Catecholamines and Cardiac Contractility

The Signal: β₁-Adrenergic Receptor Activation

Catecholamine → β₁ receptor → Gs protein → Adenylyl cyclase → ↑cAMP → Protein Kinase A (PKA)

Three Mechanisms by Which PKA Increases Contractility

1. Phosphorylation of L-type Ca²⁺ Channels (Sarcolemmal)

• PKA phosphorylates L-type Ca²⁺ channels in the sarcolemma
• → ↑ inward Ca²⁺ current during the action potential plateau
• → More trigger Ca²⁺ enters the cell
• → Greater Ca²⁺-induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR)
• → ↑ peak intracellular [Ca²⁺]

2. Phosphorylation of Phospholamban (via cAMP/PL pathway)

• Phospholamban normally inhibits the SR Ca²⁺-ATPase (SERCA pump)
• PKA phosphorylates phospholamban → relieves SERCA inhibition
• → SERCA pumps Ca²⁺ back into the SR faster and in greater amounts
• Dual effect:
  • ↑ Ca²⁺ stored in SR → more Ca²⁺ available for release on the next beat
  • Faster Ca²⁺ removal → faster relaxation (positive lusitropy) — shorter twitch, more time for diastolic filling

3. Phosphorylation of Troponin I (via cAMP/TnI pathway)

• PKA phosphorylates Troponin I (TnI)
• This increases the sensitivity/efficacy of Troponin C for activator Ca²⁺
• → The contractile apparatus responds more effectively to available Ca²⁺
• → Further contributes to increased force generation

Net Functional Effects (Positive Inotropy)

Additional Notes

• β₂-adrenergic receptors are also present in the myocardium (more in atria than ventricles), also linked to Gs/adenylyl cyclase — they help preserve catecholamine responsiveness when β₁ receptors are downregulated (as in chronic heart failure).
• Parasympathetic/ACh produces the opposite effect (negative inotropy) via Gk protein → ↓ cAMP, ↓ Ca²⁺ current, ↑ IK-ACh — but this effect is mainly in the atria (ventricles have sparse vagal innervation).
• In heart failure, β₁ receptor downregulation and abnormal Ca²⁺ homeostasis reduce the clinical efficacy of catecholamines — this is why high doses are needed and why they have diminishing returns.