Inhibitory effect of the plant-extract osthole on L-type calcium current in NG108-15 neuronal cells

The effects of osthole, a coumarin isolated from Cnidium monnieri (L.) Cusson, on ionic currents in a mouse neuroblastoma and rat glioma hybrid cell line, NG105-18, were investigated with the aid of the whole-cell voltage-clamp technique. Osthole (0.3-100 muM) caused an inhibition of voltage-dependent L-type Ca2+ current (I-Ca,I-L) in a concentration-dependent manner. Osthole produced no change in the overall shape of the current-voltage relationship of I-Ca,I-L. The IC50 value of the osthole-induced inhibition of I-Ca,I-L was 4 muM. The presence of osthole (3 muM) shifted the steady state inactivation curve of I-Ca,I-L to a more negative potential by approximately -15 mV. Osthole (3 muM) also produced a prolongation in the recovery of I-Ca,I-L inactivation. Although osthole might suppress phosophodiesterases to increase intracellular adenosine-3',5-cyclic monophosphate (cyclic AMP) or guanosine-3',5'-cyclic monophosphate (cyclic GMP), sp-cAMPS did not affect I-Ca,I-L and 8-bromo-cyclic GMP slightly suppressed it. Thus, osthole-mediated inhibition of I-Ca,I-L was not associated with intracellular cyclic AMP or GMP. However, no effect of osthole on voltage-dependent K+ outward current was observed. Under a current-clamp mode, osthole could decrease the firing frequency of action potentials. Therefore, the channel-blocking properties of osthole may, at least in part, contribute to the underlying mechanisms by which it affects neuronal or neuroendocrine function. (C) 2002 Elsevier Science Inc. All rights reserved.