Funny current discrepancy

[loveoforganic]

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I was taught in class that funny current is due to constant voltage-gated Na+ channel permeability with steadily decreasing voltage-gated K+ channel permeability, but gunnertraining states the funny current as being due to increased Na+ conductance.

Which is actually the case?

 

[FunnyCurrent]

I was taught in class that funny current is due to constant voltage-gated Na+ channel permeability with steadily decreasing voltage-gated K+ channel permeability, but gunnertraining states the funny current as being due to increased Na+ conductance.

Which is actually the case?

I was taught that spontaneous depolarization in phase 4 by the HCN channel is non-selective but mostly you get an influx of sodium.

 

[Charles_Carmichael]

^ Username win!

But yea, I learned that it was due to influx of Na+. Never heard anything about decreased permeability involving voltage-gated K+ channels.

 

[loveoforganic]

Username win indeed. Thanks for the replies. I uploaded the image I was taught from, if there was any curiosity. I'm wondering if this is one of the things where something plausible is taught that hasn't been really tested directly yet. As far as I can tell, either story is equally plausible, regardless of what the case may actually be.

I did a quick scholar search for "funny current conductance" and didn't get a direct answer, but found some stuff on blocking/inhibition of HCN channels by ivabradine to decrease heart rate, and this would only make sense, as far as I can tell, if the funny current is primarily due to Na+ influx.

Taking more opinions, at any rate 🙂

 

[link2swim06]

I was taught in class that funny current is due to constant voltage-gated Na+ channel permeability with steadily decreasing voltage-gated K+ channel permeability, but gunnertraining states the funny current as being due to increased Na+ conductance.

Which is actually the case?

There are no voltage gated Na channels in the SA and AV nodes FYI. Most research says the funny current is a combination of ca, na and k.

 

[aSagacious]

Here are the first few sentences of a relevant abstract:

The 'funny' (pacemaker, I(f)) current, first described almost 30 years ago in sinoatrial node (SAN) myocytes, is a mixed sodium/potassium inward current, activated on hyperpolarisation in the diastolic range of voltages. 'Funny' (f) channels are activated by intracellular cyclic adenosine monophosphate (cAMP) concentrations according to a mechanism mediating regulation of heart rate by the autonomic nervous system, as well as by voltage hyperpolarisation.

Source: "The funny current: cellular basis for the control of heart rate." (PMID: 17999560)

 

[loveoforganic]

There are no voltage gated Na channels in the SA and AV nodes FYI. Most research says the funny current is a combination of ca, na and k.

Yes, sorry about that, just an inaccurate conceptual way to think about the HCN conductance that stuck - I realize they're not a component of the AP in pacemakers (although I didn't realize they actually weren't in existence in them at all, though that makes sense). For the conductances the research proposes, I assume it's hyperpolarization-triggered, but what is the actual change in conductance of the Ca, Na, K after stimulated?

Thanks for the info, and thanks for the link asag

 

[aSagacious]

For the conductances the research proposes, I assume it's hyperpolarization-triggered, but what is the actual change in conductance of the Ca, Na, K after stimulated?

From another paper discussing the properties of the HCN4 channel (one thought to be responsible for If):

HCN channels conduct Na+ and K+ with permeability ratios of about 1:4... [and] also seem to display a small permeability for Ca2+... [of] about 0.5%.

Of course these measurements were probably made at resting membrane potential. A voltage-dependent change in conductance for each ion would be very difficult to measure since the affects of each are manifested in the same current (both a ↑ inward sodium flow and a ↓ outward potassium flow would contribute to an inward current). As you would imagine, the ratio would probably favor sodium conduction in the hyperpolarized state and would tend to favor potassium conduction in the depolarized state due to the enormous disparity in reversal potentials.

Source: HCN channels: Structure, cellular regulation and physiological function (PMID: 18953682)

 

[loveoforganic]

That would make sense, thank you!