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Since β-Adrenergic stimulation increases cellular uptake of potassium, this will reduce the plasma concentration of potassium. Thus, a more negative membrane voltage will arise as the potassium ions flow out of the cardiac muscle cells more easily due to the more steep ionic gradient set.

This, consequently, makes it more difficult for an action potential to fire, and as a result, a decrease in the heart rate. So now since the sympathetic nervous system increases the heart rate, then why does this beta adrenergic stimulation occur? It would decrease the heart rate, so what is happening?

This information is from the medical physiology Guyton and Hall textbook, page 384, put into my own words if I have understood it correctly. This is the entire quote from it:

"β-Adrenergic Stimulation Increases Cellular Uptake of Potassium: Increased secretion of catecholamines, especially epinephrine, can cause movement of potassium from the extracellular to the intracellular fluid, mainly by activation of β2-adrenergic receptors." Or does this mean that the stimulation causes a direct intake of potassium ions FROM THE PLASMA INTO THE CARDIAC CELLS, hence causing a less negative membrane potential and an easier firing of an action potential?

Many thanks in advance!

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It's important to keep in mind that extracellular potassium concentration is normally very tightly regulated at 4.2 mEq/L (±0.3 mEq/L in nonpathological circumstances.) It's precisely because so many cells are so sensitive to extracellular potassium levels.

Since β-Adrenergic stimulation increases cellular uptake of potassium, this will reduce the plasma concentration of potassium. [Emphasis mine.]

If this were the case, the entire body would need respond to increased K+ levels, which would present some difficulties. So your second interpretation is the correct one.

Or does this mean that the stimulation causes a direct intake of potassium ions FROM THE PLASMA INTO THE CARDIAC CELLS, hence causing a less negative membrane potential and an easier firing of an action potential?

The effect of receptor stimulation is localized to the cells which have that receptor. So β-adrenergic receptor stimulation causes in increase in heart rate and contractility, whereas β blockers decreases same.

More than I knew and more than you probably want to know at this point abot about β-adrenergic receptors in the heart

One of my mnemonics for β-adrenergic receptors was "1 heart, 2 lungs" to remind me that the heart was predominantly β1 and the lungs β2. β2 stimulation in the lungs decreases smooth muscle contractility, which is why we used to give epinephrine to those with asthma attacks. That's been replaced with albuterol and atropine, but epi is still useful if the attack is severe enough.

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  • $\begingroup$ Hey thanks very much for your help! So do you mean that the heart does not have as much of β2-adrenergic receptors? But so they still do have some, right? So during for example exercise, the SNS is activated stimulating the β2-adrenergic receptors to cause an influx of potassium, hence causing a more positive membrane voltage, and as a result, the heart muscles are able to contract more easily, increasing the heart rate? Many thanks again! $\endgroup$
    – Drita Raci
    Feb 17, 2023 at 14:34
  • $\begingroup$ Yes, they have both, but β1:β2 is about 70:30. If this answers your question, you might consider accepting the answer. Thanks. $\endgroup$ Feb 17, 2023 at 14:48
  • $\begingroup$ okay cool! And so just to summarize it, is the answer to this: "So during for example exercise, the SNS is activated stimulating the β2-adrenergic receptors to cause an influx of potassium, hence causing a more positive membrane voltage, and as a result, the heart muscles are able to contract more easily, increasing the heart rate?" a yes? $\endgroup$
    – Drita Raci
    Feb 17, 2023 at 14:54
  • $\begingroup$ Well, it's also linked to CAMP and Ca++, but for the purposes of your question, yes. See the source I linked to for how complex it is, and this was the easier of a number of papers. Most of the papers dealing only with K+ involved the treatment of heart failure... so weren't too helpful. $\endgroup$ Feb 17, 2023 at 15:04
  • $\begingroup$ great! thanks so much! $\endgroup$
    – Drita Raci
    Feb 22, 2023 at 12:49

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