Diminished expression of renal dopamine D1A receptors in the kidney inner medulla of the spontaneously hypertensive rat

BACKGROUND

Dysfunctional dopamine neurotransmission and greater than normal retention of salt have been found for renal proximal tubules of the spontaneously hypertensive rat

OBJECTIVE

To determine whether there are differences between kidney D1A dopamine receptor distributions of spontaneously hypertensive rats and Wistar-Kyoto rats.

METHODS

We examined the expression of D1A dopamine receptors in kidneys of spontaneously hypertensive rats and the normotensive Wistar-Kyoto rat through Western blots and immunocytochemistry, using highly specific antipeptide antibodies directed against the receptor.

RESULTS

The specificity of the antisera was demonstrated by Western blot studies, using proximal tubules, from Wistar-Kyoto rats. The antiserum recognized a major polypeptide with Mr of 72 kDa and a minor protein of Mr 66 kDa, which were not detected either by antigen-adsorbed or by preimmune sera. In renal cortex of both Wistar-Kyoto rats and spontaneously hypertensive rats, D1A receptors were expressed at equivalent levels. In the inner medulla of Wistar-Kyoto rat, there was diminished (by 60%) expression of D1A receptors compared with that of the renal cortex. However, the expression of D1A receptors in the inner medulla in the spontaneously hypertensive rat was even more diminished (by 83%) relative to levels found in spontaneously hypertensive rat renal cortex. Immunocytochemical studies localized the D1A receptor protein in renal cortex primarily to epithelia of tubules. Relative to renal cortex, there was an overall decrease in staining intensity in the inner medulla both of Wistar-Kyoto rats and of spontaneously hypertensive rats. Compared with that of Wistar-Kyoto rat, the intensity of staining of D1A receptors in the inner medulla of spontaneously hypertensive rats was greatly diminished, confirming the Western blot analyses. The less than normal expression of D1A receptors in the inner medulla of spontaneously hypertensive rats might be of physiologic importance in the etiology of greater than normal retention of salt and hypertension in spontaneously hypertensive rats.

Coupling of D1 and D5 dopamine receptors to multiple G proteins: Implications for understanding the diversity in receptor-G protein coupling

Dopamine receptors are a subclass of the super family of G protein-coupled receptors, that transduce their effects by coupling to specific G proteins. Within the dopamine receptor family, the adenylyl cyclase stimulatory receptors include the D1 and D5 subtypes. The D1 and D5 dopamine receptors are genetically distinct, sharing >80% sequence homology within the highly conserved seven transmembrane spanning domains, but displaying only 50% overall homology at the amino acid level. When expressed in transfected GH4C1 rat pituitary cells, both D1 and D5 receptors stimulate adenylyl cyclase and have identical affinities toward dopaminergic agonists and antagonists. In order to analyze specific signaling pathways mediated by activation of either D1 or D5 receptors, we have identified the G proteins that are coupled to these receptors. Through functional analyses and competition binding studies, and from immunoprecipitation techniques, using antisera against the various alpha subunits of G proteins, we have established that both D1 and D5 receptors couple to G(s)alpha. In addition, D1 receptors are also coupled to G(o)alpha. Since G(o)alpha has been implicated in the regulation of Ca2+, K+, and Na+ channels, this finding would suggest that D1 receptors can mediate the functional activity of these ion channels. There is also evidence to indicate that D5 receptors couple to G(z)alpha, a novel G protein abundantly expressed in neurons. Thus, despite similar pharmacological properties, such differential coupling of D1 and D5 receptors to G proteins other than G(s)alpha, indicates that dopamine can transduce varied signaling responses upon the simultaneous stimulation of both these receptors.

Photoaffinity labeling of D1 and D5 dopamine receptors

Although human D1 and D5 dopamine receptors are encoded by distinct genes and share only 50% sequence homology at the amino acid level, their pharmacological properties are identical. Using a selective D1 receptor photoaffinity radioligand, (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4-azidophenyl)-2,3,4,5-tetrahyd ro-1H-3-benzazepine ([125I]MAB), we have further probed the molecular properties of these receptors in transfected GH4C1 rat pituitary cells. Under reversible, non-covalent binding conditions, [125I]MAB bound to both the D1 and the D5 receptors with identical affinities, dopaminergic selectivity and stereospecificity. Upon photoactivation of the bound [125I]MAB, the label was incorporated into a approximately 64,000 mol. wt protein corresponding to the D1 dopamine receptor. However, there was no specific photoincorporation of the ligand observed in D5 receptors. The lack of [125I]MAB photolabeling of D5 receptors was independent of the cell line chosen, since similar results were obtained using other transfected cells. The data suggest that although both D1 and D5 receptors share structurally similar binding sites, the protein domains around the sites are different. Thus, although there are currently no specific compounds which bind preferentially to D1 or D5 receptors, these receptors can be distinguished from one another by the inability of [125I]MAB to photolabel D5, but not D1, receptors. Such selective targeting of a specific receptor may be useful in understanding the functional importance and/or interaction between closely related members of the same receptor family when co-expressed in the same cell.

Effect of oil on the level of solubilization of testosterone propionate into nonionic oil-in-water microemulsions

The level of solubilization of the drug testosterone propionate into 2% w/w oil-in-water (o/w) microemulsions, stabilized by the nonionic surfactant polyoxyethylene 10-oleyl ether (Brij 96) and containing a range of oils, has been determined. Although testosterone propionate was readily soluble in the ethyl esters ethyl oleate, ethyl caprylate, and ethyl butyrate, and the triglycerides soybean oil, Miglyol 812, and tributryin, and the alkene 1-heptene, only microemulsions containing the ethyl esters and the triglyceride oils exhibited a significant increase in solubilization over the corresponding micellar solution (i.e., surfactant solution in the absence of oil). Furthermore, the increase in drug solubility observed in the microemulsion systems was not related to the solubility of the drug in the bulk oil. That is, while the smaller molecular volume oils, such as ethyl butyrate, exhibited a greater capacity for the drug, microemulsions containing these oils were only marginally better at solubilizing the drug than the corresponding micellar solution. In contrast, microemulsions containing the larger molecular volume oil, Miglyol 812, gave levels of drug solubilization almost three times those containing ethyl butyrate, yet the bulk capacity for drug in this oil was less than half that of ethyl butyrate. Light scattering and phase inversion temperature studies suggested that the structure of the microemulsion was sensitive to the oil being used, in that, at the low oil concentrations used in this study, the smaller molecular volume oils generally penetrated the interfacial surfactant monolayer in much the same way as a cosurfactant, causing an alteration, presumably a dilution, of the relatively concentrated polyoxyethylene region close to the hydrophobic core, thereby destroying one of the main loci of drug solubilization and counteracting any advantages encountered due to the high solubility of the drug in the bulk oil.

Regulation and expression of D-1, but not D-5, dopamine receptors in human SK-N-MC neuroblastoma cells

Human SK-N-MC neuroblastoma cells endogenously express functional D-1-like dopamine receptors. The recent identification of at least two distinct type of D-1-like receptors, D-1 and D-5, lead us to investigate the precise molecular identify of the receptors expressed in these cells. By using specific primer sets and amplification of the mRNA by RT-PCR, we show that only D-1, but not D-5, dopamine receptors are expressed in these cells. Treatment of cells with 100 microM dopamine initially caused an upregulation in D-1 mRNA expression, followed by attenuation of the message compared to control, untreated cells. The D-1 receptors in SK-N-MC cells, whose expression is controlled by dopamine in a bimodal manner, may be important in understanding how these receptors are regulated at the molecular level.

Molecular and structural differences between rat brain D-1 and renal DA-1 dopamine receptors

Renal DA-1 dopamine receptors in proximal tubules (PTs) of the Wistar-Kyoto (WKY) rat display pharmacological binding properties which are different from central nervous system (CNS) striatal D-1 dopamine receptors. In general, the renal DA-1 receptors display affinity binding values of dopaminergic drugs which are 6-36-fold less than those seen for brain D-1 receptors. The renal and brain DA receptors also displayed differential sensitivity toward the alkylating agent, N-ethylmaleimide (NEM). Inactivation of 50% of DA-1 renal receptors was achieved at lower concentrations of NEM (5.2 microM), relative to brain D-1 receptors (140 microM). Western blot analyses of rat pituitary GH4C1 cells, transfected with human CNS D-1 receptor cDNA, with human anti-D-1 dopamine receptor antiserum, detected a single polypeptide with M(r) of 66 kDa. In PTs, a specific polypeptide of higher molecular weight (M(r) = 72 kDa) was seen. Surprisingly, in rat striatal membranes, the D-1 antiserum failed to detect any proteins within this molecular weight range. Photoaffinity labeling studies with a DA-1 selective photoligand, identified the identical protein by autoradiography and Western blots in kidney, but not in striate. Together, these data indicate that renal DA-1 dopamine receptors have distinct molecular properties relative to brain D-1 dopamine receptors.

Dysfunctional D1A receptor-G-protein coupling in proximal tubules of spontaneously hypertensive rats is not due to abnormal G-proteins

BACKGROUND

Dysfunctional dopamine neurotransmission and defective D1A receptor-G protein coupling exist in renal proximal tubules (RPT) of the spontaneously hypertensive rat (SHR).

OBJECTIVE

To determine whether the G proteins in SHR are abnormal, preventing formation of agonist high affinity sites in SHR.

METHODS

We examined the expression levels of the alpha-subunits of G proteins, as well as D1A receptor receptor coupling to exogenously added normal G proteins, in RPT of SHR and the normotensive Wister-Kyoto (WKY) rat.

RESULTS

In the presence of 110 mmol/l NaCl, the D1A dopamine receptor-selective agonist SKF R-38393 binds both to high- and to low-affinity sites on solubilized and reconstituted D1A receptors extracted from renal proximal tubules of normotensive Wistar-Kyoto (WKY) rats. In the spontaneously hypertensive rat (SHR), SKF R-38393 bound to a single site on the reconstituted receptor with affinity values corresponding to the low-affinity state of the receptor. Western blot analyses indicated that the alpha-subunit of the guanine nucleotide binding protein (G-protein), Gs, was expressed at similar levels, whereas G(o)alpha was not expressed in proximal tubule membranes from WKY rats and SHR. Pretreatment of proximal tubule membranes with the alkylating agent N-ethylmaleimide in the presence of SKF R-38393 inactivated alpha-subunits of endogenous G-proteins, but not D1A receptors, resulting in loss of high-affinity binding sites in WKY rats. These N-ethylmaleimide-treated D1A receptors from WKY rats, when reconstituted with exogenous sources of G-proteins, were able to couple to these exogenous G-proteins, with complete restoration of high-affinity sites. Moreover, the affinity values and the proportion of these hybrid sites were similar to those of untreated receptors, and these affinity sites were regulated by guanine nucleotide analogs. Reconstitution of D1A receptors from SHR with the same exogenous G-proteins failed to similarly induce formation of the high-affinity binding sites in the hybrid reconstituted systems, and SKF R-38393 continued to bind in a single low-affinity state of the receptor.

CONCLUSION

These results demonstrate that the absence of G-protein coupling in SHR is due to intrinsic defects within the receptor protein, rather than to any abnormalities of the endogenous G-proteins themselves.

A novel regulation of expression of the alpha-subunit of the G stimulatory protein by dopamine via D1 dopamine receptors

Exposure of human neuroblastoma SK-N-MC cells to 100 microM dopamine (DA) for 72 h caused 70% loss of immunodetectable membrane-bound levels of the alpha-subunit of Ga. The loss in Gs alpha was accompanied by reduced (64.3 +/- 0.35% of control values) NaF-mediated stimulation of adenylyl cyclase and was independent of accumulated cyclic AMP (cAMP) levels, because neither forskolin nor dibutyryl cAMP treatment of cells mimicked the DA-induced effects. The reduction in Gs alpha content was manifest at the transcriptional level; Gs alpha mRNA levels were attenuated to 56.5 +/- 10% of control values after a 24-h treatment of cells with 100 microM DA. The concentration of DA required to produce the half-maximal decrease of Gs alpha mRNA content was 20 nM, similar to the high-affinity binding value (8.5 nM) of DA to D1 sites. Gs alpha mRNA levels were also attenuated (52 +/- 3.5% of control values) by the D1-selective agonist SKF R-38393 but not by forskolin or dibutyryl cAMP. Attenuation of Gs alpha mRNA levels by agonists was blocked by the D1-selective antagonist SCH 23390. Stimulation of adenylyl cyclase-inhibitory DA receptors, which are coexpressed in these cells, failed to down-regulate Gs alpha mRNA, indicating that regulation of Gs alpha mRNA expression occurs specifically through chronic stimulation of D1 DA receptors.

Increased expression of inhibitory guanine nucleotide binding proteins in membranes from renal proximal tubules, but not brain striata, of the normotensive and the spontaneously hypertensive rat

BACKGROUND

Dysfunctional dopamine neurotransmission and defective D1A receptor-G protein coupling exist in renal proximal tubules (RPT) of the spontaneously hypertensive rat (SHR).

OBJECTIVE

To determine whether differential expression of G proteins contributes to these anomalous phenomena.

METHODS

We examined the expression levels of the alpha-subunits of G proteins, in RPT of SHR and the normotensive Wistar-Kyoto (WKY) rat, as well as in striatal membranes, where dopamine functions and receptor-G protein coupling are known to be normal.

RESULTS

In general, rat striatal membranes displayed overall higher expression levels of the different alpha-subunits, relative to those of RPT membranes. Moreover, no significant differences were observed between G protein levels in rat striata in SHR and WKY rats. However, in RPT, both subunits of Gs alpha were equally expressed in SHR and WKY rats. Gi1 alpha was expressed in RPT of SHR and WKY rats to the same levels, but neither G(o) alpha nor Gz alpha was detected in these membranes; higher (50%) expression levels of Gi2 alpha were found in SHR. The largest difference in alpha-subunit expression levels between SHR and WKY RPT was observed for Gi3 alpha. This protein was present in SHR RPT at levels 4.8-fold higher than those in WKY rat RPT. The only alpha-subunit which was slightly depressed (by 20%) in SHR was Gq alpha.

CONCLUSION

The overexpression of Gi2 alpha and Gi3 alpha in SHR RPT may be of importance in the genesis of hypertension in this animal.

Sulfhydryl groups of renal D1A dopamine receptors: differential sensitivity of receptors to N-ethylmaleimide in normotensive and hypertensive rats

OBJECTIVES

To determine whether sulfhydryl groups are present on D1A receptors of spontaneously hypertensive rats (SHR) and to test the hypothesis that failure of agonists to bind to such receptors is linked to microstructural changes involving sulfhydryl groups.

METHOD

Alkylation of renal proximal tubule membranes by N-ethylmaleimide caused 70% loss of D1A dopamine receptor binding sites in the normotensive Wistar-Kyoto (WKY) rat and the SHR.

RESULTS

The concentration of N-ethylmaleimide (IC50) required to produce half-maximal loss of receptor binding was 5.2 and 1200 mumol/l in WKY rats and SHR, respectively. Previous receptor occupancy of WKY rat D1A sites by the D1A agonist SKF R-38393 completely protected the binding sites from N-ethylmaleimide-mediated inactivation. Occupancy with the D1A antagonist SCH 23390 partially protected the binding sites and produced a 500-fold increase in the IC50 of N-ethylmaleimide. In SHR, receptor occupancy either by SKF R-38393 or by SCH 23390 failed to protect the D1A sites from N-ethylmaleimide or to alter the IC50 of N-ethylmaleimide-mediated inactivation.

CONCLUSION

These results indicate that D1A dopamine receptors both of WKY rats and of SHR contain sulfhydryl groups at or near the ligand binding site, which display differential sensitivity to N-ethylmaleimide.

Absence of DA1/DA2 dopamine receptor interactions in proximal tubules of spontaneously hypertensive rats

The impact of defective DA1 dopamine receptors in proximal tubules (PT) of the spontaneously hypertensive rat (SHR) on DA1/DA2 receptor interactions was assessed with the DA1-selective photoaffinity ligand, (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4-azidophenyl)- 2,3,4,5-tetrahydro-1H-3-benzazepine ([125I]MAB). In PT membranes from both normotensive (Wistar-Kyoto, WKY) and spontaneously hypertensive rats (SHR), [125I]MAB was specifically incorporated into a polypeptide with an M(r) of 74,000 Da, corresponding to the DA1 receptor. The labeling of this band by [125I]MAB in both SHR and WKY was not prevented by SKF-82526, a potent DA1-selective agonist. However, in the presence of the DA2 antagonist, (-)-sulpiride, but not DA2 agonist, LY-171555, SKF-82526 abolished photoincorporation of [125I]MAB into the 74,000-Da band in WKY. In SHR, (-)-sulpiride failed to enhance the ability of SKF-82526 to compete with [125I]MAB for binding to the 74,000-Da subunit. In competition binding studies with SKF-82526, (-)-sulpiride induced the formation of agonist high-affinity binding sites in WKY but not in SHR. These data suggest that in membranes of SHR, but not WKY, DA1/DA2 dopamine receptor interactions are lacking.

Alpha 1B-adrenergic receptors in rat renal microvessels

Although several alpha-adrenergic receptor genes are expressed in the rat kidney, their expression in the renal vasculature has not been studied. Since pharmacological studies have suggested that an alpha 1B-adrenergic receptor may mediate renal vasoconstriction, we studied the expression of alpha 1B-adrenergic receptors in renal microvessels, from 10- to 14-week-old male spontaneously hypertensive rats (SHR) and their normotensive control, the Wistar-Kyoto rat (WKY). In these microvessels, isolated by perfusion with iron, alpha 1B-adrenergic receptor mRNA levels (by ribonuclease protection assay) were similar in SHR and WKY rats. Photo-affinity labeling with [125I]-arylazidoprazosin demonstrated the presence of alpha 1B-adrenergic receptor protein. Maximum receptor density (determined by 3H-prazosin binding: Bmax 59.8 +/- 4.1 and 58.7 +/- 4.3; Kd 0.48 +/- 0.05 nM and 0.31 +/- 0.06 nM in SHR and WKY, respectively) and chloroethylclonidine (CEC)-sensitive binding sites (determined by [125I]-(2-beta(4-hydroxyphenyl)-ethylaminomethyl)-tetralone binding) (125I-HEAT) were similar in SHR and WKY rats. There are two novel findings in these studies: (1) the alpha 1B-adrenergic receptor gene is expressed in renal microvessels of WKY and SHR; (2) alpha 1B-adrenergic receptor gene expression in renal microvessels is not altered in adult SHR. The failure to down-regulate expression of the alpha 1B-adrenergic receptor at the mRNA and protein level in the SHR could result in persistence of alpha 1B-adrenergic receptor effects and contribute to the increased vascular resistance in hypertension.

Coupling of human D-1 dopamine receptors to different guanine nucleotide binding proteins. Evidence that D-1 dopamine receptors can couple to both Gs and G(o)

Coupling between D-1 dopamine receptors and G proteins in cell lines expressing human D-1 receptors and different G proteins was examined. Pertussis toxin (PTX) treatment of rat pituitary GH4C1 cells significantly reduced, but did not abolish, agonist high affinity binding sites of the D-1 dopamine receptor; in SK-N-MC neuroblastoma cells, PTX failed to have any effect on D-1 high affinity sites. Cholera toxin (CTX) treatment of GH4C1 cells reduced but did not abolish the high affinity sites of D-1 receptors, while in SK-N-MC cells, treatment with CTX abolished all the high affinity sites. Western blot analyses with specific antisera indicated that Gs alpha, Gi1 alpha, Gi3 alpha, and Gq alpha were expressed in both cell lines, while Gi2 alpha and G(o) alpha were expressed in GH4C1 but not SK-N-MC cells. Antisera NEI-805 (anti-Gs alpha) and 9072 (anti-G(o) alpha) immunoprecipitated 24 +/- 4.3 and 34.4 +/- 6.9%, respectively, of G protein-associated D-1 dopamine receptors. Antisera 3646 (anti-Gi1 alpha), 1521 (anti-Gi2 alpha), 1518 (anti-Gi3 alpha), and 0941 (anti-Gq alpha) failed to coimmunoprecipitate appreciable levels of soluble receptors. These data indicate that D-1 dopamine receptors are coupled to both Gs alpha and G(o) alpha but not to Gq alpha.

Differences in photoaffinity labeling of DA1 receptors in renal proximal tubules from normotensive rat and SHR

Renal DA1 dopamine receptors in proximal tubule membranes of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were characterized with the novel D1 dopamine receptor-selective photoaffinity probe, (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4-azidophenyl)-2,3,4,5- tetrahydro-1H-3-benzazepine ([125I]MAB). Under nonphotolyzing conditions, saturation studies showed that [125I]MAB bound with similar affinity to DA1 dopamine receptors in both WKY [dissociation constant (Kd) = 16.3 nM] and SHR (Kd = 19.5 nM). At photolysis, [125I]MAB was irreversibly incorporated into a single major protein of 74,000 Da in both WKY and SHR. DA1-selective antagonists blocked photolabeling of DA1 sites with similar efficiency and specificity in SHR and WKY. However, under identical assay conditions, dopaminergic agonists were unable to block photoincorporation of [125I]MAB in SHR but not in WKY. This pattern of labeling of DA1 sites by [125I]MAB may suggest the presence of defective agonist, but not antagonist, binding domains on the receptor in SHR but not in WKY rats.

Differential coupling of D1 and D5 dopamine receptors to guanine nucleotide binding proteins in transfected GH4C1 rat somatomammotrophic cells

D1 and D5 dopamine receptor genes, stably expressed in GH4C1 rat somatomammotrophic cells, display identical binding values and stimulate adenylate cyclase. Approximately 60% of D1 receptors were in the agonist high-affinity state and were converted to the low-affinity state by 100 microM guanyl-5'-ylimidodiphosphate [Gpp(NH)p]. Of the 48% of D5 receptors in the high-affinity state, only half were modulated by 100 microM Gpp(NH)p; in the presence of the G protein activator, AIF4-, the high-affinity sites of D5 receptors were abolished by Gpp(NH)p, suggesting tight coupling between D5 receptors and G proteins. The high-affinity sites of D1, but not D5, receptors were reduced after pertussis toxin treatment of cells. Thus, whereas D1 receptors in GH4C1 cells couple to both Gs, the G stimulatory protein, and a pertussis toxin-sensitive G protein, D5 receptors couple to Gs and a pertussis toxin-insensitive G protein. Neither D1 nor D5 receptors were able to stimulate phosphoinositide metabolism in these cells. The ability of D5, but not D1, receptors to couple to novel G proteins may be significant in assigning a functional role for these receptors.

Ascorbic acid inhibits 125I-SCH 23982 binding but increases the affinity of dopamine for D1 dopamine receptors

Effects of ascorbic acid (AA) on 125I-SCH 23982 binding to D1 dopaminergic receptors in membrane preparations from rat striatum were investigated. AA in the range of 0.03 microM-0.33 mM inhibited 75% of specific binding of 125I-SCH 23982 in a dose-dependent manner. At higher concentrations, this inhibition of binding activity by AA was less potent, and 3.3 mM AA inhibited only 30% of specific binding. Reduced glutathione did not alter the inhibition of binding by 0.33 mM AA, but reduced the inhibition by 3.3 mM AA to 8% of specific binding. The loss of specific binding by AA was rescued by 1 mM EDTA, an inhibitor of lipid peroxidation. In the absence of AA, competition experiments with the agonist, dopamine, revealed the presence of high-affinity (Kh = 224.9 +/- 48.9 nm) and low-affinity (Kl = 21,100 +/- 2,400 nM) binding sites. Although the maximum binding of 125I-SCH 23982 decreased to 40% without affecting the KD value in the presence of 1.67 mM AA, the value of the high-affinity site for dopamine was increased (Kh = 23.3 +/- 9.4 nM) and that of the low-affinity site was decreased (Kl = 136,800 +/- 40,900 nM). These results suggest that AA may affect D1 dopamine receptor function by lipid peroxidation, competition with dopamine for low-affinity sites, and reduced oxidation of dopamine.

Induction of G protein-independent agonist high-affinity binding sites of D-1 dopamine receptors by beta-mercaptoethanol

We have purified the D-1 dopamine receptor 8200-fold to 78% purity from rat striatal membranes. Critical to this purification was the N-ethylmaleimide (NEM)-mediated alkylation of all endogenous sulfhydryl groups, except those associated with the D-1 dopamine receptors, which were protected by the D-1 agonist SKF R-38393. Such NEM treatment of D-1 receptors abolished all agonist high-affinity binding sites of the receptors, but did not alter the antagonist binding properties. When NEM-treated D-1 receptors were affinity-purified by mercury-agarose columns, the pharmacological properties of these purified receptors were examined, after removal of beta-mercaptoethanol (beta ME), which was used for elution of receptors from the affinity column. Purified D-1 receptors displayed typical dopaminergic antagonist binding values; however, agonists bound to the purified receptors with only high-affinity binding values, despite the prior absence of high-affinity sites in crude soluble extracts of NEM-treated receptors. The agonist high-affinity binding of purified D-1 receptors was insensitive to modulation by the GTP analog Gpp(NH)p and occurred in the absence of any G proteins. These Gpp(NH)p-insensitive high-affinity sites appeared to be induced by beta ME, since similar high-affinity binding was also restored by beta ME to crude soluble and membrane-bound receptors, which had been pretreated with NEM. The ability of D-1 dopamine receptors to bind with high-affinity to agonists in the absence of functionally active G proteins may be an intrinsic property of the reduced state of D-1 dopamine receptors.

Temperature sensitivity of agonist high-affinity binding sites of solubilized and reconstituted D1 dopamine receptors

Solubilization of rat striatal membranes with sodium cholate, followed by reconstitution into phospholipid vesicles, leads to a 6.5-fold increase in the agonist high-affinity binding sites of the D1 dopamine receptor. These high-affinity binding sites display differential sensitivity toward temperature. When reconstituted receptors were preincubated for 1 h at 0-4 degrees C (on ice) or at 22 degrees C (room temperature) followed by radioligand binding assays with dopamine, neither the high-affinity values of the receptor for dopamine nor the percent receptors in the high-affinity state (31-39%) were changed from control reconstituted receptors, which were not subject to any preincubations. At 30 degrees C, there was a partial loss in the number of high-affinity D1 receptors with only 25% of the total receptor population in the high-affinity state; there was no change in the affinity values of the high-affinity binding sites. At 37 degrees C, there was a 40% loss in total number of D1 receptor binding sites. All the high-affinity binding sites were lost and the remaining 60% of binding activity represented the low-affinity binding state of the receptor. These results indicate that the high-affinity binding sites of the reconstituted D1 dopamine receptors are uniquely sensitive to higher temperatures.

Specific phospholipid requirements for the solubilization and reconstitution of D-1 dopamine receptors from striatal membranes

We have reported the solubilization and reconstitution of functional D-1 dopamine receptors from rat striatal tissue, using sodium cholate as detergent [Sidhu, A. (1988) Biochemistry 27, 8768-8776]. Critical to our method of extraction was the absolute requirement for the persistent presence of a crude extract of phospholipids (PLs) from bovine brain, during both solubilization of membranes and reconstitution of the soluble extract into PL vesicles. In the absence of PLs, fewer than 10% of the receptors were recovered, while in the presence of PLs, 40% of the receptors were reconstituted into vesicles. To probe the composition of PLs required by D-1 dopamine receptors during these extraction procedures, specific PLs of a defined composition were used during either solubilization or reconstitution alone or during both solubilization and reconstitution. Phosphatidylcholine (PC), when used during the solubilization procedure alone or during both solubilization and reconstitution, resulted in recovery of 41-48% of the D-1 dopamine receptors but was 3.7-fold less effective when present during reconstitution alone (11%). Phosphatidylethanolamine (PE), when used during reconstitution alone, resulted in recovery of nearly 25% of the D-1 dopamine receptors. When PE was present during either solubilization or both solubilization and reconstitution, 6-11% of the receptors were recovered. If PE was used with PC in ratios of 1:1 or 2:1, respectively, 28-38% of the receptors were recovered. When PL vesicles of PE:PC were present in ratios of 1:2 during both solubilization and reconstitution, the maximum theoretical (74-87%) recovery of total receptor binding sites was achieved.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of a multiple disk centrifugal pump as an artificial ventricle

A multiple-disk centrifugal pump based on the Tesla Turbine design has been modified for potential use as an artificial ventricle or ventricular assist device. The pump consists of a series of interconnected parallel disks placed within a spiral volute housing. This pump normally operates as a continuous flow device; however, a controller circuit has been developed to also allow for pulsatile operation. Frequency, systolic duration, systolic rise time, and diastolic decay time can be independently controlled to produce a wide range of pulsatile pressures and flows. This pumping system was tested in vitro on a mock circulatory system using a blood analogue. Inlet and outlet pressures, outlet flow, and motor rotations per minute were continually monitored over a wide range of physiologic operating conditions. The disk pump output was compared with that of other artificial ventricles and produced favorable results. Direct experimental comparisons were made with a Harvard Apparatus pulsatile piston pump. Unlike the Harvard pump, the disk pump does not use valves. Rather, a slight forward rotation of the disks is used to offset the adverse diastolic pressure gradient, which avoids backflow through the device.

Persistent defective coupling of dopamine-1 receptors to G proteins after solubilization from kidney proximal tubules of hypertensive rats

The natriuretic effect of dopamine-1 (DA-1) agonists is reduced in spontaneously hypertensive rat (SHR), partly because of defective DA-1 receptor-adenylate cyclase (AC) coupling in renal proximal convoluted tubules. To investigate this defective coupling, DA-1 dopamine receptors from renal proximal tubules were solubilized and reconstituted into phospholipid vesicles. The binding of DA-1-selective ligand [125I]SCH 23982 was specific and saturable, with no differences in receptor density or Kd between SHR and normotensive rats (Wistar-Kyoto rats; WKY). Competition experiments of the reconstituted DA-1 dopamine receptors in WKY with a DA-1-selective agonist, SKF R-38393, revealed the presence of high- (Kh = 350 +/- 209 nM) and low-affinity (Kl = 70,500 +/- 39,500 nM) binding sites. 100 microM Gpp(NH)p abolished the agonist high-affinity sites, converting them to a low-affinity state (Ki = 33,650 +/- 10,850 nM). In SHR, one affinity site was noted (Ki = 13,800 +/- 500) and was not modulated by Gpp(NH)p (Ki = 11,505 +/- 2,295). The absence of guanine nucleotide-sensitive agonist high-affinity sites may explain the defective DA-1/AC coupling mechanism in the SHR.

Characterization of [125I]SCH 23982 binding in human brain: comparison with [3H]SCH 23390

We studied binding of [125I]SCH 23982 in two regions of human brain, the caudate and the dorsolateral prefrontal cortex. Binding characteristics of [125I]SCH 23982 and of the non-iodinated tritiated analogue, [3H]SCH 23390, were compared. In caudate, binding of [125I]SCH 23982 was consistent with binding to D1 dopamine receptors while in frontal cortex, [125I]SCH 23982 bound mostly to serotonergic 5HT2 receptors. In contrast to [3H]SCH 23390, no evidence of binding of [125I]SCH 23982 to D1 receptors could be found in human frontal cortex. This indicates that iodination of SCH 23390 induces a decrease in its relative D1 versus 5HT2 selectivity that prohibits the use of [125I]SCH 23982 to label D1 receptors in human cortex.

D1 dopamine receptors can interact with both stimulatory and inhibitory guanine nucleotide binding proteins

Pretreatment of striatal membranes with N-ethylmaleimide in the presence of a D1-specific agonist inactivated endogenous guanine nucleotide binding proteins (G proteins), but not D1 dopamine receptors, resulting in a loss of high-affinity agonist binding sites. Such D1 receptors were solubilized, mixed with exogenous G proteins from cells not containing D1 receptors, and reconstituted into phospholipid vesicles. These reconstituted receptors were able to couple to the exogenous G proteins, and the proportion of agonist high-affinity sites of the receptor (40-57%) was similar to levels obtained with naive receptors coupling to endogenous G proteins (40%) upon solubilization and reconstitution. These hybrid high-affinity sites were fully modulated by guanine nucleotides. Pretreatment of cells with pertussis toxin prior to extraction of G proteins resulted in a 50% decrease in the proportion of high-affinity sites; these sites remained sensitive to guanine nucleotides. When D1 receptors were reconstituted with extracts of cyc- cells, which lack stimulatory G proteins, the proportion of high-affinity sites was reduced to 31% of the total. Pertussis toxin treatment of the cyc- cells completely abolished the formation of high-affinity sites. These results demonstrate that D1-dopaminergic receptors are able to couple to not only stimulatory G proteins (Gs), but also to inhibitory G proteins (Gi).

A novel affinity purification of D-1 dopamine receptors from rat striatum

When rat striatal membranes were pretreated with the sulfhydryl (-SH) modifying reagent, N-ethylmaleimide (NEM) in the presence of the D-1-specific agonist, SKF R-38393, the D-1 dopamine receptor was completely protected from NEM-mediated inactivation. The D-1 receptors, solubilized from these membranes with 1% sodium cholate in the presence of phospholipids, bound with high efficiency (greater than 90%) to mercury-agarose columns. The bound receptors were eluted from the affinity column with a -SH reducing agent, beta-mercaptoethanol. Upon removal of beta-mercaptoethanol from the eluted fractions by inclusion chromatography, the receptor was reconstituted into phospholipid vesicles and assayed for ligand binding activity. The affinity purified receptor exhibited saturable and specific binding of the D-1-specific ligand 125I-SCH 23982, with a Kd of 1.6 nM comparable to that measured in intact membranes and solubilized extracts. The binding capacity of these receptors for 125I-SCH 23982 was 11,000 pmol/mg protein, representing greater than an 8000-fold purification over the starting membrane preparation. The purity of the affinity eluted receptors was estimated to be 78%. The purified receptors retained the pharmacological properties of membrane-bound receptors, including the ability to distinguish between active and inactive enantiomers of specific dopaminergic antagonists. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining revealed the presence of two major polypeptides of 74 and 54 kDa. These two polypeptides were absent in those affinity eluted fractions which did not display 125I-SCH 23982-binding activity and also were not detected in preparations obtained from membranes which were NEM-treated in the absence of D-1-specific agonist. The molecular weights of these polypeptides were similar to those of membrane-bound D-1 receptors, when labeled with a D-1-specific photo-affinity ligand, 125I-8-hydroxy-3-methyl-1-(4-azidphenyl)-2,3,4,5-tetrahydro-1H-3-b enzazepine. These two polypeptides may represent glycosylated and deglycosylated forms of the D-1 dopamine receptor.

Comparison of the central and renal dopamine-1 receptor

The dopamine D-1 receptor from striatal membranes was compared with the dopamine DA-1 receptor from renal proximal tubules. The dopamine-1 receptors were solubilized with 1% sodium cholate and phospholipids after pretreatment with the dopamine-1 agonist, SKF R-38393. The soluble receptors were reconstituted into phospholipid vesicles after removal of sodium cholate. The receptors were studied by radioligand binding using the dopamine-1 antagonist [125I]-SCH 23982. The reconstituted dopamine D-1 and DA-1 receptor densities were similar. However, the affinity of the solubilized D-1 receptor was 17-fold greater than the solubilized DA-1 receptor. The affinity of membrane bound D-1 receptor to the radioligand was also greater than that noted for membrane bound DA-1 receptor. The mechanism for this difference remains to be determined.