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Reduced cholinergic sinus node restraint in hyperthyroidism

C W White, T J Zimmerman

    Circulation
    |December 1, 1976
    PubMed
    Summary
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    Hyperthyroidism alters heart rate by affecting the cholinergic nervous system. This study in dogs suggests reduced cholinergic activity, not sinus node issues, contributes to thyrotoxic tachycardia.

    Area of Science:

    • Cardiovascular Physiology
    • Endocrinology
    • Autonomic Nervous System Research

    Background:

    • Hyperthyroidism is associated with increased heart rate (tachycardia).
    • The role of the autonomic nervous system, particularly the cholinergic system, in this tachycardia is not fully understood.
    • Thyroid hormones directly influence cardiac function and automaticity.

    Purpose of the Study:

    • To investigate the role of the cholinergic nervous system in hyperthyroid-induced tachycardia.
    • To determine if parasympathetic control of heart rate is abnormal in hyperthyroidism.
    • To elucidate the specific mechanisms underlying cholinergic system dysfunction in thyrotoxicosis.

    Main Methods:

    • Induction of severe thyrotoxicosis in dogs using Na-L-thyroxine over 6-9 months.

    Related Experiment Videos

  • Assessment of heart rate responses to atropine (cholinergic blockade) before and after propranolol (beta-adrenergic blockade).
  • Evaluation of heart rate responses to vagal stimulation and direct sinus node artery injections in anesthetized dogs.

    • Main Results:

    • Cholinergic blockade with atropine abolished heart rate differences between control and thyrotoxic dogs.
    • These effects were observed both before and after beta-adrenergic blockade.
    • No differences in heart rate response to vagal stimulation or direct acetylcholine injection were found in anesthetized thyrotoxic dogs.

    Conclusions:

    • Hyperthyroidism involves an abnormality in parasympathetic control of heart rate beyond direct thyroid hormone effects.
    • This abnormality is not due to reduced sinus node responsiveness or impaired neurotransmitter release.
    • The findings suggest a reduction in overall cholinergic efferent activity contributes to tachycardia in the thyrotoxic state.