Comparative partial agonist activity of -adrenoceptor antagonists

Area of Science:

• Pharmacology
• Cardiovascular Physiology

Background:

• Beta-adrenoceptor antagonists are commonly used in cardiovascular medicine.
• Their effects on cardiac contractility can vary.
• Understanding these effects is crucial for therapeutic applications.

Purpose of the Study:

• To investigate the inotropic effects of various beta-adrenoceptor antagonists.
• To characterize the dose-response relationships of these antagonists on cardiac muscle contractility.
• To compare the positive inotropic activity of different antagonists and their interaction with propranolol.

Main Methods:

• Construction of inotropic dose-response curves.
• Utilizing isolated dog trabecular muscles.
• Stimulation of muscle contraction at a regular rate.
• Experiments conducted with both normal and catecholamine-depleted muscles.

Main Results:

• Propranolol exhibited only a negative inotropic effect.
• K01366, LB46, oxprenolol, and practolol demonstrated a positive inotropic response.
• The potency order for positive inotropic activity was K01366 > LB46 > oxprenolol > practolol.
• The positive inotropic effects were more prolonged than those induced by isoprenaline.
• Propranolol significantly attenuated the positive inotropic responses to the other tested antagonists.

Conclusions:

• Several beta-adrenoceptor antagonists possess intrinsic positive inotropic properties.
• These properties are independent of catecholamine levels and distinct from the effects of isoprenaline.
• Propranolol can antagonize the positive inotropic actions of other beta-blockers, suggesting complex receptor interactions.