The differential regulation of aldosterone output in hamster adrenal by angiotensinII and adrenocorticotropin

Influence of dietary sodium restriction on angiotensin II receptors in rat adrenals

We studied the distribution of angiotensin II (AII) receptors type 1 (AT1) and type 2 (AT2) and the effects of a low sodium intake on these two subtypes of receptors in male rat adrenals. Binding studies on adrenal slices, on cell membranes and on cell suspensions were performed using [125I]AII and specific analogs for AT1 (Losartan) and AT2 (PD 123319) receptors. The distribution of AT1 was also studied by immunofluorescence. Complementary approaches were necessary to reach our goal. Indeed, by autoradiography on adrenal slices, [125I]AII was shown to bind to the zona glomerulosa (ZG) and to the medulla (M). When coincubated with [125I]AII, PD 123319 displaced [125I]AII from the medulla and from the ZG, indicating the presence of AT2 receptors in both zones. Losartan partially displaced [125I]AII from the ZG, indicating the presence of AT1 receptors in that zone. Furthermore, the labeling intensity of the medulla (AT2 receptors) was much stronger in adrenal sections from rats kept on a low sodium regimen than from controls. Immunofluorescence microscopy revealed that AT1 receptors were located mainly in the ZG of control rats. After sodium restriction, AT1 receptors appeared to be uniformly distributed within an enlarged ZG; furthermore AT1 receptor-positive cells were found to a limited degree in the zona fasciculata and possibly in the zona reticularis, and a greater number of these positive cells appeared in these zones under sodium restriction. Cell suspensions from rats fed a low sodium diet showed a 2.7- and 2.1-fold increase in total AII receptors in adrenal ZG and ZFR + M cells when compared with controls. Based on Losartan displacement, we calculated that [125I]AII bound to AT1 and to AT2 receptors was increased in both ZG and ZFR + M cell preparations under sodium restriction. Results of binding studies on cell membranes were also indicative of an increasing effect of sodium restriction on AT1 and AT2 receptors binding capacity. Furthermore, Northern blotting analysis revealed 3.0- and 2.5-fold increases in the level of AT1 receptor mRNA in the ZG and the ZFR + M of rats fed a low sodium diet as compared with those fed a normal diet. The low sodium intake resulted in a weaker increase (1.5-fold) in the level of AT2 receptor messenger RNA in the ZG, with no changes in the ZFR + M preparations. In conclusion, in this study complementary approaches were needed to determine the localization of AT1 and AT2 receptors in the rat adrenal, and to show the increasing effects of a low sodium regimen on the adrenal level of these receptors. Immunofluorescence studies revealed AT1 receptors mainly in the ZG and also in some cells of the inner adrenal cortex zones; in adrenals of rats kept on a low sodium diet the ZG was markedly enlarged, and an increased number of immunoreactive cells with AT1 receptors were observed throughout that zone; also more immunoreactive cells were present in the inner zones of the adrenal cortex. Furthermore in the adrenals of rats kept on a low sodium diet, we observed: 1) an increased number of AT1 and AT2 receptors in cell suspensions from the ZG, and in cell suspensions of the ZFR + M; 2) an increased level of AT1 and AT2 receptor mRNAs in the ZG; 3) an increased level of AT1 receptor mRNA, with no changes in the AT2 mRNA level in the ZFR + M. These results suggest a role for AT1 as well as for AT2 receptors in controlling adrenal function and differentiation under normal as well as under physiological stimulation of AII production following sodium restriction.

Characterization of the adrenal cytochrome P450C17 in the hamster, a small animal model for the study of adrenal dehydroepiandrosterone biosynthesis

The hamster, like the human produces cortisol as its major glucocorticoid, rather than corticosterone, typical of most enzyme rodents. It is not known, however, if the hamster cytochrome P450C17 (P450C17), a key enzyme for cortisol formation, also exhibits 17,20-lyase activity and if it catalyzes the formation of dehydroepiandrosterone (DHEA) at the adrenal level. To study this, we isolated the cDNA of P450C17 from a hamster adrenal library. This cDNA was sequenced and was found to have an open reading frame for a protein of 511 amino acids, as compared to the human P450C17, which contains 508 amino acids. The hamster P450C17 cDNA, in the coding region, is 76% homologous with the human P450C17 cDNA. The cDNA was then cloned in the expression vector pSV-SPORT 1, which was transiently transfected into COS 1 cells. The transfected cells were used for temporal studies on the transformation of radiolabeled C21-delta5- and C21-delta4-precursors. When transfected cells were incubated with [14C]pregnenolone, rapid formation of [14C]DHEA occurred. The intermediate 17alpha-hydroxypregnenolone accumulated initially with subsequent metabolism to DHEA. Likewise, when incubated with C21-delta4-steroids, [14C]progesterone and [3H]17alpha-hydroxyprogesterone, the 17,20-lyase product androstenedione was produced efficiently. In these studies, with respect to the delta5 pathway, the expressed hamster P450C17 gave similar results to bovine P450C17 cDNA inserted in the same expression vector. However, in contrast to the bovine enzyme, which converted low amounts of progesterone to androstenedione, the expressed hamster P450C17 enzyme showed an active metabolism via the delta4 pathway. Northern blot analysis, using the complete alpha-32P labeled hamster P450C17 cDNA as the probe, demonstrated a strong presence of P450C17 mRNA in hamster adrenals, a weaker presence in testes and ovaries, and no detectable species in brain, mesentery, and kidney. Immunoblotting analysis using an anti-rat P450C17 antibody demonstrated the presence of P450C17 protein in hamster adrenals, testes, and ovaries. Hamster adrenal cell suspensions and microsomal preparations were used to demonstrate the biosynthesis of [14C]17alpha-hydroxypregnenolone and [14C]DHEA from [14C]pregnenolone; both metabolites were formed during incubations. However, the ratio of [14C]DHEA/[14C]17alpha-hydroxypregnenolone was much lower in adrenal cells than in transfected COS 1 cells, indicating the presence of putative factors in hamster adrenal cells, favoring the 17alpha-hydroxylase activity rather than that of the 17,20-lyase. In conclusion, these studies demonstrate that the hamster adrenal is both a DHEA and a cortisol producer, and, therefore, this animal could be a suitable small animal model for the study of the role of DHEA in relation to human biochemistry and physiology.

The hamster adrenal cytochrome P450C11 has equipotent 11beta-hydroxylase and 19-hydroxylase activities, but no aldosterone synthase activity

We have isolated a hamster adrenal P45OC11 cDNA which shared 90 and 84% homology, respectively, with the nucleotide sequence and the amino acid sequence of the hamster adrenal P450aldo. Both P450C11 and P450aldo cDNA coding sequences were inserted in the plasmid pBluescript SK, transcribed and then translated using a rabbit reticulocyte system in the presence of [35S]methionine. The reaction products were immunoprecipitated with an anti-bovine P450C11 antibody for P450C11 and with an anti-hamster P450aldo for P450aldo. Immunoprecipitated proteins were analyzed by polyacrylamide gel electrophoresis. A single 35S-labeled protein band was detected for P450C11 and for P450aldo, respectively. P450C11 and P450aldo cDNAs were then both inserted into the expression vector pCMV5 containing a viral sequence specific for the attachment of ribosomes to mRNA. These constructions were transfected in COS-1 cells. 24 h after transfection, the presence of P450C11 and P450aldo mRNAs was determined by Northern blot analysis. In a time study experiment we found that P450C11 transformed the labeled-steroid into [14C]corticosterone, [14C]19-OH-deoxycorticosterone and [14C]18-OH-deoxycorticosterone in ratios of 1:1.11:0.07, after 2 h of incubation; no [14C]aldosterone could be detected. Cells transfected with plasmids harboring the P450aldo cDNA transformed [14C]deoxycorticosterone to [14C]corticosterone, [14C]aldosterone, [14C]18-OH-corticosterone, [14C]18-OH-deoxycorticosterone, [14C]19-OH-deoxycorticosterone and [14C]11-dehydrocorticosterone in ratios of 1:0.25:0.45:0.04:0.04:0.04 after 12 h of incubation. These results indicate that one P450 catalyzes the ultimate step of glucocorticoid formation and a separate P450 is involved in the final steps of aldosterone formation in hamster adrenals. The capacity of the hamster adrenal P450C11 to hydroxylate at positions 11beta and 19 in nearly equal ratio makes this animal an excellent model to study the mechanism of synthesis and inhibition of 19-OH-deoxycorticosterone, the precursor of 19-nor-deoxycorticosterone, a very potent mineralocorticoid involved in the development of essential hypertension.

The presence of two cytochrome P450 aldosterone synthase mRNAs in the hamster adrenal

We isolated a cDNA from a hamster adrenal cDNA library which was similar in sequence to those of the mouse and rat P450c18 cDNAs. The hamster P450c18 cDNA, however, was shorter than the rat and mouse P450c18 cDNAs at its 5'-end and the peptide leader sequence was absent. From a hamster genomic library we isolated and sequenced the first seven exons and a 5'-flanking region of the first P450c18 gene exon. With this information we were able to generate a P450c18 cDNA containing the peptide leader sequence using the polymerase chain reaction. Northern analyses were performed on adrenals from hamsters maintained on a low sodium diet for 0, 4, 7 and 10 days using a 32P-labeled sequence specific to P450c18; two mRNA bands were found at 2 and 3.4 kb. The intensity of both bands was increased about 3- to 5-fold under sodium restriction compared to controls. A distinct mRNA band of 2.3 kb hybridized with an oligonucleotide specific to P450(11) beta and its intensity did not change following low sodium intake. Immunoblotting analyses were performed using an antibovine adrenal P450(11) beta antibody that does not discriminate between P450(11) beta and P450c18 proteins. Three bands were detected at 52, 48 and 45 kDa in homogenate preparations of entire glands. Furthermore, the 45 kDa protein band was present in homogenates of the zona glomerulosa and absent in homogenates of the zone fasciculata-reticularis. In conclusion, these results show that the hamster adrenals express P450c18 as do mouse, rat and human adrenal glands. Furthermore, two P450c18 mRNAs, which are inducible by a low sodium intake, are present in the hamster adrenal vs one for the rat. The physiological role of these two hamster adrenal mRNA species remains to be elucidated.