omega-agatoxin-iva and hyperforin

Whole-cell, patch-clamp recordings from acutely isolated cerebellar Purkinje neurons demonstrate a two-stage modulation of P-type high-voltage-activated (HVA) Ca2+ current by a constituent of St. John's wort, hyperforin (0.04-0.8 microM). The first stage of modulation was voltage dependent and reversible. It comprised slow-down of the activation kinetics and a shift in the voltage dependence of P-current to more negative voltages. Hyperforin (0.8 microM) shifted the maximum of the current/voltage (I/V) relationship by -8+/-2 mV. The second, voltage-independent stage of modulation was manifested as a slowly developing inhibition of P-current that could not be reversed within the period of study. Neither form of modulation was abolished by intracellular guanosine 5'-O-(2-thiodiphosphate) (GDPPS) or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) or by strong depolarising pre-pulses, indicating that modulation via guanine nucleotide-binding proteins (G proteins) is not involved in the observed phenomenon. Calmidazolium (0.5 microM), an antagonist of the intracellular Ca2+-binding protein calmodulin significantly inhibited the hyperforin-induced shift of the IIV curve maximum and the slow-down of the activation kinetics. It did not, however, affect the delayed inhibition of P-current, indicating that the two stages of modulation are mediated by separate mechanisms.

Topics: Animals; Anti-Bacterial Agents; Bridged Bicyclo Compounds; Calcium Channel Blockers; Calcium Channels, P-Type; Guanosine 5'-O-(3-Thiotriphosphate); Guanosine Diphosphate; Hypericum; Ion Channel Gating; Membrane Potentials; omega-Agatoxin IVA; omega-Conotoxin GVIA; Patch-Clamp Techniques; Phloroglucinol; Plants, Medicinal; Purkinje Cells; Rats; Rats, Wistar; Second Messenger Systems; Terpenes; Thionucleotides