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Re: resting potential and K+ [HAPP-L]



Alan Magid wrote:

In thinking about these matters, IGNORE any changes in the bulk
concentrations of the ions. They hardly change and so the reasoning in your
last paragraph is unrealistic. The ion movements are trivial, requiring only
the miniscule quantities of charge to change the charge on the membrane
capacitance.


on 9/5/03 4:53 PM, Pat Bowne at Pat.Bowne@xxxxxxxxxxx wrote:



But muscle firing, as in seizures or heavy exercise, is given as a cause of
hyperkalemia. So surely the amount of K+ crossing the cell membrane is not
always negligible.


You have it backwards. Hyperkalemia, usually caused by renal insufficiency,
may produce seizure, for the reasons I detailed earlier.



"If one of the principals of a general anesthetic drug like ether is to open
K+ channels wider than usual in order to prevent most action potentials, is
this a contributing factor to why patients are infused with intravenous
solutions containing K+, in order to introduce suffient amounts of K+ back
into the system so that the NA+/K+ pumps can get the K+ back into nerve
cells?"


A big IF. General anesthetics such as ether, cycloprone, etc act at synapses
not on axonal conduction. Ionic channels don't open "wider" but the open
state may be preferred leading to higher fluxes. The K+ in IV fluids is most
often at physiological levels that mimic normal concentrations and thus will
not change K+ concentrations. OTOH, normal ionic distribution requires an
intact pumping system which in turn requires normal blood flow to support
ATP production.

Let me reiterate, before undertaking to explain something a student believes
to be true it is essential to demand clear reasoning of a causal nature, and
an ACCOUNT OF THE EVIDENCE! (sorry for shouting, but it seems warranted.)

-Alan


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