microtubule elongation

[mark at uucp.nibr]

I am working on a model concerning the elongation of microtubules
in neurons during outgrowth. My model so far shows that stopping
neuritic outgrowth by stopping microtubule outgrowth demands a stop
of the input of material in the outgrowing neuritic tips. I have two
questions about this, which I could not trace out in literature:

1 - Is elevated calcium able to (locally) block transport of soluble
    proteins at the site of the growth cone ?

2 - Is the production of tubulin (all isoforms) at the soma regulated
    by the concentration of tubulin ? More specifically: is there a
    negative feedback system ?

All responses would be welcome,

Mark van Veen
Neth. Inst. Brain Research
mark%nibr at nluug.nl