Dear Andi:

I can't help you with "Krebb's" but if you can get hold of a copy of
"Data for Biochemical Research", by Dawson, Elliott, Elliott and Jones
(Oxford Science Publications, 3rd paperback Edition is ISBN 0 19 855299
8), there is a collection of recipés for "Krebs" buffers.  They are all
derivatives of Ringer's solutions, and are mainly buffered NaCl, with
small amounts of other cations and anions.  They are all approximately
isotonic with serum.  The simplest is made by mixing:

100 parts 0.9% NaCl (0.154M)
4 parts 1.15% KCl (0.154M)
3 parts 1.22% CaCl2 (0.11M)
1 part 2.11% KH2PO4 (0.154M)
1 part 3.8% MgSO4.7H2O (0.154M)
21 parts 1.3% NaHCO3 (gassed with 100% CO2 for 1 hour before adding to
the other components).

As you can see, "simplest" is relative.  Unless you specifically need
all the minor ions and the carbon dioxide, I would try simple PBS first
to see if that works.

Legend had it at Oxford (Krebs was Biochemistry professor there in the
1950s and 1960s) that it really should be Hems' buffer (Reg Hems was
Krebs' lab assisstant), but Reg didn't write the paper.

Good luck,

Adrian Leek.