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.
Sidach
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Department of Pharmacology and Experimental Therapeutics, Boston University Medical Center, Boston, Massachusetts 02118, USA.
Mintz
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D000818:Animals; D001464:Barium / Q000378:metabolism / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020864:Calcium Channels, N-Type / Q000187:drug effects / Q000378:metabolism*; D020865:Calcium Channels, P-Type / Q000187:drug effects / Q000378:metabolism; D002469:Cell Separation; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005728:Ganglia, Sympathetic / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D011689:Purkinje Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism; D020531:Subthalamic Nucleus / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D020867:omega-Agatoxin IVA / Q000494:pharmacology*
D002121:Calcium Channel Blockers; D020864:Calcium Channels, N-Type; D020865:Calcium Channels, P-Type; D020867:omega-Agatoxin IVA; D001464:Barium; D012964:Sodium; D011188:Potassium