Cardiovascular receptors and calcium

Area of Science:

• Cardiovascular Physiology
• Molecular Pharmacology

Background:

• The sarcolemma plays a crucial role in regulating intracellular calcium in cardiac muscle.
• Understanding the interaction of drugs with sarcolemmal components is vital for cardiovascular therapeutics.

Purpose of the Study:

• To investigate the effects of verapamil on calcium binding and associated enzyme activity in cardiac sarcolemma.
• To determine the specific isomer of verapamil responsible for these effects.
• To elucidate the binding site of verapamil within the sarcolemma.

Main Methods:

• Isolation of sarcolemma-rich microsomal fractions from rabbit ventricular muscle using differential centrifugation and sucrose gradients.
• Characterization of fractions via calcium binding assays, Ca2+-activated ATPase activity measurements, gel electrophoresis, and ultrastructural analysis.
• Use of 14C-labeled verapamil to study binding interactions and effects of enzymatic pretreatment (trypsin, phospholipase C).

Main Results:

• Racemic verapamil (1 microM) significantly reduced (P < 0.001) Ca2+-binding and Ca2+-activated ATPase activity in sarcolemmal fractions.
• The inhibitory activity was attributed to the l-isomer of verapamil.
• 14C-verapamil binding occurred at carbohydrate residues within the membrane, and this binding was reduced by trypsin or phospholipase C pretreatment.
• Verapamil administration to isolated hearts under low-flow conditions prevented mitochondrial Ca2+ overload.

Conclusions:

• The l-isomer of verapamil directly inhibits calcium binding and Ca2+-activated ATPase activity in cardiac sarcolemma.
• Verapamil interacts with carbohydrate residues on the sarcolemma, suggesting a specific binding mechanism.
• These actions contribute to verapamil's cardioprotective effects by preventing mitochondrial calcium overload during ischemic stress.