Direct demonstration of D1 dopamine receptors in the bovine parathyroid gland using the D1 selective antagonist [125I]‐SCH 23982

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Abstract
The presence of D1 dopamine receptors in the parathyroid gland has been proposed based on the demonstration of dopaminergic regulation of adenylate cyclase activity and parathyroid hormone release in dispersed bovine parathyroid cells. Using a radioiodinated D1 selective antagonist [125I]‐SCH 23982, we have now directly labeled and characterized the D1 dopamine receptors in bovine parathyroid gland membranes. [125I]‐SCH 23982 binds in a saturable manner with high affinity and low nonspecific binding to membranes prepared from bovine parathyroid glands. D1 dopamine receptors are present in this preparation at a concentration of ≈︁130 fMoles/mg protein and [125I]‐SCH 23982 binding increases with increasing protein concentration in a linear fashion. Determination of the Kd using the association (k1) and dissociation (k‐1) rate constants revealed good agreement with the Kd determined by saturation analysis (390 pM vs. 682 pM, respectively). Inhibition of 0.3 nM [125I]‐SCH 23982 binding by a series of dopaminergic antagonists verified the D1 nature of this binding site, exhibiting appropriate affinities and rank order of potency. The competition curves of all antagonists exhibited Hill coefficients that were not significantly different from 1. Inhibition of [125I]‐SCH 23982 binding by dopamine and other dopaminergic agonists revealed the presence of high and low affinity agonist binding sites. Addition of 200 μM GppNHp effected a complete conversion of high affinity dopamine binding sites to a homogeneous population of low affinity dopamine sites. The D1 receptors identified in the parathyroid gland with [125I]‐SCH 23982 appear to be pharmacologically identical with those previously characterized in the central nervous system. These results demonstrate that the bovine parathyroid gland, which possesses well‐characterized functional responses to dopaminergic stimulation, may provide a single cell‐type model system for future investigation of the molecular events involved in D1 dopamine receptor/response coupling and regulation.