Progesterone release of lipid second messengers at the amphibian oocyte plasma membrane: role of ceramide in initiating the G2/M transition

Treatment with either sphingomyelinase (SMase), or soluble forms of ceramide, has been reported to induce meiosis in oocytes from Xenopus laevis, a species which can breed throughout most of the year. In this paper the sphingomyelin-derived second messenger, ceramide, is compared with progesterone for its ability to induce meiosis in oocytes from the seasonal breeder, Rana pipiens. Serum gonadotropin levels normally rise as Rana females emerge from hibernation in the spring, stimulating follicular synthesis of progesterone and subsequent ovulation. Injection of gonadotropins can induce earlier meiosis and ovulation, effective from the previous October through the following spring. During the same period, defolliculated oocytes respond to exogenous progesterone by meiosis, as indicated by nuclear breakdown. We find that in the spring, treatment of defolliculated Rana oocytes with exogenous C2- or C8-ceramide or SMase did induce meiosis, but not during the fall or winter. A 50% response was seen by late April and a 100% response by early May. Exposure of [3H]palmitate-labeled Rana oocytes to either exogenous progesterone or to SMase produced a rapid and comparable release of intracellular [3H]ceramide within 1-2 min in fall, winter or spring. Our results from this and from previous experiments indicate that increased ceramide is not the initiating event in meiotic induction in Rana, but is associated with a subsequent pathway which depends upon a threshold level of progesterone.

Mg2+ modulates membrane lipids in vascular smooth muscle: a link to atherogenesis

Epidemiological studies associate low dietary magnesium intake with an increased incidence of ischemic heart disease and sudden cardiac death. We have used proton-magnetic resonance (1H-NMR) techniques and Mg2+-selective electrodes to monitor changes in lipid extracts of aortic and cerebrovascular smooth muscle as extracellular ionized magnesium ion concentration ([Mg2+]o) is lowered. We have found that, within the pathophysiological range of Mg2+ concentrations, fatty acid chain length and double bond content are progressively reduced as [Mg2+]o is lowered. In contrast, the plasmalogen content is progressively increased. A concomitant decrease in fatty acid chain length and double bonds indicates oxidation of double bonds resulting in truncation of the fatty acids. A decrease in lipid oxidation in the presence of elevated Mg2+ could contribute to the apparent protective role of increased Mg2+ intake on vascular function in humans.

Progesterone binding to plasma membrane and cytosol receptors in the amphibian oocyte

We have found a single class of progesterone binding sites at the amphibian oocyte plasma membrane, whereas two progesterone receptor forms, similar to those in chick and human, are present in the cytosol. In this study both plasma membranes and 105,000 x g cytosol from Rana pipiens oocytes were photoaffinity labeled with the synthetic progestin [3H]R5020. SDS-polyacrylamide gel electrophoresis of the photolabeled proteins in the oocyte cytosol indicate that the two forms have molecular weights essentially identical to that found for human breast tissue and chick oviduct, i.e., 80 and 110 kDa, and that the forms were present in approximately equimolar ratios. In contrast, the plasma membrane form is present as a single 110 kDa species and accounts for at least 50% of the total 110 kDa species. The presence of large amounts of the 110 kDa protein in both membrane and cytosol suggests that the plasma membrane receptor may not be unique, and that the 110 kDa form may function both in membrane and cytosol and/or that part of the cytosolic 110 kDa form represents progesterone receptor in the process of being transported to or from the plasma membrane.

Progesterone triggers the rapid activation of phospholipase D in the amphibian oocyte plasma membrane when initiating the G2/M transition

Previous reports indicate that, in the Rana pipiens oocyte, progesterone triggers a rapid rise in 1,2-diacylglycerol (DAG) derived from phosphatidylcholine (PC) in the plasma membranes. This DAG transient, which appears and is terminated within 60-90 s, is derived both from a phospholipase which we assumed to be phospholipase C and from sphingomyelin (SM) synthase. We now find that progesterone stimulates PC and DAG turnover primarily via the phospholipase D (PLD) and phosphatidic acid phosphohydrolase (PAP) pathways as well as via the SM-ceramide pathway. Rana oocytes were prelabeled with [3H]choline chloride under conditions in which about 70% is incorporated into PC of the plasma membrane of the intact oocyte or with [3H]lysoplatelet activating factor (1-O-octadecyl-sn-glycero-3-phosphocholine, lysoPAF) which is selectively incorporated into plasma membrane PC. Progesterone induced the release of [3H]choline from intact oocytes into the medium within 60-90 s. This choline release was dose-dependent and was not inhibited by a putative PC-specific phospholipase C inhibitor, D609. Progesterone also induced a transient rise in [3H]lysoPAF-derived [3H]DAG within 1-2 min followed by a rise in [3H]PA. In the presence of 20 mM ethanol, progesterone stimulated formation of [3H]lysoPAF-derived phosphatidylethanol, indicating progesterone activation of PC-specific PLD and concomitant formation of PA. A DGK inhibitor (D102) reduced the level of [3H]PA, produced a sustained rise in [3H]DAG and was a weak inducer of meiosis in oocytes not exposed to progesterone. A PA phosphohydrolase inhibitor (propranolol) elevated [3H]PA and completely inhibited the progesterone-induced rise in DAG. Progesterone thus acts at oocyte plasma membrane receptors to release PC-derived DAG via both SM synthase and PC-PLD. The duration of the DAG signal is regulated by the coordinate action of DGK and PAP.

Differences in hydration state of nucleus and cytoplasm of the amphibian oocyte

Nuclear magnetic resonance (NMR) microimaging and proton relaxation times were used to monitor differences between the hydration state of the nucleus and cytoplasm in the Rana pipiens oocyte. Individual isolated ovarian oocytes were imaged in a drop of Ringer's solution with an in-plane resolution of 80 microm. Proton spin echo images of oocytes arrested in prophase I indicated a marked difference in contrast between nucleoplasm and cytoplasm with additional intensity gradations between the yolk platelet-rich region of the cytoplasm and regions with little yolk. Neither shortening taue (spin echo time) to 9 msec (from 18 msec) nor lengthening taur (spin recovery time) to 2 sec (from 0.5 sec) reduced the observed contrast between nucleus and cytoplasm. Water proton T1 (spin-lattice) relaxation times of oocyte suspensions indicated three water compartments that corresponded to extracellular medium (T1 = 3.0 sec), cytoplasm (T1 = 0.8 sec) and nucleoplasm (T1 = 1.6 sec). The 1.6 sec compartment disappeared at the time of nuclear breakdown. Measurements of plasma and nuclear membrane potentials with KCl-filled glass microelectrodes demonstrated that the prophase I oocyte nucleus was about 25 mV inside positive relative to the extracellular medium. A model for the prophase-arrested oocyte is proposed in which a high concentration of large impermeant ions together with small counter ions set up a Donnan-type equilibrium that results in an increased distribution of water within the nucleus in comparison with the cytosol. This study indicates: (i) a slow exchange between two or more intracellular water compartments on the NMR time-scale, (ii) an increased rotational correlation time for water molecules in both the cytoplasmic and nuclear compartments compared to bulk water, and (iii) a higher water content (per unit dry mass) of the nucleus compared to the cytoplasm, and (iv) the existence of a large (about 75 mV positive) electropotential difference between the nuclear and cytoplasmic compartments.

Progesterone-induced phospholipid N-methylation and sphingomyelin synthesis in the amphibian oocyte plasma membrane: a second source of the 1,2-diacylglycerol second messenger associated with the G2/M transition

The effects of progesterone and GTP gamma S on phospholipid N-methylation and sphingomyelin synthesis were studied in plasma-vitelline membranes isolated from amphibian (Rana pipiens) oocytes. Plasma-vitelline membranes were preincubated with S-adenosyl-L-[methyl-3H]methionine for 2 min at 20 degrees C and total phospholipids extracted at 0, 15, 30 and 60 s after addition of progesterone and/or GTP gamma S. Progesterone levels (3 microM) that induce meiosis in the intact oocyte stimulated [3H-methyl]incorporation into phosphatidylmonomethylethanolamine (PME) 9-10-fold over the first 60 s, with smaller increases in phosphatidyldimethylethanolamine (PDE) and phosphatidylcholine (PC). [methyl-3H] labeling of sphingomyelin (SM) rises after 30 s, approaching that of [methyl-3H]PME by 60 s. 17 beta-Estradiol, a noninducer of meiosis, was inactive. When oocytes were prelabeled with [3H]palmitic acid, it was found that a fall in [3H]ceramide coincides with the transient increase in [3H]SM, indicating that the end product of N-methylation (PC) undergoes a transfer reaction with ceramide to form SM and 1,2-DG. GTP gamma S levels previously reported to stimulate PC-specific phospholipase C activity in oocyte plasma membranes (5 microM) also stimulated both [methyl-3H]PME and [methyl-3H]SM formation. An inhibitor of phospholipid N-methylation, 2-(methyl-amino)ethanol, blocked stimulation of [methyl-3H]SM synthesis by both progesterone and GTP gamma S as well as induction of meiosis by progesterone. Progesterone thus acts at the oocyte plasma membrane to stimulate PE N-methyltransferase and SM synthase. The finding that GTP gamma S mimics progesterone suggests that N-methyltransferase is mediated by G-protein(s). The transient increase in 1,2-DG which we had previously reported to occur within 1-2 min following progesterone stimulation of the Rana oocyte appears to arise from PC by two different pathways: SM synthesis and hydrolysis of PC by phospholipase C.

Cyclic AMP binding to the amphibian oocyte plasma membrane: possible interrelationship between meiotic arrest and membrane fluidity

Cyclic AMP, which maintains the vertebrate oocyte in prophase arrest under physiological conditions, exhibits specific and saturable binding to the cytoplasmic face of the prophase-arrested Rana pipiens oocyte plasma membrane. Scatchard type analyses of [3H]cAMP binding to isolated plasma membranes indicate a single class of binding sites with a Kd = 19.3 +/- 7.0 nM at cAMP concentrations below 10(-6) M and additional low affinity site(s) and/or non-specific binding at concentrations above 10(-6) M. Photoaffinity labeling of prophase oocyte plasma membranes with [32P]-8-N3cAMP demonstrates cAMP/cGMP-displacable binding of 8-N3[32P]cAMP to a 100-110 kDa peptide doublet. Plasma membrane fluidity was monitored by electron spin resonance in isolated plasma-vitelline membranes using a 5-doxyl stearic acid probe. Exogenous dibutyryl cAMP (dbcAMP) produces an increase in membrane fluidity within minutes and blocks and/or reverses the progesterone-induced decrease in plasma membrane fluidity. The dbcAMP concentration that produced half-maximal fluidity increase (10 microM) corresponds to the half-maximal inhibiting dose of dbcAMP for progesterone induction of meiosis. Cholera toxin, which elevates intracellular cAMP and blocks meiosis, also increases membrane fluidity and inhibits progesterone-induced decrease in membrane fluidity. Elevated levels of intracellular cAMP thus appear to maintain meiotic arrest by binding to specific plasma membrane site(s) and maintaining the plasma membrane in a relatively fluid state. The progesterone-induced fall in intracellular cAMP first reported in Rana thus appears to be responsible for the progesterone-induced increase in membrane fluidity and further suggests that the change in membrane order is essential for the resumption of the meiotic divisions.

Steroid action at the plasma membrane: progesterone stimulation of phosphatidylcholine-specific phospholipase C following release of the prophase block in amphibian oocytes

Progesterone, acting at the amphibian oocyte plasma membrane, triggers the progression of the prophase oocyte nucleus through the first meiotic metaphase. We previously reported a transient increase in 1,2-diacylglycerol (1,2-DG) within the first 1-2 min after exposure of Rana pipiens oocytes to progesterone. We have now investigated the source of the 1,2-DG, using this highly synchronous oocyte population. Phospholipid pools of intact prophase-arrested oocytes were labeled with [3H]glycerol, [methyl-3H]choline chloride or 1-O-[3H]octadecyl-sn-glycero-3-phosphocholine (lyso platelet activating factor, lysoPAF). [3H]LysoPAF is selectively taken up into the plasma membrane of the intact oocyte and esterified to form the [3H]alkyl-analogue of phosphatidylcholine (PC). Intact oocytes and/or isolated plasma membranes were then stimulated with progesterone and the changes in [3H]DG, [methyl-3H]phosphocholine and [3H]phospholipids were monitored as a function of time. Progesterone induced a transient increase in [3H]glycerol-derived DG, [methyl-3H]phosphocholine and [3H]alkyl-2-acylglycerol from [3H]alkyl-PC within the first 2 min, indicating activation of a PC-specific phospholipase C. Different pulse-labeling conditions indicate a biphasic rise in [3H]DG from [3H]glycerol-labeled oocytes; the first rise (1-2 min) when phospholipid labeling in the plasma membrane is enriched followed by an approximately 3-fold larger rise at 5-15 min when phospholipids of intracellular membranes are preferentially labeled. An early transient increase in [3H]DG or [3H]alkyl-2-acylglycerol was also seen when progesterone and/or guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) were added to isolated plasma-vitelline membranes prepared from oocytes prelabeled with either [3H]glycerol or [3H]lysoPAF. Progesterone thus appears to activate a G-protein-linked PC-specific phospholipase C in the oocyte plasma membrane which is followed by much larger DG release from intracellular membranes. The transient character of the hydrolysis suggests that this may represent a mechanism for transducing a membrane event into a meiotic signal.

Progesterone-induced second messengers at the onset of meiotic maturation in the amphibian oocyte: interrelationships between phospholipid N-methylation, calcium and diacylglycerol release, and inositol phospholipid turnover

The steady-state turnover in phospholipid N-methylation, 1,2-diacylglycerol and inositol phospholipids in prophase-arrested Rana pipiens oocytes was compared with changes occurring in these pathways immediately following progesterone induction of the first meiotic division. Oocytes were preincubated with [3H-methyl]methionine, [3H]glycerol, [3H]myo-inositol or [3H]arachidonic acid. Ca2+ efflux was measured in oocytes preloaded with 45Ca2+. Membrane phospholipids and cytosolic levels of radiolabeled 1,2-diacylglycerol (DAG), inositol bis- (InsP2), tris- (InsP3), and tetrakisphosphate (InsP4) were monitored immediately following induction with progesterone. A transient increase in both N-methylation of ethanolamine phospholipids and in [3H]DAG coincides with a release of 45Ca2+ from the oocyte surface during the first minute. At least 80% of the total phospholipid N-methylation is associated with the plasma membrane. 45Ca2+ and [3H]DAG release occur prior to a rise in intracellular InsP3, the latter beginning 2-3 min after exposure to the hormone and reaching a maximum by 15-30 min. Progesterone induces rapid and successive changes in ethanolamine, choline, and inositol-containing phospholipids, which represent three of the four major phospholipid classes found in membranes. The maintenance of higher levels of DAG and InsP3 during the first 90 min might be expected to sustain the previously observed increase in protein kinase C activity.

Progesterone induces transient changes in plasma membrane fluidity of amphibian oocytes during the first meiotic division

Progesterone acts at the surface of the amphibian oocyte to induce resumption of the meiotic divisions. Progesterone binding leads to a transient dose-dependent decrease in the fluidity (increase in order parameter) of the Rana oocyte plasma membrane, which was detected by electron spin resonance in isolated plasma membranes using either 5- or 16-DOXYL stearic acid probes. The 5-DOXYL probe, which inserts into the membrane with the spin label nearest the surface, showed an increase in the order parameter within minutes, a maximum change by 2 h, and a return to control levels by 6 h. The order parameter for the 16-DOXYL probe, which reflects the fluidity deeper within the plasma membrane, increased slowly and remained elevated during the first meiotic division. RU 38486, a synthetic steroid that blocks progesterone receptors, prevents progesterone-induced fluidity changes. These findings indicate that the binding of progesterone to its receptor changes the oocyte plasma membrane structure resulting in a differential decrease in mobility near the membrane surface compared to that deeper in the membrane.

Calcium-dependent phosphorylation of the amphibian oocyte plasma membrane: an early event in initiating the meiotic divisions

Plasma membranes isolated from Rana oocytes showed a 7-10 fold increase in the Ca2+-dependent phosphorylation of endogenous protein following exposure to meiotic stimuli (progesterone, insulin) either in vivo or in-vitro. Exogenous phosphatidylmonomethylethanolamine (PME) was effective in stimulating Ca2+-dependent membrane phosphorylation and also induced meiosis. Induction of phosphorylation was blocked by the protease inhibitor leupeptin, as are all other responses to meiotic stimuli. Phosphatidylserine was inactive when added to intact oocytes, but stimulated membrane phosphorylation nearly 15-fold when added to isolated membranes. The results indicate a link between phospholipid methylation and protein kinase C activation.

Factors affecting the response of the female rat reproductive system to cannabinoids

Chronic oral administration of either crude marihuana extract (CME) or delta 9-tetrahydrocannabinol (THC) to female Fischer rats for 64-72 days, at a dose approximating heavy usage by humans, reduces food intake by about 8%. Pair-feeding studies demonstrate that this decreased food intake accounts for previously described decreases in uterine and ovarian weights, which are much more affected by food restriction than is body weight. THC-treated rats lost weight initially which was not regained. Pair-fed rats gained only about one-half of the weight of the untreated control or vehicle-treated control rats over a 64-day period. Although long-term cannabinoid administration leads to tolerance and the resumption of the estrous cycle, the onset of estrus is often delayed when cannabinoid is administered 5-6 hr before the proestrus luteinizing hormone (LH) surge. Our results indicate that although chronic exposure to cannabinoids can continue to affect the rat estrous cycle, they do not have a direct effect on growth of the reproductive organs. The results reemphasize the need for adequate nutritional controls in marihuana and other toxicological research.

Progesterone induction of phospholipid methylation and arachidonic acid turnover during the first meiotic division in amphibian oocytes

Progesterone is the physiological stimulus that acts at the amphibian oocyte plasma membrane to induce the meiotic divisions. Rana oocytes were preincubated with [3H]-arachidonic acid, [3H]-methionine and/or [14C]choline. Total and plasma membrane phospholipids were monitored during the first 2 h after induction with progesterone. A transient increase in methylation of phosphatidylethanolamine during the first 10 minutes coincided with an increased Ca2+ efflux and was followed by increased arachidonic acid incorporation into phosphatidylcholine during a period of increasing membrane conductance. The labeled phospholipids disappeared sequentially 5-90 min after the hormone stimulus, suggesting that activation of phospholipases A2 and/or C occur as part of a cascade of membrane events.

The effect of intragastric administration of delta 9-tetrahydrocannabinol on the growth and development of fetal mice of the A/J strain

Pregnant A/J mice were intubated with vehicle (sesame oil:Tween 80:water) or 60, 120, or 240 mg/kg of delta 9-tetrahydrocannabinol on Days 11 and 12, 12 and 13, or 13 and 14 (vehicle and 240 mg doses only) of gestation. Mice were killed on Day 20 of gestation, and examined for number of corpora lutea and live and resorbed fetuses. Fetuses were weighed and examined for gross external and internal malformations. Each treatment group consisted of a minimum of 10 litters with about 10 pups per litter. In a few groups the effects of feed deprivation on Day 12 or of glucocorticoid administration on Days 12 and 13 (positive control) were assessed. Intubation with vehicle or delta 9-tetrahydrocannabinol, or feed deprivation did not affect number of live fetuses, incidence of resorption, fetal weights, or gross malformations other than cleft palate. Intubation of delta 9-tetrahydrocannabinol on gestational Days 12 and 13 or 13 and 14 increased the mean frequency of cleft palate formation. The increase was 2- to 2.5-fold at the 240-mg dose, being significant (p = 0.05) in the Days 12 and 13 group. Cortisone acetate and corticosterone injection induced both resorption and cleft palate formation. Other developmental or reproductive parameters were not influenced by delta 9-tetrahydrocannabinol treatment. We conclude that delta 9-tetrahydrocannabinol administered by gavage during Days 12 and 13 of gestation retards normal palatal development.

NMR studies of intracellular sodium ions in amphibian oocytes, ovulated eggs, and early embryos

23Na NMR, in combination with an anionic paramagnetic shift reagent dysprosium bis(tripolyphosphate), has been used to study intracellular Na+ in Rana oocytes, ovulated eggs, and early cleavage embryos. The technique allows accurate and simultaneous determination of both extracellular space and intracellular Na+ concentration. In prophase-arrested, follicle-enclosed oocytes, only about 17% of the total oocyte Na+ (approximately 40 mmol/kg of cells) was NMR-visible. Homogenizing oocytes in 0.24 M sucrose did not significantly affect the 23Na resonance. About 30% of the total oocyte Na+ was associated with the yolk platelets isolated at room temperature by differential centrifugation. NMR analysis, however, did not yield a detectable 23Na signal from these intact platelets. Thus, while yolk platelets are rich in Na+, this Na+ does not contribute to the oocyte 23Na NMR signal. Denuded oocytes, obtained by removing the follicular epithelium, gained about 10 mmol of total Na+/kg of cells and exhibited a comparable increase in NMR-visible Na+, suggesting the existence of compartments with varying degree of NMR visibility within the oocyte. Partially relaxed 23Na Fourier transform NMR spectra revealed the existence of at least two major intracellular compartments of NMR-visible Na+ with different magnetic environments and relaxation behavior in denuded oocytes. Since platelet Na+ appears to be NMR-invisible, one of the two observed compartments may be the nucleus. Progesterone action on the amphibian oocyte caused measurable changes in NMR-visible Na+. By ovulation (second metaphase), there is a gain in total egg Na+, and the NMR-visible Na+ is also increased. Following fertilization, however, there is some loss of total cell Na+ but, by the 2-4 cell stage, about 70% of the total Na+ becomes NMR-visible. These results indicate that a sizable fraction of the Na+ in follicle-enclosed, prophase oocyte is sequestered and located in NMR-invisible compartments and that changes in NMR-visible intracellular Na+ occur following hormonal and developmental stimuli.

Studies of insulin action on the amphibian oocyte plasma membrane using NMR, electrophysiological and ion flux techniques

Insulin (0.1-10 microM) reinitiates the meiotic divisions in Rana oocytes and produces a 14-20 mV negative-going hyperpolarization of the plasma membrane as well as a 0.25 unit increase in intracellular pH during the first 90 min. During hyperpolarization, the Na+ conductance of the membrane decreases by 40-50% with a concomitant increase in 22Na+ uptake from the medium. The increased uptake of Na+ during a period of decreasing Na+ conductance is apparently due to an increase in fluid phase turnover associated with insulin-mediated endocytosis. Both membrane hyperpolarization and increase in pHi are Na+-dependent and are blocked by the serine proteinase inhibitor, phenylmethylsulfonyl fluoride. The membrane potential of the prophase oocyte has a significant electrogenic component with potential but not conductance sensitive to glycosides and substitution of Li+ for Na+. Insulin hyperpolarizes Li+ or glycoside-treated oocytes whereas glycosides do not affect insulin-hyperpolarized oocytes. [3H]Ouabain binding by the plasma membrane of the untreated oocyte shows at least two K+-sensitive components (Kd = 42 and 2000 nM) linked to inhibition of the Na+ pump. Insulin-treated oocytes show a single class of intermediate-affinity ouabain sites (Kd = 490 nM) which appear to result from insulin-induced internalization of membrane-bound ouabain. [125I]Insulin binding to the plasma membrane shows a class of high-affinity sites (Kd = 87 nM) with 40-50 pump sites per insulin-binding site. Our results suggest that insulin-induced mediator peptides stimulate Na+-H+ exchange resulting in an increase in intracellular pH and Na+ uptake concomitant with an increase in receptor-mediated endocytosis and a decrease in Na+ conductance and associated membrane hyperpolarization. The net result appears to be a down-regulation of the Na+ pump which together with a decrease in Na+ conductance may divert high-energy phosphate compounds from cation regulation to anabolic processes of meiosis.

Effect of long-term administration of delta 9-tetrahydrocannabinol on hepatic mixed-function oxidase systems in the rat

Chronic oral treatment of young female Fischer rats with 25 mg/kg delta 9-tetrahydrocannabinol (THC) per day inhibited aminopyrine demethylation and significantly increased benzo[a]pyrene oxidation by the liver. THC treatment also elevated serum corticosterone levels and produced a significant loss of body weight. The weight loss was not due to vehicle or food intake (pair-feeding). Pair-feeding did, however, produce a stimulation of both mixed function oxidase pathways as well as a marked elevation in serum corticosterone levels. The results indicate that THC has a differential effect on mixed function oxidase pathways in the liver that is not directly related to food intake or corticosterone levels.

Biochemical correlates of progesterone-induced plasma membrane depolarization during the first meiotic division in Rana oocytes

Changes in protein synthesis, protein phosphorylation and lipid phosphorylation in the amphibian oocyte plasma membrane have been correlated with electrical changes following steroid induction of the completion of the first meiotic division. The oocyte first depolarizes from about -60 mV (inside negative) to about -25 mV 1 to 2 hr before breakdown of the large nucleus followed by a further depolarization beginning 3 to 6 hr after nuclear breakdown. The initial depolarization is associated with appearance of previously described cycloheximide-sensitive cytoplasmic factor(s) which induce both nuclear breakdown and plasma membrane depolarization. We found a similar ED50 (0.4 microM) for cycloheximide inhibition of nuclear breakdown, membrane depolarization, and [3H]-leucine incorporation. Emetine (1 nM to 1 mM) was inactive. The period of cycloheximide sensitivity (first 5 hr) is essentially the same for plasma membrane depolarization and nuclear breakdown. The onset of the second depolarization phase following nuclear breakdown is associated with a marked increase in the rate of [3H]-leucine and [32PO4] incorporation into membrane protein and lipid. Polyacrylamide gel electrophoresis of membrane protein and lipoprotein indicated that a major newly synthesized membrane component is proteolipid. An increase in [32PO4] incorporation into membrane phosphatidylserine and phosphatidylethanolamine (with a decrease in phosphatidylcholine [32PO4] begins during the second depolarization phase and coincides with the appearance of excitability in the oocyte plasma membrane. In toto, the bulk of the biochemical changes (proteins, phosphoproteins, proteolipids, phospholipids) appear to be associated with plasma membrane components and coincide with stepwise changes in membrane permeability to specific ions (e.g. Cl-).

Role of calcium in regulating intracellular pH following the stepwise release of the metabolic blocks at first-meiotic prophase and second-meiotic metaphase in amphibian oocytes

31P-NMR has been used to monitor changes in intracellular pH following the sequential release of the block at first-meiotic prophase by hormones and the block at second-meiotic metaphase by fertilization in Rana eggs and oocytes. The broad phosphoprotein signal was eliminated by a combination of spin-echo and deconvolution techniques. pHi was determined from the pH-dependent separation of intracellular Pi and phosphocreatine resonances. Agents that release the prophase block (progesterone, insulin, D-600, La3+) increased pHi from 7.38 to 7.7-7.8 within 1-3 h. Noninducers such as 17 beta-estradiol were without effect. By second-metaphase arrest (ovulated, unfertilized) the pHi had fallen to 7.1-7.2. pHi underwent a transient increase to about 7.7 within the first 30 min at fertilization, with a slow 0.1-0.2 pH unit oscillation during early cleavage. The progesterone-induced elevation of intracellular pH is not blocked by amiloride and occurs in Na+-free medium. A transient rise in pHi occurs when the prophase-arrested oocyte is transferred to Ca2+-free medium or when ionophore A23187 is added to the Ca2+-containing medium. Agents that inhibit the resumption of the first meiotic division either block the rise in pHi (procaine, PMSF) or shorten the time-course of the rise in pHi (ionophore A23187). Conditions that elevate intracellular Ca2+ levels and/or increase Ca2+ exchange produce an increase in pHi, whereas those conditions that decrease intracellular Ca2+ levels and/or exchange produce a fall in pHi within 1 h. The time-course of the increase in pHi both following release of the prophase block and at fertilization coincide with a fall in intracellular cAMP and release of surface and/or intracellular Ca2+. These results suggest that: (1) pHi is a function of cytosolic free Ca2+ levels and/or Ca2+ exchange across the oocyte plasma membrane, and (2) meiotic agonists (progesterone, insulin, D-600) and mitogens (sperm, ionophore A23187) modulate intracellular and/or membrane Ca2+ with the resulting changes in pHi and cAMP and resumption of the meiotic divisions.

Endocytosis in the amphibian oocyte. Effect of insulin and progesterone on membrane and fluid internalization during the meiotic divisions

Endocytosis has been studied in the denuded Rana pipiens oocyte using 3H-labeled inulin. Internalization of labeled inulin is linear after the first 10-15 min and uptake into the cytoplasm is temperature-dependent and is blocked by 15 microM cyanide. Uptake occurs without hydrolysis of the inulin and varies exponentially with the concentration of inulin in the medium. Based on specific activity of the medium and inulin uptake into the cytoplasm, it is estimated that a fluid volume of about 20-25 nl is internalized per oocyte per hour. This fluid phase uptake corresponds to a half-time of about 35 h for turnover of the oocyte fluid phase. An estimate of membrane area based on endocytotic vesicle size from electron micrographs suggests that the entire oocyte plasma membrane recycles several times an hour. A fraction (15-20%) of the inulin taken up is associated with the plasma-vitelline membrane complex and uptake into the membrane complex parallels uptake into the cytoplasm. Insulin (a meiotic agonist) concentrations that induce plasma membrane hyperpolarization over the first h also stimulate [3H]inulin uptake into both the oocyte cytoplasm and membrane complex over the same time period. Progesterone (the physiological inducer) has no effect on inulin uptake during the first hour, but by 16-17 h after exposure to progesterone, inulin uptake is significantly enhanced. These results suggest that hormones such as insulin and progesterone may regulate membrane permeability by a programmed internalization and possible recycling of the plasma membrane components.

Effects of cannabinoids on function of testis and secondary sex organs in the Fischer rat

Chronic oral treatment of young adult male Fischer rats with delta 9-tetrahydrocannabinol (THC), 1, 5 and 25 mg kg-1 day-1, or crude marihuana extract (CME), 3, 15 and 75 mg kg-1 day-1, reduced body weight gain by about 50-80% at the high CME or THC dose and was correlated with decreased food intake. When cannabinoid was administered early in the light cycle (9-11 a.m.), cauda epididymis sperm count and seminal vesicle fluid and fructose content were depressed to 50-65% at the high dosages but were not significantly different from those of pair-fed controls. Administration late in the light cycle (4-5 p.m.) depressed epididymal sperm count, seminal vesicle fluid content, and weight of testis, seminal vesicles and epididymis to 40-80% below that seen for pair-fed controls. 24 h after the last treatment, serum testosterone was unchanged in intubated control and low-dose treated rats, compared with untreated controls, but was elevated nearly twofold in medium-dose-treated rats (p less than 0.05). The results indicate that time of cannabinoid administration as well as feeding pattern are critical in studies of the rat reproductive system.

Specific binding of progesterone to the cell surface and its role in the meiotic divisions in Rana oocytes

Progesterone is believed to act at the cell surface to induce the resumption of the meiotic divisions in amphibian oocytes. Analysis of [3H]- and [14C] progesterone uptake and exchange by the plasma-vitelline membrane complex, nucleus and cytoplasm of the isolated Rana oocyte indicates that progesterone uptake by the plasma membrane is saturable, specific and temperature-dependent, and has a slow off-rate. Estradiol (a noninducer) did not compete with progesterone, whereas testosterone (an inducer) blocked progesterone uptake by the membrane complex. Scatchard-type plots indicate an apparent Kd of 5.1.10-7 M over the [progesterone]0 range of 0.01-1.0 microM with maximum binding at about 70 fmol per oocyte. Membrane uptake at higher [progesterone]0 (2-40 microM) indicates apparent cooperative binding, with saturation up to 10 pmol per oocyte. Cytoplasmic uptake was apparently nonspecific and less temperature-dependent than membrane uptake and steroid concentrations (progesterone and pregnanediones) exceeded water solubility by 30-60 min. Nuclear uptake was saturable and specific but uptake was independent of temperature. A comparison of membrane binding and a physiological response (nuclear breakdown) indicated only about 10% of the membrane sites need be filled to initiate a 50% response.

Progesterone-induced down-regulation of electrogenic Na+, K+-ATPase during the first meiotic division in amphibian oocytes

Progesterone initiates the resumption of the meiotic divisions in the amphibian oocyte. Depolarization of the Rama pipiens oocyte plasma membrane begins 6-10 hr after exposure to progesterone (1-2 hr before nuclear breakdown). The oocyte cytoplasm becomes essentially isopotential with the medium by the end of the first meiotic division (20-22 hr). Voltage-clamp studies indicate that the depolarization coincides with the disappearance of an electrogenic Na+, K+-pump, and other electrophysiological studies indicate a decrease in both K+ and Cl- conductances of the oocyte plasma membrane. Measurement of [3H]-ouabain binding to the plasma-vitelline membrane complex indicates that there are high-affinity (Kd = 4.2 x 10-8M), K+-sensitive ouabain-binding sites on the unstimulated (prophase-arrest) oocyte and that ouabain binding virtually disappears during membrane depolarization. [3H]-Leucine incorporation into the plasma-vitelline membrane complex increased ninefold during depolarization with no significant change in uptake or incorporation into cytoplasmic proteins or acid soluble pool(s). This together with previous findings suggest that progesterone acts at a translational level to produce a cytoplasmic factor(s) that down-regulates the membrane Na+, K+-ATPase and alters the ion permeability and transport properties of both nuclear and plasma membranes.