The motile major sperm protein (MSP) of Ascaris suum forms filaments constructed from two helical subfilaments

The amoeboid motility nematode sperm is mediated by cytoskeletal filaments composed of major sperm protein (MSP). We have used electron microscopy and image processing to show that MSP filaments are constructed from two subfilament strands which are themselves formed from a helical arrangement of subunits. The subfilaments are based on left-handed helices of pitch 9 nm that then coil along right-handed helical tracks of pitch 22.5 nm to form filaments. The subfilaments appear to be indistinguishable from the helices present in orthorhombic crystals of MSP. Because in filaments the subfilaments are themselves helical, not all subunits are able to participate in protein-protein interactions between different strands. One consequence of this interaction geometry is that the same molecular interactions that function to assemble subfilaments into filaments can also be used to assemble filaments into larger supramolecular assemblies such as the macrofibres formed in vitro and the fibre bundles found in vivo in sperm pseudopods. These results indicate the importance of filament bundling in addition to vectorial filament assembly in amoeboid cell motility.

Supramolecular assemblies of the Ascaris suum major sperm protein (MSP) associated with amoeboid cell motility

Sperm of the nematode, Ascaris suum, are amoeboid cells that do not require actin or myosin to crawl over solid substrata. In these cells, the role usually played by actin has been taken over by major sperm protein (MSP), which assembles into filaments that pack the sperm pseudopod. These MSP filaments are organized into multi-filament arrays called fiber complexes that flow centripetally from the leading edge of the pseudopod to the cell body in a pattern that is intimately associated with motility. We have characterized structurally a hierarchy of helical assemblies formed by MSP. The basic unit of the MSP cytoskeleton is a filament formed by two subfilaments coiled around one another along right-handed helical tracks. In vitro, higher-order assemblies (macrofibers) are formed by MSP filaments that coil around one another in a left-handed helical sense. The multi-filament assemblies formed by MSP in vitro are strikingly similar to the fiber complexes that characterize the sperm cytoskeleton. Thus, self-association is an intrinsic property of MSP filaments that distinguishes these fibers from actin filaments. The results obtained with MSP help clarify the roles of different aspects of the actin cytoskeleton in the generation of locomotion and, in particular, emphasize the contributions made by vectorial assembly and filament bundling.

Effects of kistrin on bone resorption in vitro and serum calcium in vivo

In many cell systems, cell-cell and cell-matrix interactions are mediated by integrins, a family of cell surface heterodimeric glycoprotein receptors. Osteoclast integrins may play a role in the process of bone resorption. Osteoclasts express the alpha v and beta 3 subunits of the vitronectin receptor and adhere to a wide range of proteins in vitro, all which contain the amino acid sequence Arg-Gly-Asp (RGD), an adhesion site recognition sequence common to many protein ligands that bind to integrins. The effect of kistrin, an RGD-containing snake venom protein, on osteoclast-mediated bone resorption was investigated in vivo and in vitro. When kistrin was infused into normocalcemic and hypercalcemic mice, serum calcium was significantly lowered at 3 and 6 h after the start of infusion, indicating an inhibitory effect on osteoclast activity in vivo. In vitro, kistrin potently inhibited bone resorption by isolated rat osteoclasts cultured on slices of bovine bone, and kistrin also inhibited the attachment of 293 cells expressing recombinant human alpha v beta 3 to fibrinogen (IC50 = 1 nM). These results indicate the potential therapeutic use of RGD-containing molecules for hypercalcemia of malignancy or for other disorders associated with bone loss.

Age-dependent increase of mitochondrial DNA deletions together with lipid peroxides and superoxide dismutase in human liver mitochondria

We previously reported an age-dependent deterioration of mitochondrial respiration as well as two age-associated mitochondrial DNA (mtDNA) deletions in the human liver. In this study, we further determined the relative quantities of the deleted mtDNAs in liver biopsies from 64 subjects of different ages. The results showed that both mtDNA deletions increase in frequency and quantity with age. Moreover, we measured hepatic lipid peroxides (malondialdehyde; MDA) of isolated mitochondria and manganese superoxide dismutase (Mn-SOD) activity of submitochondrial particles. We found a significant age-dependent increase in both MDA and Mn-SOD levels in liver mitochondria. These results confirm the previous contention that enhanced generation of lipid peroxides in the mitochondria during the aging process may damage mtDNA, and mtDNA deletions may be one of the important factors contributing to aging in humans.

Regulation of the Ascaris major sperm protein (MSP) cytoskeleton by intracellular pH

The development and locomotion of the amoeboid sperm of the nematode, Ascaris suum, depend on precise control of the assembly of their unique major sperm protein (MSP) filament system. We used fluorescence ratio imaging of cells loaded with BCECF to show that intracellular pH (pHi) is involved in controlling MSP polymerization in vivo. Spermatogenesis is marked by a cycle of MSP assembly-disassembly-reassembly that coincides with changes in pHi. In spermatocytes, which contain MSP in paracrystalline fibrous bodies, pHi was 6.8, 0.6 units higher than in spermatids, which disassemble the fibrous bodies and contain no assemblies of MSP filaments. Activation of spermatids to complete development resulted in rapid increase in pHi to 6.4 and reappearance of filaments. Treatment of spermatocytes with weak acids caused the fibrous bodies to disassemble whereas incubation of spermatids in weak bases induced MSP assembly. The MSP filaments in spermatozoa are organized into fiber complexes that flow continuously rearward from the leading edge of the pseudopod. These cells established a pseudopodial pH gradient with pHi 0.15 units higher at the leading edge, where fiber complexes assemble, than at the base of the pseudopod, where disassembly occurs. Acidification of these cells caused the MSP cytoskeleton to disassemble and abolished the pH gradient. Acid removal resulted in reassembly of the cytoskeleton, re-establishment of the pH gradient, and re-initiation of motility. MSP assembly in sperm undergoing normal development and motility and in cells responding to chemical manipulation of pHi occurs preferentially at membranes. Thus, we propose that filament assembly in sperm is controlled by pH-sensitive MSP-membrane interaction.

Bile duct injury and bile leakage in laparoscopic cholecystectomy

Between January 1991 and July 1992, 350 laparoscopic cholecystectomies were performed. There were six biliary complications (1.7 per cent): common bile duct (CBD) injury (one patient), delayed CBD necrosis (one), immediate postoperative bile leakage (one) and delayed bile leakage (three). All six patients required laparotomy. Primary repair with long-arm T tube splinting for 3 months was performed for the CBD injury. Religation of the cystic duct was carried out after immediate postoperative bile leakage. Laparotomy with T tube choledochostomy only was performed in the three patients with delayed bile leakage, and hepatojejunostomy Roux-en-Y was undertaken for CBD necrosis. The patient with primary repair of the CBD injury required choledochojejunostomy Roux-en-Y 18 months later for stenosis. Episodes of intermittent cholangitis occurred in the patient with CBD necrosis. The outcome for the four patients with bile leakage was good. There were no deaths.

Crystallization of the motile major sperm protein (MSP) of the nematode Ascaris suum

We have obtained orthorhombic crystals (space group P2(1)2(1)2(1), a = 72 A, b = 78 A, c = 452 A) of the major cytoskeletal protein associated with the amoeboid motility of Ascaris sperm. These crystals diffract past 3.5 A and appear to be constructed from arrays of helical fibres of the major sperm protein. The fibres within the crystals appear to be closely analogous to those seen in vitro and in vivo.

Retroperitoneal cystic lymphangioma in adults

Retroperitoneal lymphangioma is an unusual disease, more customarily reported in infants, only occasionally in adults. This paper reports four cases of histologically-confirmed retroperitoneal cystic lymphangiomas detected in adulthood. All four patients presented with palpable abdominal mass. The diagnostic imaging methods used included abdominal ultrasonogram and CT scan. Correct preoperative diagnosis was made in only one case, but four tumors were totally excised. Biochemical analysis of the lymphangioma content from three of the patients proved protein and fat rich. The CT attenuation numbers of both serous and chylous lymphangioma were similar and near that of water and so were not helpful in differential diagnosis with other cystic lesions with watery density content.

The potential for avermectins to affect the nutrient economy of grazed pastures

This examination of the potential ecotoxic effects of the avermectins in temperature pastures grazed by sheep is based on a community approach and is focussed on one important aspect of ecosystem function, the nutrient cycle. Data on the amount and distribution of sheep dung on pastures grazed at different stocking rates indicated that areas of high stocking and sheep camps would be affected by avermectin residues to the greatest extent. Mineral losses from sheep dung which does not contain ivermectin, have been examined to provide a background against which the potential effects of avermectins on nutrient cycling can be appraised. The source of the diet is important; for example, dung from sheep grazing on improved pasture loses sulphur faster, and has higher microbial activity, than that of native pasture, regardless of whether it is fresh or old. Dung-dwelling fauna such as dung beetles and microarthropods are most abundant in areas of high dung concentration and microbial activity is greatest in sheep camps where a large quantity of excreta is voided. However, while there is evidence that avermectin does affect certain of the larger dung-dwelling fauna, little is known of its effects on the smaller invertebrate biota such as free-living nematodes and microarthropods. Calculations for phosphorus budgets on both native and improved pastures indicate that the amount of phosphorus recycled in these systems could be reduced by up to 5% the dosage levels currently recommended for the drug.

Methods for assessing the impact of avermectins on the decomposer community of sheep pastures

This paper outlines methods which can be used in the field assessment of potentially toxic chemicals such as the avermectins. The procedures focus on measuring the effects of the drug on decomposer organisms and the nutrient cycling process in pastures grazed by sheep. Measurements of decomposer activity are described along with methods for determining dry and organic matter loss and mineral loss from dung to the underlying soil. Sampling methods for both micro- and macro-invertebrates are discussed along with determination of the percentage infection of plant roots with vesicular-arbuscular mycorrhizal fungi. An integrated sampling unit for assessing the ecotoxicity of ivermectin in pastures grazed by sheep is presented.

Prenatal diagnosis of fetal pelvic kidney. A case report

Published experience on prenatal diagnosis of pelvic kidney is limited. We present the ultrasonographic characteristics of a case of ectopic pelvic kidney and discuss the significance of prenatal diagnosis.

Metabolism of arachidonic acid by isolated rabbit ciliary epithelium

We examined the ability of rabbit ciliary epithelium to metabolize arachidonic acid in vitro. The epithelium was homogenized and incubated with 14C-labeled arachidonic acid. 14C-labeled metabolites were extracted and then separated by thin layer chromatography. The range of arachidonic acid metabolites synthesized by ciliary epithelium was compared to the metabolites generated by rabbit iris-ciliary body. Ciliary epithelium produced substantial amounts of arachidonic acid metabolites that comigrated with 5-HETE and 12-HETE. Authenticity of the 12-HETE produced by ciliary epithelium was confirmed by gas chromatography/mass spectrometry. The ciliary epithelium generated only small amounts of the cyclooxygenase products, PGF2 alpha, PGE2, PGD2 and 6k-PGF1 alpha. In contrast, the iris-ciliary body produced large amounts of cyclooxygenase products such as PGF2 alpha and PGD2. The ability of the ciliary epithelium to generate 12-HETE is noteworthy since 12(R)-HETE is known to be capable of lowering intraocular pressure.

Structure and macromolecular assembly of two isoforms of the major sperm protein (MSP) from the amoeboid sperm of the nematode, Ascaris suum

Ascaris sperm are amoeboid cells that crawl by extending pseudopods. Although amoeboid motility is generally mediated through an actin-based cytoskeleton, Ascaris sperm lack this system. Instead, their major sperm protein (MSP) forms an extensive filament system that appears to fulfil this function. Because their motility appears to be essentially the same as that of their actin-rich counterparts, Ascaris sperm offer a simple alternative system for investigation of the molecular mechanism of amoeboid movement. To examine the structure and composition of the cytoskeleton, we stabilized the extremely labile native MSP filaments by detergent lysis of sperm in the presence of either glutaraldehyde or polyethylene glycol (PEG). Biochemical analysis showed that the cytoskeleton contained two isoforms of MSP, designated alpha- and beta-, that we purified and sequenced. Both contain 126 amino acids and have an acetylated N-terminal alanine, but differ at four residues so that alpha-MSP is 142 Da larger and 0.6 pH unit more basic than beta-MSP. Neither isoform shares sequence homology with other cytoskeletal proteins. In ethanol, 2-methyl-2,4-pentanediol (MPD), and other water-miscible alcohols each isoform assembled into filaments 10 nm wide with a characteristic substructure repeating axially at 9 nm. These filaments were indistinguishable from native fibers isolated from detergent-lysed sperm. Pelleting assays indicated a critical concentration for assembly of 0.2 mM for both isoforms in 30% ethanol, but alpha-MSP formed filaments at lower solvent concentration than beta-MSP. When incubated in polyethylene glycol, both isoforms formed thin, needle-shaped crystals that appeared to be constructed from helical fibers, with a 9 nm axial repeat that matched that seen in isolated filaments. These crystals probably contained a parallel array of helical filaments, and may enable both the structure of MSP molecules and their mode of assembly into higher aggregates to be investigated to high resolution.

Age-dependent 6kb deletion in human liver mitochondrial DNA

Using PCR technique, restriction mapping and DNA sequencing, we analyzed liver mitochondrial DNA (mtDNA) of 2 stillborn babies and 62 Chinese subjects with non-liver disease from 27 to 86 years old. The results showed an age-dependent 6,063 bp deletion in the liver mtDNA of older subjects. We found a TAACAGAC sequence flanking the 5'-end breakpoint at 7,842 nucleotide position and an imperfect repeat sequence CAACATAC flanking the 3'-end breakpoint at 13,905 nucleotide position. The incidence of the deleted mtDNA was found to increase with age. The deleted mtDNA was not detected in the liver of the stillbirth or blood cells of all the subjects. This is the first account that an age-related 6,063 bp deletion occurs in the liver mtDNA of old humans. The occurrence of this and previously reported 4,977 bp deletions is consistent with our recent finding that liver mitochondrial respiratory functions decline with age and support the hypothesis that continuous accumulation of mtDNA mutations is an important contributor to ageing process in the human.

The influence of cyclic AMP upon Na,K-ATPase activity in rabbit ciliary epithelium

ATPase activity was measured in samples of freshly dissected rabbit ciliary epithelium. The epithelium was ruptured in distilled water, frozen briefly, and incubated at 37 degrees C in a buffer containing 100 mM NaCl and 32P-labeled adenosine triphosphate (ATP). The rate of ATP hydrolysis by the epithelium was linear for as long as 45 min. Ouabain (1 mM) reduced the ATP hydrolysis rate by approximately 50%. When the epithelium was preincubated for 10 min. in the presence of 1 mM dibutyryl cyclic adenosine monophosphate (cAMP), the ouabain-sensitive (Na,K-ATPase) activity was diminished; ouabain-insensitive ATPase activity was not reduced. Preincubation of the epithelium with forskolin with isobutylmethylxanthine also reduced ouabain-sensitive ATPase activity. These observations suggest that the ciliary epithelium may have a mechanism for short-term modulation of Na,K-ATPase activity by cAMP. Such a mechanism could be linked to the ability of cAMP-dependent protein kinase to reduce Na,K-ATPase activity in the tissue.

Thrombin and parathyroid hormone mobilize intracellular calcium in rat osteosarcoma cells by distinct pathways

The mechanisms by which PTH and thrombin mobilize intracellular Ca2+ (Cai2+) were examined in UMR 106-H5 rat osteosarcoma cells. Bovine PTH-(1-34) (24 pM to 240 nM) produced a dose-dependent increase in Cai2+ (EC50, 3 nM), which returned to baseline within 75 sec. Human alpha-thrombin produced an increase in Cai2+ (ECmax, 10 U/ml) which was similar to that of PTH with respect to both magnitude and time course. Chelation of extracellular calcium with 5.0 mM EGTA did not alter the Cai2+ response to either PTH or thrombin. When added together at maximally effective concentrations, PTH and thrombin produced additive effects on Cai2+ in the presence and absence of EGTA. The additive effects of PTH and thrombin on Cai2+ were confirmed at the single cell level, using laser-based image analysis. Bradykinin (1 microM) produced a significant increase in Cai2+ in UMR 106-H5 cells which was of lesser magnitude than the peak 2- to 3-fold increase elicited by PTH or thrombin. Preexposure of cells to 10 U/ml thrombin for 2 min abolished the Cai2+ response to bradykinin, whereas preexposure to 240 nM PTH had no effect on the Cai2+ response to bradykinin. Thrombin elicited a rapid increase in the accumulation of 3H-labeled inositol phosphates (IP2 and IP3) in UMR 106-H5 cells, with increases in [3H]1,4,5-IP3 detectable as early as 15 sec after the addition of thrombin. Bradykinin increased [3H]IP production to a lesser extent than thrombin, whereas PTH neither increased [3H]IP accumulation nor potentiated the [3H]IP response to thrombin. The results suggest that thrombin and bradykinin mobilize Cai2+ from a shared IP3-responsive calcium pool, whereas PTH may use signals in addition to 1,4,5-IP3 to mobilize calcium from a distinct cellular calcium pool. Alternatively, specific calcium compartmentalization exists, and there is differential coupling of these agonists to the 1,4,5-IP3/Cai2+ pathway.

Playground injuries: a scientific appraisal of popular concerns

The widespread public concern in Britain over childhood injuries arising from playground accidents is examined using a new national data base on leisure accidents managed by the Department of Trade and Industry. Popular beliefs about the level of risk, the nature and cause of injury, and remedial measures are found to be unsupported by the analysis. Playground risks are comparatively low; accident causes are diverse and most involve long bone injuries and not head injuries as has been widely reported; and the cost of some popular risk reduction measures would seem to be incommensurate with the reasonably-anticipated risk reduction which they might afford. It is suggested that for those who wish to reduce risk further, consideration should be given to increasing public awareness of the many causes of risk, of the opportunities which the individual has to control risk, and of the inevitability of residual risk.

The influence of 12(R)-hydroxyeicosatetraenoic acid on ciliary epithelial sodium, potassium-adenosine triphosphatase activity and intraocular pressure in the rabbit

Ciliary epithelium was isolated from the rabbit eye and used as a source of plasma membrane material for sodium, potassium-adenosine triphosphatase (Na,K-ATPase) measurements. In the presence of 12(R)-hydroxyeicosatetraenoic acid [12(R)HETE], Na,K-ATPase (ouabain-sensitive ATPase) activity was reduced from 22.5 to 16.3 microM phosphate released/mg protein/hr. Ouabain-insensitive ATPase activity was not altered by 12(R)HETE. No changes in ciliary epithelium ATPase activity were observed in the presence of 12(S)HETE. In parallel studies with conscious rabbits, 12(R)HETE applied topically to the eye was found to lower intraocular pressure (IOP). It is possible that the IOP-lowering effect of 12(R)HETE may be, in part, associated with its ability to suppress the Na,K-ATPase activity of the ciliary epithelium.

Ageing-associated 5 kb deletion in human liver mitochondrial DNA

Using PCR technique and restriction mapping, we analyzed liver mitochondrial DNA (mtDNA) of 2 stillborn babies and 55 Chinese subjects from 27 to 86 years old and blood cell mtDNA from 20 subjects of various ages. An ageing-associated 4,977-bp deletion was detected between nucleotide position 8,469 and 13,447 (or between 8,482 and 13,460) in the liver mtDNA of older subjects. In the region containing the junction fragment, we observed a 13 bp repeat "ACCTCCCTCACCA". Moreover, the incidence of the deleted mtDNA of each of the study subjects was found to increase with age. The deletion was found in 5 out of 8 patients of the 31-40 age group and 9 out of 11 patients of the 41-50 age group, and in all the patients over 50 years old. The deletion was not observed in either the mtDNA of the liver of the stillbirth or the blood cells of subjects of all the age groups. These results support our previous contention that liver mitochondrial respiratory functions decline with age and the hypothesis that continuous accumulation of mitochondrial DNA mutation is an important contributor to ageing process.

Characterization of an anti-p185HER2 monoclonal antibody that stimulates receptor function and inhibits tumor cell growth

The HER2 protooncogene encodes a growth factor receptor-like transmembrane protein tyrosine kinase (p185HER2) whose ligand remains to be fully characterized. The overexpression of p185HER2 is implicated in aggressive forms of breast and ovarian cancers. The role of p185HER2 in aggressive malignancy, as well as its cell surface localization, makes it an attractive target for therapeutic monoclonal antibodies. In this report we have studied the modulation of p185HER2 function with 2 monoclonal antibodies, termed 4D5 and 6E9, which bind the extracellular domain of p185HER2. 4D5 inhibited proliferation of p185HER2 overexpressing SK-BR-3 human breast carcinoma cells (ED50 of approximately 1 nM) but did not inhibit proliferation of cultured human breast carcinoma MCF7 cells, low expressors of p185HER2. Monoclonal antibody 6E9 does not inhibit the growth of either cell line. Antibody binding studies revealed 2 populations of p185HER2 molecules on SK-BR-3 cells: one of high abundance (approximately 2 x 10(6) sites/cell) recognized by 4D5 (Kd approximately 6 nM) and the other of low abundance (2 x 10(4) sites/cell) recognized by 6E9 (Kd approximately 0.1 nM). 4D5, in an agonistic manner, downregulated SK-BR-3 cell surface p185HER2, was internalized, and stimulated p185HER2 phosphorylation in intact cells. Phosphoamino acid analysis of p185HER2 derived from SK-BR-3 cells incubated with the 4D5 monoclonal antibody demonstrated increased tyrosine, serine and threonine phosphorylation. 4D5, on short term (5 min) exposure to SK-BR-3 cells, stimulated inositol lipid hydrolysis as evidenced by increased intracellular levels of inositol polyphosphates (InsP) and sn-1,2-diacylglycerol (sn-1,2-DAG). On longer (24 h) exposure to the cells, the antibody appeared to downregulate this signalling pathway since the intracellular levels of InsP and sn-1,2-DAG decreased by 30 to 40%. 6E9 did not inhibit SK-BR-3 cell proliferation, downregulate surface p185HER2, stimulate receptor phosphorylation, or stimulate the second messenger pathway. Despite these agonistic properties, 4D5 was an inhibitor of SK-BR-3 cell proliferation at all concentrations tested (0.7 to 70 pM). The data suggest that 4D5 is a partial or weak agonist and thus may inhibit cell proliferation by mimicking ligand-like receptor downregulation.

Calcium transport, Ca2(+)-ATPase, and lipid order in rabbit ocular lens membranes

Calcium transport was monitored by measuring ATP-dependent 45Ca uptake into membrane vesicles prepared from rabbit lens cortex. Calcium-stimulated adenosinetriphosphatase (Ca2(+)-ATPase) activity was also measured in the same membrane preparation. Both uptake and Ca2(+)-ATPase activity were inhibited by vanadate. Calcium activation of the uptake process was similar to that of the Ca2(+)-ATPase. Calcium uptake was prevented by calcium ionophore A23187, suggesting that the calcium transported into the vesicles remains diffusible. The ATP-dependent calcium uptake probably represents the transport of calcium into "inside-out" membrane vesicles by the Ca2(+)-ATPase mechanism that normally shifts calcium outward from the lens cytoplasm. The temperature dependence of the Ca2(+)-ATPase and the calcium uptake process was determined. Because lipid order can influence Ca2(+)-ATPase function, we attempted to correlate calcium transport with the physical state of the membrane lipids. Infrared spectroscopy was used to determine the temperature dependence of the CH2 symmetric stretching frequency (an order parameter) in the lipids. A similarity was noted between the temperature-dependence curves for lipid order, Ca2(+)-ATPase, and calcium uptake rate. Entropy, enthalpy, and transition temperature calculated for the Ca2(+)-ATPase and calcium uptake process were in the same range as those parameters calculated for the lipid-phase transition.

Centripetal flow and directed reassembly of the major sperm protein (MSP) cytoskeleton in the amoeboid sperm of the nematode, Ascaris suum

The cytoskeleton of the amoeboid spermatozoa of Ascaris suum consists of major sperm protein (MSP) filaments arranged into long, branched fiber complexes that span the length of the pseudopod and treadmill rearward continuously due to assembly and disassembly at opposite ends of the complexes (Sepsenwol et al., Journal of Cell Biology 108:55-66, (1989)). Examination by video-enhanced microscopy showed that this cytoskeletal flow is tightly coupled to sperm locomotion. The fiber complexes treadmilled rearward at the same rate (10-50 microns/min) as the cell crawled forward. Only fiber complexes with their plasmalemmal ends within a limited sector along the leading edge of the pseudopod underwent continuous assembly. Thus, the location of this sector, which occupies about 50% of the pseudopod perimeter, determined the direction of sperm locomotion. Treatment of sperm with agents that lower intracellular pH, such as weak acids and protonophores, caused the fiber complexes to disassemble completely in 4-5 sec. Removal of these compounds resulted in reassembly of the cytoskeleton in a pattern that mimicked treadmilling in intact sperm. The fiber complexes were reconstructed by assembly at their plasmalemmal ends so that within 30-60 sec the entire filament system reformed and the cell resumed locomotion. Both cytoskeletal reassembly and treadmilling required exogenous HCO3-. These results suggest that variation in intracellular pH may help regulate cytoskeletal treadmilling and thereby play a significant role in sperm locomotion.

Growth hormone stimulates the formation of sn-1,2-diacylglycerol in rat hepatocytes

The transmembrane signaling events of GH were investigated in the liver, a major target organ of GH action. Recombinant human GH when added to freshly isolated rat hepatocytes rapidly stimulated the production of sn-1,2-diacylglycerol (DAG). The generation of DAG was biphasic with the first transient peak observed at 2 min and the second peak at 15 min (1.2-fold and 1.4-fold over control, respectively). Levels of DAG continued to be elevated above those in control cells at 30 min. The response was dose-dependent with an EC50 of 0.15 nM. Both bovine GH and rat GH, which bind to the rat GH receptor but not to the PRL receptor, also stimulated DAG production. Similarly, human PRL, which binds to the PRL but not GH receptor, stimulated DAG formation to a comparable extent. These results suggest that production of DAG may be an early signaling event mediated by hormone stimulation of both the GH and PRL receptors.