Salbutamol cause potassium influx or efflux
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salbutamol potassium influx efflux mechanism beta2 agonist hypokalemia
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Salbutamol Causes Potassium Influx (into cells)
Salbutamol causes potassium influx - meaning K+ moves from the extracellular space (blood) into cells. This results in a fall in serum potassium (hypokalemia).
Mechanism
Salbutamol (albuterol) is a selective beta-2 adrenergic agonist. It lowers serum K+ through two transporters:
- Na+/K+-ATPase activation - the main pump; it moves 3 Na+ out and 2 K+ in per cycle. Beta-2 stimulation via cAMP increases its activity, pumping more K+ into skeletal muscle and hepatocytes.
- NKCC1 (Na+-K+-2Cl- cotransporter) - also activated, further driving K+ into cells.
The net result: K+ shifts into hepatocytes and skeletal myocytes, reducing plasma [K+].
Clinical Relevance
This mechanism is exploited therapeutically in hyperkalemia management:
| Route | Dose | K+ reduction | Onset | Duration |
|---|---|---|---|---|
| IV (not available in USA) | 0.5 mg over 10-15 min | ~0.9-1.4 mmol/L | Few minutes | 2-6 hours |
| Nebulized | 10-20 mg in 4 mL NS over 10 min | ~0.5-1.0 mmol/L | ~30 min (peak 90 min) | 2-6 hours |
| MDI with spacer | - | ~0.4 mmol/L | - | - |
- Salbutamol + insulin/glucose together have an additive effect, reducing K+ by ~1.2-1.5 mmol/L.
- About 20-40% of ESKD patients do not respond adequately, so it should not be used as a single agent in hyperkalemia.
- Side effects: tachycardia (~6-10 bpm with inhaled, ~20 bpm with IV), mild hyperglycemia.
Memory Aid
Beta-2 → cAMP → Na+/K+-ATPase ↑ → K+ goes IN → serum K+ falls
This is why salbutamol overdose or high-dose nebulization can cause hypokalemia as a side effect - and why it is used as a treatment for hyperkalemia.
Source: Brenner and Rector's The Kidney, 2-Volume Set - β2-Adrenergic Agonists section
Salbutamol causes bronchodilation by potassium efflux
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salbutamol bronchodilation mechanism potassium efflux KATP channel smooth muscle
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beta2 agonist bronchodilation mechanism PKA MLCK myosin light chain potassium channel airway smooth muscle
The statement is correct in a narrow sense - potassium efflux does play a role in bronchodilation - but it is not the primary or main mechanism. Here is the complete picture:
Salbutamol Bronchodilation: Full Mechanism
Primary Mechanism (Main pathway)
Beta-2 receptor → Gs protein → Adenylyl cyclase → ↑cAMP → PKA activation
PKA then causes bronchodilation through:
- Phosphorylation of MLCK (myosin light chain kinase) - inactivates it, so myosin light chain (LC20) cannot be phosphorylated → smooth muscle cannot contract → relaxation
- ↓ Intracellular Ca²+ - PKA phosphorylates and inhibits IP₃ receptors and promotes Ca²+ reuptake into the sarcoplasmic reticulum
- Large-conductance Ca²+-activated K+ channels (BKCa) opening → K+ efflux → membrane hyperpolarization → voltage-gated Ca²+ channels close → less Ca²+ entry → smooth muscle relaxation
Where Potassium Efflux Fits
The K+ efflux mechanism in bronchodilation is real but secondary:
| Step | What happens |
|---|---|
| Beta-2 activation → PKA | Opens BKCa (maxi-K) channels in airway smooth muscle |
| K+ flows out of the cell (efflux) | Down its concentration gradient |
| Membrane hyperpolarizes | Further away from threshold for Ca²+ channel opening |
| Less voltage-gated Ca²+ entry | Smooth muscle relaxes → bronchodilation |
This is distinct from the systemic hypokalemia effect, where salbutamol causes K+ influx into skeletal muscle via Na+/K+-ATPase.
The Key Distinction to Remember
| Context | Direction | Mechanism | Effect |
|---|---|---|---|
| Airway smooth muscle (bronchodilation) | K+ efflux (out of cell) | BKCa channel opening → hyperpolarization | Relaxation → bronchodilation |
| Skeletal muscle / liver (systemic) | K+ influx (into cell) | Na+/K+-ATPase stimulation | ↓ Serum K+ (hypokalemia) |
So the statement "salbutamol causes bronchodilation by potassium efflux" is not wrong - K+ efflux from airway smooth muscle does contribute to hyperpolarization and relaxation. However, it is not the dominant mechanism. The primary driver of bronchodilation is cAMP/PKA-mediated MLCK phosphorylation (calcium-independent smooth muscle relaxation), with K+ efflux acting as a supporting hyperpolarizing mechanism.
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