Dopamine D1 receptor mRNA in rat kidney: localization by in situ hybridization

The Renin-Angiotensin and Renal Dopaminergic Systems Interact in Normotensive Humans

The renin-angiotensin-aldosterone (RAAS) and renal dopaminergic systems interact to maintain sodium balance. High NaCl intake increases renal synthesis of dopamine and dopaminergic receptor activity, decreasing epithelial sodium transport, whereas sodium deficit activates the RAAS, increasing epithelial sodium transport. We tested the hypothesis that attenuation of the natriuretic effect of dopamine D1-like receptors during salt restriction results in part from increased RAAS activity in seven salt-resistant normotensive adults using a double-blind placebo-controlled balanced crossover design. All subjects attained sodium balance on low (50 mmol Na(+)/day) and high (300 mmol Na(+)/day) NaCl diets, administered 4 weeks apart. Sodium, potassium, lithium, para-aminohippurate, and creatinine clearances were measured before, during, and after a 3-hour infusion of fenoldopam, a D1-like receptor agonist, with and without pretreatment with enalapril, an angiotensin converting enzyme inhibitor. On the high NaCl diet, fenoldopam-induced natriuresis was associated with the inhibition of renal proximal and distal tubule sodium transport. On the low NaCl diet, fenoldopam decreased renal distal tubule sodium transport but did not cause natriuresis. The addition of enalapril to fenoldopam restored the natriuretic effect of fenoldopam and its inhibitory effect on proximal tubule sodium transport. Thus, on a high NaCl diet fenoldopam causes natriuresis by inhibiting renal proximal and distal tubule transport, but on a low NaCl diet the increased RAAS activity prevents the D1-like receptor from inhibiting renal proximal tubule sodium transport, neutralizing the natriuretic effect of fenoldopam. These results demonstrate an interaction between the renin-angiotensin and renal dopaminergic systems in humans and highlight the influence of dietary NaCl on these interactions.

Dopamine D4 receptor isoform mRNA and protein are expressed in the rat cortical collecting duct

We reported previously [Am. J. Physiol. 271 (Renal Fluid Electrolyte Physiol. 40): F391-F400, 1996] that dopamine inhibits vasopressin (AVP)-dependent water permeability and Na+ transport in the rat cortical collecting duct (CCD) apparently through a D4 dopamine receptor. The present experiments used RT-PCR of total RNA extracted from microdissected rat CCD to determine whether the D4 and D1A dopamine receptor isoforms are expressed. Specific primers were used to amplify three regions of the D4 cDNA. All three gave products with 98-100% nucleotide identity to the known rat D4 sequence; however, there was an extra 6-bp insert at the 3' end of the second transmembrane region that was identical to the human and mouse sequences but which had not been documented in the rat sequence. D4 receptor protein was also localized exclusively to the CCD and medullary collecting ducts by immunohistochemistry. Two regions of the D1A dopamine receptor message were also amplified by RT-PCR of RNA from rat CCD and were verified by sequencing and immunohistochemistry. We conclude that both D4 and D1A dopamine receptors are expressed in the rat CCD, but the physiological effects are attributable to a D4 receptor.

Dopamine-induced recruitment of dopamine D1 receptors to the plasma membrane

The recruitment of G protein-coupled receptors from the cytoplasm to the plasma membrane generally is believed to be a constitutive process. We show here by the use of both confocal microscopy and subcellular fractionation that, for at least one such receptor, this recruitment is regulated and not constitutive. Cells from a proximal tubular-like cell line, LLCPK1 cells, were incubated with either a D1 agonist, a dopamine precursor, or an inhibitor of dopamine metabolism to increase dopamine availability in the cell. Each of the three procedures led to a rapid translocation of dopamine D1 receptors from the cytosol to the plasma membrane.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Dopaminergic defect in hypertension

Reverse genetics and the candidate gene approach have been utilized to identify the genetic defect(s) in hypertension. We have proposed the dopamine receptor gene as one candidate in the pathogenesis of hypertension. Because some forms of hypertension are sodium dependent or aggravated by sodium loading and because dopamine is important in aiding the organism to eliminate "excess" sodium, an abnormality in the renal dopaminergic system may be responsible for the sodium retention in hypertension. Both human and animal models of hypertension are associated with renal dopamine production and/or post first messenger defects. The Dahl salt-sensitive rat, which has a decreased ability to generate renal dopamine, and the spontaneously hypertensive rat (SHR), which has no such limitation, have a defective coupling of a D1 receptor to a G protein/adenylyl cyclase complex. This coupling defect is: (1) genetic, since it precedes the onset of hypertension and co-segregates with the hypertensive phenotype, (2) receptor specific, since it is not shared by other humoral agents, and (3) organ and nephron segment selective, since it occurs in proximal tubules but not in cortical collecting ducts or the brain striatum. A consequence of the defective dopamine receptor/adenylyl cyclase coupling in the SHR is a decreased ability of D1 agonists to inhibit Na+/H+ exchange activity. A resistance to the natriuretic effect of dopamine and D1 agonists in the SHR is due mainly to decreased cyclic AMP production, although with maturation a post cyclic AMP defect is acquired. Radioligand binding studies suggest a "loss" of the high-affinity D1 binding site in the SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

New insights into dopamine receptors in the central nervous system

Whereas biochemical and pharmacological studies indicated that there were two subclasses of dopamine receptor (D1, D2) the application of molecular biology techniques has defined at least six dopamine receptor isoforms. These may be divided into D1-like (D1, D5) and D2-like (D2(short), D2(long), D3, D4) subfamilies on the basis of their structural and pharmacological properties. In this commentary the common properties of these dopamine receptor species are described, including the predicted structures of seven transmembrane alpha-helices, amino acid homologies and conserved amino acids that may play important structural and functional roles. The D1-like and D2-like receptor isoforms have individual properties and these are described in terms of their structures, pharmacological and biochemical properties and localizations in different brain regions. The existence of multiple dopamine receptor isoforms is important for understanding how certain drugs achieve their therapeutic effects and how unwanted side effects arise. This is considered for the anti-parkinsonian and anti-schizophrenic drugs. The localization of D1 and D2 dopamine receptors to particular cell types in the neostriatum allows new insights to be made into the normal mode of action of dopamine to control motor function and how this is disturbed in disease stages e.g. Parkinson's disease, Huntington's disease. The detailed mode of action of anti-parkinsonian drugs can also be better understood from this. The availability, from molecular biology studies, of the amino acid sequences of the receptor isoforms allows predictions to be made of the structures of these species. In particular it is possible to produce speculative models of the three dimensional structures of the ligand-binding sites of these receptors. These speculations can be complemented by chemical modification, pH dependency and mutagenesis studies which provide information on the amino acid residues at the ligand binding site that actually interact with the ligand. In time it should be possible to understand in some detail the mechanism of receptor-ligand interaction and this will be important for the design of drugs targeted at specific isoforms.