Inhibition of L-type Ca2+ current by ginsenoside Rd in rat ventricular myocytes

Background: Ginsenoside Rd (GSRd), one of the most abundant ingredients of Panax ginseng, protects the heart via multiple mechanisms including the inhibition of Ca2+ influx. We intended to explore the effects of GSRd on L-type Ca2+ current (ICa,L) and define the mechanism of the suppression of ICa,L by GSRd. Methods: Perforated-patch recording and whole-cell voltage clamp techniques were applied in isolated rat ventricular myocytes. Results: (1) GSRd reduced ICa,L peak amplitude in a concentration-dependent manner [half-maximal inhibitory concentration (IC50) = 32.4 ± 7.1 μmol/L] and up-shifted the current–voltage (I–V) curve. (2) GSRd (30 μmol/L) significantly changed the steady-state activation curve of ICa,L (V0.5: −19.12 ± 0.68 vs. −16.26 ± 0.38 mV; n = 5, p < 0.05) and slowed down the recovery of ICa,L from inactivation [the time content (ζ) from 91 ms to 136 ms, n = 5, p < 0.01]. (3) A more significant inhibitive effect of GSRd (100 μmol/L) was identified in perforated-patch recording when compared with whole-cell recording [65.7 ± 3.2% (n = 10) vs. 31.4 ± 5.2% (n = 5), p < 0.01]. (4) Pertussis toxin (Gi protein inhibitor) completely abolished the ICa,L inhibition induced by GSRd. There was a significant difference in inhibition potency between the two cyclic adenosine monophosphate elevating agents (isoprenaline and forskolin) prestimulation [55 ± 7.8% (n = 5) vs. 17.2 ± 3.5% (n = 5), p < 0.01]. (5) 1H-[1,2,4]Oxadiazolo[4,3-a]-quinoxalin-1-one (a guanylate cyclase inhibitor) and N-acetyl-l-cysteine (a nitric oxide scavenger) partly recovered the ICa,L inhibition induced by GSRd. (6) Phorbol-12-myristate-13-acetate (a protein kinase C activator) and GF109203X (a protein kinase C inhibitor) did not contribute to the inhibition of GSRd. Conclusion: These findings suggest that GSRd could inhibit ICa,L through pertussis toxin-sensitive G protein (Gi) and a nitric oxide–cyclic guanosine monophosphate-dependent mechanism