Cardiac beta-adrenoceptor changes in experimental hyperthyroidism in dogs

Area of Science:

• Cardiovascular Physiology
• Endocrinology
• Molecular Pharmacology

Background:

• Hyperthyroidism is associated with cardiovascular changes.
• The role of beta-adrenoceptor subtypes in mediating these changes requires further elucidation.

Purpose of the Study:

• To investigate the impact of triiodothyronine (T3)-induced hyperthyroidism on cardiovascular function and beta-adrenoceptor density in dogs.
• To examine the relationship between altered beta-adrenoceptor density and cardiac function in experimental hyperthyroidism.

Main Methods:

• Administration of triiodothyronine (T3) to dogs for 14 days, with saline-treated dogs as controls.
• Measurement of beta-adrenoceptor subtype density using radioligand binding assays.
• Assessment of in vivo chronotropic responses to isoprenaline.
• Evaluation of adenylate cyclase activity in cardiac membranes.

Main Results:

• T3 treatment resulted in physiological signs of hyperthyroidism and significantly increased chronotropic responses to isoprenaline.
• A substantial increase in beta-1 adrenoceptor density was observed in both right atrial (93%) and left ventricular (141%) membranes.
• Beta-2 adrenoceptor densities in atrial, ventricular, and lung membranes remained unchanged.
• Basal and maximally stimulated adenylate cyclase activities in left ventricular membranes were not affected by T3 treatment.

Conclusions:

• Experimental hyperthyroidism in dogs enhances chronotropic responses to isoprenaline, correlating with increased cardiac beta-1 adrenoceptor density.
• The lack of change in adenylate cyclase activity suggests that downstream signaling pathways may not be solely responsible for the observed functional changes.
• These findings contribute to understanding the complex cardiovascular adaptations during hyperthyroidism.