Chiral recognition of pinacidil and its 3-pyridyl isomer by canine cardiac and smooth muscle: antagonism by sulfonylureas

• Published in: The Journal of pharmacology and experimental therapeutics Vol. 256; no. 1; pp. 222 - 229
• Main Authors: STEINBERG, M. I, WIEST, S. A, ZIMMERMAN, K. M, ERTEL, P. J, BEMIS, K. G, ROBERTSON, D. W
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Pharmacology and Experimental Therapeutics 01.01.1991
• Subjects: 550201 - Biochemistry- Tracer Techniques; Adenosine Triphosphate - physiology; ALKALI METAL ISOTOPES; Animals; Antihypertensive agents; ATP; BASIC BIOLOGICAL SCIENCES; BETA DECAY RADIOISOTOPES; BETA-MINUS DECAY RADIOISOTOPES; BIOCHEMICAL REACTION KINETICS; Biological and medical sciences; BIOLOGICAL FUNCTIONS; BLOOD VESSELS; BODY; CARDIOVASCULAR AGENTS; CARDIOVASCULAR SYSTEM; CELL CONSTITUENTS; CELL MEMBRANES; DAYS LIVING RADIOISOTOPES; Dogs; Dose-Response Relationship, Drug; DOSE-RESPONSE RELATIONSHIPS; DRUGS; Female; FUNCTIONS; Glipizide - pharmacology; Glyburide - pharmacology; Guanidines - pharmacology; HEART; Heart - drug effects; INTERMEDIATE MASS NUCLEI; ISOMERIC TRANSITION ISOTOPES; ISOMERS; ISOTOPE APPLICATIONS; ISOTOPES; KINETICS; Male; Medical sciences; MEMBRANES; MINUTES LIVING RADIOISOTOPES; Muscle Relaxation - drug effects; Muscle, Smooth, Vascular - drug effects; Muscle, Smooth, Vascular - physiology; MUSCLES; NUCLEI; NUCLEOTIDES; ODD-ODD NUCLEI; ORGANIC COMPOUNDS; ORGANS; Pharmacology. Drug treatments; Phenylephrine - pharmacology; Pinacidil; Potassium Channels - drug effects; Purkinje Fibers - drug effects; Purkinje Fibers - physiology; RADIOISOTOPES; REACTION KINETICS; RUBIDIUM 86; RUBIDIUM ISOTOPES; Rubidium Radioisotopes; Stereoisomerism; Sulfonylurea Compounds - pharmacology; TRACER TECHNIQUES; VASODILATION; Vasodilator Agents - pharmacology; VASODILATORS; VEINS; Heart; Fissipedia; Carnivora; Agonist; Interaction; Smooth muscle; Ionic channel; Guanidines; Selectivity; Isomer; Vertebrata; Mammalia; Animal; Enantiomer; Blood vessel; Sulfonylureas; Antihypertensive agent; Circulatory system; Mechanism of action; Dog; Potassium
• ISSN: 0022-3565; 1521-0103

Pinacidil, a potassium channel opener (PCO), relaxes vascular smooth muscle by increasing potassium ion membrane conductance, thereby causing membrane hyperpolarization. PCOs also act on cardiac muscle to decrease action potential duration (APD) selectively. To examine the enantiomeric selectivity of pinacidil, the stereoisomers of pinacidil (a 4-pyridylcyanoguanidine) and its 3-pyridyl isomer (LY222675) were synthesized and studied in canine Purkinje fibers and cephalic veins. The (-)-enantiomers of both pinacidil and LY222675 were more potent in relaxing phenylephrine-contracted cephalic veins and decreasing APD than were their corresponding (+)-enantiomers. The EC50 values for (-)-pinacidil and (-)-LY222675 in relaxing cephalic veins were 0.44 and 0.09 microM, respectively. In decreasing APD, the EC50 values were 3.2 microM for (-)-pinacidil and 0.43 microM for (-)-LY222675. The eudismic ratio was greater for the 3-pyridyl isomer than for pinacidil in both cardiac (71 vs. 22) and vascular (53 vs. 17) tissues. (-)-LY222675 and (-)-pinacidil (0.1-30 microM) also increased 86Rb efflux from cephalic veins to a greater extent than did their respective optical antipodes. The antidiabetic sulfonylurea, glyburide (1-30 microM), shifted the vascular concentration-response curve of (-)-pinacidil to the right by a similar extent at each inhibitor concentration. Glipizide also antagonized the response to (-)-pinacidil, but was about 1/10 as potent with a maximal shift occurring at 10 and 30 microM. Glyburide antagonized the vascular relaxant effects of 0.3 microM (-)-LY222675 (EC50, 2.3 microM) and reversed the decrease in APD caused by 3 microM (-)-LY222675 (EC50, 1.9 microM). Nitroprusside did not alter 86Rb efflux, and vascular relaxation induced by sodium nitroprusside was unaffected by sulfonylureas. Thus, the enantiomers of the 3-pyridyl isomer of pinacidil demonstrate enhanced stereospecificity in both canine cardiac and vascular tissues compared to the enantiomers of pinacidil. However, the relative selectivity of pinacidil and its 3-pyridyl isomer for cardiac and vascular smooth muscle remains unaltered. Sulfonylureas antagonize the more potent enantiomers in both tissues, supporting the involvement of an ATP-sensitive potassium channel in the action of PCOs; however, antagonism in canine vascular smooth muscle by sulfonylureas does not resemble classical competitive antagonism.