Expression using metallothonein promoter in CHO cells

[Harry Witchel]

Hallo Hallo Kings of DNA --
   I have had inconsistent expression of a K+ channel driven by 
a metallothonein promoter which is in a vector which also has hygromycin 
resistance driven by a cytomegalovirus promoter.  The cells are screened 
for a month using hygromycin, so all the cells should have the plasmid by 
now.  When I add 10 uM Cd2+ to cells (CHO) grown on glass coverslips 
(using Ham's F12 medium plus 10% FCS) and let expression go for 48 hours, 
sometimes I get expression of my K+ channels, but often I don't.  Also, I 
find that my cells get very sick in the Cd2+, and they often round up, 
and while patching they easily explode.
   Is there a better way to induce the metallothionein promoter?
   Is there a better inducible promoter (ie another vector)?
Thanks,
Harry