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Toxicology – Cardiac Physiology (Key Mechanisms)
Beta-Adrenergic Activation
Beta-agonists bind to β-receptors on cardiac cells, leading to activation of intracellular G proteins that initiate downstream signaling.
Adenylyl Cyclase Activation
The activated G protein stimulates adenylyl cyclase, an enzyme that converts ATP into cyclic AMP (cAMP), a key second messenger.
Glucagon Pathway
Glucagon can independently stimulate adenylyl cyclase, increasing cAMP levels through an alternative (“bypass”) pathway that does not rely on β-receptors.
Calcium Influx
Elevated cAMP activates protein kinase A, which enhances calcium channel opening and increases intracellular calcium entry, strengthening cardiac contraction.
cAMP Breakdown
cAMP is eventually degraded into inactive 5′-AMP by phosphodiesterase enzymes, terminating its effects.
Digitalis Effect
Digitalis inhibits the Na⁺/K⁺-ATPase pump, leading to increased intracellular sodium and secondary rise in intracellular calcium, which enhances cardiac contractility.
Key Points
Beta-Adrenergic Activation
Beta-agonists bind to β-receptors on cardiac cells, leading to activation of intracellular G proteins that initiate downstream signaling.
Adenylyl Cyclase Activation
The activated G protein stimulates adenylyl cyclase, an enzyme that converts ATP into cyclic AMP (cAMP), a key second messenger.
Glucagon Pathway
Glucagon can independently stimulate adenylyl cyclase, increasing cAMP levels through an alternative (“bypass”) pathway that does not rely on β-receptors.
Calcium Influx
Elevated cAMP activates protein kinase A, which enhances calcium channel opening and increases intracellular calcium entry, strengthening cardiac contraction.
cAMP Breakdown
cAMP is eventually degraded into inactive 5′-AMP by phosphodiesterase enzymes, terminating its effects.
Digitalis Effect
Digitalis inhibits the Na⁺/K⁺-ATPase pump, leading to increased intracellular sodium and secondary rise in intracellular calcium, which enhances cardiac contractility.
Key Points
- Beta-blockers inhibit β-adrenergic receptors, reducing cardiac stimulation.
- Glucagon is useful in beta-blocker overdose because it increases cAMP independently of β-receptors.
- Calcium channel blockers reduce calcium entry into cells, decreasing contractility and conduction.
- Phosphodiesterase inhibitors increase cAMP levels, thereby promoting calcium influx and enhancing cardiac function.
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