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Dopamine (DA) produces a natriuretic/diuretic response in the kidney by mechanisms that are still not well understood. There is some indication that DA2 receptors may be involved in mediating the effects of DA, but little is known regarding the nature of this receptor in the kidney. Autoradiographic localization of [3H]spiperone, a DA2 antagonist, indicated that high-density binding was restricted to inner medullary collecting ducts (IMCDs). [3H]Spiperone binding was saturable, high affinity (Kd, 17.2 +/- 1.65 nM), and high density (Bmax, 935 +/- 83 fmol per mg of protein). The photosensitive spiperone analogue N-(p-azido-m-[125I]iodophenethyl)spiperone labeled similar sized proteins of Mr = 120,000 in membranes prepared from the kidney inner medulla, striatum, and pituitary. However, the rank-order competition profile for the [3H]spiperone binding in the kidney inner medulla differed from the DA2 receptor in striatum and pituitary and, furthermore, RNA (Northern) blot analyses of kidney inner medullary RNA with brain DA2 receptor oligonucleotide probes were negative. Functionally, DA stimulated prostaglandin E2 production by IMCD cells, an effect that could be blocked by the DA2 antagonist domperidone. These results indicate that the kidney inner medulla expresses a functional DA receptor that may represent a newly identified DA receptor subtype (here designated DA2K). Moreover, these results suggest that the kidney inner medulla may be a significant site at which DA, either directly or indirectly, influences water and electrolyte excretion.

This article summarizes the recent progress our laboratory has made in understanding the molecular characteristics of the D1 dopamine receptor. The D1 dopamine receptor from rat striatum has been purified to near homogeneity using a combination of several chromatographic steps. Furthermore, the gene for the human D1 dopamine receptor has been cloned, sequenced, and expressed. The cloned receptor has all the pharmacologic and biochemical properties of the classical D1 receptor coupled to adenylyl cyclase which has been previously described in the central nervous system.

Significant new information on the D2 dopamine receptor has recently become available from a combination of protein chemical and molecular genetic analyses. Molecular genetic studies have shown the receptor to be a member of the family of receptors that are linked to G proteins and that have structures predicted to contain seven transmembrane domains. Two distinct species of D2 dopamine receptor have been found which may differ in their coupling to G proteins; their distributions have been mapped at the nucleic acid level. The D2 dopamine receptor has been purified from brain and anterior pituitary and characterized. Chemical modification of the brain receptor provides evidence for the importance of a carboxyl group that interacts with ligands at the receptor binding site. Here, Philip Strange discusses these points and proposes models of receptor-ligand interaction based on the conservation of several aspartic acid residues in receptors that bind cationic amines.