Interaction between gallopamil and cardiac ryanodine receptors

Area of Science:

• Cardiovascular Physiology
• Molecular Pharmacology
• Biochemistry

Background:

• The ryanodine receptor (RyR) in the sarcoplasmic reticulum is crucial for calcium release in cardiac muscle.
• Understanding the binding kinetics of ryanodine and the effects of modulators like gallopamil is key to elucidating RyR function.

Purpose of the Study:

• To investigate the binding characteristics of [3H]-ryanodine to rat heart sarcoplasmic reticulum fractions.
• To determine the effects of gallopamil on [3H]-ryanodine binding and dissociation kinetics.
• To explore the molecular mechanisms underlying gallopamil's action on the ryanodine receptor.

Main Methods:

• Equilibrium [3H]-ryanodine binding assays were performed on isolated rat heart sarcoplasmic reticulum fractions.
• Kinetic analysis of ryanodine dissociation rates in the presence and absence of gallopamil was conducted.
• Inhibition studies using varying concentrations of [3H]-ryanodine and gallopamil were analyzed.

Main Results:

• Two affinity sites for [3H]-ryanodine were identified (high: KD=1.3 nM, low: KD=2.8 µM).
• Gallopamil inhibited low-affinity [3H]-ryanodine binding (IC50 in µM range) and decreased ryanodine dissociation rates.
• Gallopamil, particularly with micromolar ryanodine, significantly slowed the dissociation of nanomolar ryanodine.

Conclusions:

• Results support a model of the ryanodine receptor as a negatively cooperative oligomer.
• Gallopamil exhibits complex actions, inhibiting low-affinity binding and modulating receptor cooperativity and conformational transitions.
• These findings provide molecular insight into gallopamil's effects on cardiac sarcoplasmic reticulum calcium release.