Cellular mechanisms of nitrate action.

Area of Science:

• Biochemistry
• Pharmacology
• Cardiovascular Physiology

Background:

• Organic nitrates are widely accepted to exert vasodilator effects via nitric oxide (NO) production.
• The precise cellular localization of the enzymes responsible for this critical metabolic conversion remains undefined.
• Understanding nitrate metabolism is crucial for elucidating mechanisms of nitrate action and tolerance.

Purpose of the Study:

• To determine the cellular location of enzyme activity responsible for nitric oxide generation from organic nitrates.
• To investigate the role of extracellular pathways and guanylate cyclase activity in nitrate tolerance.

Main Methods:

• Chemiluminescence detection was used to measure nitric oxide production in various cellular fractions of bovine coronary artery.
• Hemodynamic effects of nitroglycerin were assessed in rats with and without exogenous thiol administration.
• In vitro relaxation studies were performed on blood vessels rendered tolerant to nitroglycerin.

Main Results:

• The highest nitric oxide production activity was localized to the plasma membrane fraction.
• Cellular-impermeant thiols, like glutathione, potentiated the hemodynamic effects of nitroglycerin, suggesting an extracellular pathway.
• Nitroglycerin-tolerant blood vessels maintained full responsiveness to nitric oxide and S-nitrosothiols, indicating preserved guanylate cyclase activity.

Conclusions:

• The plasma membrane is identified as the primary site for organic nitrate conversion to nitric oxide.
• The beneficial effects of thiols on nitrate action may involve extracellular S-nitrosothiol formation.
• Nitrate tolerance does not appear to involve significant alterations in guanylate cyclase responsiveness to nitric oxide or S-nitrosothiols.