I am about to undergo a study looking at voltage operated calcium studies,
using brain slice preparations. I wanted to know whether anyone could
advise on concentrations of omega-conotoxin GVIA (to block N channels), and
omega-conotoxin MVIIC (to block N, P and Q channels).
I wish to use a single maximal concentration which is selective to the
channels stated above. I have seen reports in a number of publications,
that (functionally at least) there is no further effect beyond 10-30nM and
M for GVIA and MVIIC respectively. Although there is a large range, a lot
of studies use these toxins at 1microM for GVIA and 2-3microM for MVIIC.
I believe, and this is confirmed in the literature, that the concentration
required in slices should not be greater than for isolated cells, but the
time for peak response will be slower (due to slowed drug equilibration). I
have also read that these toxins bind best to calcium channels, in the
hyperpolarised state. I am not using voltage/patch clamp, and so the cells
are at their normal resting potential.
My most important criteria is that the drug effects are maximal and
selective, but on the other hand, since these toxins are expensive, I don't
want to go too overboard.
Secondly, I will also be using omega-agatoxin IVA at two concentrations,
one which is selective to P channels, and one which will hit Ps and Qs.
I've read that the EC50s are as low as 1nM and 89nM for P and Q
respectively, and so clearly 100nM, which is sometimes reported as
selective for P, won't be! Any recommendations for concentrations?
There are many conflicting views in the literature, and so any help on this
matter will be greatly appreciated.
Paul E.M. Phillips
Royal London Hospital
London E1 1BB
tel: +44 171 377 7725
fax: +44 171 377 7126