Rat atrial muscle responses with caffeine: dose-response, force frequency, and postrest contractions

Area of Science:

• Cardiology
• Pharmacology
• Cell Physiology

Background:

• Caffeine's inotropic effects on cardiac muscle are complex, with both positive and negative effects reported.
• The influence of cardiac rhythm and beat interval on caffeine's cardiac response requires further investigation.

Purpose of the Study:

• To investigate the impact of varying stimulation frequencies and post-rest intervals on rat atrial contractility in the presence of caffeine.
• To elucidate the mechanism underlying caffeine's inotropic effects, particularly concerning calcium handling in cardiac cells.

Main Methods:

• Isolated rat atria were subjected to controlled electrical stimulation at different frequencies (0.1-3.0 Hz).
• Caffeine (1.0-10 mM) was administered, and its effects on contractile force and relaxation were measured.
• Post-rest potentiation was assessed by introducing variable pauses in stimulation.

Main Results:

• Caffeine induced a biphasic inotropic response (positive then negative) at 0.5 and 2.0 Hz, leading to contracture due to incomplete relaxation.
• At low frequency (0.1 Hz), caffeine exerted a purely negative inotropic effect, more pronounced with 1 mM than 5 mM.
• Post-rest potentiation was significantly reduced by caffeine (2.0 mM), while steady-state contraction amplitude remained unaffected across frequencies (0.5-3.0 Hz).

Conclusions:

• Caffeine's effects on cardiac contractility are dependent on stimulation frequency and beat intervals.
• Results suggest caffeine may induce a sarcoplasmic reticulum calcium leak, with subsequent cellular extrusion via Na+/Ca2+ exchange.
• Calcium uptake and translocation within the sarcoplasmic reticulum appear unaffected by caffeine at concentrations up to 5 mM.