(-)-[I-125-LABELED IODOPINDOLOL, A NEW HIGHLY SELECTIVE RADIOIODINATED BETA-ADRENERGIC-RECEPTOR ANTAGONIST - MEASUREMENT OF BETA-RECEPTORS ON INTACT RAT ASTROCYTOMA-CELLS

  • 1 January 1980
    • journal article
    • research article
    • Vol. 6  (4) , 297-307
Abstract
(-)-Pindolol, potent .beta.-adrenergic receptor antagonist, was radioiodinated using chloramine-T oxidation of carrier-free Na125I and separated from unreacted pindolol to yield 2200 Ci/mmole (-)-[125I]-iodopindolol ((-)-[125I]-IPin). Mass and UV spectra confirmed that the iodination occurred on the indole ring, presumably at the 3 position. The binding of (-)-[125I]-IPin to .beta.-adrenergic receptors was studied using intact C6 rat astrocytoma cells (2B subclone) grown in monolayer cultures. Binding of (-)-[125I]-IPin was saturable with time and concentration. Using 13 pM (-)-[125I]-IPin, binding equilibrium was reached in 90 min at 21-22.degree. C. The reverse rate constant was 0.026 min-1 at 21.degree. C. Specific binding (expressed as 1 .mu.M (-)-propranolol displaceable counts) of (-)-[125I]-IPin was 95% of total binding. Scatchard analysis of (-)-[125I]-IPin binding revealed .apprx. 4300 receptors/cell and a dissociation constant of 30 pM. This was in excellent agreement with the kinetically determined dissociation constant of 35 pM. Displacement by propranolol and isoproterenol showed that (-)-[125I]-IPin binding sites were pharmacologically and stereospecifically selective. (-)-[125I]-IPin, a pure (-)-stereoisomer, high specific activity radioligand, selectively binds to .beta.-adrenergic receptors in whole cells with a high percentage of specific binding and should therefore be useful in the study and measurement of cellular .beta.-adrenergic receptors.