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Biomedical subjects

E R Simpson

Publications and source records attributed to E R Simpson.

At least 307 records · Page 17Linked to original sources
PubMed1980

Metabolism of high density lipoprotein by human fetal adrenal tissue.

The role of lipoproteins as a source of the cholesterol utilized for steroidogenesis by human fetal adrenal (HFA) tissue was investigated previously. It was found that low density lipoprotein (LDL) was the lipoprotein preferred as a source of cholesterol for steroidogenesis by the HFA. [125I]Iodo-LDL was taken up and degraded by HFA tissue in organ culture, and the degradation of [125I]iodo-LDL was stimulated when ACTH (1 microgram X ml-1) was present in the culture medium. Others have shown that high density lipoprotein (HDL) is utilized as a source of cholesterol for steroidogenesis by rat adrenocortical cells in vitro and by the adrenals of the adult rat in vivo. In the present investigation we evaluated the metabolism of [125I]iodo-HDL by HFA tissue. [125I]iodo-HDL uptake by the HFA tissue increased in a linear manner with time and as the concentration of [125I]iodo-HDL in the culture medium was increased. However, there was little degradation of [125I]iodo-HDL by HFA. Moreover, preincubation of HFA tissue in medium containing ACTH (1 microgram X ml-1) or HDL, in various concentrations, did not affect the rate of uptake and degradation of [125I]iodo-HDL. The rate of degradation of [125I]iodo-LDL was found to decrease to low levels as the concentration of nonradiolabeled LDL in the culture medium was increased, whereas nonradiolabeled HDL had little effect on the degradation of [125I]iodo-LDL. HFA tissue fragments were incubated in medium containing ACTH plus lipoprotein-poor serum (LPPS) alone or LPPS plus HDL in various concentrations (50-1000 microgram X ml-1). The medium was changed daily and assayed for dehydroisoandrosterone sulfate and cortisol. In the presence of HDL, steroid secretion rates were no greater than those attained by HFA maintained in medium containing LPPS. It is concluded that the HFA utilizes cholesterol derived from LDL for steroidogenesis and that HDL is not metabolized efficiently by the human fetal adrenal.

Adrenal Glands↗
PubMed1980

Uptake and degradation of lipoproteins by human trophoblastic cells in primary culture.

It has been shown previously that human placental trophoblastic cells use principally lipoprotein-cholesterol for progesterone synthesis. In the present investigation, human placental trophoblastic cells in primary culture were employed to study the kinetics of uptake and degradation of lipoproteins by trophoblastic tissue. Maximal uptake of [125I]iodo-low density lipoprotein ([125I]LDL) by these cells was achieved by 5-6 h, while degradation of [125I]iodo-LDL was linear from 30 min to 32 h. Uptake and degradation of [125I]iodo-LDL as a function of substrate concentration was studied. The apparent dissociation constants (KD) for both uptake and degradation processes were calculated from these data and were found to be similar (1.03 X 10(-8) and 6.8 X 10(-9) M, respectively), a finding which is suggestive that these processes are causally related. Chloroquine inhibited [125I]iodo-LDL degradation by these cells, a finding that is indicative that LDL degradation is dependent on lysosomal proteolytic activity. When nonradiolabeled high density lipoprotein (HDL) was added to the culture medium, there was little effect on either uptake or degradation of [125I]iodo-LDL; however, when nonradiolabeled LDL was added to the culture medium the net uptake and degradation of [125I]iodo-LDL were diminished progressively as the concentration of nonradiolabeled LDL was increased, findings which are indicative of specificity of the LDL receptor. Uptake and degradation of [125I]iodo-HDL by these cells as a function of substrate concentration were examined. Uptake of [125I]iodo-HDL increased in a linear fashion as the concentration of [125I]iodo-HDL was increased up to 1000 microgram protein/ml; however, degradation of [125I]iodo-HDL was negligible. It is concluded that utilization of lipoproteins by human placental trophoblastic cells is mediated by high affinity, low capacity receptors specific for LDL. These cells do not degrade the protein component of HDL.

Biological Transport↗
PubMed1980

Regulation of steroid secretion by adrenal tissue of a human anencephalic fetus.

ACTH-stimulated adrenal tissue of a human anencephalic fetus, when maintained in organ culture, secreted appreciable quantities of cortisol but little dehydroisoandrosterone sulfate or pregnenolone sulfate. In the absence of ACTH, cortisol secretion was severely attenuated. Arginine vasopressin or alpha MSH, when added to the culture medium, did not stimulate steroid secretion. When whole human serum was present in the culture medium bathing the adrenal tissue of the anencephalic fetus, the rate of cortisol secretion was similar to that attained when lipoprotein-poor serum was in the medium. Based on these findings, it is concluded that in the presence of ACTH, the adrenals of the anencephalic fetus secrete principally cortisol, and the failure of dehydroisoandrosterone sulfate and pregnenolone sulfate secretion is due to the absence of the fetal zone. The lack of stimulation of cortisol secretion by lipoprotein is probably due to a reduced number of low density lipoprotein receptors resulting from diminished ACTH stimulation before birth.

Adrenal Glands↗
PubMed1980

Production rate of deoxycorticosterone in women during the follicular and luteal phases of the ovarian cycle: the role of extraadrenal 21-hydroxylation of circulating progesterone in deoxycorticosterone production.

The fractional conversion of plasma progesterone to deoxycorticosterone (DOC) ([rho]BU P-DOC) has been determined in pregnant, nonpregnant, and adrenalectomized women and men. The [rho]BU P-DOC varies widely among persons, but the [rho]BU P-DOC in a given subject is invariant regardless of plasma progesterone concentrations. In the present investigation we sought to ascertain 1) the production rate of DOC (PR-DOC) during the follicular and luteal phases of the ovarian cycle of women and 2) the role of extraadrenal 21-hydroxylation of progesterone as a source of DOC during the luteal phase of the ovarian cycle. Four subjects were given iv infusions of [3H]progesterone and [14C]DOC in order to determine [rho]BU P-DOC and the PR-DOC during both phases of the ovarian cycle. The [rho]BU P-DOC found in these subjects were within the range of values previously reported, viz. 0.007 +/- 0.001 (mean +/- SEM), and did not change during the cycle. The mean daily PR-DOC in these subjects during the follicular phase of the menstrual cycle was 58 microgram/24 h, whereas the mean daily PR-DOC during the luteal phase was 318 microgram/24 h. In each subject the PR-DOC during the luteal phase was 4--7 times greater than the PR-DOC during the follicular phase of the cycle. The increase in plasma concentrations of DOC during the luteal phase of the ovarian cycle apparently results from extraadrenal formation of DOC from circulating progesterone.

Adult↗
PubMed1980

Plasma lipoproteins in follicular fluid of human ovaries.

Human corpus luteum tissue, maintained in organ culture, is dependent upon low-density lipoprotein (LDL) as a source of cholesterol for maximal progesterone secretion whereas high-density lipoprotein (HDL) does not support progesterone biosynthesis by human corpus luteum tissue. In the present investigation, follicular fluid of human ovaries was found to contain little or no LDL or very low-density lipoprotein. The concentration of HDL in follicular fluid approached that found in plasma. We suggest that one reason that granulosa cells of the preovulatory follicle produce limited quantities of progesterone is because of the absence of LDL-cholesterol in follicular fluid. Following ovulation, vascularization of the corpus luteum provides a means by which LDL is available to the luteinized granulosa cells and thence progesterone biosynthesis can commence.

Body Fluids↗
PubMed1979

The role of serum lipoproteins in steroidogenesis by the human fetal adrenal cortex.

In the present investigation it was found that human fetal adrenal tissue maintained in organ culture secreted appreciable quantities of dehydroisoandrosterone sulfate (DS) and cortisol. Pregnenolone was also secreted in significant amounts, principally as the sulfate ester. The highest rate of secretion of these steroids by fetal adrenal tissue occurred when both ACTH and whole human serum were present in the culture medium. In the absence of ACTH, steroid secretion was low. When whole serum was replaced by lipoprotein-poor serum, the steroidogenic response to ACTH was markedly attenuated but not abolished. On the basis of these findings, it is concluded (1) that the human fetal adrenal can synthesize steroid hormones de novo from cholesterol, (2) that ACTH is an important stimulant of steroidogenesis by the human fetal adrenal, and (3) that plasma lipoproteins are a major source of the cholesterol utilized by the human fetal adrenal for steroidogenesis. Hence, it is likely that factors which regulate the production of fetal plasma lipoproteins are important determinants of fetal adrenal steroidogenic activity.

Adrenal Cortex Hormones↗
PubMed1978

Regulation by plasma lipoproteins of progesterone biosynthesis and 3-hydroxy-3-methyl glutaryl coenzyme a reductase activity in cultured human choriocarcinoma cells.

The regulation of both the activity of 3-hydroxy-3-methyl glutaryl coenzyme A (HMG CoA) reductase [mevalonate-NADP+ oxidoreductase (CoA-acylating) EC 1.1.1.34] and the secretion of progesterone by human plasma lipoproteins has been investigated in human choriocarcinoma cells in culture. HMG CoA reductase activity was computed from the rate of formation of [14C]mevalonolactone from [14C]HMG CoA. The activity of HMG CoA reductase was expressed as nanomoles of mevalonolactone formed/min . mg solubilized cell protein. An inverse relationship was found between the presence of lipoprotein in the culture medium and the activity of HMG CoA reductase in these cells. In cells maintained in the presence of lipoprotein-enriched culture medium containing 840 micrograms cholesterol/ml, the average activity of HMG CoA reductase was 0.25 nmol/min . mg protein. After removal of lipoprotein, the activity of HMG CoA reductase increased to 1.3 nmol/min . mg protein. The average activity of HMG CoA reductase in cells maintained in lipoprotein-deficient culture medium was 1.5 nmol/min . mg protein but fell to 0.3 nmol/min . mg protein after addition of lipoprotein to the medium. When cells were maintained in the presence of lipoprotein, the rates of section of progesterone and pregnenolone into the culture medium were 2-8 times greater than the rates of secretion of these steroids by cells maintained in the absence of lipoprotein. On the basis of these results, it is concluded that lipoproteins control the rate of cholesterol biosynthesis in cultured choriocarcinoma cells by regulating the activity of HMG CoA reductase, and control the rate of synthesis of progesterone by providing the precursor, cholesterol. We suggest that progesterone synthesis by the trophoblast of the human placenta may also be regulated by the uptake of lipoprotein from maternal blood.

Cells, Cultured↗
PubMed1978

Investigations into the role of calcium ions in the control of steroid production by isolated adrenal zona glomerulosa cells of the rat.

An increase in the concentration of extracellular potassium from 3.6 to 8.4 mmol/1 had only a small delayed effect on the uptake of radioactive calcium by isolated adrenal glomerulosa cells. However, the same stimulus had a rapid and highly significant effect on the efflux of radioactive calcium from glomerulosa cells preloaded with 45Ca2+. Cells incubated in medium containing 8.4 mmol potassium/1 had retained approximately 15% more radioactivity than conrol cells after 2.5 min and this difference was maintained for up to 90 min. There was an increase in the production of steroids by the glomerulosa cells in the presence of 8.4 mM-potassium. No effect on calcium efflux was observed in similar experiments with isolated fasciculata cells; it has been established that this concentration of potassium does not affect steroidogenesis in fasciculata cells, indicating that the effect on glomerulosa cells may be causally linked to steroidogenesis. There was no significant change in the total calcium content of glomerulosa cells in the presence of 8.4 mM-potassium. Exchangeable calcium in these cells was found to be 60% of the total calcium content.

Adrenal Cortex↗
PubMed1976

Electron paramagnetic resonance spectra of mitochondrial and microsomal cytochrome P-450 from the rat adrenal.

The electron paramagnetic resonance (EPR) spectra of rat adrenal zona fasciculate mitochondria showed peaks corresponding to low spin ferric cytochrome P-450 with apparent g values of 2.424, 2.248 and 1.917, and weak signals due to high spin ferric cytochrome P-450 with gx values of 8.08 and 7.80. The former is attributed to cholesterol side chain cleavage cytochrome P-450, the latter to 11beta-hydroxylase cytochrome P-450. On addition of deoxycorticosterone the g = 7.80 signal was elevated and there was an associated drop in the low spinal signal. As the pH was reduced from 7.4 to 6.1, the g = 8.08 signal increased with again a drop in intensity of the low spin signal. Mitochondria from the zona glomerulosa showed similar spectral properties to those described above. Addition of succinate, isocitrate or pregnenolone caused a loss of the g = 8.08 signal. Addition of calcium increased the magnitude of the g = 8.08 signal, and caused a slight reduction in the magnitude of the low spin signal. Also, addition of deoxycorticosterone, pregnenolone, succinate or isocitrate caused slight shifts of the outer lines of the low spin spectrum. Interaction of mitochondrial cytochrome P-450 with metyrapone and aminoglutethimide modified the low spinal parameters. Adrenal microsomal cytochrome P-450 had low spin ferric g values of 2.417, 2.244 and 1.919 and a high spin ferric gxy values of 7.90 and 3.85, distinct from the values obtained with mitochondria.

Adrenal Glands↗
PubMed1976

Electron paramagnetic resonance studies of cytochrome P-450 and adrenal ferredoxin in single whole rat adrenal glands. Effect of corticotropin.

Low and high spin ferric cytochrome P-450 and reduced adrenal ferredoxin (adrenodoxin) have been directly studied by EPR techniques in whole rat adrenal glands. The spectra obtained correspond closely to those obtained from sub-cellular fractions except in the case of low spin ferric cytochrome P-450, where there are differences in the shape of the g = 2.41 line. The relative magnitudes of these peaks in anaerobic and aerobic rapidly frozen adrenals from control and corticotropin stimulated hypophysectomised rats were used to investigate the control and rate limiting steps in adrenal steroid biosynthesis via cytochrome P-450. All adrenals showed a close to maximal level of reduced adrenodoxin and aerobic and anaerobic glands from control rats and aerobic glands from corticotropin stimulated rats showed similar quantities of low spin ferric cytochrome P-450. On anaerobiosis the quantity of low spin ferric cytochrome in adrenals from corticotropin stimulated rats dropped to 30--40% of the aerobic level. Treatment of the rats with cycloheximide prior to administration of corticotropin prevented these changes. Approximately 0.4% of the total cytochrome P-450 was high spin ferric in control adrenals and in aerobic stimulated adrenals this rose to approximately to 0.6%. These results demonstrate that association of substrate with cytochrome P-450 is the rate limiting step in adrenal steroidogenesis via cytochrome P-450. It is suggested on the basis of these and mitochondrial optical and EPR experiments that the limiting step being observed is cholesterol binding to cholesterol side chain cleavage cytochrome P-450, and that the rate of this association is stimulated by corticotropin.

Adrenal Glands↗