Reevaluation of the role of cyclic adenosine 3',5'-monophosphate and protein kinase in the stimulation of steroidogenesis by luteinizing hormone in bovine corpus luteum slices

Incubation of bovine corpus luteum slices at 37 C with luteinizing hormone showed that 10.0 microng LH/ml caused a maximum rise in tissue cyclic AMP content within 15 min. Slices incubated with 1.0 or 0.1 microng LH/ml showed a much more gradual accumulation of this nucleotide. In the absence of added LH, a marked decline in the amount of cyclic AMP was observed during the first 60 min. The possible role of cyclic AMP in the action of LH was reexamined by studying the dose-response effect of LH on the stimulation of progesterone synthesis, cyclic AMP-dependent protein kinase activation, and cyclic AMP accumulation. After a 2-h incubatin, the results showed that the dose required to elicit a minimal significant stimulation of steroidogenesis was 0.01 microng/ml LH. At this and higher concentrations of LH, a concomitant stimulation of protein kinase activity and progesterone synthesis was also consistently observed. However, significant accumulation of cyclic AMP became consistently detectable only at 0.1 microng/ml LH. This report is the first to show a positive correlation between the activation of cyclic AMP-dependent protein kinase and the stimulation of steroidogenesis in the corpus luteum at the same minimal effective level of LH. These results indicate that cyclic AMP and the cyclic AMP-dependent protein kinase probably play important intermediary roles in the stimulation of steroidogenesis by LH in the bovine corpus luteum.

Adenosine-3',5'-monophosphate in the plasma from human pregnancy

Adenosine-3',5'-monophosphate (cyclic AMP) concentration was measured in plasma from nonpregnant women; women in the 7-41 weeks of normal pregnancy; during labor; and 5-7 h postpartum. The cyclic AMP levels in the course of normal pregnancy showed an initial peak volume at 14 weeks. After falling to nonpregnant level at 18 weeks, it began to rise steadily and reached a second peak at 34 weeks. A gradual decline was then followed until labor. The postpartum plasma concentration was significantly lower than the nonpregnant level. A similar pattern was found in serial studies in 4 women of normal pregnancy. Sequential cyclic AMP measurement in 5 hypertensive pregnancies showed a markedly elevated level during 16-26 weeks, but became comparable to normal pregnancy values thereafter. In the only preeclamptic patient studied, cyclic AMP was elevated in the 16-27th weeks although no clinical symptom was found until the 31st week. The study showed that the plasma cyclic AMP level in normal pregnancy becomes elevated above nonpregnant level at the end of the first and during the third trimesters. However, this profile appeared to be altered in pregnancies complicated by hypertension.

Dynamics of adenosine-3', 5'- monophosphate transfer among mother, fetus, and amniotic fluid in the rhesus monkey

Cyclic AMP exchange among the mother, amniotic fluid, and fetus was studied in normal rhesus monkeys at term pregnancy. Following a pulse intravenous administration of 3H-cyclic AMP into the mother, a small fraction of the dose appeared in less than 1 minute in fetal blood. It appeared in the amniotic fluid after 5 minutes and reached maximum level in 20 minutes. The accumulation of 3H-cyclic AMP in the amniotic fluid in 1 hour was 0.03 per cent of the injected dose. The amount and time course of 3H-cyclic AMP accumulation in the fluid were not altered by tying the umbilical vessels. The transfer of 3H-cyclic AMP injected in utero into the fetal femoral artery resembled that in the mother; in 1 hour the amniotic fluid contained 0.22 per cent of the injected dose. When injected directly into the amniotic sac, more than 65 per cent of the injected dose remained unchanged after 1 hour, with minimal transfer into the maternal and fetal compartments. This study showed rapid bidirectional exchange of cyclic AMP between the mother and the fetus. Both of these compartments can contribute cyclic AMP to the amniotic fluid, independently or in concert. It remained fairly stable in the fluid and was not readily metabolized or transported out.

The role of protein synthesis in the stimulation by LH of prostaglandin accumulation in rat preovulatory follicles in vitro

Preovulatory follicles isolated from immature rats, treated in vivo with pregnant mare's serum gonadotropin, were incubated in vitro and the accumulation of prostaglandin E measured. The addition of luteinizing hormone (5 mug/ml) increased this accumulation, after a lag period of 3 hours. This delay suggested the involvement of macromolecular synthesis in the mechanism of prostaglandin stimulation by luteinizing hormone. When the synthesis of protein was inhibited by the addition of puromycin (100 muM), the luteinizing hormone stimulation of prostaglandin E in these follicles was completely abolished. This inhibition was not seen with an analogue of puromycin, which does not inhibit protein synthesis, puromycin amino-nucleoside. These data suggest that concomitant protein synthesis is required for the luteinzing hormone stimulation of prostaglandin accumulation in rat follicles.

The role of prostaglandins in the development of refractoriness to LH stimulation by Graafian follicles

The possibility that prostaglandins might be responsible for the development of the pre-ovulatory refractoriness to the stimulation by LH of cyclic AMP accumulation in vitro in the Graafian follicle was examined. Isolated rabbit Graafian follicles were obtained at estrus and at 0.5, 5 and 9 hours after an ovulatory dose of LH. The follicles were incubated in vitro in the presence of (8-3H)adenine and the accumulation of (8-3H)cyclic AMP measured. Follicles from estrous animals responded to the in vitro addition of LH with a marked increase of cyclic AMP accumulation and lost this response as the time of ovulation approached. Animals pretreated with indomethacin, which inhibits the usual pre-ovulatory rise of follicular prostaglandin levels, showed essentially the same loss of responsiveness. Indomethacin alone was without effect. It is concluded that prostaglandins are not the major factor in the development of refractoriness to LH stimulation in vitro which has been observed in pre-ovulatory follicles.

Subcellular localization and partial characterization of bovine corpus luteum adenylate cyclase

The subcellular localization of adenylate cyclase (ATP pyrophosphatelyase (cyclizing), EC 4.6.1.1) in bovine corpus luteum was studied using isotonic and hypotonic homogenization and fractionation conditions. All fractions prepared were assayed for adenylate cyclase, marker enzymes and DNA. Only plasma membrane marker enzyme, 5'-nucleotidase paralleled the distribution of adenylate cyclase under both isotonic and hypotonic conditions (conditionsoth isotonic and hypotonic conditions (coefficient of correlation = 0.95). Two main fractions prepared under hypotonic conditions were subfractionated by discontinuous sucrose gradient centrifugation. The highest amount of adenylate cyclase was found in a fraction having a density approximately equal to 1.13 g/cm3. The specific activity of this fraction was 4--6 times higher than that of the homogenate. The electron microscopic study of this fraction revealed the presence of a single type of particulate material consisting of small vesicles exhibiting a typical unit membrane structure. It is concluded that this adenylate cyclase is primarily localized in the plasma membranes. Basal adenylate cyclase activity of plasma membranes was stimulated 2--3 times by luteinizing hormone (10 mug/ml), 3--4 times by prostaglandin E2 (10 mug/ml), 4--6 times by NaF (0.01 M) and two times by methanol (0.2%).

Protein kinase stimulation of a reconstituted cholesterol side chain cleavage enzyme system in the bovine corpus luteum

A solubilized preparation of cytochrome P-450, obtained by treatment of mitochondria from bovine corpora lutea with phospholipase A, contained all of the necessary components for the cholesterol side chain cleavage activity. The solubilized cytochrome -450 preparation could be isolated essentially free of endogenous cholesterol side chain cleavage activity by various fractionation techniques. A cholesterol side chain cleavage enzyme system was reconstituted using the isolated cytochrome P-450 preparation and purified adrenodoxin and adrenodoxin reductase (components of the enzyme system purified from the adrenal cortex). Protein kinase was partially purified from the cytosol fraction of bovine corpora lutea. It was purified 43-fold and the activity was highly dependent on cyclic adenosine 3:5-monophosphate (cyclic AMP). When ATP and this partially purified cyclic AMP-dependent protein kinase were added to the reconstituted cholesterol side chain cleavage enzyme assay in which cytochrome P-450 was limiting, a stimulation (20 to 74%) of the conversion of cholesterol into pregnenolone was observed. This stimulation was statistically significant with p value less than 0.001. The stimulatory effect of the protein kinase appeared to be dependent on ATP and was not mimicked by bovine serum albumin, indicating that the effect was specific for protein kinase. Protein kinase caused a phosphorylation of the cytochrome P-450 preparation when large amounts of this preparation were used in the assay. It is concluded from these results that the direct activation of the cytochrome P-450 component of the cholesterol side chain cleavage by protein kinase may be one of the ways by which cyclic AMP mediates the effect of luteinizine.

Interrelationships between prostaglandins, cyclic AMP and steroids in ovulation

Ovulation is a complex phenomenon, involving a series of biochemical events within the ovary, leading to the rupture of the follicle. This paper summarizes recent studies in our laboratory of some of these biochemical changes using the rabbit as an experimental model. It has been shown in our laboratory that isolated Graafian follicles obtained from oestrous rabbits synthesize steroids and cyclic AMP when incubated in vitro. Luteinizing hormone added to the incubation medium increased steroidogenesis and cyclic AMP synthesis many fold. When follicles were isolated from rabbits at different times following the ovulatory stimulus (mating or HCG injection) it was found that the in vitro response to LH in terms of steroidogenesis and cylcic AMP synthesis was lost as ovulation approached. In contrast, when prostaglandins (PGF and PGE) were measured in rabbit Graafian follicles it was found that the PGF and PGE levels increased as ovulation approached. From these data and from reports in the literature, we have developed a hypothetical model for ovulation in the rabbit which may help in a better understanding of the ovulatory process.

Pre and post ovulatory changes in the concentration of prostaglandins in rat graafian follicles

The concentrations of prostaglandin F (PGF) and prostaglandin E (PGE) were measured by radioimmunoassay in isolated Graafian follicles of mature female rats during the pre and post ovulatory period of the estrous cycle. The levels of these prostaglandins were low and relatively constant from 8 a.m. to 4 p.m. on the day of proestrus, but there was a marked increase at 8 p.m. of proestrus reaching an apparent maximum at midnight (PGF 18 fold, PGE 70 fold). By 4 a.m. to 8 a.m. on the morning of estrus these prostaglandins declined rapidly to levels similar to those observed between 8 a.m. and 4 p.m. on the day of proestrus. The increases in prostaglandin levels occurred after the LH peak and apparently before the time of ovulation. These data confirm the role of PGF and PGE in the local mechanism of ovulation in the normal adult of a spontaneously ovulating animal species.