Membrane-bound phosphodiesterases in rat myocardium

Area of Science:

• Cardiovascular Research
• Enzymology
• Molecular Biology

Background:

• Phosphodiesterase (PDE) isoenzyme-specific inhibitors are investigated as positive inotropic drugs.
• Understanding PDE isoenzyme distribution in tissues like rat myocardium is crucial for drug development and physiological studies.
• Previous reports suggested rat myocardium lacks the particulate cGMP-inhibited cAMP-PDE (type III isoenzyme).

Purpose of the Study:

• To re-evaluate the phosphodiesterase isoenzyme profile of rat myocardium.
• To determine the presence and characteristics of type III PDE in rat heart tissue.
• To reconcile conflicting reports on type III PDE in rat myocardium.

Main Methods:

• Ion-exchange chromatography was employed to analyze cAMP-PDE isoenzyme patterns.
• Specific PDE inhibitors, siguazodan (type III) and rolipram (type IV), were used.
• Western blot analysis with type III-specific antibodies was performed.

Main Results:

• Type III phosphodiesterase isoenzyme was found to be abundant in the particulate fraction of rat myocardium.
• PDE III-specific antibodies inhibited PDE activity and identified bands at 64 and 71 kDa in Western blots.
• Type III isoenzyme in myocardial membranes exhibited instability at 37°C, potentially explaining prior negative findings.
• Rat heart particulate fraction contains equal activities of membrane-bound type III and type IV low Km PDE isoenzymes.

Conclusions:

• Rat myocardium possesses considerable amounts of membrane-bound type III PDE isoenzyme.
• The instability of myocardial type III PDE at physiological temperatures may have led to previous underestimation of its presence.
• This study corrects the record, confirming the presence of type III PDE in rat myocardium, vital for understanding cardiac function and drug targeting.