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

Matt Jones jonesmat at
Tue Aug 3 16:01:48 EST 2004

tehgabriel at (tomte) wrote in message news:<1adcf6cc.0408030642.bbb6e66 at>...
> Hi!
> 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!
> Thanks,
> Thomas
> 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


Most people use sucrose to increase osmolarity, and distilled
deionized water (e.g., from a MilliQ system) to reduce osmolarity.
Sucrose isn't charged, so you don't have to worry about it messing
with your reversal potentials, etc. It's also not a salt, so 1 mM of
sucrose increases osmolarity by 1 mOsmol.

I wonder why your solutions are coming out so low, when the calculated
osmolarity should be 286? some possibilities are:

1) You osmometer needs to be calibrated: check it against a solution
with a known, standard osmolarity. You can probably purchase standards
from the manufacturer of the osmometer.

2) Your salts are old and have taken on moisture, which adds to the
weight so that the weighed amount contains less salt than it should.


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