Stage-synchronized seminiferous epithelium in rats after manipulation of retinol levels.

Optimal conditions for obtaining stage-synchronization of the seminiferous epithelium were investigated. In this study, 147 rats were subjected to protocols in which vitamin A deficiency was induced by feeding a diet without retinol (R-ol) or retinoic acid (RA), followed by maintenance on a diet containing RA and supplementation of R-ol by injection and diet. An acceptable degree of stage synchronization and recovery of the seminiferous epithelium was observed in 90 (61%) of the 147 rats. The effects on synchrony of variations in the protocol, including the degree of deficiency before RA maintenance, the dose and duration of RA maintenance, and the manner of injection of R-ol, were tested. Initiation of maintenance on RA when a medium degree of deficiency was achieved (4-12 g of weight loss, 3-6 days without growth) resulted in a more reliable (80% of the rats) induction of synchrony than did initiation of maintenance on RA at either a less (70% synchronized rats) or more severe (50-60% synchronized rats) deficiency. Maintenance on food containing 10 mg/kg RA gave better and more reliable synchrony (70%) than maintenance on food containing 5 mg/kg RA (less than 40%). Although the duration of this maintenance did not influence the degree of synchrony, the reliability was lower when maintenance was continued for a month or more (54%). During the interval from 33 to 128 days after resupplementation, the degree of synchronization decreased, as did the predictability of the stages, while the restoration of spermatogenesis increased. Linear regression, performed on the location of the median point of synchronization, indicated that spermatogenesis progressed at a rate of 12.4 days per cycle. The median stage of synchronization, predicted by this regression line, differed by an average of 8% of the cycle from the actual location in individual rats. Extrapolation of the regression line indicated that spermatogenesis was reinitiated in mid-to-late stage VII.     

Disruption of sustentacular (Sertoli) cell tight junctions and regression of spermatogenesis in vitamin-A-deficient rats

The relationship between the intactness of sustentacular (Sertoli) cell tight junctions and the status of spermatogenesis was examined in rats fed a vitamin-A-deficient diet after weaning (VAD rats). Both serum and testicular retinol concentrations of the VAD rats declined to a nadir by 80 days of age. At this time, it was observed that Sertoli cell tight junctions of the VAD animals were intact and complete spermatogenesis was maintained. Leakage in Sertoli cell tight junctions, as demonstrated by the presence of lanthanum in the adluminal compartment of the seminiferous epithelium, was first observed in 90-day-old VAD rats. Severe regression of spermatogenic cells was noted in 100-day or older VAD animals. These results suggest that severe germ cell loss observed during chronic vitamin A deficiency may result from abnormal intratubular environment due to the disruption of the blood-testis barrier.     

Specific mRNAs in Sertoli and germinal cells of testes from stage synchronized rats

A treatment which used vitamin A depletion followed by vitamin A repletion was used to synchronize seminiferous tubules to a few related stages of the cycle of the seminiferous epithelium. The success of the synchronization procedure was dependent on the age and size of the rat at the initiation of the experiment (20 days of age and 35-40 g) and the extent to which the vitamin A deficiency had progressed. Administration of retinol was done when the only viable germinal cells in the testis were preleptotene spermatocytes and type A spermatogonia but if the deficiency was prolonged spermatogenesis did not recover. Once established synchrony appeared to be sustained at least through several consecutive cycles. A combination of molecular probes was used to determine if the synchronized testes displayed stage specific variations in Sertoli cell and germinal cell mRNA levels as has been reported for normal asynchronized rats. Sertoli cells in the synchronized testes were shown by quantitative in situ hybridization and by Northern blot analysis to have stage specific variations in the levels of mRNA for transferrin, sulfated glycoprotein-1, and sulfated glycoprotein-2. The mRNA levels in the different stages were qualitatively similar to those in equivalent stages previously reported for testes from asynchronous rats. The germinal cell content of the synchronized testes were examined with Northern blots probed with nick-translated protamine 1 and transition protein 1 cDNAs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vitamin A status regulates hepatic lecithin: retinol acyltransferase activity in rats

The activity of lecithin:retinol acyltransferase (LRAT) was determined in microsomes from the liver and small intestine of rats with differing vitamin A status. In animals depleted of retinol, as judged by undetectable liver vitamin A stores and low plasma retinol concentrations, hepatic LRAT activity was almost undetectable, whether assayed with retinol bound to cellular retinol-binding protein or solvent-dispersed retinol. In contrast, neither the activity of intestinal LRAT nor that of acyl-CoA:retinol acyltransferase in either liver or intestine differed from that of vitamin A-adequate rats. During the course of vitamin A depletion, liver LRAT activity fell progressively, nearly in parallel to the decrease in plasma retinol concentration. Oral repletion of vitamin A-depleted rats with 0.8 mg of retinol resulted in a very rapid restoration of plasma retinol concentration and full recovery of hepatic LRAT activity within 24 h, together with deposition of retinyl ester in the liver. These data strongly implicate LRAT activity in liver as responsible for the storage of hepatic retinyl esters. Retention of the intestine's capacity to esterify retinol during vitamin A deficiency provides a mechanism for capture of dietary vitamin A, while reduced hepatic LRAT activity may function to redirect retinol in liver from storage to other metabolic pathways.     

Modification of Sertoli cell functions in vitamin A-deficient rats

FSH binding and cAMP responses to FSH in Sertoli cell-enriched testes were not affected by the vitamin A (retinol) status of the animals. These results indicate that changes in Sertoli cell functions during vitamin A deficiency are independent of FSH-Sertoli cell interactions. Concentrations of serum androgen binding protein (ABP) in vitamin A-deficient rats were consistently higher than those of control animals throughout the study period. The accumulation of testicular fluid after efferent duct ligation, an indication of Sertoli cell secretory function, was normal in vitamin A-deficient rats at least until 70 days of age, but declined thereafter. ABP concentrations in seminiferous tubular fluid of vitamin A-deficient rats increased transitorily during the 70-80-day age period but returned to normal by 90 days. The increment of ABP in seminiferous tubular fluid after efferent duct ligation, and ABP concentrations in interstitial fluid were consistently lower in vitamin A-deficient rats. The higher serum ABP in vitamin A-deficient rats therefore cannot be explained by an increase in the permeability of Sertoli-cell tight junctions or basement membrane.     

Transfer of retinoic acid from its complex with cellular retinoic acid-binding protein to the nucleus

Cellular retinoic acid-binding protein (CRABP), a potential mediator of retinoic acid action, enables retinoic acid to bind in a specific manner to nuclei and chromatin isolated from testes of control and vitamin A-deficient rats. The binding of retinoic acid was followed after complexing [3H]retinoic acid with CRABP purified from rat testes. The binding was specific, saturable, and temperature dependent. If CRABP charged with nonlabeled retinoic acid was included in the incubation, binding of radioactivity was diminished, whereas inclusion of free retinoic acid, or the complex of retinol with cellular retinol binding protein (CRBP) or serum retinol binding protein had no effect. Approximately 4.0 X 10(4) specific binding sites for retinoic acid were detected per nucleus from deficient animals. The number of binding sites observed was influenced by vitamin A status. Refeeding vitamin A-deficient rats (4 h) with retinoic acid lowered the amount of detectable binding sites in the nucleus. CRABP itself did not remain bound to these sites, indicating a transfer of retinoic acid from its complex with CRABP to the nuclear sites. Further, CRBP, the putative mediator of retinol action, was found to enable retinol to be bound to testicular nuclei, in an interaction similar to the binding of retinol to liver nuclei described previously.     

A method for quantifying synchrony in testes of rats treated with vitamin A deprivation and readministration

Using a variation of a previously published method for manipulating vitamin A levels, we obtained synchronized rat testes and determined the frequency of stages of the seminiferous epithelium in each rat. In this study, we have demonstrated a method for quantitative analysis of the synchrony. The degree of synchronization was expressed as a fraction of the cycle of the seminiferous epithelium, and thus in terms not influenced by the different durations of the stages of this cycle. The median stage about which the tubules were synchronized was calculated. This method may be used to compare the effects of different synchronizing treatments, which may be subtle, and to study various aspects of spermatogenesis in the synchronized testes. For example, the duration of the cycle of the seminiferous epithelium in synchronized testes is estimated to be 12.5 days.     

Effect of hexachlorobiphenyl on vitamin A homeostasis in the rat

Vitamin A status and turnover were examined in rats that had been exposed to chronic dietary treatment of 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), 1 mg/kg diet. HCB caused hepatic depletion and renal accumulation of vitamin A, and a 1.7-fold increase in the serum retinol concentration. Intravenously administered [3H]retinol bound to retinol binding protein-transthyretin complex (RBP-TTR complex) was used to study the dynamics of circulatory retinol in these rats. In HCB-treated rats, the plasma turnover rate of retinol was increased compared to vitamin A-adequate untreated controls. HCB caused a 50% reduction of total radioactivity in liver, and, except for 0.5 h after the [3H]retinol-RBP-TTR dose, the specific activity of the hepatic retinyl ester pool was greater compared to control rats. The kidneys of HCB-treated rats accumulated radioactivity in the retinyl ester fraction. HCB also caused a 50% reduction in adrenal radioactivity compared with control rats. Urinary and fecal excretion of radioactivity was 3-fold higher in HCB-treated rats as compared to controls. Our findings demonstrate that chronic HCB feeding results in expansion of plasma vitamin A mass, in changes of liver and kidney retinol and retinyl ester pool dynamics and in an increased metabolism of vitamin A.     

Utilization of vitamin A in rats with inflammation

Reductions in the concentration of retinol (vitamin A) in serum, lung and kidney were observed in rats subjected to inflammation-inducing treatments (turpentine oil injection of thermal injury). At the same time, the liver showed an almost normal vitamin A content. Feeding of retinol to vitamin A-depleted rats with inflammation revealed that intestinal absorption of retinol was still active in the inflamed state, and the livers of these rats showed good incorporation of retinol. The livers of normal and vitamin A-depleted rats subjected to the inflammatory treatments showed a normal RBP content (retinol-binding protein) and hepatic release of holo-RBP into the serum was not impaired functionally. These results suggest the possibility that the decreases of vitamin A in the lung, serum and kidney may be due primarily to enhanced local consumption of vitamin A related to the inflammation, rather than to a reduced supply of vitamin A from the liver or to decreased intestinal absorption. In bovine serum albumin (BSA)-sensitized rats produced by direct intubation of BSA into the lungs, the level of vitamin A in the lung decreased prior to that in the liver or serum, supporting the hypothesis that the decrease in vitamin A in the inflamed lungs of these rats may be due mainly to the consumption of vitamin A in the lung in response to inflammation.     

Changes in vitamin A status following prolonged immobilization (simulated weightlessness).

A study was conducted to investigate the effects of a simulated weightlessness induced by chronic immobilization on vitamin A status. To simulate the stress condition of weightlessness, rats were suspended for 10 days in a special jacket to which metal chains were attached. Animals received a commercial stock diet. Control rats were pair-fed in reference to the suspended rats. As compared with the control, prolonged immobilization resulted in a decrease in body weight gain and an increase in adrenal weight occurred. In the suspended rats, serum concentrations of retinol and retinol-binding protein (RBP) declined. Hepatic retinyl palmitate content increased, and the hepatic retinol level was decreased. The prolonged immobilization led to significantly reduced retinyl palmitate levels in the testis and lung as well as lowered testicular retinol levels. The results suggest that the stress state induced by prolonged immobilization caused accumulation of hepatic retinyl palmitate, decreasing the serum retinol concentration and retinyl ester content in the extrahepatic tissues.     

The majority of vitamin A is transported as retinyl esters in the blood of most carnivores

1. In canines and mustelides total vitamin A was 10-50 times higher compared to other species due to a high amount of retinyl esters (40-99% of total vitamin A) in blood plasma. The dominant vitamin A ester was in most species retinyl stearate. 2. In Ursidae, Procyonidae, Viveridae and Felidae, total vitamin A was much lower. When present, however, retinyl esters also represented 10-65% of total vitamin A in plasma. 3. Only retinol was detected in plasma of the family, Hyaenidae, and the suborder, Pinnipedia. 4. In maned wolf cubs it was found that retinol, retinyl esters and alpha-tocopherol increased with the age of the animals, reaching values comparable to adult animals at the age of 5 months.     

Effects of vitamin A and insulin on the antioxidative state of diabetic rat heart: a comparison study with combination treatment

Because elevated oxidative stress may exacerbate cardiovascular complications of diabetes mellitus, the current study aimed to investigate the effects of treatment with either vitamin A, an antioxidant, or with insulin on lipid peroxidation products and antioxidant enzyme activities of diabetic rat heart. Also to evaluate whether a combination of vitamin A and insulin exerts more beneficial effects than treatment with each agent alone. Rats were made diabetic with a single injection of streptozotocin (STZ, 55 mg kg(-1) i.p.). Two days after STZ-injection, one group of diabetic rats was treated with vitamin A (retinol acetate, 30 mg kg(-1) day(-1) i.o.) for 12 weeks. A second group of diabetic rats was untreated for 6 weeks and then treated for another 6 weeks with insulin (8-10 IU rat(-1) day(-1) s.c.). Both therapies were applied to another group of diabetic rats for assessment of combined therapy with vitamin A plus insulin. Hearts from 12-week untreated diabetic animals showed about a four-fold increase in the level of thiobarbituric acid reactive substances (TBARS), indicative of increased lipid peroxidation. This was accompanied by approximately 100% increase in both catalase and glutathione peroxidase (GSHPx) enzyme activities. Therapy with insulin alone caused a small but significant improvement in plasma TBARS as well as GSHPx activities, but no significant change in plasma catalase in diabetic animals. Diabetes-induced disturbance in TBARS was almost completely prevented by vitamin A therapy. Although, a similar degree of activities for GSHPx was determined in diabetic animals treated with each agent alone, combination therapy was found to be more effective than single therapies in the recovery of GSHPx of diabetic heart. In contrast to insulin single therapy, vitamin A alone significantly prevented an increase in catalase activity of diabetic heart, and a combination of these agents did not supply any further benefit. Superoxide dismutase (SOD) activity was not found significantly different among the experimental groups. STZ-diabetes also resulted in less plasma retinol and retinol-binding protein (RBP), which was significantly improved by insulin single therapy while vitamin A used alone, failed to increase plasma retinol and RBP levels of diabetic animals. Our findings suggest that single therapy with insulin is unable to preclude oxidative reactions in diabetic heart to the same extent as obtained by vitamin A therapy alone, in spite of allowing recovery of normal growth rate and improved vitamin A metabolism in diabetic rats. A combination of insulin with vitamin A may provide more benefits than use of either agent alone in the treatment of general characteristics of diabetes and the maintenance of antioxidant defence of diabetic heart and thus in the reduction of peroxidative stress-induced cardiac injury.     

Retinol esterification in Sertoli cells by lecithin-retinol acyltransferase

Esterification of retinol occurs during the metabolism of vitamin A in the testis. An acyl-CoA:retinol acyltransferase (ARAT) activity has been described for microsomes isolated from testis homogenates. That activity was also observed here in microsomal preparations obtained from cultured Sertoli cells from 20-day-old (midpubertal) rats. ARAT catalyzed the synthesis of retinyl laurate when free retinol and lauroyl-CoA were provided as substrates. However, in the absence of exogenous acyl-CoA, retinol was esterified by a different activity in a manner similar to the lecithin:retinol acyltransferase (LRAT) activity described recently for liver and intestine. Microsomal preparations obtained from enriched Sertoli cell fractions from the adult rat testis had 75-fold higher levels of LRAT than the preparations from midpubertal animals, but ARAT activity was the same in both these preparations. LRAT utilized an endogenous acyl donor and either unbound retinol or retinol complexed with cellular retinol-binding protein (CRBP) to catalyze the synthesis of retinyl linoleate, retinyl oleate, retinyl palmitate, and retinyl stearate. The addition of exogenous dilaurylphosphatidylcholine (DLPC) resulted in the synthesis of retinyl laurate. The esterification from both exogenous DLPC and endogenous acyl donor was inhibited by 2 mM phenylmethanesulfonyl fluoride (PMSF). ARAT activity was not affected by similar concentrations of PMSF. Furthermore, retinol bound to CRBP, a protein known to be present in Sertoli cells, was not an effective substrate for testicular ARAT. When retinol uptake and metabolism were examined in cultured Sertoli cells from 20-day-old rats, the cells synthesized the same retinyl esters that were produced by microsomal LRAT in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of high doses of retinol acetate on the 3-beta-ol-steroid dehydrogenase and alkaline phosphatase content in mouse ovaries

The author reports on the effects of different doses of retinol acetate on ovarian steroidogenesis. Two groups of CBA/C57BL mice with a mean body weight of 18-20 g received 3.44% oily retinol acetate per os in daily doses of 50 000 and 80 000 IU for 10 days. After completion of the experiments the quick-frozen sections of the ovaries were subjected to a histochemical assay for the content of 3-beta-ol-steroid dehydrogenase and alkaline phosphatase. Administration of 50 000 IU vitamin A was found to stimulate ovarian steroidogenesis. The effect of vitamin A was the most demonstrable in the interstitial tissue, atretic corpora, and, in the internal theca of the follicles. Administration of 80 000 IU retinol acetate inhibited ovarian steroidogenesis. The estrous cycle in animals ceased. Administration of vitamin A (80 000) primarily affected the follicular apparatus of the ovaries, namely the epithelium of the follicles and yellow bodies. At the same time secretory function of atretic corpora and interstitial tissue remained within normal, which was regarded as a compensatory-adaptive mechanism under toxic hypervitaminosis A.     

Stability of spermatogenic synchronization achieved by depletion and restoration of vitamin A in rats

Studies of synchronization of spermatogenesis following vitamin A deficiency have suggested that this may provide an in vivo model for the study of stage-dependent changes in hormonal action and protein secretion within the seminiferous epithelium. However, until now, no information on the stability or durability of this condition has been available. In this study, 200 seminiferous tubules from each of 40 rats (including controls) were classified according to their spermatogenic stage after withdrawal and replenishment of vitamin A. Following 15 wk withdrawal and subsequent replenishment of vitamin A, spermatogenesis was initiated in a synchronous fashion. This synchrony remained stable for more than 10 cycles of the seminiferous epithelium (2.5 spermatogenic cycles). In association with the extended period of vitamin A deficiency, a proportion of tubules (30%) showed morphological characteristics of either Sertoli cells only or Sertoli cells plus spermatogonia with occasional pachytene spermatocytes. During the 11-wk period of observation in this study, no significant change in proportions of damaged tubules were observed. Testicular testosterone concentrations, although elevated with respect to controls, showed no correlation with the stage of the cycle of the seminiferous epithelium observed, whereas pituitary and serum follicle-stimulating hormone levels were elevated, probably due to the number of damaged tubules observed. The persistence of synchrony in spermatogenesis following vitamin A treatment suggests that this model is applicable for studies of paracrine actions within the testis. However, the decreased ratio of synchrony observed with time may provide evidence that duration of the individual stages of the cycle of the seminiferous epithelium might be subject to temporal variation, leading to a progressive desynchronization of spermatogenesis in this model system.     

SPERMATOGONIAL CHALONE(S): EFFECT ON THE PHASES OF THE CELL CYCLE OF TYPE A SPERMATOGONIA IN THE RAT

Adult rats with X-irradiated testes were used to analyze the effect of the spermatogonial chalone(s) on the phases of the cell cycle of type A spermatogonia. Twelve days after irradiation, the animals were used in two experiments designed to test the existence of hypothetical G₂ and S phase chalones. For the G₂ assay, rats injected twice with testicular extract (Group I), liver extract (Group II) or physiological saline (Group III) were killed 10 hr after the initial injection. Mitoses of type A, Intermediate and type B spermatogonia were counted in whole mounts of dissected seminiferous tubules. To test for an S phase inhibitor, two groups of rats were given multiple injections of either testicular extract (Group IV) or saline solution (Group V). Twenty-two hr after the first injection they were injected with [³H]thymidine and killed 2 hr later. Silver grains over labelled type A nuclei were counted in radioautographed sections of testes from these animals. The average grain counts were identical in Groups IV and V, indicating that the testicular extract did not affect type A spermatogonia during the S phase. Counts of type A mitoses in Groups I, II and III revealed that in the animals injected with the testicular extract (Group I) the number of divisions was 50% lower than in the control groups (Groups II and III). In contrast, mitotic activity of differentiating spermatogonia (In + B) was similar in all three groups of animals. This result is attributed to a testicular chalone which specifically inhibits type A spermatogonia during the G₂ phase of the cell cycle. Indirect evidence for a G₁ spermatogonial chalone is also presented, as a result of an analysis of published data (Clermont & Mauger, 1974).     

Vitamin A metabolism in rats chronically treated with 3,3',4,4',5,5'-hexabromobiphenyl

Chronic dietary administration of 3,3',4,4',5,5'-hexabromobiphenyl (HBB), 1 mg/kg diet, caused a decrease in retinol (20-fold) and retinyl esters (23-fold) in the livers of female rats, but resulted in a 6.4-fold increase in retinol and 7.4-fold increase in retinyl esters in the kidneys. Liver acyl-CoA:retinol acyltransferase and retinyl palmitate hydrolase activities were reduced while serum concentration of retinol was unaffected by HBB feeding. Metabolism of a physiological dose of [11-3H]retinyl acetate (10 micrograms), was examined in rats fed either vitamin A-adequate diet, or marginal amounts of vitamin A, or vitamin A-adequate diet containing HBB. A 13-fold greater amount of the administered vitamin A was found in kidneys of HBB-treated rats. In rats fed adequate or low amounts of vitamin A, kidney radioactivity was primarily in the retinol fraction, while in HBB-fed rats the radioactivity was associated mostly with retinyl esters. Fecal and urinary excretion of radioactivity was greatly increased in HBB-treated rats. Chronic HBB feeding results in a loss of ability of liver to store vitamin A, and severely alters the uptake and metabolism of vitamin A in the kidneys. We conclude that HBB causes major disturbances in the regulation of vitamin A metabolism.