madman
Super Moderator
Introduction
The function of the Leydig cell is the synthesis of testosterone. During the process of biosynthesis, steroid precursors as well as metabolites of testosterone enter the spermatic vein and are secreted. At present, there is no convincing evidence that the secretion rate of any of the biologically active steroids is high enough to exert a significant physiologic effect. Analysis of the secretory patterns does, however, give clues about the biosynthetic pathways in the testis and can provide additional indexes and information about Leydig cell performance.
Measurement
It is necessary to discuss techniques for identifying steroid secretion and measuring secretion rates. In the simplest case, the demonstration of a gradient between plasma steroid concentration of an effluent vein and that of a peripheral vein constitute proof of secretion. However, since measurements of blood flow to the glands are difficult to obtain in man,estimates of secretory rates cannot be made with confidence from catheterization data. Further, the procedure of introducing a catheter may alter blood flow.
When it can be shown that one gland secretes essentially all of the steroid entering the circulation, then measurement of the production rate* of that steroid will approximate secretion rate. To do this, it is necessary to measure plasma steroid concentration and the metabolic clearance rate (MCR) of the steroid (1). Given the imprécisions of measurement of both variables, the fluctuations of steroid concentrations during the day and alterations of MCR with posture (2-4), production rate estimates by this method may have an associated 25% error.
When the steroid in question is secreted by both adrenal cortex and gonad, then selective suppression of each gland and measurement of residual production rate will approximate the contribution of that gland to the production rate. This technique rests on the assumption that the agent used for suppression, such as dexamethasone to suppress adrenalf unction does not alter the function of the other gland. This assumption was shown to be incorrect in at least one case (5) when it was noted that gonadal androgen secretion in hirsute women decreased with dexamethasone administration.
Finally, the steroid may be secreted by both glands and synthesized in peripheral tissues from any of several peripheral tissues from any of several precursors also secreted by these glands. An example of this is testosterone production in women; the testosterone is secreted by both glands and synthesized in several other tissues from circulating steroids such as androstenedione and dehydroepiandrosterone. It is possible to measure the contribution of each precursor to the steroid production rate(6) although these methods are tedious and subject to errors greater than those associated with measurement of production rates alone.
* The term production rate is used to signify the irreversible rate of entry of a steroid into the circulation from ail sources; the secretion rate is the rate of entry from adrenal cortex or gonad.
Testosterone Secretion
In adult men, testosterone is secreted by the testis in amounts approaching the production rate of about 7 mg/24 hr. Possible precursors such as androstenedione (7) and dehydroepiandrosterone (8) are not present at high enough concentrations to contribute significantly to testosterone production rates by peripheral conversion. It has been accepted that the Leydig cell is the source of testosterone synthesis. However, recent studies have shown that the seminiferous tubule can synthesize testosterone from C-19-steroids but not from cholesterol (9-12). The quantitative significance of this is unknown but it seems doubtful that the tubule is an important source of plasma testosterone. For example, destruction of the tubule by radiation does not alter plasma testosterone levels in the rat. It has been shown recently that germ cells can synthesize testosterone from pregnenolone (13) but plasma testosterone levels are normal in germinal cell aplasia (14) so that the germ cell is not a significant source of testosterone in man.
Spermatic vein testosterone concentrations range between 20 and 50 µg% . Thus it is necessary to postulate a testicular venous plasma flow of about 10 ml/min to give a secretion rate of 6 mg/day. This is somewhat higher than the blood flows recorded in other species.
Dihydrotestosterone
The high probability that dihydrotestosterone (17/5-hydroxy-5a-androstan-3-one) is the effective intracellular androgen in tissues such as prostate, seminal vesicle, and epididymis led to studies of its origin and production rate. Plasma concentration of dihydrotestosterone is higher in men than in women (54 vs 15 ng/100 ml) (15). The production rate in men is only 400 μg per 24 hr, however (16). By examining the rate at which testosterone is converted to dihydrotestosterone in peripheral tissues (17) it was concluded that all o rmost of plasma dihydrotestosterone was not secreted. A more recent study suggests that the secretion role may be as much as one-third of the production rate (16). In either case it is clear that the secretion of dihydrotestosterone is unimportant quantitatively when compared with that of testosterone for androgenic effect.
Estrogens
Twenty years ago, hCG was shown to increase urinary estrogens (18) and these investigators concluded that estrogens were both secreted by the testis and produced by conversion from testosterone. These early conclusions have been amply confirmed. Using constant infusion techniques, Longcopeet al. (19) measured the peripheral conversion of testosterone and androstenedione to estradiol and estrone, respectively ,and concluded that less than half of the estradiol production rate was due to secretion. Using methods involving the specific activity of urinary estrogens, others (2,20) concluded that little estradiol was secreted. This controversy was settled by catheterizing the spermatic vein and demonstrating secretion of estradiol (E ) (21-24). A comparison of the two techniques is shown in Table I. Because of the problems of quantitation discussed above, these numbers cannot be taken as signifying either agreement or disagreement although the production rate of E exceeds the E derived from testosterone by the calculated secretion rate of E . What is certain, however, is that the testis secretes estradiol and that the spermatic venous estradiol levels are sufficient to account for an appreciable fraction of the estradiol production rate.
When hCG has been given acutely to men, estradiol and testosterone rise proportionately (25). Thus, unless interconversion rates change with hCG administration, and this is unlikely, the ratio of secreted estradiol testosterone in spermatic venous blood must remain constant. With chronic administration of hCG in the rat (26) and in man (27), plasma estradiol levels do not increase proportionally to testosterone suggesting a limited aromatizing capacity in the testis. This is in accord with the difficulty of measuring aromatization in the testis in vitro.
Small amounts of estrone are secreted by the testis (22-24), but in normal men the conversion of plasma androstenedione to estrone is the main source of estrone entering the blood.
It is interesting to note that the equilibrium of the 17-oxidoreductase differs in testis and adrenal. Thus int he testis, the secretory products are testosterone and estradiol; the adrenal cortex secretes the 17-oxoderivatives,androstenedione and estrone.
17 - hydroxyprogesterone
The biosynthetic sequence from pregnenolone to testosterone is shown in Figure 1. Each steroid in this sequence has been measured in blood and estimates have been made of their production rates and sites of origin. These are summarized in Table II. The secretion rate of 17-hydroxyprogesterone by the testis is one-third that of testosterone (28) and it is thus an important testicular secretion. In fact the excretion of its principal metabolite 5a-pregnane-3a,17a, 20a-triol is an adequate index of Leydig cell function. The relatively high secretion rate of 17-hydroxyprogesterone indicates that the biosynthetic step involving the C17 -C20 lyase is rate-limiting so that 17-hydroxyprogesterone escapes conversion to androstenedione and is secreted. Since both the 17a-hydroxylase and the lyase are found in the smooth surfaced microsomal fraction in the testis (29) geographical separation of the enzymes is not the responsible factor. In the adrenal cortex 17-hydroxyprogesterone also serves as a precursor of cortisol and this alternate pathway means that the lyase is not rate-limiting.
Δ5 -3β-hydroxysteroids
The plasma concentrations of pregnenolone (30, 31) and 17-hydroxypregnenolone (32) are the same in men and women, suggesting that the adrenal cortex is the source of these steroids. During administration of dexamethasone, 17-hydroxyprognenolone concentrations become undectectable confirming its origin in the adrenal cortex (32).
Dehydroepiandrosterone has been identified in spermatic venous blood (33,34) but the gradient between spermatic and peripheral vein is low (34). Thus the testis contributes little to the dehydroepiandrosterone production rate. In the above two studies, secretion of dehydroepiandrosterone sulfate could not be demonstrated. The finding that plasma dehydroepiandrosterone concentrations are the same in men and women confirms the conclusion that the testis is an unimportant source of dehydroepiandrosterone.
The concentration of androsteinediol, the proximate precursor of testosterone in the Δ5 -pathway is higher in men than in women (161 vs 100 ng/100 ml) (35,36). The ratio androstenediol:testosterone in spermatic vein blood was 0.2 (33), approximately the same as in peripheral blood. These data suggest that little androstenediol in blood is derived from dehydroepiandrosterone. This conclusion needs to be tested since the conversion factor for a similar transformation at C17 , androstenedione to testosterone is 0.12 (6). If this is assumed for the conversion of dehydroepiandrosterone to androstenediol, then most of blood androstenediol would be the result of peripheral transformation of dehydroepiandrosterone.
It is doubtful that further studies of steroid secretion by the testis will yield significant new information. The important steroid problems are those concerned with intratesticular steroid synthesis and metabolism. These are: What is the concentration of testosterone in testicular interstitial fluid necessary to maintain spermatogenesis? Does intratesticular estradiol have physiologic significance? How does the seminiferous tubule contribute to androgen secretion and is its synthesis of testosterone important for tubular function? What is the target cell in the tubule for testosterone action and how is this action manifested? Further study of these questions will be necessary to understand important physiologic aspects of the role of the Leydig cell in steroid secretion.
The function of the Leydig cell is the synthesis of testosterone. During the process of biosynthesis, steroid precursors as well as metabolites of testosterone enter the spermatic vein and are secreted. At present, there is no convincing evidence that the secretion rate of any of the biologically active steroids is high enough to exert a significant physiologic effect. Analysis of the secretory patterns does, however, give clues about the biosynthetic pathways in the testis and can provide additional indexes and information about Leydig cell performance.
Measurement
It is necessary to discuss techniques for identifying steroid secretion and measuring secretion rates. In the simplest case, the demonstration of a gradient between plasma steroid concentration of an effluent vein and that of a peripheral vein constitute proof of secretion. However, since measurements of blood flow to the glands are difficult to obtain in man,estimates of secretory rates cannot be made with confidence from catheterization data. Further, the procedure of introducing a catheter may alter blood flow.
When it can be shown that one gland secretes essentially all of the steroid entering the circulation, then measurement of the production rate* of that steroid will approximate secretion rate. To do this, it is necessary to measure plasma steroid concentration and the metabolic clearance rate (MCR) of the steroid (1). Given the imprécisions of measurement of both variables, the fluctuations of steroid concentrations during the day and alterations of MCR with posture (2-4), production rate estimates by this method may have an associated 25% error.
When the steroid in question is secreted by both adrenal cortex and gonad, then selective suppression of each gland and measurement of residual production rate will approximate the contribution of that gland to the production rate. This technique rests on the assumption that the agent used for suppression, such as dexamethasone to suppress adrenalf unction does not alter the function of the other gland. This assumption was shown to be incorrect in at least one case (5) when it was noted that gonadal androgen secretion in hirsute women decreased with dexamethasone administration.
Finally, the steroid may be secreted by both glands and synthesized in peripheral tissues from any of several peripheral tissues from any of several precursors also secreted by these glands. An example of this is testosterone production in women; the testosterone is secreted by both glands and synthesized in several other tissues from circulating steroids such as androstenedione and dehydroepiandrosterone. It is possible to measure the contribution of each precursor to the steroid production rate(6) although these methods are tedious and subject to errors greater than those associated with measurement of production rates alone.
* The term production rate is used to signify the irreversible rate of entry of a steroid into the circulation from ail sources; the secretion rate is the rate of entry from adrenal cortex or gonad.
Testosterone Secretion
In adult men, testosterone is secreted by the testis in amounts approaching the production rate of about 7 mg/24 hr. Possible precursors such as androstenedione (7) and dehydroepiandrosterone (8) are not present at high enough concentrations to contribute significantly to testosterone production rates by peripheral conversion. It has been accepted that the Leydig cell is the source of testosterone synthesis. However, recent studies have shown that the seminiferous tubule can synthesize testosterone from C-19-steroids but not from cholesterol (9-12). The quantitative significance of this is unknown but it seems doubtful that the tubule is an important source of plasma testosterone. For example, destruction of the tubule by radiation does not alter plasma testosterone levels in the rat. It has been shown recently that germ cells can synthesize testosterone from pregnenolone (13) but plasma testosterone levels are normal in germinal cell aplasia (14) so that the germ cell is not a significant source of testosterone in man.
Spermatic vein testosterone concentrations range between 20 and 50 µg% . Thus it is necessary to postulate a testicular venous plasma flow of about 10 ml/min to give a secretion rate of 6 mg/day. This is somewhat higher than the blood flows recorded in other species.
Dihydrotestosterone
The high probability that dihydrotestosterone (17/5-hydroxy-5a-androstan-3-one) is the effective intracellular androgen in tissues such as prostate, seminal vesicle, and epididymis led to studies of its origin and production rate. Plasma concentration of dihydrotestosterone is higher in men than in women (54 vs 15 ng/100 ml) (15). The production rate in men is only 400 μg per 24 hr, however (16). By examining the rate at which testosterone is converted to dihydrotestosterone in peripheral tissues (17) it was concluded that all o rmost of plasma dihydrotestosterone was not secreted. A more recent study suggests that the secretion role may be as much as one-third of the production rate (16). In either case it is clear that the secretion of dihydrotestosterone is unimportant quantitatively when compared with that of testosterone for androgenic effect.
Estrogens
Twenty years ago, hCG was shown to increase urinary estrogens (18) and these investigators concluded that estrogens were both secreted by the testis and produced by conversion from testosterone. These early conclusions have been amply confirmed. Using constant infusion techniques, Longcopeet al. (19) measured the peripheral conversion of testosterone and androstenedione to estradiol and estrone, respectively ,and concluded that less than half of the estradiol production rate was due to secretion. Using methods involving the specific activity of urinary estrogens, others (2,20) concluded that little estradiol was secreted. This controversy was settled by catheterizing the spermatic vein and demonstrating secretion of estradiol (E ) (21-24). A comparison of the two techniques is shown in Table I. Because of the problems of quantitation discussed above, these numbers cannot be taken as signifying either agreement or disagreement although the production rate of E exceeds the E derived from testosterone by the calculated secretion rate of E . What is certain, however, is that the testis secretes estradiol and that the spermatic venous estradiol levels are sufficient to account for an appreciable fraction of the estradiol production rate.
When hCG has been given acutely to men, estradiol and testosterone rise proportionately (25). Thus, unless interconversion rates change with hCG administration, and this is unlikely, the ratio of secreted estradiol testosterone in spermatic venous blood must remain constant. With chronic administration of hCG in the rat (26) and in man (27), plasma estradiol levels do not increase proportionally to testosterone suggesting a limited aromatizing capacity in the testis. This is in accord with the difficulty of measuring aromatization in the testis in vitro.
Small amounts of estrone are secreted by the testis (22-24), but in normal men the conversion of plasma androstenedione to estrone is the main source of estrone entering the blood.
It is interesting to note that the equilibrium of the 17-oxidoreductase differs in testis and adrenal. Thus int he testis, the secretory products are testosterone and estradiol; the adrenal cortex secretes the 17-oxoderivatives,androstenedione and estrone.
17 - hydroxyprogesterone
The biosynthetic sequence from pregnenolone to testosterone is shown in Figure 1. Each steroid in this sequence has been measured in blood and estimates have been made of their production rates and sites of origin. These are summarized in Table II. The secretion rate of 17-hydroxyprogesterone by the testis is one-third that of testosterone (28) and it is thus an important testicular secretion. In fact the excretion of its principal metabolite 5a-pregnane-3a,17a, 20a-triol is an adequate index of Leydig cell function. The relatively high secretion rate of 17-hydroxyprogesterone indicates that the biosynthetic step involving the C17 -C20 lyase is rate-limiting so that 17-hydroxyprogesterone escapes conversion to androstenedione and is secreted. Since both the 17a-hydroxylase and the lyase are found in the smooth surfaced microsomal fraction in the testis (29) geographical separation of the enzymes is not the responsible factor. In the adrenal cortex 17-hydroxyprogesterone also serves as a precursor of cortisol and this alternate pathway means that the lyase is not rate-limiting.
Δ5 -3β-hydroxysteroids
The plasma concentrations of pregnenolone (30, 31) and 17-hydroxypregnenolone (32) are the same in men and women, suggesting that the adrenal cortex is the source of these steroids. During administration of dexamethasone, 17-hydroxyprognenolone concentrations become undectectable confirming its origin in the adrenal cortex (32).
Dehydroepiandrosterone has been identified in spermatic venous blood (33,34) but the gradient between spermatic and peripheral vein is low (34). Thus the testis contributes little to the dehydroepiandrosterone production rate. In the above two studies, secretion of dehydroepiandrosterone sulfate could not be demonstrated. The finding that plasma dehydroepiandrosterone concentrations are the same in men and women confirms the conclusion that the testis is an unimportant source of dehydroepiandrosterone.
The concentration of androsteinediol, the proximate precursor of testosterone in the Δ5 -pathway is higher in men than in women (161 vs 100 ng/100 ml) (35,36). The ratio androstenediol:testosterone in spermatic vein blood was 0.2 (33), approximately the same as in peripheral blood. These data suggest that little androstenediol in blood is derived from dehydroepiandrosterone. This conclusion needs to be tested since the conversion factor for a similar transformation at C17 , androstenedione to testosterone is 0.12 (6). If this is assumed for the conversion of dehydroepiandrosterone to androstenediol, then most of blood androstenediol would be the result of peripheral transformation of dehydroepiandrosterone.
It is doubtful that further studies of steroid secretion by the testis will yield significant new information. The important steroid problems are those concerned with intratesticular steroid synthesis and metabolism. These are: What is the concentration of testosterone in testicular interstitial fluid necessary to maintain spermatogenesis? Does intratesticular estradiol have physiologic significance? How does the seminiferous tubule contribute to androgen secretion and is its synthesis of testosterone important for tubular function? What is the target cell in the tubule for testosterone action and how is this action manifested? Further study of these questions will be necessary to understand important physiologic aspects of the role of the Leydig cell in steroid secretion.
