Mechanism of glyceryl trinitrate-induced vasodilation. II. Lack of evidence for specific binding of GTN to bovine pulmonary vein.

ABSTRACT

Radioligand binding studies were conducted to test the hypothesis that specific receptor sites for glyceryl trinitrate (GTN) exist in vascular smooth muscle. The radioligand, [3H]GTN (9.98 Ci/mmol), was incubated with whole homogenate or subcellular fractions (10,000 X g pellet, 100,000 X g pellet and 100,000 X g supernatant) of bovine pulmonary vein (BPV) at 4 degrees C, 37 degrees C or room temperature. After incubation for 5, 15, 30 or 60 min, unbound and bound ligand were estimated by physical separation and liquid scintillation spectrometry. Separation of bound from free ligand was accomplished by vacuum filtration, centrifugation, equilibrium dialysis, gel filtration or precipitation. No evidence of specific binding of [3H]GTN to BPV whole homogenate or any subcellular fraction was observed. The viability of the BPV preparations was verified by demonstrating specific binding of [3H]nitrendipine to the 100,000 X g pellet. To ensure that the ligand was present throughout the incubation procedure in sufficient concentration to detect specific binding, the ability of BPV whole homogenate and subcellular fractions to biotransform GTN to glyceryl dinitrate was measured. After 60-min incubation at 37 degrees C, greater than 95% of the GTN added was unaltered; at 90 min, more than 83% of the radioactivity was associated with GTN. Our data are interpreted as not supporting the concept of a GTN receptor in BPV.