L-type Ca 2+-current by 2.5-fold by shifting the voltagedependence 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 �M) exhibiting differential pharmacological interactions with three classes of Ca 2+-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 Ca 2+-channel agonists exhibiting unique features, which set it apart from other presently known L-type Ca 2+-channel agonists