Targeting protein arginine methyltransferase 5 (PRMT5) suppresses radiation-induced neuroendocrine differentiation and sensitizes prostate cancer cells to radiation

Prostate cancer remains the second leading cause of cancer death among American men. Radiation therapy (RT) is a potentially curative treatment for localized prostate cancer, and failure to control localized disease contributes to the majority of prostate cancer deaths. Neuroendocrine differentiation (NED) in prostate cancer, a process by which prostate adenocarcinoma cells transdifferentiate into neuroendocrine-like (NE-like) cells, is an emerging mechanism of resistance to cancer therapies and contributes to disease progression. NED also occurs in response to treatment to promote the development of treatment-induced neuroendocrine prostate cancer (NEPC), a highly-aggressive and terminal stage disease. We previously demonstrated that by mimicking clinical RT protocol, fractionated ionizing radiation (FIR) induces prostate cancer cells to undergo NED in vitro and in vivo. Here, we performed transcriptomic analysis and confirmed that FIR-induced NE-like cells share some features of clinical NEPC, suggesting that FIR-induced NED represents a clinically-relevant model. Further, we demonstrated that protein arginine methyltransferase 5 (PRMT5), a master epigenetic regulator of the DNA damage response and a putative oncogene in prostate cancer, along with its cofactors pICln and MEP50, mediate FIR-induced NED. Knockdown of PRMT5, pICln, or MEP50 during FIR-inhibited NED sensitized prostate cancer cells to radiation. Significantly, PRMT5 knockdown in prostate cancer xenograft tumors in mice during FIR prevented NED, enhanced tumor killing, significantly reduced and delayed tumor recurrence, and prolonged overall survival. Collectively, our results demonstrate that PRMT5 promotes FIR-induced NED and suggests that targeting PRMT5 may be a novel and effective radiosensitization approach for prostate cancer RT.

Loss of smarcad1a accelerates tumorigenesis of malignant peripheral nerve sheath tumors in zebrafish

Malignant peripheral nerve sheath tumors (MPNSTs) are a type of sarcoma that generally originates from Schwann cells. The prognosis for this type of malignancy is relatively poor due to complicated genetic alterations and the lack of specific targeted therapy. Chromosome fragment 4q22-23 is frequently deleted in MPNSTs and other human tumors, suggesting tumor suppressor genes may reside in this region. Here, we provide evidence that SMARCAD1, a known chromatin remodeler, is a novel tumor suppressor gene located in 4q22-23. We identified two human homologous smarcad1 genes (smarcad1a and smarcad1b) in zebrafish, and both genes share overlapping expression patterns during embryonic development. We demonstrated that two smarcad1a loss-of-function mutants, sa1299 and p403, can accelerate MPNST tumorigenesis in the tp53 mutant background, suggesting smarcad1a is a bona fide tumor suppressor gene for MPNSTs. Moreover, we found that DNA double-strand break (DSB) repair might be compromised in both mutants compared to wildtype zebrafish, as indicated by pH2AX, a DNA DSB marker. In addition, both SMARCAD1 gene knockdown and overexpression in human cells were able to inhibit tumor growth and displayed similar DSB repair responses, suggesting proper SMARCAD1 gene expression level or gene dosage is critical for cell growth. Given that mutations of SMARCAD1 sensitize cells to poly ADP ribose polymerase inhibitors in yeast and the human U2OS osteosarcoma cell line, the identification of SMARCAD1 as a novel tumor suppressor gene might contribute to the development of new cancer therapies for MPNSTs.

Protein Arginine Methyltransferase 5 Promotes pICln-Dependent Androgen Receptor Transcription in Castration-Resistant Prostate Cancer

The majority of advanced prostate cancer therapies aim to inhibit androgen receptor (AR) signaling. However, AR reactivation inevitably drives disease progression to castration-resistant prostate cancer (CRPC). Here we demonstrate that protein arginine methyltransferase 5 (PRMT5) functions as an epigenetic activator of AR transcription in CRPC, requiring cooperation with a methylosome subunit pICln. In vitro and in xenograft tumors in mice, targeting PRMT5 or pICln suppressed growth of CRPC cells. Full-length AR and AR-V7 transcription activation required both PRMT5 and pICln but not MEP50. This activation of transcription was accompanied by PRMT5-mediated symmetric dimethylation of H4R3 at the proximal AR promoter. Further, knockdown of PRMT5 abolished the binding of pICln (but not vice versa) to the AR proximal promoter region, suggesting that PRMT5 recruits pICln to the AR promoter to activate AR transcription. Differential gene expression analysis in 22Rv1 cells confirmed that PRMT5 and pICln both regulate the androgen signaling pathway. In addition, PRMT5 and pICln protein expression positively correlated with AR and AR-V7 protein expression in CRPC tissues and their expression was highly correlated at the mRNA level across multiple publicly available CRPC datasets. Our results suggest that targeting PRMT5 or pICln may be explored as a novel therapy for CRPC treatment by suppressing expression of AR and AR splice variants to circumvent AR reactivation. SIGNIFICANCE: This study provides evidence that targeting PRMT5 can eliminate expression of AR and can be explored as a novel therapeutic approach to treat metastatic hormone-naïve and castration-resistant prostate cancer.

PRMT5 Cooperates with pICln to Function as a Master Epigenetic Activator of DNA Double-Strand Break Repair Genes

DNA double-strand break (DSB) repair is critical for cell survival and genome integrity. Upon recognition of DSBs, repair proteins are transiently upregulated to facilitate repair through homologous recombination (HR) or non-homologous end joining (NHEJ). We present evidence that PRMT5 cooperates with pICln to function as a master epigenetic activator of DNA damage response (DDR) genes involved in HR, NHEJ, and G₂ arrest (including RAD51, BRCA1, and BRCA2) to upregulate gene expression upon DNA damage. Contrary to the predominant role of PRMT5 as an epigenetic repressor, our results demonstrate that PRMT5 and pICln can activate gene expression, potentially independent of PRMT5's obligate cofactor MEP50. Targeting PRMT5 or pICln hinders repair of DSBs in multiple cancer cell lines, and both PRMT5 and pICln expression positively correlates with DDR genes across 32 clinical cancer datasets. Thus, targeting PRMT5 or pICln may be explored in combination with radiation or chemotherapy for cancer treatment.

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PRMT5 activates transcription of DSB repair genes upon DNA damage

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pICln cooperates with PRMT5 to activate transcription of DSB repair genes

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Targeting PRMT5 is effective to sensitize multiple cancer types to radiation

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PRMT5 expression positively correlates with DSB repair genes in cancer tissues

X-irradiation reduces lesion scarring at the contusion site of adult rat spinal cord

Spinal cord injury (SCI) results in cell death and tissue destruction, and ultimately cavitation followed by the formation of lesion scars at the injury site. The lesion scars include an astrocytic component (glial scar) and a fibroblastic component (connective tissue scar). The purpose of the present study is to determine if X-irradiation could minimize the formation of lesion scars and reduce the levels of chondroitin sulfate proteoglycans (CSPGs) in the contusion SCI model of the adult rat. Two weeks after SCI, a connective tissue scar formed at the injury site consisting primarily of fibroblasts and exhibits strong CSPG immunoreactivity. The fibroblasts might originate from the connective tissue of pia mater or arachnoid mater. At the same time, reactive astrocytes in the spared tissue accumulate surrounding the lesion cavity to form a thick glial scar with significant enhancement of glial fibrillary acidic protein (GFAP) and CSPG immunoreactivity. After X-irradiation (40 Gy) of the injury site 2 days post-injury, that results in an attenuated dose to the lesion, the connective tissue scar was not observed, and accordingly, almost no CSPG immunoreactivity was detected at this area. Meanwhile, the glial scar and its CSPG immunoreactivity were prominently reduced. X-irradiation did not show significant improvement in locomotor recovery, but resulted in a slight delay of body weight recovery following injury. This preparative treatment could be used to reduce secondary scarring in the lesion resulting in an enriched site for further treatment such as growth related transplantation.

Improved drug delivery to cancer cells: a method using magnetoliposomes that target epidermal growth factor receptors

The toxin delivery system described herein would allow for the selective killing of tumor cells overexpressing the epidermal growth factor receptor (EGFR). Tumor cells often overexpress EGFR, because it allows the cells to divide more quickly. Past delivery systems targeting this receptor have been ineffective due to a lack of specificity that results in harm to surrounding tissue and damage to organs such as the liver. The technique presented here is different, because it presents the possibility of delivering toxin only to the tumor area and almost exclusively to the tumor cells. Delivery is localized to the tumor tissue through the use of EGF conjugated magnetoliposomes. These are liposomes that have magnets imbedded in their bilayer, allowing for selective heating and release of a drug when the magnetoliposome is under an oscillating magnetic field. To create an additional level of specificity, the delivery system will consist of two EGF-bound components that must interact within the endosome of a cell for the toxin to be released. If a tumor cell overexpresses EGFR by 5-fold, then each of its endosomes will have 5 times more receptors than those of a normal cell. Therefore, the tumor cell's endosome has a 5 times greater chance of containing one EGF-bound component and a 25 times greater chance of containing both components. Since both components are necessary for toxin release, the tumor cells will receive 25 times more toxin than the normal cells. Theoretically, it is possible to produce a three or four component system that would deliver 125 or 625 times more toxin to the tumor cells.

Role of M2 domain residues in conductance and gating of acetylcholine receptors in developing Xenopus muscle

1. The contributions of specific residues in gamma- and epsilon-subunits to the developmental changes in conductance and open time of Xenopus muscle acetylcholine receptors (AChRs) were investigated. This study was directed primarily at residues in the M2 domains of gamma- and epsilon-subunits; however, the results of additional mutations in the extracellular region flanking M2 and in the amphipathic region between M3 and M4 are also described. 2. The M2 domains of gamma- and epsilon-subunits differ at only three amino acid residues, two of which are adjacent to each other and located near the narrowest part of the pore. These two residues (NI in gamma, SV in epsilon) were found to be major determinants of the difference in conductance and open time of AChRs bearing gamma- or epsilon-subunits. 3. Mutation of N to S in the gamma-subunit converted the long open time of receptors bearing the gamma-subunit (gamma-AChRs) to the brief open time characteristic of receptors bearing an epsilon-subunit (epsilon-AChRs). Conversely, epsilon-AChRs with SV mutated to NI in the epsilon-subunit exhibited a long open time characteristic of gamma-AChRs. 4. Mutation of N to S in the gamma-subunit increased the conductance of gamma-AChRs but did not confer the full conductance of wild-type epsilon-AChRs. Conversely, mutation of SV to NI in the epsilon-subunit reduced the conductance of epsilon-AChRs, but not completely to the level of wild-type gamma-AChRs.

Transfecting neurons and glia in the rat using pH-sensitive immunoliposomes

Immunoliposomes were constructed using antibody 5-113 (directed to an antigen on the external surface rat glial cells), the antibody Thy 1.1, and a non-immune antibody. The antibodies were conjugated to N-gluytaryl-phosphatidylethanolamine. Liposomes were constructed with these conjugated antibodies, other lipids and a beta-galactosidase plasmid under the control of the cytomegalovirus promoter. When immunoliposomes decorated with one of three different antibodies were injected into the brain or spinal cord of adult rats, the X-gal reaction product was observed in neurons, astrocytes and vascular elements. There was an increase in neuronal labeling when animals were injected with Thy 1.1 conjugated liposomes and there was an increase in glial labeling in animals injected with 5-113 liposomes. In spinal cords, the immunoliposomes appear to penetrate a substantial distance, transfecting neurons several centimeters from the site of delivery. These data suggest that immunoliposomes may provide an effective transfection system for gene delivery in the CNS.

Embryonic expression of motoneuron topography in the rat diaphragm muscle

The phrenic motoneuron pool of the rat projects onto the diaphragm muscle with a distinct rostrocaudal bias. This bias is detectable at birth and is reestablished following denervation. In an effort to define the mechanisms underlying this topographic bias, we asked whether growing phrenic motoneurons select their muscle contacts initially upon first contact or whether the initial neuromuscular distribution is random, to be specified later through synaptic rearrangement. The onset of neurotransmission in embryonic diaphragm muscles aged E-14 to E-18 was studied using focal extracellular microelectrodes. Two important phenomena were observed. First, motoneurons from all three cervical ventral roots (C4, C5, and C6) establish functional innervation at the same time. Second, already at E-15, when the earliest synaptic potentials could be recorded, a distinct rostrocaudal bias was detected. This bias was amplified as innervation progressed to rostral and caudal sectors during E-16 to E-18. These results suggest that growing phrenic motoneurons make topographic choices as they navigate toward their muscle targets. Moreover, the results indicate that further research into the mechanisms for topographic selectivity should focus on initial nerve-muscle contacts in the embryo, rather than secondary processes of error correction.

Sequential expression of acetylcholine receptor isoforms in mesodermalized Xenopus animal caps

Exposure of Xenopus animal pole explants to transforming growth factor beta 2 (TGF-beta 2) induced the sequential expression of muscle nicotinic acetylcholine receptor (AChRs) isoforms and their corresponding mRNAs in cells which normally give rise to ectoderm. Single channel recordings revealed two functional classes of receptors with properties similar to those expressed during normal development of skeletal muscle in vivo. The predominant class of receptors in all patches corresponded to those of embryonic myotomal muscle. Additional receptors resembling those of mature myotomal muscle were observed in older explants. Levels of transcripts encoding the embryonic and adult AChR subunit isoforms varied accordingly. TGF-beta 2 appears to initiate a developmental program of AChR gene expression which is similar to that found in normally developing muscle.

Delivery of plasmid DNA to glial cells using pH-sensitive immunoliposomes

Immunoliposomes were constructed with an antibody specific to glial cells. They were used to examine the specificity and efficacy of cell type plasmid transfection. Liposomes contained a beta-galactosidase gene under control of an SV-40 promotor. Two different monoclonal antibodies of a different subclass, IgM and IgG, were examined for their targeting ability using immunoliposomes. Cultured C6 glioma (specific target cell type) and NIH 3T3 (control cell type, fibroblast) cells were transfected using these immunoliposomes. Results indicate a three-fold increase in transfection by the glial specific immunoliposomes, "gliasomes", in glial cell culture over control liposomes. Gliasomes were exposed to NIH 3T3 cells and showed no enhanced transfection over control liposomes. Gliasomes were tested for their specificity by the addition of excess antibody to the cell culture in order to saturate specific receptors on C6 glioma cells. Results indicate a reduced transfection, nearly three-fold, in cells that were saturated with excess antibody prior to exposure to the immunoliposomes.

Evaluation of neurosyphilis in human immunodeficiency virus-infected individuals

The diagnosis of neurosyphilis in patients infected with the human immunodeficiency virus (HIV) remains problematic. We examined the use of the Treponema pallidum hemagglutination (TPHA) index and quantitative tests of CSF by means of microhemagglutination-T. pallidum for diagnosis of neurosyphilis in 58 HIV-infected persons with latent syphilis who had not recently received therapy for syphilis. Five patients (9%) had reactive CSF VDRL tests and thus had proven neurosyphilis. For 13 patients (22%), CSF findings were normal and revealed no evidence of neurosyphilis. For 40 patients (69%), abnormal CSF findings were characteristic of neurosyphilis, but their CSF VDRL tests were nonreactive. Twenty-five of the 40 patients with possible neurosyphilis had pleocytosis and elevated CSF levels of protein and/or IgG. Five (12.5%) of these 40 patients had positive TPHA indices that indicated intrathecal antitreponemal antibody production, a finding that provided greater support for the diagnosis of active neurosyphilis. With use of the TPHA index, patients with CSF abnormalities can be better classified in regard to their need for therapy for neurosyphilis.

Junctional acetylcholine receptor channel open time is not presynaptically regulated in developing muscle

The role of motor innervation in controlling the development of acetylcholine receptor (AChR) channel open time was tested by examining synaptic current durations in transplanted muscles of Xenopus tadpoles. The presumptive lower jaw region, which gives rise to the interhyoideus muscle, was transplanted to the tail, overlying the myotomal muscle cells. The transplanted muscles became innervated, presumably by spinal nerves which normally innervate myotomal muscle. Despite development in the presence of foreign innervation, synaptic currents in the transplanted interhyoideus were predominantly long in duration and resembled those in the normally innervated interhyoideus. They did not resemble those in the myotomal muscle, where synaptic currents are brief. The apparent lack of neural influence on development of AChR function in muscle contrasts with the evidence for presynaptic control of AChR open time in frog sympathetic ganglia. This may reflect a fundamental difference between nerve and muscle in the regulation of postsynaptic function.

Multiple conductance classes of mouse nicotinic acetylcholine receptors expressed in Xenopus oocytes

Acetylcholine receptor (AcChoR) subunit mRNAs transcribed from mouse BC3H-1 cDNAs were injected into Xenopus oocytes and the expressed AcChoR channels were examined by single channel recording. Injection of alpha-, beta-, gamma-, and delta-subunit mRNAs produced two predominant channel classes with conductances of approximately 50 and approximately 12 pS, while infrequent openings of approximately 25-pS channels were also observed. Injection of alpha-, beta-, and gamma-subunit mRNAs produced a single class of approximately 12-pS AcChoR channels, which resembled the smallest conductance channels present in alpha beta gamma omega-injected oocytes. Assembly of delta-less channels may thus explain the lowest conductance AcChoR channels in alpha beta gamma delta-injected oocytes and might also account for similar channels that have been observed in vertebrate skeletal muscle.

Expression of nicotinic acetylcholine receptors in aneural Xenopus embryos

During gastrulation in vertebrate embryos, the mesoderm moves inward and under the ectoderm and these two cell layers subsequently differentiate in close proximity to each other, providing an opportunity for the exchange of inductive signals. This study examines whether the activation of muscle nicotinic acetylcholine receptor (AChR) genes and the subsequent expression of receptors in Xenopus myotomal muscle are dependent on interaction between the ectoderm and the mesoderm, or their derivatives, after the onset of gastrulation. We eliminated such interaction by inducing total exogastrulation of Xenopus embryos. During exogastrulation, the mesoderm moves away from the ectoderm, and the nervous system fails to develop. Single channel recordings from the myotomal muscle of exogastrulated embryos revealed the presence of two major classes of AChRs, which could be distinguished on the basis of channel conductance. The current amplitudes, conductances, reversal potentials, and open times of these channels closely resembled those reported for the two major classes of AChR channels normally expressed in vivo. We conclude that interaction between ectoderm and mesoderm following the onset of gastrulation is not required for the future expression of the major classes of AChRs in myotomal muscle.

Three conductance classes of nicotinic acetylcholine receptors are expressed in developing amphibian skeletal muscle

Two previously described classes of nicotinic AChRs in vertebrate skeletal muscle have conductances of 40 and 60 pS. In addition, a third conductance class of AChR channels is present in developing Xenopus muscle. This class appears to represent an independent channel type, rather than a subconductance state of the larger conductance channels. The channel has a slope conductance of 25 pS and a reversal potential of about 0 mV membrane potential. Its kinetic properties resemble those of the 40 pS channels present in early embryonic myotomal muscle. The channel has a mean open time of about 6 msec (at 40 mV applied potential). The open time is dependent on membrane potential and increases e-fold for every 60 mV of hyperpolarization. Consecutive openings were often separated by brief closures of about 0.4 msec in duration. The identity of the channel as a nicotinic AChR was established by blocking the channel openings with alpha-BTX and by demonstrating bursting and desensitization in the presence of high agonist concentrations. In some muscles (e.g., extraocular), this channel may be a predominant form at early developmental stages and could therefore be important to the function of developing synapses in those muscles.

In vivo development of nicotinic acetylcholine receptor channels in Xenopus myotomal muscle

The development of acetylcholine receptor (AChR) channel function in Xenopus myotomal muscle was studied by single-channel recordings from cell-attached patches of nonjunctional membrane in vivo. AChR channels were studied from the time of their first appearance on the muscle membrane until the time of full maturity of the muscle. Two predominant amplitude classes of AChR channels were observed with slope conductances of about 40 and 60 pS. During the first day after their initial appearance on the membrane, the small-conductance channels were the most numerous class on the muscle membrane. The large-conductance channels then began to be expressed in significant numbers and, over the next 2 d, became the predominant channel type. The large-conductance class had an apparent mean open time of approximately 0.7 msec at resting potential, which remained constant throughout development. The small channel initially had an apparent open time of approximately 3 msec at resting potential, which decreased during development by about 50%. The decrease in open time of the small channel was correlated in time with the increased expression of the large-conductance channels. Openings of the large-conductance channels were generally separated by closed intervals of more than 1 msec, whereas openings of the small-conductance channels were commonly interrupted by brief gaps of about 0.2 msec duration. The duration of the brief gaps did not change during development.

Comparative development of end-plate currents in two muscles of Xenopus laevis

The development of miniature end-plate currents (m.e.p.c.s) was studied in the superior oblique and interhyoideus muscles of Xenopus laevis. An analysis of m.e.p.c. decays shows that each muscle possesses its own characteristic programme of end-plate current development. In the superior oblique, the exponential decay constants of m.e.p.c.s were initially about 3 ms; they declined within half a day to 1 ms and remained at that value for six weeks. They then gradually became longer, reaching a mean value of 1.7 ms at late metamorphosis. In the interhyoideus, m.e.p.c. decay constants were initially about 6 ms. They declined in less than one day to a mean value of 2.6 ms and remained there for the following seven weeks. Upon completion of metamorphosis, the decay constants underwent a further decrease to about 1 ms. In both muscles, the changes in m.e.p.c. decays were correlated with developmental changes in muscle contraction speeds, as measured by maximum twitch frequencies. The above changes in end-plate currents in the superior oblique and interhyoideus muscles are discussed in terms of the development of acetylcholine receptor channel gating and acetylcholinesterase activity.

Endocytosis precedes dissolution of basic calcium phosphate crystals by murine macrophages

Murine peritoneal macrophages were incubated with 45Ca-labeled basic calcium phosphate (BCP) crystals in the presence or absence of cytochalasin B. Untreated macrophages solubilized 30-50% of 45Ca-BCP in 24 hours. Dissolution began within 3 hours and was linear thereafter. Twenty-three percent of BCP was cell-associated by 3 hours. Endocytosis of crystal occurred continuously throughout the incubation. Endocytosis of crystal did not affect the migration of macrophages through Percoll density gradients. Addition of cytochalasin B did not prevent cell-association of BCP, but inhibited solubilization in a dose-dependent manner. Virtually all cell-associated BCP was removed when cytochalasin B-treated cells were washed with EDTA, suggesting that the crystals were only bound to the surface. In contrast, cell-associated BCP in untreated cells was only partially removed by EDTA, suggesting that endocytosis of crystal had occurred. We conclude that cell association of BCP is not sufficient for its dissolution, and that endocytosis precedes solubilization of BCP crystals by macrophages.

Development of synaptic currents in immobilized muscle of Xenopus laevis

The effect of chronic immobilization on the development of synaptic currents was studied in myotomal muscle of Xenopus laevis. Embryos and tadpoles were immobilized by rearing them in the presence of tetrodotoxin (TTX) after removal of the egg membranes. Immobilization did not affect the developmental change in duration of miniature end-plate currents (m.e.p.c.s). Rise times decreased from about 3 to 0.6 ms in both immobilized and control muscle, and decay constants decreased from about 7 to 1 - 2 ms in both conditions. M.e.p.c.s with double exponential decays were recorded in both immobilized and control muscle at intermediate and late developmental stages. The fast and slow decay constants were 0.7 ms and slightly less than 3 ms in older muscle of both groups. These values are comparable to the apparent open times of fast and slow ACh receptors present on Xenopus muscle. Application of an anticholinesterase (methanesulphonyl fluoride) lengthened the duration of m.e.p.c.s comparably in immobilized and control muscle. These data indicate that the deposition of junctional acetylcholinesterase and the reduction in open time of acetylcholine receptor channels in developing Xenopus myotomal muscle are independent of contractile activity of muscle and TTX-blockable action potentials in muscle or motoneurones.

Effect of litter size on blood haematocrit and liveweight in Booroola Merino and Merino ewes during pregnancy and the post-partum period

Blood haematocrit and liveweight were determined throughout pregnancy and the post-partum period in 217 Booroola Merino and Merino ewes in order to relate these parameters to litter size at birth. In pregnant ewes, haematocrit declined from three until five months gestation, rose immediately after parturition then declined until two months post-partum. During the third to fifth month of gestation, haematocrit decreased in proportion to litter size. Nonpregnant ewes, measured at similar intervals, did not show the same pattern. Haematocrit of nonpregnant animals was higher than that of triplet-bearing ewes at three, four and five months gestation, but was only significantly different to single- and twin-bearing ewes at five months. The liveweight of pregnant ewes increased up to parturition and then declined until two months post-partum. The liveweight of nonpregnant ewes increased over the experimental period. It was concluded that the number of foetuses a ewe carried had significant effects on the decline in haematocrit during pregnancy. Haematocrit was not a precise indicator of litter size in sheep. Haematocrit, ewe liveweight and ovulation rate together in a multiple regression only accounted for 37% of the variation in litter size.

Energetics of coupled Na+ and Cl- entry into epithelial cells of bullfrog small intestine

Na+, K+ and Cl- concentrations (cij) and activities (aij), and mucosal membrane potentials (Em) were measured in epithelial cells of isolated bullfrog (Rana catesbeiana) small intestine. Segments of intestine were stripped of their external muscle layers, and bathed (at 25 degrees C and pH 7.2) in oxygenated Ringer solutions containing 105 mM Na+ and Cl- and 5.4 mM K+. Na+ and K+ concentrations were determined by atomic absorption spectrometry and Cl- concentrations by conductometric titration following extraction of the dried tissue with 0.1 M HNO3. 14C-labelled inulin was used to determine extracellular volume. Em was measured with conventional open tip microelectrodes, aiCl with solid-state Cl-selective silver microelectrodes and aiNa and aiK with Na+ and K+-selective liquid ion-exchanger microelectrodes. The average Em recorded was -34mV. ciNa, ciK and ciCl were 51, 105 and 52 mM. The corresponding values for aiNa, aiK and aiCl were 18, 80 and 33 mM. These results suggest that a large fraction of the cytoplasmic Na+ is 'bound' or sequestered in an osmotically inactive form, that all, or virtually all the cytoplasmic K+ behaves as if in free solution, and that there is probably some binding of cytoplasmic Cl-. aiCl significantly exceeds the level corresponding to electrochemical equilibrium across the mucosal and baso-lateral cell membranes. Earlier studies showed that coupled mucosal entry of Na+ and Cl- is implicated in intracellular Cl- accumulation in this tissue. This study permitted estimation of the steady-state transapical Na+ and Cl- electrochemical potential differences (deltamuNa and deltamuCl). deltamuNa (-7000 J . mol-1; cell minus mucosal medium) was energetically more than sufficient to account for deltamuCl (1000--2000 J . mol-1).

Glucose metabolism in isolated fat cells: enhanced response of larger adipocytes from older rats to epinephrine and adrenocorticotropin

The effects of insulin and of two lipolytic hormones (epinephrine and ACTH1) on the rate and pattern of glucose metabolism were compared during incubation of isolated fat cells, obtained from epididymal fat pads of rats of varying age and degrees of adiposity. Glucose metabolism and the intracellular free fatty acid levels were expressed on a per cell basis and in relation to adipocyte size. The data for total glucose metabolism show that, in contrast to the declining insulin effect observed with adipocyte enlargement, the stimulation of glucose uptake and metabolism by these lipolytic hormones was significantly greater in the larger fat cells from the older fatter rats than in the smaller ones from the younger leaner rats. Lipolytic hormones suppressed, whereas insulin enhanced, fatty acid synthesis; moreover the lipolytic hormones stiumlated glucose ce effect of epinephrine on the intracellular free fatty acid levels was greater in the small fat cells than in the large ones; this effect of epinephrine was markedly curtained by the presence of glucose in the incubation medium, making it unlikely that acceleration of glucose metabolism by the lipolytic stimulus was mediated by an elevation of the intracellular free fatty acid level. The present results show a markedly enhanced capacity of the large adipocytes to accelerate glucose metabolism in response to these liplytic hormones. Thus, in contrast to prevailing notions of declining hormonal responsiveness with expanding fat cell size in older and more obese animals, this study documents an instance of increased hormonal response in enlarged adipocytes and points to the need for a more comprehensive reevaluation of the various hormonal effects in adipocytes of different size.

Metabolic patterns and insulin responsiveness of enlarging fat cells

The rate and pattern of glucose metabolism, basal lipolysis, and intracellular concentration of free fatty acids were determined in isolated epididymal fat cell preparations (mean volume 30-800 pl) from rats on the basis of fat cell number and in relation to the cell volume. The effects of increasing glucose concentrations in the medium and of insulin on the cellular metabolic activities were compared. Expanding fat cell volume correlated positively and significantly (P < 0.001) with the synthesis of glyceride glycerol from glucose (correlation coefficient, r = 0.919), with rates of basal lipolysis (r = 0.663), and with intracellular free fatty acid accumulation (r = 0.796); it correlated negatively and significantly with glucose conversion to glyceride fatty acids (r = -0.814, P < 0.01). The differences in patterns of glucose metabolism and basal lipolysis between small (<100 pl) and large (>400 pl) fat cells were not modified by insulin or by increments in glucose concentration. The results indicate that the reduced capacity of the large fat cells to respond to insulin cannot be attributed solely to a limited capacity of the cells to take up and metabolize increasing amounts of glucose. The acquired unresponsiveness of the large cells to insulin may result from an alteration in the mechanism of action of insulin and may be related to an intracellular metabolic derangement with increased basal lipolysis, free fatty acid accumulation, and accelerated glyceride synthesis resulting from the accumulation of triglyceride.