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(Hypertension. 1996;28:847-853.)
© 1996 American Heart Association, Inc.

Articles

Differential Expression and Autoradiographic Localization of Atrial Natriuretic Peptide Receptor in Spontaneously Hypertensive and Normotensive Rat Testes: Diminution of Testosterone in Hypertension

Aditi A. Kapasi; Ravindra Kumar; James R. Pauly; Kailash N. Pandey

the Departments of Biochemistry and Molecular Biology and of Pharmacology and Toxicology (J.R.P.), Medical College of Georgia School of Medicine, Augusta.

Correspondence to Dr Kailash N. Pandey, Department of Biochemistry and Molecular Biology, Medical College of Georgia School of Medicine, Augusta, GA 30912-2100.


*    Abstract
up arrowTop
 *Abstract
down arrowIntroduction
 down arrowMethods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
Previous studies have shown that the diuretic hormone atrial natriuretic peptide (ANP) also regulates the steroidogenic responsiveness in isolated Leydig cells from mouse and rat testes. In the present study, we examined the distribution of specific receptors for ANP and C-type natriuretic peptide (CNP) in the testicular compartments of 12-week-old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). We used an in vitro autoradiographic procedure on slide-mounted frozen testicular sections to localize the receptors of the natriuretic peptide hormone family using 125I-ANP and 125I-CNP as radioligands. A high level of specific 125I-ANP binding sites was localized largely in the Leydig cells of the interstitial compartment; other testicular cells were not significantly labeled. On the other hand, no significant difference was observed in 125I-CNP binding sites in the testicular cells of SHR and WKY. Semiquantitative analysis of the binding sites indicated that the density of 125I-ANP receptor binding in Leydig cells of WKY testis was ninefold higher than in those of SHR testis. A moderate level of 125I-ANP binding was also observed in seminiferous tubules, particularly in the spermatids of both SHR and WKY. 125I-ANP binding in WKY spermatids was approximately 2.5-fold higher than in SHR spermatids. Northern blot analysis showed that mRNA specific for guanylyl cyclase type A (Npra) was expressed at approximately twofold higher levels in WKY than in SHR testis. ANP (1x10-8 mol/L) stimulated fourfold to fivefold increased levels of testosterone production in isolated Leydig cells from normotensive WKY compared with those from SHR. These findings support a new physiological role of ANP in Leydig cells, in which a functional relationship seems to exist between testicular ANP receptor expression and testosterone production and the state of hypertension in SHR.


Key Words: atrial natriuretic factor • guanylate cyclase • gene expression • rats, inbred SHR • testis • Leydig cells • testosterone


*    Introduction
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 up arrowAbstract
 *Introduction
down arrowMethods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
The three natriuretic peptide hormones ANP, BNP, and CNP belong to a family of closely related hormones that exert profound effects on multiple physiological responses, regulating fluid volume, blood pressure, steroidogenesis, and cell growth (for review, see References 1 through 5). ANP and BNP are considered the circulating hormones and are predominantly secreted from the cardiac atrium and ventricle, respectively.6 7 8 CNP was originally identified in the brain but has also been found in other tissues, including the vasculature.9 10 The physiological effects of the natriuretic peptide hormones are regulated by binding to specific cell-surface receptors, with membrane-bound forms of guanylyl cyclases representing the biologically active natriuretic peptide receptors. Thus, cGMP has been considered a second messenger of this hormone family. Two different types of the natriuretic peptide receptors with guanylyl cyclase activities have been designated GC-A and GC-B, also referred to as Npra and Nprb, respectively.11 Both GC-A and GC-B cDNAs have been cloned and corresponding amino acid sequences determined from human,12 13 rat,14 15 and mouse.16 The evidence suggests that both ANP and BNP selectively bind to Npra, and CNP has been shown to bind primarily to Nprb.17 18 Both Npra and Nprb consist of an extracellular ligand binding domain, transmembrane region, and the intracellular protein kinase–like and guanylyl cyclase catalytic domains. The extracellular domain of the guanylyl cyclase–linked natriuretic peptide receptor family is homologous to the ANP clearance receptor, which has been designated Nprc; it apparently is not coupled to guanylyl cyclase activation but has been postulated to clear ANP from the circulation.19 20

Among the three natriuretic peptides, ANP has been studied in the greatest detail and seems to participate in multiple physiological responses, such as the inhibition of aldosterone secretion from adrenal gland, renin from kidney, and vasopressin from posterior pituitary as well as stimulation of androgen secretion from normal Leydig cells, progesterone from granulosa cells, and luteinizing hormone from anterior pituitary.1 2 In addition to their hemodynamic roles and steroidogenic effects, the natriuretic peptides have also been shown to contain antimitogenic activity.21 22 23 24 25 26 Our previous studies have shown that both ANP and BNP potently stimulated androgen synthesis and release from isolated normal Leydig cells, whereas CNP exerted only minimal effect.27 The receptor binding studies showed that isolated normal Leydig cells contained both Npra and Nprb species; however, Nprc was absent. To better understand the distribution, expression, and role of testicular natriuretic peptide receptors and their relationships with the state of hypertension, we have studied these receptors in the testes of hypertensive and normotensive rats.


*    Methods
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 up arrowAbstract
 up arrowIntroduction
 *Methods
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Animals
Male SHR and WKY (11 to 12 weeks of age) were obtained from Harlan Sprague Dawley, Inc (Indianapolis, Ind). The average blood pressures of SHR and WKY were recorded as 161 and 94 mm Hg, respectively. Rats were housed under a 12-hour light/dark cycle and provided with food and water ad libitum. The procedures used in these experiments conformed to animal use guidelines established by the National Institutes of Health and were approved by the institutional committee for Animal Use in Research and Education.

Materials
ANP (rat-28) and CNP (porcine-22) were purchased from Peninsula Laboratories, Inc. [125I]NaI (14 to 17 mCi/µg) and Hyperfilm-ECL were from Amersham Corp. Leupeptin, aprotinin, bacitracin, phenylmethylsulfonylfluoride (PMSF), and guanidine isothiocyanate were from Sigma Chemical Co. Formamide was from Life Technologies, and [32P]dCTP was from ICN. The radioimmunoassay kit for the testosterone assay was obtained from Pantex. Percoll was from Pharmacia. All other chemicals were reagent grade.

Iodination
ANP and CNP were radioiodinated by the chloramine T method28 as previously described.29 30 125I-ANP and 125I-CNP were purified by application of the reaction mixture on PD-10 columns (Pharmacia) and elution with 0.05 mol/L sodium phosphate buffer (pH 7.4) containing 0.25% bovine serum albumin (BSA). The specific activities of 125I-ANP and 125I-CNP ranged from 600 to 900 µCi/µg.

Tissue Preparation
Rats were decapitated and the testicles rapidly removed and frozen in isopentane (-35°C) and then stored at -70°C for at least 24 hours. The tissue was sectioned (16 µm thick) on an International Equipment Company minotome cryostat and thaw-mounted on slides coated with gelatin and chrome alum. Sections were collected such that ANP and CNP receptor binding could be evaluated on adjacent sets of sections from the same tissue sample.

Receptor Binding and Autoradiography
Tissue sections were brought to room temperature under vacuum over 1 to 2 hours. Sections were preincubated in a buffer solution containing 0.05 mol/L Tris-HCl (pH 7.4) and 0.005 mol/L MgCl2 for 20 minutes at room temperature. For radioligand binding, the sections were transferred to a buffer solution containing 2x10-10 mol/L 125I-ANP or 125I-CNP, 0.05 mol/L Tris-HCl (pH 7.4), 0.005 mol/L MgCl2, 40 µg/mL bacitracin, 4 µg/mL leupeptin, 4 µg/mL aprotinin, 0.002 mol/L PMSF, and 0.5% BSA and were incubated for 60 minutes at room temperature. All slides were then washed three times in preincubation buffer (2 minutes per wash, 4°C), once in 10-fold diluted preincubation buffer (10 seconds, 4°C), and once in deionized water (10 seconds, 4°C). The sections were gently air-dried and desiccated overnight at room temperature under a vacuum. Nonspecific binding was determined on approximately every eighth section by including an excess of unlabeled peptide (1x10-6 mol/L) in the binding incubation. Slides were exposed to Amersham Hyperfilm (Bmax film) and stored in Wolf x-ray cassettes for 4 days for 125I-ANP binding or 2 weeks for 125I-CNP binding. Films were processed in Kodak D-19 developer and fixed with Kodak fixer. Testicular sections were stained with hematoxylin and eosin so that the autoradiograms could be compared and/or aligned with the corresponding tissue section.

Once the x-ray films were developed, selected slides were prepared for emulsion autoradiography with methods similar to those described by Simmons et al.31 Slides were dehydrated by immersion in 95% ethanol (one time, 2 minutes) and absolute ethanol (three times, 2 minutes each) and then delipidated in two changes of xylene (30 minutes each). Sections were coated with a thin layer of Kodak NTB-2 liquid autoradiography emulsion, dried, and stored in airtight containers for 2 weeks. Slides were developed in Kodak D-19 developer, stopped in a distilled water bath, and then fixed. After rinsing in distilled water, sections were counterstained with toluidine blue and coverslipped.

Image Analysis
Tissue paste standards were included in all film exposures so that a semiquantitative estimation of 125I-ANP and 125I-CNP binding could be performed. Rat brain tissue homogenates were spiked with various concentrations of 125I–{alpha}-bungarotoxin (0.15 to 15 nCi/mg wet tissue wt), and the exact specific activity of each standard was determined by weighing a tissue aliquot and then measuring the amount of radioactivity in the sample in a gamma counter essentially as previously described.32 The tissue standards were frozen in isopentane and stored at -70°C until they were sectioned at the same thickness as the testicular sections (16 µmol/L). Receptor binding in various parts of the testicular tissue was measured with video densitometry and Image software (National Institutes of Health). The components of the image-analysis system included a Macintosh Quadra 700 microcomputer, Sony XC-77 CCD video camera, Quick Capture frame grabber (Data Translation Inc), and Northern Light precision illuminator (Imaging Research). Molar quantities of ligand bound were determined with values interpolated from the optical density versus tissue radioactivity standard curve (fitted to a third-degree polynomial).

RNA Isolation and Northern Analysis
Total RNA was isolated from SHR and WKY testes by the guanidine isothiocyanate method.33 RNA was resolved on a 1% denaturing formaldehyde agarose gel, transferred onto a nylon membrane (Duralan UV-membrane, Stratagene), and fixed by UV cross-linking. Ribosomal RNA on the membranes was stained with methylene blue for detection of potential differences in loading and/or transfer efficiencies. Prehybridization and hybridization were performed at 42°C in a solution containing 50% deionized formamide, 5x SSC, 5x Denhardt's solution, 1% salmon sperm DNA (included only in the prehybridization solution), and 1% sodium dodecyl sulfate (SDS) (included only in the hybridization solution). The membranes were hybridized with a 1.2-kb EcoRI restriction fragment of Npra cDNA23 that was labeled with [32P]dCTP with the random primers oligolabeling kit (Pharmacia). The membranes were washed twice with 2x SSC/0.1% SDS at 22°C for 15 minutes and once with 0.1x SSC/0.1% SDS at 65°C for 15 minutes. RNA bands containing Npra mRNA homology were made visible by exposing Kodak X-Omat AR film to the washed membrane at -80°C with an intensifying screen.

Dissociation of Testes and Purification of Leydig Cells
Testicular cells were prepared by general methods described previously.34 Six SHR and WKY were used in each preparation, and four testes were treated in a group. The testes were quickly excised, decapsulated, and digested for 30 minutes at 37°C in medium 199 containing 0.1% BSA, 0.025 mol/L HEPES buffer, and 0.4 mg/mL type I collagenase in a total volume of 10 mL with constant agitation in a rotary shaker bath at 120 cycles per minute. All centrifugations of cell suspensions were done at 1500 rpm for 10 minutes, unless otherwise indicated. The collagenase-dissociated cells were washed in medium 199 containing 0.1% BSA and 0.025 mol/L HEPES. Two milliliters of cell suspension was layered over a Percoll solution, and a continuous gradient was generated by centrifugation at 15 000 rpm for 60 minutes at 4°C. The fraction containing Leydig cells was removed from the Percoll gradient between a density of 1.064 and 1.070 g/mL. Percoll was removed by washing the cells with medium 199 at 1500 rpm for 10 minutes. Cell viability was determined by the trypan blue exclusion test.

Testosterone Assay
Purified Leydig cells from WKY and SHR testes were treated with ANP and incubated at 37°C for 3 hours in an atmosphere of 5% CO2/95% O2 in a shaking water bath at 80 rpm. The reaction was stopped by centrifugation at 1500 rpm for 10 minutes. Testosterone in the medium was measured by direct radioimmunoassay.27

Statistical Analysis
Receptor binding in testicular tissue sections was determined by a semiquantitative method as nanocuries bound per milligram wet tissue weight on 20 to 30 sections from each rat. These values were averaged to yield one binding value per rat. A total of five to six rats were included for both SHR and WKY for determination of 125I-ANP and 125I-CNP binding sites. All binding data were analyzed by one-way ANOVA.


*    Results
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 *Results
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 down arrowReferences
 
Distribution of 125I-ANP Binding Sites in Testicular Cells of SHR and WKY
Results obtained from semiquantitative autoradiography demonstrated the presence of high-density 125I-ANP binding sites in the serial testicular tissue sections of normotensive WKY compared with SHR (Fig 1Down). The x-ray film autoradiograms showed that a high intensity of specific 125I-ANP labeling was present in the Leydig cells of the testicular interstitial compartments of the tissues surrounding the seminiferous tubules of WKY (Fig 1ADown). No labeling was seen in the control tissue sections for nonspecific binding in the presence of 100-fold excess molar concentrations of unlabeled ANP (Fig 1BDown). In contrast, 125I-ANP binding was drastically reduced in serial tissue sections of SHR testes (Fig 1CDown), and no labeled cells were observed in the control tissue sections used for nonspecific binding (Fig 1DDown). Unrelated hormones such as angiotensin II, endothelin, and arginine vasopressin did not displace 125I-ANP binding in serial tissue sections of either SHR or WKY.



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  		Figure 1. Pseudocolor representation of 125I-ANP binding sites in SHR and WKY testes. A, Total 125I-ANP binding in WKY testis. Most labeling is present in the interstitial compartments between the seminiferous tubules. B, Nonspecific 125I-ANP binding in semiadjacent tissue section of WKY testis displaced by 100-fold excess molar concentrations of unlabeled ANP. C and D, Total and nonspecific 125I-ANP binding in SHR testis, respectively. A significantly reduced level of 125I-ANP binding in SHR testis is apparent compared with WKY testis (magnification x10).

Comparisons of the autoradiograms of testicular tissue sections from SHR with corresponding autoradiograms of WKY revealed the presence of high-density binding sites of 125I-ANP in the Leydig cells. A moderate level of 125I-ANP binding was also present in the seminiferous tubules, especially in the spermatids of both SHR and WKY testes. Semiquantitative analysis of the receptor binding sites in testicular tissue sections with the use of video densitometry and Image software revealed that the magnitude of 125I-ANP binding in the Leydig cells of WKY testis was approximately ninefold higher than in cells of SHR testis. Analyses of the binding data indicated that 125I-ANP binding in seminiferous tubules was approximately fourfold greater in WKY than SHR (TableDown). Emulsion autoradiography revealed a very high density of silver grain covering the interstitial cells in the testicular tissue sections of WKY, whereas such grains were almost completely absent in the interstitial cells of SHR testis (Fig 2Down).


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  		Table 1. Quantitative Distribution of 125I-ANP and 125I-CNP Binding Constants in Testicular Tissue Sections of SHR and WKY



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  		Figure 2. Emulsion autoradiographic localization of 125I-ANP binding sites in SHR and WKY testes. A, High concentrations of silver grains are distributed overlying Leydig cells in the interstitial compartments of WKY testis. B, Grain distribution is almost negligible in the interstitial compartment of SHR testis (magnification x200).

Distribution of 125I-CNP in Serial Tissue Sections of SHR and WKY Testes
The x-ray film autoradiograms demonstrated the presence of specific but low-density binding sites of 125I-CNP in serial tissue sections of both SHR and WKY testes (Fig 3A and 3CDownDown). 125I-CNP binding was absent in the control testicular tissue sections used for nonspecific binding from both SHR and WKY (Fig 3B and 3DDownDown). Quantitative analysis of the receptor binding sites did not show a significant difference in 125I-CNP binding in either the Leydig cells or seminiferous tubules of the SHR and WKY testes. The magnitude of 125I-CNP binding was drastically reduced compared with 125I-ANP binding in the Leydig cells of both SHR and WKY. Similarly, semiquantitative analysis showed that 125I-CNP binding was also substantially lower than 125I-ANP binding in the seminiferous tubules of SHR and WKY testes, respectively (TableUp). The specificity of CNP binding was examined, as unrelated peptides such as angiotensin II, endothelin, and arginine vasopressin were unable to displace 125I-CNP binding in these testicular cells from both SHR and WKY.



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  		Figure 3. Pseudocolor representation of 125I-CNP binding sites in SHR and WKY testes. A and B, Total and nonspecific 125I-CNP binding in WKY testis, respectively. C and D, Total and nonspecific binding in SHR testis, respectively. 125I-CNP binding is not significantly different between SHR and WKY testes (magnification x10).

Expression of Npra mRNA in SHR and WKY Testes
Northern blot analysis revealed that expression of Npra mRNA was higher in WKY than SHR testis (Fig 4aDown). Methylene blue staining of the 28S ribosomal bands showed equal loading and transfer efficiency of mRNA in the samples (Fig 4bDown). The positive signals of hybridized products from Fig 4aDown were measured by densitometric scan and indicated that Npra mRNA levels in WKY rat testis were more than twofold as high as levels in SHR testis (Fig 4cDown).



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  		Figure 4. Northern blot hybridization of Npra mRNA in SHR and WKY testes. Total RNA (20 µg) was used for determination of the expression level of indicated mRNAs by Northern blot analysis performed as described in "Methods." a, Autoradiogram of WKY and SHR testes mRNA hybridized with 1.2-kb fragment of murine Npra cDNA (exposure time, 24 hours). b, Methylene blue staining of 28S ribosomal bands representing equal loading and transfer efficiency of mRNA samples. c, Quantitation of mRNA expression as determined by scanning autoradiograms shown in panel a by soft laser densitometry. Autoradiograms are representative of three independent experiments.

Stimulation of Testosterone in Isolated Leydig Cells of SHR and WKY
ANP treatment of isolated Leydig cells from WKY stimulated approximately 15-fold testosterone production in a dose-dependent manner (Fig 5Down). However, similar treatments of SHR Leydig cells with ANP showed only a fivefold to sixfold stimulation of testosterone. Luteinizing hormone and human chorionic gonadotropin (LH/hCG) both stimulated testosterone in WKY Leydig cells, comparable to ANP, although the LH/hCG-stimulated levels of testosterone were only 35% to 40% higher in WKY than in SHR Leydig cells. Similarly, serum testosterone level was also lower by 25% to 30% in SHR than WKY.



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  		Figure 5. Stimulation of testosterone production in SHR and WKY Leydig cells by ANP. In each tube, 1x106 cells were taken in 0.5 mL assay medium and incubated in the presence of increasing ANP concentrations for 3 hours at 37°C in 5% CO2/95% O2. The reaction was stopped by placing the tubes on ice and centrifuging at 1500 rpm for 10 minutes. Testosterone in the medium was estimated by direct radioimmunoassay. Data indicate the mean of triplicate treatments from SHR and WKY Leydig cells.


*    Discussion
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 up arrowIntroduction
 up arrowMethods
 up arrowResults
 *Discussion
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The present data clearly show that 12-week-old SHR contain significantly reduced levels of specific 125I-ANP binding sites in the testes compared with age-matched WKY. The magnitude of 125I-ANP binding in WKY testis was approximately sixfold higher than in SHR testis. Specific 125I-CNP binding did not differ significantly between the serial tissue sections from SHR and WKY testes. Intriguing was the finding that the majority of the 125I-ANP binding sites were localized in Leydig cells. As determined by semiquantitative analysis, the presence of 125I-ANP binding sites in the Leydig cells as well as in the spermatids of WKY was significantly greater than 125I-CNP binding sites in these cells. In agreement with these findings, Northern blot analysis indicated that Npra mRNA expression was 50% lower in SHR than in WKY testis. The demonstration of high-density 125I-ANP binding sites in the Leydig cells of intact testis from normotensive WKY is comparable with data from previous investigations suggesting high densities of ANP receptors in cultured murine Leydig tumor (MA-10) cells and freshly prepared Leydig cells27 29 30 and autoradiographic localization in rat Leydig cells.35 Cultured murine Leydig tumor (MA-10) cells predominantly overexpress GC-A (Npra) at a density of 1x106 to 2x106 receptor sites per cell and produce more than 1500- to 2000-fold cGMP in response to ANP, more than any other cell or tissue reported to date.36 37

The finding that ANP (1x10-8 mol/L) stimulated fourfold to fivefold higher testosterone levels in isolated WKY Leydig cells compared with SHR cells is also intriguing. The ANP-dependent testosterone production in WKY Leydig cells was comparable to that stimulated by gonadotropins. Both luteinizing hormone and human chorionic gonadotropin also stimulated slightly higher levels of testosterone production in WKY than in SHR Leydig cells. Similarly, serum testosterone level was 30% to 40% lower in SHR than WKY. These results suggest that ANP possibly plays a compensatory physiological role in the production and maintenance of testosterone levels in a receptor-mediated manner. ANP dramatically stimulates testosterone production in freshly prepared mouse and rat Leydig cells, both independently and in combination with gonadotropins.27 38 39 Our recent studies have shown that the magnitude of the testosterone production by ANP and BNP was equivalent to that stimulated by gonadotropins; however, the CNP-stimulated testosterone production remained minimal.27 It is known that ANP and BNP dramatically stimulate GC-A (Npra), whereas CNP stimulates GC-B (Nprb) in different cell systems.4 23 24 Nevertheless, the exact physiological roles and consequences of the testosterone production stimulated by the natriuretic peptide hormones in Leydig cells are not yet completely understood.

The present results clearly demonstrate that 125I-ANP binding in Leydig cells of normotensive WKY is significantly higher than 125I-CNP binding, which seems to be in agreement with previous findings suggesting that CNP is not actively involved in the physiological regulation of Leydig cells27 and ovarian cells.40 On the other hand, lower densities of ANP receptors in SHR testis raise the question of whether Npra is downregulated in SHR. Our previous studies and results from other laboratories have shown the localization of ANP in both mouse and rat testes, suggesting that ANP may play an autocrine or paracrine regulatory role, or both, in testicular function.41 42 The demonstration that immunoreactive ANP–like substances are localized in spermatids and elongating spermatozoa indicates that the synthesis and processing of the prohormone to biologically active peptide hormone occurs in the male germ line.41 43 It was postulated that target cells for such a locally produced ANP in testis could be the Leydig cells because ANP has been shown to stimulate testosterone synthesis and release in a receptor-mediated manner in these cells.27 38 39 The present results demonstrate a direct localization of ANP binding sites in situ in the interstitial cells, supporting previous findings of high densities of ANP receptors in membrane preparations and intact Leydig cells.27 30 44 125I-ANP binding in the spermatids was also detected at significantly higher levels in WKY than SHR testis. With radioreceptor analysis and autoradiography, specific ANP binding sites were previously localized in viable human spermatozoa.43 ANP receptor guanylyl cyclase activation has been demonstrated in sperm cells, and cGMP was suggested to stimulate the lipid metabolism and respiration of spermatozoa.45 46 However, a direct relationship between the receptor-mediated ANP action and the mammalian sperm function remains to be investigated. Nevertheless, these findings show that ANP receptors are present in Leydig cells as well as in developing and mature spermatozoa, suggesting a role of ANP in both gonadal steroidogenesis and gamete physiology.

The present results demonstrating the low expression of Npra mRNA with a diminution in ANP binding sites in SHR testis closely correlate with a low number of ANP receptors detected in neuronal and astrocyte glial cultures of SHR compared with WKY.47 Earlier epidemiological and experimental evidence has suggested that androgen contributes to hypertension,48 and castration has been shown to retard the development of hypertension in SHR.49 50 It has also been reported that testosterone modulates norepinephrine level in the sympathetic fibers innervating the rat vas deferens, and testosterone has been postulated to play a role in the development and/or maintenance of hypertension.51 On the contrary, our present results have provided new and intriguing evidence that ANP binding sites are drastically reduced in SHR, probably hampering the ability of ANP to stimulate testosterone production in isolated SHR Leydig cells compared with WKY cells. These findings support the physiological importance of testicular ANP receptors, which seem to be drastically affected by the state of hypertension in these animals. Our results indicate that ANP receptor expression in SHR testis is probably regulated by both transcriptional and translational events.

In conclusion, the present results show that ANP receptor density was drastically reduced in SHR compared with WKY testis, whereas CNP receptor density was at a low level but almost equivalent in both SHR and WKY testes. These findings are consistent with low expression levels of Npra mRNA in SHR compared with WKY testis. Since ANP weakly stimulated testosterone synthesis in isolated Leydig cells of SHR, we speculate that this might be due to the reduced density of ANP receptor in SHR compared with WKY. Further studies of ANP-dependent stimulation of testosterone in Leydig cells of SHR and WKY in an age-dependent manner can be important for assessment of the more direct physiological roles of testicular ANP receptors in this hypertensive rat model. Since ANP regulates the steroidogenic responsiveness in SHR testis, these data support the concept that a defect at one or more loci in ANP and/or its receptor system may contribute to the development of hypertension in SHR.


*    Selected Abbreviations and Acronyms
 
ANP = atrial natriuretic peptide
BNP = brain natriuretic peptide
CNP = C-type natriuretic peptide
Npra, Nprb, Npre = natriuretic peptide receptors with guanylyl cyclase activities
SHR = spontaneously hypertensive rat(s)
WKY = Wistar-Kyoto rat(s)


*    Acknowledgments
 
This research was supported by grants from the National Institutes of Health (DA08443 to J.R.P. and HD 25527 to K.N.P.) and the American Heart Association (Established Investigatorship Award 900260 and Grant-in-Aid 9201011 to K.N.P.). We thank Willie Cartledge and Kelli Agee for excellent technical assistance. We also thank Sarah A. Taylor for expert secretarial assistance.

Received October 3, 1995; first decision November 30, 1995; first decision May 27, 1996;
*    References
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 up arrowResults
 up arrowDiscussion
 *References
 

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