How to adjust osmolarity of patch pipette filling solution ???

tomte tehgabriel at
Tue Aug 3 09:42:17 EST 2004

How may one adjust the osmolarity of the solution used to fill patch
electrodes? Though i am using a receipe that seems to be quite
conventional i always obtain a solution with low osmolarity compared
to the values i found in literature. I use a standard recipe (see
below) for which i would expect an osmolarity of 280 +/- 5 mOsm. But
what i get after checking osmolarity is a value of 255 mOsm. Now i
could increase osm. by simply adding more k+-gluconate. but to reach a
final level of 285 mOsm i would have to add ~ 25 mM k-gluconate,
thereby increasing the final [k+] of the solution to ~160 mM!!!

Is there a better way to increase osmolarity without affecting [K+]
Or is it a kind of 'common knowledge' that a adjusting osmolarity is
accompanied by an alterarion of e.g. [K+]?

Any comment would be appreciated!

Here is the recipe i  am using (all in mM):
K - Gluconate	115
KCl	                20
MgCl2	               2
HEPES	             10
Mg - ATP	     2
Na2 - ATP	     2
Tris - GTP	      0,3

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