Hypericin activates L-type Ca²⁺ channels in cardiac myocytes

The effects and the mode of action of hypericin (1) were studied, in the dark, on the action potential (AP) and the L-type Ca²⁺ channel of frog atrial heart muscle, using intracellular microelectrode and patch-clamp techniques, respectively. In the presence of Ca²⁺ in Ringer solution, hypericin (1 to 4 μM) did not markedly modify the AP. Total replacement of Ca²⁺ by Sr²⁺ in the solution (Ringer Sr²⁺) revealed that hypericin (4 μM) prolonged the AP duration (APD). Hypericin dose-dependently increased the magnitude of the Sr²⁺ current, which develops through L-type Ca²⁺ channels in the Ringer solution containing tetrodotoxin (0.7 μM) and tetraethylammonium (10 mM), but did not modify the kinetics of activation and inactivation. This revealed that hypericin increased L-type Ca²⁺ channel conductance, which accounted for the APD lengthening. The hypericin-induced APD lengthening recorded in the Ringer Sr²⁺ was not prevented by (i) a blockade of α- and β-adrenoceptors by yohimbine (1 μM), urapidil (1 μM), and propanolol (50 μM), respectively, and (ii) PKC blockade by staurosporine (1 μM). The hypericin-induced APD lengthening recorded in the Ringer Sr²⁺ was prevented by blocking soluble guanylate cyclase (sGC) activity by ¹H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (13 μM), which mimicked the effects of hypericin. Hypericin decreased the cellular cGMP level by 69% in atrial myocytes. The compound also decreased the cellular cGMP level by inhibiting sGC, thus cancelling the nucleotide inhibitory effect on the cardiac L-type Ca²⁺ channel.