Effects of aging and caloric restriction on mitochondrial energy production in gastrocnemius muscle and heart

Mitochondria are chronically exposed to reactive oxygen intermediates. As a result, various tissues, including skeletal muscle and heart, are characterized by an age-associated increase in reactive oxidant-induced mitochondrial DNA (mtDNA) damage. It has been postulated that these alterations may result in a decline in the content and rate of production of ATP, which may affect tissue function, contribute to the aging process, and lead to several disease states. We show that with age, ATP content and production decreased by approximately 50% in isolated rat mitochondria from the gastrocnemius muscle; however, no decline was observed in heart mitochondria. The decline observed in skeletal muscle may be a factor in the process of sarcopenia, which increases in incidence with advancing age. Lifelong caloric restriction, which prolongs maximum life span in animals, did not attenuate the age-related decline in ATP content or rate of production in skeletal muscle and had no effect on the heart. 8-Oxo-7,8-dihydro-2'-deoxyguanosine in skeletal muscle mtDNA was unaffected by aging but decreased 30% with caloric restriction, suggesting that the mechanisms that decrease oxidative stress in these tissues with caloric restriction are independent from ATP availability. The generation of reactive oxygen species, as indicated by H2O2 production in isolated mitochondria, did not change significantly with age in skeletal muscle or in the heart. Caloric restriction tended to reduce the levels of H2O2 production in the muscle but not in the heart. These data are the first to show that an age-associated decline in ATP content and rate of ATP production is tissue specific, in that it occurs in skeletal muscle but not heart, and that mitochondrial ATP production was unaltered by caloric restriction in both tissues.

Selection of lymphocyte gating protocol has an impact on the level of reliability of T-cell subsets in aging specimens

BACKGROUND

In the past decade, human immunodeficiency virus (HIV) lymphocyte immunophenotyping has evolved significantly. New fluorochromes, new multicolor reagents, enhanced instruments, and the capacity to provide absolute cell counts using the single-platform technique have all contributed to the reliability of T-cell subset measurements. In this study, four gating protocols were evaluated to select the most robust method for T-cell subset enumeration.

METHODS

Peripheral blood specimens from 21 HIV(+) and 20 HIV(-) individuals were monitored up to 96 h. Aliquots of specimens were stored at room temperature and analyzed at 6 (baseline), 48, 72, and 96 h. Aliquots were stained with CD45-fluorescein isothiocyanate (FITC)/CD3PC5/CD4RD1/CD8ECD. Data analysis was performed with all four gating protocols.

RESULTS

Only with fresh blood did all protocols provide similar results. From samples that were 48 h old, the choice of gating strategy had a dramatic impact on immunophenotyping results. The largest deviations from baseline values occurred at 96 h and gating protocols that included dual light scatter gates provided the greatest shift of T-cell subset values over time. The gating protocols that were based exclusively on cell lineage-specific gates gave the most robust T-cell values up to 96 h.

CONCLUSION

By selecting the appropriate gating protocol, the temporal integrity of specimens can be extended up to 4 days.

Apoptosis and exercise

This brief review will discuss an exciting new area in exercise science, namely the role of apoptosis or programmed cell death in exercise. Apoptotic cell death differs morphologically and biochemically from necrotic cell death, although both appear to occur after exercise. Accelerated apoptosis has been documented to occur in a variety of disease states, such as AIDS and Alzheimer's disease, as well as in the aging heart. In striking contrast, failure to activate this genetically regulated cell death may result in cancer and certain viral infections. We will discuss factors that may activate apoptosis during and after exercise and the importance of cell turnover after exercise. We will also discuss differences in apoptosis between lymphocyte and skeletal muscle cells. We speculate that exercise-induced apoptosis is a normal regulatory process that serves to remove certain damaged cells without a pronounced inflammatory response, thus ensuring optimal body function.

Loss of myometrial oxytocin receptors during oxytocin-induced and oxytocin-augmented labour

Oxytocin is used widely for the induction and augmentation of labour, but there is little information about the dynamics of oxytocin receptors in human myometrium during parturition, and the possible effect of oxytocin infusion. This information is important because G protein-coupled receptors, such as the oxytocin receptor, undergo desensitization after prolonged or repeated stimulation. The concentration of myometrial oxytocin receptors and the steady state of its mRNA were measured in patients undergoing Caesarean sections before or during spontaneous or induced labour. The concentration of receptors before labour was 477 (175-641) fmol mg(-1) protein (median, quartile range), and decreased to 140 (72-206; P < 0.05) and 118 (69-75; P < 0.01) fmol mg(-1) protein during prolonged oxytocin-augmented and oxytocin-induced labour, respectively. The corresponding oxytocin receptor mRNA concentrations decreased by 60- and 300-fold, respectively. The decrease in receptor binding and mRNA in women receiving oxytocin infusion indicates that homologous receptor desensitization occurs in vivo.

Loss of myometrial oxytocin receptors during oxytocin-induced and oxytocin-augmented labour

Oxytocin is used widely for the induction and augmentation of labour, but there is little information about the dynamics of oxytocin receptors in human myometrium during parturition, and the possible effect of oxytocin infusion. This information is important because G protein-coupled receptors, such as the oxytocin receptor, undergo desensitization after prolonged or repeated stimulation. The concentration of myometrial oxytocin receptors and the steady state of its mRNA were measured in patients undergoing Caesarean sections before or during spontaneous or induced labour. The concentration of receptors before labour was 477 (175-641) fmol mg(-1) protein (median, quartile range), and decreased to 140 (72-206; P < 0.05) and 118 (69-75; P < 0.01) fmol mg(-1) protein during prolonged oxytocin-augmented and oxytocin-induced labour, respectively. The corresponding oxytocin receptor mRNA concentrations decreased by 60- and 300-fold, respectively. The decrease in receptor binding and mRNA in women receiving oxytocin infusion indicates that homologous receptor desensitization occurs in vivo.

Adenylyl cyclase isoforms in pregnant and non-pregnant human myometrium

The precise factors involved in the transition of the relaxed pregnant uterus to the contractile state at the onset of parturition remain unclear, but it is accepted that cAMP-generating pathways contribute to uterine relaxation. We have previously reported an increased expression of the adenylyl cyclase (AC)-stimulating protein Galphas in human myometrium during gestation, with a corresponding increase in GTP-stimulated AC activity. However, little is known about the predominating AC isoforms expressed during pregnancy. This information is important, because although all AC isoforms are stimulated by Galphas, their regulation by other signalling molecules is very different. In the present study we have identified the isoforms of AC expressed in both pregnant and non-pregnant myometrium by mRNA analysis and immunoblotting. mRNA encoding for AC I, II, III, VIII and IX was present in non-pregnant and pregnant myometrium, and in cultured myometrial cells. Differing levels of AC protein could be detected in myometrial plasma membranes, with decreased levels of Group 1 (isoforms I, III and VIII) and Group 4 (IX) ACs allied with increased levels of Group 2 (II, IV and VII) and 3 (V and VI) ACs during pregnancy. These findings imply a role for Group 2-activating pathways, e.g. G-protein betagamma-subunits and protein kinase C, in the maintenance of uterine quiescence, whilst suggesting a lesser involvement of calcium-calmodulin complex, an activator of Group 1 AC isoforms, in uterine relaxation during gestation. These data may provide an alternative pharmacological approach for the attenuation of preterm labour.

Expression of G-protein-coupled receptor kinases in pregnant term and non-pregnant human myometrium

There is evidence for hormonal receptor desensitisation in human myometrium, but little is known about the mechanisms involved in the loss of myometrial response to agonists such as beta(2)-adrenergic agonists, prostaglandin gamma and oxytocin. It is well known that the receptors for these hormones are coupled to G-proteins. The first step of receptor desensitisation is the phosphorylation of activated receptors by a G-protein-coupled receptor kinase (GRK). GRKs are members of a multigene family and the various subtypes differ in their localisation, regulation and mode of action. We have used Western blotting and reverse transcription PCR to identify the GRKs present in human myometrium from pregnant and non-pregnant women as well as in cultured human myometrial cells. We have found that human myometrium expresses the GRK subtypes 2, 4gamma, 5 and 6. On the other hand, GRK3 and the isoforms GRK4alpha, beta and delta were not found in myometrial tissue. Our data indicate that GRK2 is only expressed in pregnant term myometrium and is not found in non-pregnant tissue. Moreover, GRK6 appears to be expressed at a much higher level in pregnant term tissue than in non-pregnant myometrium. Our observations suggest that GRK2 and GRK6 may contribute to the regulation of uterine contractility at term. Further work is necessary to determine whether GRKs and receptor desensitisation play a role in disorders of uterine contractility.

Bromocriptine/SKF38393 treatment ameliorates dyslipidemia in ob/ob mice

Our previous studies have shown that the dopaminergic D1 receptor agonist SKF38393 (SKF) plus the D2 receptor agonist bromocriptine (BC) act synergistically to reduce obesity in obese C57BL/6J (ob/ob) mice. The present study investigated the effects of this combination on dyslipidemia in ob/ob mice. Female ob/ob mice were treated daily with vehicle or SKF (20 mg/kg body weight [BW]) plus BC (16 mg/kg BW [BC/SKF]) for 14 days. The animals were then used for the characterization of plasma lipoprotein profiles, hepatic triacylglycerol synthesis and secretion, adipocyte lipolysis, adipose and muscle lipoprotein lipase (LPL) activity, and muscle triglyceride (TG) content. The treatment significantly reduced plasma glucose 54%, TG 41%, cholesterol 21%, phospholipid 20%, and free fatty acid (FFA) 36% (P < .01). Hepatic triacylglycerol synthesis was 55% lower in treated mice versus control mice (P < .01). The cell size of isolated adipocytes was significantly reduced (41%) by treatment. LPL activity was increased in soleus skeletal muscle (25%, P < .05) but was sharply reduced in adipose tissue (91%, P < .01) in treated versus control mice. The TG content of hindlimb muscle was about 49% lower in treated versus control mice (P < .05). The basal and isoproterenol-stimulated lipolytic rate was decreased (approximately 53%) in adipocytes from treated animals compared with the control (P < .01). In conclusion, BC/SKF normalized the hypertriglyceridemia likely via its simultaneous antilipogenic action in liver tissue and antilipolytic action in adipose tissue. Decreased plasma flux of FFA partially contributed to the reduced hepatic lipogenesis, plasma very-low-density lipoprotein (VLDL)-TG, and TG in skeletal muscle. The above-described effects of BC/SKF treatment are largely independent of its effect to normalize hyperphagia in ob/ob mice.

Biochemical mechanisms responsible for the attenuation of diabetic and obese conditions in ob/ob mice treated with dopaminergic agonists

OBJECTIVE

We previously reported that a two week treatment with SKF 38393 (SKF, a dopamine D1 receptor agonist), plus bromocriptine (BC, a dopamine D2 receptor agonist) acted synergistically to normalize hyperphagia, body fat, hyperglycaemia and hyperlipidaemia in ob/ob mice. The present study further investigates the biochemical mechanisms triggered by this drug treatment.

DESIGN

Six week old female C57BL/6J ob/ob mice were divided into three groups and treated for two weeks with either BC and SKF, vehicle (control), or vehicle and pair fed to match the drug-treated group's daily food intake.

RESULTS

BC/SKF treatment reduced food consumption by 55%, and treated mice weighed less than either pair fed or ad libitum fed controls after two weeks of treatment. Moreover, oxygen consumption was increased by 2.4-fold and the respiratory quotient (RQ) decreased from 1.23 to 0.96 (indicating a reduction in de novo lipogenesis) by drug treatment relative to ad libitum fed controls, but these parameters were unaffected by pair feeding control mice. The treatment also reduced blood glucose and free fatty acids (FFA) relative to pair fed and ad libitum fed controls. BC/SKF treatment (but not pair feeding) concurrently reduced lipolysis, lipogenic enzyme activities and hepatic gluconeogenic enzyme activities. Treatment also increased hepatic concentrations of glycogen and xylulose-5-phosphate (X-5-P), a key stimulator of glycolysis. Finally, BC/SKF, but not pair feeding, reduced the circulating concentrations of thyroxine and corticosterone, two hormones known to increase lipolysis, lipogenesis and hyperglycaemia. Drug treatment also increased serum dehydroepiandrosterone (DHEA) sulfate concentrations, an inhibitor of body fat store accumulation.

CONCLUSION

These findings demonstrate that BC/SKF treatment not only normalizes hyperphagia of ob/ob mice, but also redirects several metabolic and endocrine activities, independent of its effects on feeding to improve the obese-diabetic syndrome in ob/ob mice.

Levels of corticotrophin-releasing hormone receptor subtype 1 mRNA in pregnancy and during labour in human myometrium measured by quantitative competitive PCR

It is suggested that corticotrophin-releasing hormone (CRH) is involved in parturition. We have previously reported the presence of the CRH receptor subtype 1 (CRH R1) in human uterine myocytes. The aim of the present study was to investigate whether expression of the CRH R1 in myometrial tissue changes in pregnancy and labour. We used a quantitative competitive PCR method to measure the mRNA levels of this receptor in non-pregnant and in term pregnant myometrium before and at different stages of labour. The levels of mRNA for the housekeeping gene for glucocerebrosidase (GCB) were also determined. The results were expressed as a ratio of CRH R1 and GCB mRNA levels. We have found that in pregnancy the CRH R1 is down-regulated from a ratio of 0.093+/-0.011 in non-pregnant myometrium to 0.012+/-0.005 (P<0.001) in term non-labouring myometrium. No significant changes were observed in the CRH R1:GCB ratio in tissues sampled within 13 h (0.013+/-0.004) from the start of labour. In summary, normalised levels of CRH R1 are down-regulated in pregnancy and do not change during labour. We speculate that our results do not support a direct role for the CRH R1 receptor in myometrial stimulation.

Impact of unified procedures as implemented in the Canadian Quality Assurance Program for T lymphocyte subset enumeration. Participating Flow Cytometry Laboratories of the Canadian Clinical Trials Network for HIV/AIDS Therapies

The objective of the Canadian Quality Assurance Program (CQAP) is to provide the most reproducible and accurate T-cell subset enumeration for individuals living with HIV who are enrolled in the Canadian Clinical Trial Network for Human Immunodeficiency Virus (HIV) and Acquired Immune Deficiency Syndrome (AIDS) Therapies (abbreviated as CTN). The Canadian National Laboratory for Analytical Cytology, within the Laboratory Centre for Disease Control, is part of the Health Protection Branch of Health Canada. For the past eight years, the Laboratory for Analytical Cytology has been responsible for delivering a bilingual quality assurance program for CD4 T-cell enumeration. This federal program, which integrates biotechnology transfer with quality assessment, was achieved through the organization of workshops focused on technology transfer and essential skill-building techniques. Two training sessions were conducted for the CTN flow cytometer operators. The first introduced the concept of window of analysis, to demonstrate the practical benefits of unified quantitative fluorescent measurement. As a follow-up to the first workshop, participants performed a series of quantitative assays that monitored the expression of CD69, an early activation marker. This quantitative fluorescence protocol was performed with acceptable inter-laboratory variation using modified commercial kits. The second workshop focused on a absolute count method based on a single platform. Four preserved whole-blood preparations were tested with this approach. The combined effort reduced inter-laboratory variation. The direct impact was monitored as related to the frequency of participation. Over the years, the standard deviation of average accumulated variation decreased dramatically with increased frequency of participation, from 10% to <4%.

Desensitization of oxytocin receptors in human myometrium

In the present study, we investigated the possible mechanisms by which oxytocin might regulate oxytocin receptor (OTR) density. Exposure of cultured myometrial cells to oxytocin for a prolonged period caused desensitization: the steady-state level of oxytocin binding was 210 x 10(3) binding sites/cell, but this was time-dependently reduced to 20.1 x 10(3) sites/cell by exposing the cells to oxytocin for up to 20 h. In contrast, Western blotting data showed that the total amount of OTR protein was not affected by oxytocin treatment for up to 24 h. Flow cytometry experiments demonstrated that OTRs were not internalized during this treatment. However, RNase protection assays and Northern analysis showed that in cultured myometrial cells OTR mRNA was reduced by oxytocin treatment to reach a new low steady-state concentration. Analysis of this mRNA in myometrial biopsies from 17 patients undergoing emergency Caesarean section showed how it decreased with advancing labour. Samples obtained after 12 h of labour contained approximately 50 times less OTR mRNA than samples obtained from patients in labour for less than 12 h. We speculate that this decrease in OTR mRNA represents in-vivo OTR desensitization.

Urocortin in pregnancy

OBJECTIVES

The aim of this study was to investigate the possibility that urocortin is the ligand that displaces corticotropin-releasing hormone from its binding protein in the maternal circulation during pregnancy and, if so, to determine whether urocortin, like corticotropin-releasing hormone, is synthesized in substantial quantities in the placenta.

STUDY DESIGN

A radioimmunoassay specific for urocortin was developed and used for measurement of the peptide in chorionic villi and fetal membranes (amnion and chorion) from normal and preeclamptic pregnancies. These tissues were also assayed for corticotropin-releasing hormone. Assays for urocortin were also carried out on normal term pregnant and nonpregnant myometrium and on plasma from nonpregnant individuals, and assays for both peptides were performed on sequential normal pregnancy plasma samples taken from mid gestation until term.

RESULTS

Corticotropin-releasing hormone was present in normal term (1904 +/- 489 pg/g) and preeclamptic placentas (5897 +/- 1526 pg/g) and in normal term fetal membranes (645 +/- 155 pg/g, n = 6 in all cases). Urocortin was not detected in any of the tissues studied, nor was it found in the normal human plasma samples. Unlike the situation for corticotropin-releasing hormone, no pregnancy-related pattern was seen for urocortin in the plasma from pregnant women.

CONCLUSIONS

Urocortin is not translated to any great extent in the pregnancy tissues investigated, nor is it present in the circulation of pregnant women in detectable amounts. Furthermore, it is unlikely that urocortin is responsible for the high maternal plasma levels of free corticotropin-releasing hormone circulating in the latter stages of pregnancy, but this does not preclude the possibility that another, as yet uncharacterized, corticotropin-releasing hormone-like peptide may be.

Prostaglandin E receptors in myometrial cells

Prostaglandins (PGs) exert their effects via binding to specific cell surface receptors and influencing second messenger systems through G-proteins. PGE2 may interact with at least four receptor subtypes (EP1, EP2, EP3, EP4), each showing different pharmacological profiles. The second messengers calcium, inositol phosphates (InsPs) and cyclic nucleotides play decisive roles in uterine contractility. The question in this investigation was, which EP receptors, G-proteins and second messenger systems transmit PGE2 induced signals in human myometrium. We have measured changes in InsPs and cAMP formation and also in intracellular calcium concentration ([Ca2+]i) induced by PGE2 and receptor subtype selective analogues in cultured human myometrial cells. PGE2 increased cAMP level and this effect was shared by the EP2 receptor subtype selective agonist Butaprost and by Misoprostol (EP3 > EP2 > EP1). Sulprostone (EP3 > EP1) did not stimulate adenylyl cyclase activity per se, but inhibited forskolin-stimulated adenylyl cyclase in a pertussis toxin (PT) sensitive way. PGE2, GR63799X (EP3 selective), Sulprostone and Misoprostol activated phospholipase-C (PLC), this effect was resistant to PT treatment. PGE2 also elevated [Ca2+]i from the resting level of 60-90 nM up to 350 nM. Low concentrations (1-300 nM) of PGE2 increased [Ca2+]i without PLC activation. The selective EP1 inhibitor AH6809, Nifedipine, Verapamil and PT treatment inhibited this effect of PGE2. In cultured human myometrial cells PGE2 interacts with EP1 receptors, which elevate [Ca2+]i independently from PLC, but involving a Gi protein and plasmamembrane calcium channels; EP2 receptors which stimulate adenylyl cyclase; EP3A receptors, which inhibit adenylyl cyclase activity through Gi activation and EP3D receptors which activate PLC through a PT-insensitive pathway and also elevate [Ca2+]i.

Expression of corticotrophin-releasing hormone receptor mRNA and protein in the human myometrium

The reported effects of corticotrophin-releasing hormone (CRH) on human myometrium support the existence of specific receptors for the hormone in this tissue. We have used the reverse transcriptase-polymerase chain reaction (RT-PCR) technique to study the expression of mRNA coding for the CRH R1 and R2 receptors. RT-PCR of total RNA from both nonpregnant and pregnant myometrium using specific primers resulted in amplification products of the expected sizes for the R1 alpha and R2 alpha CRH receptors. The identity of these amplification products was confirmed by specific restriction digests and sequencing. Immunohistochemistry using a rabbit antibody raised against a specific domain of the CRH R1 receptor demonstrated that the R1 mRNA is translated into protein and confirmed that it is the uterine smooth muscle cells from both nonpregnant and pregnant women that bear this receptor. Our results suggest that CRH may play a role in human pregnancy at the myometrium.

Activation of the prostaglandin FP receptor in human granulosa cells

Prostaglandin F2 alpha (PGF2 alpha) has regulatory (mainly luteolytic) effects in the ovary but the mechanism of action is not completely understood. Reverse transcriptase-polymerase chain reaction (RT-PCR) techniques were used to demonstrate the presence of mRNA encoding the PGF2 alpha receptor (FP receptor) in human granulosa-lutein cells. Specific primers for the amplification of cDNA were designed and yielded a single product of 696 bp corresponding to the FP receptor. The identity of this product was verified by sequencing. Fluprostenol, a selective FP receptor agonist, activated phospholipase C (PLC) and increased intracellular free calcium concentration, confirming the functional activation of the receptor. We have demonstrated by Western blotting that granulosa cells express PLC-beta and PLC-gamma isoforms. The cells responded to pervanadate with increased PLC activity and increased tyrosine phosphorylation, demonstrating a functional PLC-gamma tyrosine kinase pathway. However, fluprostenol did not provoke any detectable tyrosine phosphorylation. Moreover, the effect of fluprostenol was inhibited through protein kinase C stimulation by phorbol 12, 13-dibutyrate, and was not affected when cells were treated with phenylarsine oxide, which blocks tyrosine phosphorylation. These results suggest that the FP receptor activates PLC-beta rather than PLC-gamma isoforms. Fluprostenol-induced activation was pertussis toxin resistant. Granulosa cells express G proteins of the Gq family (resistant to pertussis toxin) and mRNA for both G alpha q and G alpha 1 l has been identified by RT-PCR. In conclusion, human granulosa cells have a functional FP receptor the effects of which are mediated through PLC-beta activation probably via Gq/1 l.

The desensitization of oxytocin receptors in human myometrial cells is accompanied by down-regulation of oxytocin receptor messenger RNA

We have recently provided evidence for the desensitization of oxytocin receptors in human myometrial cells. In the present study, we have investigated the possible mechanisms by which oxytocin (OT) might regulate OT receptor density. The steady state level of OT binding in cultured myometrial cells was 220 x 10(3) binding sites/ cell, but this was time-dependently reduced to 27 x 10(3) sites/cell by exposure to OT for up to 20 h. Similarly, OT exposure decreased the binding of OT to cell membranes. In contrast, Western blotting data showed that the total amount of OT receptor protein was not affected by OT treatment for up to 48 h. Flow cytometry experiments demonstrated that OT receptors are not internalized during prolonged exposure of the cells to OT. However, RNase protection assays and Northern analysis showed that OT receptor mRNA was reduced by OT treatment to reach a new low steady state level with a time course similar to that of the disappearance of cell surface OT binding sites. Possible mechanisms involved in mRNA down-regulation include transcriptional suppression and destabilization of mRNA by RNA binding proteins.

Expression of human myometrial G alpha s messenger ribonucleic acid transcript during pregnancy and labour: involvement of alternative splicing pathways

We have shown previously that expression of 46 and 54 kDa human myometrial G alpha s protein isoforms is increased during gestation and then subsequently decreased during labour. These proteins appear to be coded for by G alpha s-Small (with a serine residue at position 72) and G alpha s-Large (with a serine residue at position 87) mRNA splice variants respectively. In the study presented here we have used a G alpha s cDNA template to generate [32P]cytidine cRNA riboprobes for use in RNase protection assays, so as to measure total myometrial G alpha s mRNA levels in relation to the pattern of expression of G alpha s mRNA splice variants during pregnancy and labour. We report that total levels of human myometrial G alpha s mRNA remain similar in non-pregnant and pregnant women but are substantially reduced during parturition. Our data also provide strong evidence that alternative splicing of G alpha s precursor mRNA has a primary role in regulating expression of G alpha s protein isoforms during pregnancy and labour. The inclusion of an additional serine codon in G alpha s mRNAs during pregnancy involves a switch in alternative splicing pathways. We speculate that this switch may be due to a change in specificity of splicing factors that are modulated during pregnancy and labour.

Fluprostenol activates phospholipase C and Ca2+ mobilization in human myometrial cells

The objective of this study was to investigate the mechanism of action of PGF2 alpha in cultured human myometrial cells. We measured the effects of PGF2 alpha and fluprostenol, a selective PGF2 alpha receptor (FP receptor) agonist, on phospholipase C(PLC) activation, on changes in the intracellular free calcium concentration ([Ca2+]i), and on protein tyrosine phosphorylation. PGF2 alpha and fluprostenol activated PLC (determined by measuring the formation of inositol phosphates) and increased [Ca2+]i in a concentration-dependent manner. The apparent affinity of the FP receptor for fluprostenol was higher than that for PGF2 alpha when measuring PLC activation, but the receptor displayed similar affinities for both agonists when measuring increases in [Ca2+]i. These effects were not altered by treating the cells with pertussis toxin (PT), suggesting that the FP receptor is linked to PLC activation by a G protein of the Gq family. By contrast, the effect of oxytocin on PLC activation involved both PT-resistant and PT-sensitive pathways. Human myometrial cells responded to pervanadate and epidermal growth factor with increased PLC activity and increased tyrosine phosphorylation, demonstrating a functional PLC-gamma tyrosine kinase pathway. However, neither fluprostenol nor oxytocin stimulated tyrosine phosphorylation, but the effects of both agonists were inhibited after protein kinase C stimulation. These data suggest that fluprostenol and oxytocin activate PLC-beta rather than PLC-gamma isoforms. The effect of fluprostenol is Ca2+ dependent, but is unlikely to involve a direct effect of Ca2+ on PLC activity.

Multiple G proteins and phospholipase C isoforms in human myometrial cells: implication for oxytocin action

Although a physiological role for oxytocin during parturition is well accepted, the mechanisms by which it activates myometrial contractility during labor have not been completely elucidated. We have previously shown the presence of Gq and two pertussis toxin (PT) substrates of the Gi family in human myometrial cells. In the present study, we have identified by Western blotting the G protein and phospholipase C (PLC) isoforms present in these cells and investigated their implication in oxytocin signaling by measuring the formation of inositol phosphates (IPs) and mobilization of intracellular calcium. We found G protein subunits alpha(q), alpha(11), alpha(i1), alpha(i2), alpha(i3), alpha(z), and two splice variants of alpha(s)- and beta-subunits. We have also detected the presence of five PLC isoforms: beta 1, beta 2, beta 3, gamma 1, and gamma 2. Oxytocin-induced IPs formation and intracellular Ca2+ mobilization were inhibited to approximately 50% after pretreatment of the cells with PT, suggesting that oxytocin activates PLC beta by interacting with at least two types of G proteins: a member of the Gq family (PT resistant) and a member of the Gi family (PT sensitive). The tyrosine phosphatase inhibitor pervanadate stimulated IPs formation in myometrial cells. Using the protein kinase inhibitors staurosporine, phenylarsine oxide, and Ro 31-8220 and the protein kinase C activator phorbol dibutyrate, we have shown that pervanadate and oxytocin activate PLC by different mechanisms. Furthermore, oxytocin did not activate tyrosine phosphorylation in human myometrial cells, as measured with an antiphosphotyrosine antibody, indicating that it does not activate a PLC gamma isoform. We conclude that oxytocin activates human myometrium by interacting with at least two G proteins and possibly three PLC beta isoforms.

Prostaglandin E2 activates phospholipase C and elevates intracellular calcium in cultured myometrial cells: involvement of EP1 and EP3 receptor subtypes

PGE2 is a powerful modulator of uterine contractility, but there is uncertainty as to which receptor subtypes (EP1, EP2, EP3, or EP4), G proteins, and second messenger systems are activated by PGE2 in myometrium. Here we show that in cultured human myometrial cells, PGE2 (1-100 microM) activates phospholipase C (PLC) up to 500% over the control level and elevates intracellular calcium ([Ca2+]i) from the resting level of 60-90 nM up to 350 nM in a concentration-dependent manner. Stimulation by the receptor subtype-selective analogs GR63799X (EP3), sulprostone (EP3 > EP1), and misoprostol (EP3 > EP2 > EP1) indicates that these effects are transmitted through EP3 receptors. Both effects are resistant to pertussis toxin (PT). Lower concentrations of PGE2 (1-300 nM) increase [Ca2+]i via a PT-sensitive pathway, without PLC activation. This [Ca2+]i increase occurs after an inverse dose-related delay and is inhibited by the selective EP1 antagonist AH6809 and calcium channel blockers. By comparison, oxytocin stimulates PLC up to 1000% over the control level and elevates [Ca2+]i up to 800 nM in a concentration-dependent manner without any measurable delay; both effects are partly sensitive to PT. These data provide functional evidence for the presence of different stimulatory mechanisms for PGE2 in myometrium: 1) a low affinity receptor (probably EP3D) that activates PLC through a PT-insensitive pathway; and 2) a high affinity receptor (probably EP1), independent from PLC and involving a PT-sensitive G protein (G(i)?). Both pathways lead to elevation of [Ca2+]i.

Human myometrial G alpha s-small (with serine) and Gs-large (with serine) messenger ribonucleic acid splice variants promote the increased expression of 46- and 54-kilodalton G alpha s protein isoforms in pregnancy and their down-regulation during labor

In previous studies using specific G alpha s antibodies we have identified several human myometrial G alpha s protein isoforms with molecular masses of 45, 46, 47, 54, and 58 kDa, respectively. During pregnancy, levels of the 46- and 54-kDa proteins are significantly increased compared to those in nonpregnant myometrium and then decreased at the onset of labor. In this study we investigated the expression of G alpha s messenger ribonucleic acid (mRNA) splice variants, which are generated as a result of alternative splicing of a single mRNA precursor, in term pregnancy and parturition to determine whether there was any correlation with the observed changes in G alpha s protein isoforms. A myometrial G alpha s complementary DNA was synthesized using RT-PCR and cloned into pCRtmII suitable for preparation of riboprobes for use in ribonuclease protection assays. Using this technique, we identified at least three myometrial G alpha s mRNAs, including two forms of G alpha s-Large (with or without the serine at amino acid 87) and one form of G alpha s-Small (with the serine at amino acid 72). G alpha s Small (with the serine) and G alpha s-Large (with the serine) mRNAs encode for the 46- and 54-kDa G alpha s protein isoforms, respectively, and were increased in term pregnancy and then subsequently decreased after the onset of labor. Our data suggest that posttranscriptional regulation of G alpha s mRNAs may be important in the differential expression of G alpha s protein isoforms during pregnancy and labor.

G protein expression and second messenger formation in human granulosa cells

The expression of heterotrimeric (alpha beta gamma subunits) GTP-binding regulatory proteins (G proteins) and the activation of G protein-linked receptors in human granulosa cells were investigated. The cells were obtained from stimulated follicles in women undergoing in vitro fertilization and were cultured in serum-supplemented medium. Immunoblotting with specific antibodies showed that granulosa cell membranes express alpha s, alpha i3 alpha i1,2, alpha q,11 and beta subunits. Three antibodies against alpha o failed to detect this protein. The cells responded to hCG and to prostaglandin E2 with a dose-dependent increase in cAMP formation, confirming the functional activation of G alpha s. The alpha 2 adrenoceptor agonist, clonidine, inhibited hCG-stimulated cAMP formation and this effect was blocked with pertussis toxin, thus involving a Gi-type protein, most likely G alpha i2. Oxytocin provoked an increase in formation of inositol phosphates and intracellular calcium concentration, which was partly pertussis toxin resistant, providing evidence of G alpha q,11 activation. However, a significant component of the response to oxytocin could be blocked by pertussis toxin, indicating Gi-mediated phospholipase C activation (by either alpha i or beta gamma subunits). These data demonstrate the presence of G proteins in granulosa cells and suggest a complex regulation of hormonal signalling. The concentration of cAMP in these cells depended on the balance of G alpha s:G alpha i activation, whereas activation of the inositol phospholipid pathway and rises in intracellular calcium involved both Gq,11 and Gi pathways.

Preterm labour: a pharmacological challenge

Preterm labour is a major cause of perinatal mortality and morbidity, but its prevention is difficult because most of the available drugs lack uterine selectivity and have potentially serious side-effects for the mother or the foetus. In this article, Andrés López Bernal and colleagues discuss new evidence that shows pregnancy is associated with changes in G protein signalling and second messenger formation in human myometrium. During gestation uterine relaxation is favoured by a pronounced increase in G alpha s levels, thereby facilitating the effect of agonists that increase cAMP formation. The change in G alpha s is reversed in spontaneous labour enabling the uterus to become responsive to contractile agents. Although it is not established that these changes in G protein function are causally related to the spontaneous onset of labour, nevertheless they provide a novel viewpoint towards increased understanding of the cellular mechanisms of uterine contractility, which may result in better drugs for the management of preterm labour.

Effects of oestradiol and tamoxifen on oxytocin-induced phospholipase C activation in human myometrial cells

The role of oestradiol in the control of uterine responsiveness to oxytocin was investigated by measuring oxytocin-induced phospholipase C activation in [3H]inositol-labelled cultured human myometrial cells. Addition of oestradiol to steroid-free culture medium (10% (v/v) fetal calf serum treated with dextran-coated charcoal in phenol red-free medium) enhanced formation of inositol phosphates and this effect was completely abolished by the anti-oestrogen tamoxifen. The inhibitory effect of tamoxifen on oxytocin-induced phospholipase C activation occurred in both steroid-free and complete culture medium; it was time- and concentration-dependent and was only partly reversed by oestradiol. When phospholipase C was activated with PGF2 alpha or fluoroaluminate instead of oxytocin, oestradiol and tamoxifen had the same stimulatory and inhibitory effects, respectively. The inhibitory effect of tamoxifen could not be prevented by treating the cells with pertussis toxin. Moreover, the effect of tamoxifen was not mediated by inhibition of protein kinase C, since the use of staurosporine (a protein kinase inhibitor) resulted in potentiation of phospholipase C activation by oxytocin. Both oestradiol and tamoxifen increased [3H]inositol incorporation into cellular lipids and cell proliferation. These results suggest that oestradiol enhances myometrial responsiveness to oxytocin and other agonists by facilitating phospholipase C activation at a post-receptor level. This effect is antagonized by tamoxifen; however, tamoxifen also has oestrogen-independent inhibitory effects.

Down-regulation of G alpha s in human myometrium in term and preterm labor: a mechanism for parturition

We have previously reported that G alpha s is expressed at considerably higher levels in myometrium taken from pregnant than from nonpregnant women. In the present study we have determined adenylyl cyclase activity in myometrial membranes by measuring the conversion of [alpha-32P]ATP to [32P]cAMP and have measured guanosine triphosphate-binding protein expression by immunoblotting with specific antibodies. Here we report that the increase in G alpha s expression in pregnant myometrium is associated with a significant increase in G alpha s-coupled adenylyl cyclase activity, as estimated by incubating myometrial membranes in the presence of 5'-guanylyl-imidodiphosphate with and without prostaglandin E2. Moreover, in myometrium from women in spontaneous labor G alpha s levels and G alpha s-coupled adenylyl cyclase activity are reduced to the levels observed in nonpregnant tissue. There was no apparent change in forskolin-stimulated adenylyl cyclase activity in nonpregnant, pregnant, and laboring tissue. The increase in G alpha s expression in pregnant myometrium may facilitate agonist-induced cAMP formation, resulting in prolonged relaxation of the uterus during gestation. Down-regulation of G alpha s would decrease the relaxing effect exerted by cAMP and may be a triggering mechanism for the initiation of labor.

Effect of oxytocin antagonists on the activation of human myometrium in vitro: atosiban prevents oxytocin-induced desensitization

OBJECTIVE

Our purpose was to investigate whether the sensitivity of myometrial cells to oxytocin is affected by prolonged exposure to oxytocin antagonists.

STUDY DESIGN

Tissue slices or cultured myometrial cells were exposed to peptides in vitro. Myometrial activation was studied by measuring the formation of inositol phosphates and the changes in intracellular calcium. Oxytocin binding was measured by saturation analysis.

RESULTS

Atosiban and related peptides inhibited oxytocin-induced myometrial activation as pure antagonists (inhibition constant 10 nmol/L) but had no effect on prostaglandin E2-induced activation. Long-term (> or = 24 hours) exposure to atosiban had no residual effect on oxytocin sensitivity. However, long-term exposure to oxytocin resulted in homologous desensitization and loss of oxytocin receptors. Oxytocin-induced desensitization was prevented by coincubation with atosiban.

CONCLUSIONS

Atosiban is a pure oxytocin antagonist and has a specific, reversible effect on myometrial cells in vitro. Its potential use for the management or even prevention of idiopathic preterm labor or to reverse uterine hypertony during oxytocin-induced labor should be tested in controlled clinical trials.

Biochemistry and physiology of preterm labour and delivery

Human parturition is associated with profound changes in uterine connective tissue affecting mainly the cervix, but the endocrine control of cervical ripening remains obscure. Connective tissue changes are also implicated in premature rupture of the membranes, a problem often associated with preterm delivery, and it is believed that local inflammatory infiltration may play a role in both this condition and cervical ripening, but it is difficult to define which changes precede parturition and which are a consequence of the trauma of labour. Chorioamnionitis can cause preterm labour by provoking the release of inflammatory mediators in the decidua/fetal membranes area and it is likely that activation of prostaglandin release by decidual macrophages is involved in triggering labour. However, the role of macrophages and other bone marrow derived cells in normal labour and in labour associated with chorioamnionitis needs to be defined. It is likely that treatment with a combination of antibiotics and prostaglandin synthase inhibitors and/or other anti-inflammatory drugs is the most appropriate therapeutic approach. Idiopathic preterm labour and spontaneous labour at term are probably due to changes in the sensitivity of the myometrium to endogenous agonists. Recent progress in cell signalling pathways, such as the characterization of regulatory G proteins and the cloning of hormone receptors, should clarify the mechanism of action of relaxing and contracting agents on myometrial cells and should provide the means for the development of new therapeutic agents of high effectiveness and selectivity. This approach should result in better management of both term and preterm labour.

Identification and expression of G-proteins in human myometrium: up-regulation of G alpha s in pregnancy

We report that human myometrium contains G alpha i1, G alpha i3, and G alpha q, and G alpha 11, which are expressed at similar levels in tissues from pregnant and nonpregnant women. G alpha i2 is also expressed, but at a slightly reduced level, in tissue taken from pregnant compared to nonpregnant donors. The major finding of this investigation is the substantial increase in G alpha s expression in pregnant myometrium. The increase in G alpha s levels may play a crucial role in maintaining relaxation of the uterus by favoring cAMP formation during pregnancy.

Forskolin inhibits platelet-activating factor binding to platelet receptors independently of adenylyl cyclase activation

The effect of forskolin on platelet-activating factor (PAF) receptor was investigated. Rabbit platelets treated with forskolin showed approximately a 9-fold increase in cAMP levels over the control. After treatment of platelets with forskolin prior to PAF binding, a 30-40% (P < 0.005) decrease in PAF binding was observed. The decrease in PAF binding caused by forskolin was concomitant with a decrease in the physiological responses of platelets induced by PAF. However, this forskolin-induced decrease in PAF binding was not a consequence of cAMP formation as the addition of a cAMP analog could not mimic the action of forskolin. Additionally, the inactive analog of forskolin, dideoxyforskolin, which does not activate adenylyl cyclase, also reduced PAF binding to its receptor. Reduction of PAF binding by forskolin and dideoxyforskolin was also observed with isolated platelet membranes. To understand the mechanism of forskolin induced changes in PAF binding, the involvement of a G-protein in this process was investigated. Cells treated with GTP gamma S showed approximately a 25% reduction in PAF binding. Addition of forskolin to the GTP gamma S treated cells resulted in a further reduction in PAF binding, suggesting the action of forskolin was independent of G-protein activation. The data suggests that the action of forskolin was independent of adenylyl cyclase or G-protein involvement. It is speculated that the action of forskolin on PAF binding was due to a direct effect of this molecule and its analog on the PAF receptor itself or to components of the post-receptor signalling for PAF.

Oxytocin-stimulated phosphoinositide hydrolysis in human myometrial cells: involvement of pertussis toxin-sensitive and -insensitive G-proteins

Phosphoinositide hydrolysis is important in mediating the actions of oxytocin and prostaglandin (PG) F2 alpha on uterine contractions during labour. We have measured the effect of oxytocin, PGF2 alpha and other agents on the formation of inositol phosphates (IPs) in cultured human myometrial cells labelled with [3H]inositol and on changes in intracellular free Ca2+ concentration ([Ca2+]i) in cells loaded with Fura-2. Oxytocin induced the formation of [3H]IPs in a concentration-dependent manner with an EC50 (concentration of agonist producing 50% of the maximal response) of 1.4 +/- 0.5 nmol/l (mean +/- S.E.M.). The maximal response was obtained with 1 mumol oxytocin/l and represented a stimulation of 670% over basal. PGF2 alpha also stimulated the formation of [3H]IPs and the response at 1 mumol/l was a 204% stimulation over basal. The effects of PGF2 alpha were independent of extracellular Ca2+ but the effect of oxytocin was reduced with low extracellular Ca2+. Cyclic AMP formation, induced by forskolin or PGE2, had no effect on the stimulated levels of [3H]IPs. Pertussis toxin (PT) reduced the oxytocin-stimulated formation of [3H]IPs in a concentration-dependent manner. The maximal effect of PT resulted in an 80% reduction in the formation of [3H]IPs. However, PGF2 alpha stimulation was not affected by PT treatment. To analyse the action of PT further, we studied its effect on oxytocin-induced changes in [Ca2+]i. The basal [Ca2+]i was 112 +/- 4 nmol/l (n = 225 cells) and was not affected by PT treatment (109 +/- 3 nmol/l; n = 200 cells). In the absence of PT, 1 mumol oxytocin/l increased [Ca2+]i to a peak of 522 +/- 26 nmol/l, and in PT-treated cells, the [Ca2+]i peak was reduced to 348 +/- 16 nmol/l. Similar inhibitory effects of PT were obtained at oxytocin concentrations ranging from 1 to 100 nmol/l. Our data suggest that in human myometrial cells, the oxytocin-induced production of [3H]IPs and increase in [Ca2+]i are mediated by a PT-sensitive G-protein. However, a significant fraction of the oxytocin response appears to be mediated by a PT-insensitive G-protein, possibly a member of the Gq family.

Polyphosphoinositide hydrolysis and protein kinase C activation in guinea pig tracheal smooth muscle cells in culture by leukotriene D4 involve a pertussis toxin sensitive G-protein

Leukotriene D4 (LTD4) at concentrations greater than 1 nM induced phosphatidylinositol bisphosphate (PIP2) hydrolysis and protein kinase C (PKC) activation in primary culture of airway smooth muscle cells. Within seconds of activation, an increase in inositol 1,4,5-trisphosphate (IP3) was observed reaching a maximum at 5 min. The level of IP3 decreased after 5 min and was followed by an increase in inositol 1,4-bisphosphate (IP2) and inositol 1-monophosphate (IP1). LTD4-induced PIP2 hydrolysis was inhibited by 1 h pretreatment of cells with 10 micrograms/ml of pertussis toxin (PTX). LTD4 activated both soluble and particulate forms of PKC by 2-3-fold. The LTD4-induced PKC activation was blocked by treatment of cells with PTX, suggesting the involvement of a PTX-sensitive G-protein. To assess the involvement of G(i) in smooth muscle cell receptor activation, the modulation of adenylyl cyclase activity was investigated. LTD4 did not stimulate cAMP formation in smooth muscle cells, and did not inhibit forskolin-induced cAMP formation. These data suggest that the LTD4 receptor in airway smooth muscle cells is coupled to a PTX-sensitive G-protein, possibly G(o).

Metabolically 35S-labeled recombinant calmodulin as a ligand for the detection of calmodulin-binding proteins

We have developed a simplified procedure for the production of metabolically labeled calmodulin. We used bacterial clones (Escherichia coli) that were found to express VU-1 calmodulin, a calmodulin that is fully active with a variety of calmodulin-regulated enzymes. VU-1 calmodulin was labeled with sulfur-35 in bacteria maintained in a sulfur-free medium. Calmodulin was then purified by chromatography on phenyl-Sepharose. Under these conditions, the specific activity of the proteins was 150 to 400 cpm/fmol of calmodulin. To demonstrate the utility of this labeled VU-1 calmodulin, we examined the calmodulin-binding proteins in aortic myocyte preparation from Day 0 and Day 15 cultures by using both the gel and the nitrocellulose overlay protocols. The results showed that calmodulin-binding proteins are easily detected by the two procedures and that the profile of these target proteins changed in myocyte with time in culture. While most of these calmodulin-binding proteins have not been identified, the relative mobility on SDS-PAGE gels suggests that myosin light chain kinase (Mr approximately 137,000) was detected by these methods. We demonstrated here that the nitrocellulose overlay was faster than the gel overlay and that this technique can be useful for the study of calmodulin-binding proteins.

Modulation of the accumulation of inositol phosphates and the mobilization of calcium in aortic myocytes

In vascular smooth muscle cells the phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate (TPA), a potent activator of C-kinase, inhibited the accumulation of inositol phosphates and the mobilization of calcium produced by several agonists. In the same way, TPA inhibited the fluoride-induced activation of phosphoinositide metabolism. These results suggest a C-kinase action at a post-receptor level. Moreover, the fluoride-induced accumulation of inositol phosphates shows the presence of one or more guanine nucleotide-binding proteins (G-proteins) in the regulation of receptor-phospholipase C coupling. This was confirmed by the use of N-ethylmaleimide and pertussis toxin. These results support the view that, in addition to the induction of sustained contractions, C-kinase can activate negative feedback mechanisms in aortic myocytes.

Phorbol ester modulation of cyclic AMP accumulation in a primary culture of rat aortic smooth muscle cells

Over the past few years, the importance of calcium and cyclic AMP in the regulation of vascular smooth muscle tone has been well documented. We used a primary culture of rat aortic myocytes to study the effect of protein kinase C on isoproterenol- and forskolin-stimulated cyclic AMP production. Addition of the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA) to these cells, but not an inactive analog, increased the stimulation of cyclic AMP production induced with isoproterenol or forskolin without changes in the apparent affinity of these compounds but did not affect the basal cAMP level. TPA also enhanced the cholera toxin-stimulated cyclic AMP accumulation. Isoproterenol and cholera toxin increased the forskolin apparent potency suggesting that interaction of activatory GTP-dependent protein with the catalytic subunit of adenylate cyclase facilitates forskolin interaction to the catalytic subunit. Treatment of myocytes with pertussis toxin had no effect on the basal level of cyclic AMP production and did not significantly modify isoproterenol- and forskolin-induced stimulation. Pertussis toxin treatment of cells did not affect the TPA-enhanced isoproterenol or forskolin stimulations suggesting that pertussis toxin and TPA actions would not share a common target of myocyte adenylate cyclase system. Our data would be in agreement with a possible direct interaction of protein kinase C with the catalytic subunit of adenylate cyclase system.

The effects of maitotoxin on phosphoinositides and calcium metabolism in a primary culture of aortic smooth muscle cells

Maitotoxin, a potent marine toxin isolated from toxic tropical dinoflagellates and poisonous fishes induces contraction of different smooth muscle preparations. Actions of maitotoxin on phosphoinositides and calcium metabolism were studied using a primary culture of aortic smooth muscle cells. Maitotoxin induced a very large increase of cytosolic calcium concentration as evaluated by fura-2 acetoxymethyl ester fluorescence. This increase was concomitant with stimulation of inositol-phosphate accumulation and loss of viability of aortic smooth muscle cells. These responses to maitotoxin were abolished in Ca2+-free medium, and were mimicked by saponin. Calcium ionophores or K+ depolarisation did not induce inositol-phosphate formation. These results suggest that maitotoxin acts by altering smooth muscle cells permeability allowing a sustained calcium influx which is able to activate inositol-phosphate formation and which is lethal for the cells.

ATP stimulates inositol phosphates accumulation and calcium mobilization in a primary culture of rat aortic myocytes

The effects of extracellular ATP on phosphoinositide metabolism and intracellular Ca2+ concentration were studied in a primary culture of rat aortic myocytes. ATP increases the level of inositol phosphates, the putative second messenger for Ca2+ mobilization. No saturation of inositol phosphates accumulation is obtained (up to 10(-2) M ATP). Under the same conditions, ATP rapidly mobilizes intracellular Ca2+ in fura-2 loaded myocytes. The mobilization of intracellular Ca2+ is dose-dependent (maximal at 10(-4) M ATP), and is not affected by addition of EGTA. It is concluded that the receptors mediating the cytosolic increase of Ca2+ are of the P2-purinoceptor subtype. The physiological functions of these receptors are not presently known.

Calcium binding properties of purified zymogen granule membrane of pig pancreas. Evidence for calcium binding proteins

Ca2+ binding properties of purified zymogen granule membranes of pig pancreas have been measured: Binding increased linearly with Ca2+ concentration in the medium up to the micromolar range; in the millimolar range a sharp rise in binding capacity was observed. Binding increased with pH both at low and high concentrations of Ca2+. It was insensitive to Na+ and K+ ions at concentrations up to 100 mM. Mg2+ was inhibitory in the millimolar range whereas La2+ and Tb3+ were inhibitory in the micromolar range. The Ca2+ binding components of zymogen granule membranes were identified by two methods: (1) by measuring 45Ca2+ binding after counter-ion electrophoresis and (2) by Stain's-all (forms a complex with Ca2+ binding proteins absorbing maximally at 600 nm), after SDS-polyacrylamide gel electrophoresis. The first method, counter-ion electrophoresis, indicated that most of the 45Ca2+ was associated with an acidic band which could be subsequently subfractionated by SDS-polyacrylamide gel electrophoresis in five bands: 66, 57, 30, 27 and 22.5 kDa. The second method, Stain's-all, revealed six positive polypeptides after SDS-polyacrylamide gel electrophoresis of native zymogen granule membranes' two were unreactive after neuraminidase treatment (130 and 92 kDa, respectively), whereas four other bands were still reactive (66, 57, 43, 30 kDa, respectively.) Ca2+ binding was also measured on intact zymogen granules: the binding capacity was higher than for zymogen granule membranes. Among the Ca2+ binding proteins of the zymogen granule membrane only one is apparently located on the granule external surface: the 30 kDa polypeptide. If Ca2+ directly facilitates fusion of zymogen granules with plasma membrane by a Ca2+-protein interaction, then this protein is a presumptive candidate to play such a key role.

Electrophoretic and cytological evidence for heterogeneity of pancreatic acinar cell responsiveness to carbachol, caerulein and secretin

Incubation of rat pancreatic lobules for 90 min with optimal concentrations of caerulein, carbachol or secretin caused the release of about 30% of the amylase content. Combination of secretin with carbachol or caerulein increased the amylase output to about 40%. With secretin, as with carbachol or caerulein, heterogeneity of cellular responsiveness was observed, some acini being partially or completely depleted of their zymogen granules, whereas others appeared to be resting. When secretin was combined with carbachol or caerulein, granule depletion, originally confined to small groups of neighbouring acini, spread to form large areas of degranulated cells, sometimes comprising a whole section of a lobule. In dispersed acini, under the same conditions, carbachol caused the release of about 60% of the amylase content, and secretin 40%. When both secretagogues were combined, a significant increase to 78% was observed. Under these conditions, there was some important cellular damage, as indicated by the release of 20% of the amylase content and between 6 and 12% of lactate dehydrogenase into the media, in the absence of stimulus. These results were corroborated by cytological observations. On the basis of their secretory response two groups of acini can be distinguished, those that respond to carbachol, caerulein or secretin and those that respond to the combination of secretin with carbachol or caerulein. Electrophoretic patterns of secretory proteins released by lobules stimulated by these different types of secretagogues were essentially similar. The pattern was quite different, however, in the absence of a stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytological effects of ionophore-induced stimulation on the exocrine pancreas of the rat

Rat-pancreas lobules were incubated with the ionophore A-23187 in the presence of Ca2+. After 90 min, some of the acini were partially or almost completely depleted of their zymogen granules while others had the appearance of resting acini. With few exceptions, the cells of a given acinus were degranulated to a comparable level. Slight dispersion of the zymogen granules was noticed in cells incubated in a Ca2+-free medium containing EGTA with or without A-23187. In the presence of Ca2+ the secretory response obtained with the ionophore was comparable to that observed with 10(-5)M urecholine. The results obtained provide cytological evidence that the secretory response is only partially determined at the membrane-receptor level and that other mechanisms intervene between cytosol Ca2+ increase and exocytosis.

Characterization and purification of a calcium-sensitive ATP diphosphohydrolase from pig pancreas

An ATP diphosphohydrolase (EC 3.6.1.5) from the pancreas of the pig has been characterized and purified. The enzyme which has an optimum pH between 8 and 9 is specific for diphospho- and triphosphonucleosides. The Km values for ADP and ATP are 7.4 and 7.3 x 10(-4) M, respectively, and the purified enzyme has specific activities of 13 and 15.2 mumol of Pi/min/m of protein, respectively. It requires calcium or magnesium ions and it is insensitive to ATPase inhibitors, namely oligomycin, ouabain, and ruthenium red, and to levamisole, an inhibitor of alkaline phosphatase. Denaturation experiments, by heat and trypsin treatments, indicated that only one enzyme is involved. This is confirmed by the solubilization and purification process and by polyacrylamide gel electrophoresis. A 270-fold purification was obtained by centrifugation and successive column chromatography on Sepharose 4B and Affi-Gel blue. It is a glycoprotein with a molecular weight of 65,000 as estimated by polyacrylamide gel electrophoresis.