α Adrenergic receptors in rat parotid cells. I. Correlation of [3
H] dihydroergocryptine binding and catecholamine stimulated potassium efflux
α-Adrenergic stimulation of parotid acinar cells elicits a release of intracellular K+. The binding of [3H]dihydroergocryptine was used to characterize the α-adrenergic receptor binding sites in both parotid membranes and dissociated parotid acinar cells. [3H]Dihydroergocryptine binds rapidly to both the membranes and dissociated cells. The association rate constant in membranes was 1.7 x 107 liters mol-1 min-1 at 20°. In cells [3H]dihydroergocryptine binding reached equilibrium within 10 min. [3H]Dihydroergocryptine binding was saturable in membranes with 29.7 fmol bound/mg of protein and a K(D) of 10.5 nM. [3H]Dihydroergocryptine binding to cells rendered hypoxic, or frozen and thawed, was saturable with approximately 15,000 sites/cell. By contrast, binding to incubated, oxygenated cells did not saturate at concentrations of [3H]dihydroergocryptine up to 20 nM. [3H]Dihydroergocryptine binding closely correlated with measurements of K+ release. The ability of eight adrenergic agents to displace [3H]dihydroergocryptine correlated (r = 0.98) with their ability to stimulate or block K+ release and had the typical potency series expected for an α-adrenergic receptor (epinephrine > norepinephrine much greater than isoproterenol; (-)-epinephrine > (+)-epinephrine; (-)-norepinephrine > (+)-norepinephrine). K+ efflux generated by α-adrenergic stimulation reached maximum at 30 to 60 s. K+ was then taken up by the cells with a half-time of 4 min. This uptake could be blocked by ouabain. The maximum release of K+ by 10-3M epinephrine in the presence of 10-3M ouabain was 2.3 μg of K+/106 cells. Since each cell possesses 15,000 binding sites, 2.3 x 106 molecules of K+ are released per binding site. If 60 s are required for release, 4 x 104 molecules of K+/binding site/s are released.
Strittmatter, WJ; Davis, JN; Lefkowitz, RJ
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