Biophysical and pharmacological characterization of a class A calcium channel

Molecular diversity of spider venom

Spider venom, a factor that has played a decisive role in the evolution of one of the most successful groups of living organisms, is reviewed. Unique molecular diversity of venom components including substances of variable structure (from simple low molecular weight compounds to large multidomain proteins) with different functions is considered. Special attention is given to the structure, properties, and biosynthesis of toxins of polypeptide nature.

Low-affinity blockade of neuronal N-type Ca channels by the spider toxin omega-agatoxin-IVA

The recognition of neuronal Ca channel diversity has led to considerable efforts to identify useful classification criteria. Here, we revisit the pharmacological definition of P- and Q-type Ca channels, which is based on their respective high and low sensitivity to the spider omega-agatoxin-IVA (omega-Aga-IVA), using whole-cell recordings of the Ca channel currents carried by 5 mM Ba(2+) in isolated rat subthalamic and sympathetic neurons. In subthalamic neurons, omega-Aga-IVA (1 microM) targeted multiple Ca channels. One population was blocked with high potency. These channels carried 50.4 +/- 3.4% (n = 5) of the control current and showed the same inactivation kinetics and voltage-dependent high affinity for omega-Aga-IVA as do prototypic P-type Ca channels. Other Ca channels were targeted with weaker potency. This heterogeneous population contributed to 14.0 +/- 1.7% (n = 5) of the control current. It included N-type Ca channels as well as high-threshold Ca channels that displayed the pharmacological signature of Q-type Ca channels but resembled P-type Ca channels in their gating properties. N-type Ca current block by omega-Aga-IVA (1 microM) was further investigated in sympathetic neurons, which mainly express this Ca channel type. Block was incomplete ( approximately 30% of the control current). Its relief at positive potentials was consistent with omega-Aga-IVA acting as a channel-gating modifier. These effects did not reflect a complete loss of selectivity, because omega-Aga-IVA (1 microM) had no effect on subthalamic Na and K currents or their T- and L-type Ca currents. Our data confirm that omega-Aga-IVA is a selective P-type Ca channel blocker. However, its diminished selectivity in the micromolar range limits its usefulness for functional studies of Q-type Ca channels.

Voltage and calcium use the same molecular determinants to inactivate calcium channels

During sustained depolarization, voltage-gated Ca2+ channels progressively undergo a transition to a nonconducting, inactivated state, preventing Ca2+ overload of the cell. This transition can be triggered either by the membrane potential (voltage-dependent inactivation) or by the consecutive entry of Ca2+ (Ca2+-dependent inactivation), depending on the type of Ca2+ channel. These two types of inactivation are suspected to arise from distinct underlying mechanisms, relying on specific molecular sequences of the different pore-forming Ca2+ channel subunits. Here we report that the voltage-dependent inactivation (of the alpha1A Ca2+ channel) and the Ca2+-dependent inactivation (of the alpha1C Ca2+ channel) are similarly influenced by Ca2+ channel beta subunits. The same molecular determinants of the beta subunit, and therefore the same subunit interactions, influence both types of inactivation. These results strongly suggest that the voltage and the Ca2+-dependent transitions leading to channel inactivation use homologous structures of the different alpha1 subunits and occur through the same molecular process. A model of inactivation taking into account these new data is presented.

Biophysical and pharmacological characterization of voltage-dependent Ca2+ channels in neurons isolated from rat nucleus accumbens

The nucleus accumbens (NA) has an integrative role in behavior and may mediate addictive and psychotherapeutic drug action. Whole cell recording techniques were used to characterize electrophysiologically and pharmacologically high- and low-threshold voltage-dependent Ca2+ currents in isolated NA neurons. High-threshold Ca2+ currents, which were found in all neurons studied and include both sustained and inactivating components, activated at potentials greater than -50 mV and reached maximal activation at approximately 0 mV. In contrast, low-threshold Ca2+ currents activated at voltages greater than -64 mV with maximal activation occurring at -30 mV. These were observed in 42% of acutely isolated neurons. Further pharmacological characterization of high-threshold Ca2+ currents was attempted using nimodipine (Nim), omega-conotoxin-GVIA (omega-CgTx) and omega-agatoxin-IVA (omegaAga), which are thought to identify the L, N, and P/Q subtypes of Ca2+ currents, respectively. Nim (5-10 muM) blocked 18%, omegaCgTx (1-2 muM) blocked 25%, and omegaAga (200 nM) blocked 17% of total Ca2+ current. Nim primarily blocked a sustained high-threshold Ca2+ current in a partially reversible manner. In contrast, omegaCgTx irreversibly blocked both sustained and inactivating components. omegaAga irreversibly blocked only a sustained component. In all three of these Ca2+ channel blockers, plus 5 muM omega-conotoxin-MVIIC to eliminate a small unblocked Q-type Ca2+ current (7%), a toxin-resistant high-threshold Ca2+ current remained that was 32% of total Ca2+ current. This current inactivated much more rapidly than the other high-threshold Ca2+ currents, was depressed in 50 muM Ni2+ and reached maximal activation 5-10 mV negative to the toxin-sensitive high-threshold Ca2+ currents. Thus NA neurons have multiple types of high-threshold Ca2+ currents with a large component being the toxin-resistant "R" component.