Receptors in the Heart and Lungs

The heart and lungs are regulated by various receptors that modulate their functions in response to neural and hormonal signals.

1. Adrenergic Receptors

Adrenergic receptors are crucial in the sympathetic nervous system’s regulation of cardiac and pulmonary functions. They respond to catecholamines like adrenaline and noradrenaline.

  • Beta-1 (β1) Adrenergic Receptors:
    • Location: Predominantly in the atria and ventricles of the heart.
    • Function: Increase heart rate (chronotropy), increase force of contraction (inotropy), and accelerate relaxation (lusitropy).
    • Mechanism: Activation increases cyclic AMP (cAMP) levels, enhancing calcium influx into cardiac cells, thereby increasing cardiac output.
  • Beta-2 (β2) Adrenergic Receptors:
    • Location: Present in both the heart and the lungs. In the heart, they are found in the atria and ventricles, though in smaller numbers compared to β1 receptors. In the lungs, they are predominantly located in the smooth muscle of the bronchioles.
    • Function:
      • Heart: Contribute to increasing heart rate and contractility, although their role is less significant than β1 receptors.
      • Lungs: Mediate bronchodilation, relaxing bronchial smooth muscle and widening the airways, which facilitates easier breathing.
    • Mechanism: Activation increases cAMP levels in both heart and lung tissues, leading to enhanced calcium influx in the heart and relaxation of smooth muscle cells in the bronchioles​.
  • Alpha-1 (α1) Adrenergic Receptors:
    • Location: Mainly in the vascular smooth muscle but also present in the heart.
    • Function: Cause vasoconstriction in peripheral vessels, indirectly affecting cardiac workload and oxygen demand.
    • Mechanism: Activation leads to increased intracellular calcium, promoting contraction of the smooth muscle cells.

2. Cholinergic Receptors

Cholinergic receptors are part of the parasympathetic nervous system, primarily responding to acetylcholine.

  • Muscarinic-2 (M2) Receptors:
    • Location: Primarily in the atria and conduction tissue such as the sinoatrial (SA) node and atrioventricular (AV) node of the heart.
    • Function: Decrease heart rate (negative chronotropy) and reduce conduction velocity through the AV node (negative dromotropy).
    • Mechanism: Activation reduces cAMP levels, which decreases calcium influx and increases potassium efflux, leading to hyperpolarization and reduced cardiac activity​.

3. Other Important Receptors

  • Adenosine Receptors:
    • Location: Throughout the heart, especially in the atria and SA node.
    • Function: Slow heart rate and AV nodal conduction.
    • Mechanism: Activation of A1 receptors leads to the opening of potassium channels and hyperpolarization of the cell membrane, slowing down the heart rate.
  • Angiotensin II Receptors (AT1):
    • Location: Found in both cardiac myocytes and fibroblasts.
    • Function: Promote vasoconstriction, sodium retention, and cell growth, contributing to hypertrophy and fibrosis in the heart.
    • Mechanism: Activation increases intracellular calcium, which leads to contraction and growth signaling.
  • Natriuretic Peptide Receptors:
    • Location: Predominantly in the atria.
    • Function: Mediate the effects of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), promoting diuresis, natriuresis, and vasodilation.
    • Mechanism: Activation increases cyclic GMP (cGMP), causing vasodilation and reduced blood volume.
  • Histamine Receptors:
    • H2 Receptors:
      • Location: Found in the myocardium of the heart.
      • Function: Increase heart rate and force of contraction.
      • Mechanism: Similar to β-adrenergic receptors, they increase cAMP levels in cardiac cells.

Summary

The heart and lungs are regulated by a complex interplay of receptors that respond to various neurotransmitters and hormones. Adrenergic receptors (β1 and β2) in the heart enhance cardiac output during stress, while β2 receptors in the lungs facilitate bronchodilation. Cholinergic receptors (M2) help to reduce heart rate and promote relaxation. Other receptors like adenosine, angiotensin II, natriuretic peptide, and histamine receptors play supplementary roles in modulating cardiac function and maintaining homeostasis.