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CH Adler, E Meller and M Goldstein
N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), an irreversible alpha-2 antagonist, was used to establish and quantitate the receptor reserve at the alpha-2 adrenergic autoreceptor mediating inhibition of [3H]norepinephrine ([3H]NE) release in rat cerebral cortical slices. EEDQ treatment had no effect on [3H]NE uptake or base-line release. Four hours after EEDQ treatment (0.8 mg/kg i.p.), the EC50 was shifted 7-fold to the right and there was a 21.5% decrease in the maximal response to the full alpha-2 agonist UK-14304. Using the double- reciprocal plot analysis, the equilibrium activation constant (KA) was calculated to be 1.41 +/- 0.8 microM. Similar analysis of alpha-2 autoreceptor response at various times after 1.6 mg/kg of EEDQ gave similar values for the KA. Therefore, evaluation of either the response of the remaining native receptors after partial irreversible inactivation or the response of newly synthesized receptors after nearly complete irreversible inactivation can be used to determine the KA of the receptor. Comparison of repopulation kinetics analyses for alpha-2 receptor response and estimated receptor number revealed that recovery of maximal response was much faster than actual receptor recovery. By examining the relationship between alpha-2 autoreceptor occupancy and response it was possible to determine that there is approximately a 60 to 70% receptor reserve; only 1.5% of the receptors need to be occupied by UK-14304 in order to obtain 50% of the maximal inhibition of [3H]NE release. The presence of a large receptor reserve must be taken into account when evaluating alpha-2 adrenergic autoreceptor regulation in the rat cerebral cortex.
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