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

BackgroundGinsenoside 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.MethodsPerforated-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 (Giprotein 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.ConclusionThese findings suggest that GSRd could inhibit ICa,L through pertussis toxin-sensitive G protein (Gi) and a nitric oxide–cyclic guanosine monophosphate-dependent mechanism.