Myosin isoforms and functional diversity in vertebrate smooth muscle

The expression of fast and slow myosin isoforms in individual cells is associated with differences in shortening velocities and power output in fully differentiated vertebrate striated muscle. This paradigm in which shortening velocity is determined by the myosin isoform (and load) is inappropriate for smooth muscle. Smooth muscle tissues express multiple myosin heavy and light chain isoforms, and it is not currently possible to separate and identify chemically distinct native myosin hexamers (i.e., isoforms). It is not known if different isoforms are localized in subpopulations of cells or in specific cellular domains nor whether they combine preferentially to form a small number of native myosin hexamer isoforms. Potentially, thick filaments are aggregates of many different combinations of heavy and light chain isoforms that may or may not exhibit different kinetics. Shortening velocities in smooth muscle are regulated by Ca(2+)-dependent crossbridge phosphorylation of the myosin regulatory light chains. Much of the observed diversity in power output in smooth muscle may be attributed to regulatory mechanisms modulating crossbridge cycling rates rather than contractile protein isoform expression.

Detection of brain-derived neurotrophic factor in a vesicular fraction of brain synaptosomes

The mRNA encoding brain-derived neurotrophic factor (BDNF) is widely distributed in central nervous system neurons, including in hippocampus and cortex. However, little is known about the physiology of BDNF protein within neurons, including how it is processed or packaged and the mechanisms that control its release. In this study, we have used antibodies to monitor the subcellular distribution of BDNF in cortical extracts from adult rats treated with kainic acid. BDNF immunoreactivity is elevated in rat cortex 12 h after kainic acid treatment. The protein is enriched in a vesicular fraction isolated from lysed synaptosomes, its distribution being similar to that of synaptotagmin, which is associated with synaptic vesicles and large dense core vesicles at nerve terminals. The vesicular pool of BDNF is digested by proteinase K only in the presence of Triton X-100 suggesting localization of BDNF in membrane fractions. Immunocytochemistry detects diffuse and punctate BDNF staining within cell bodies and processes of cortical neurons from kainic acid-treated rats, as well as in mossy fiber terminals of rat hippocampus. Taken together, these data show that BDNF can accumulate axonally within a vesicular compartment of brain neurons. Results support the idea that endogenous BDNF may be transported anterogradely and released by regulated secretory mechanisms.

Experimental apexigenesis in baboons

Apexification with calcium hydroxide is a routine procedure. However, some clinical reports suggest that root completion can occur by controlling the infection without use of a catalyst. The present study investigated the use of tetracycline treatment (in root canals) on root growth in immature teeth, rendered non-vital experimentally. Incisors in 3 young baboons were exposed and canals were left open. After 2 months all canals were cleaned and treated with either tetracycline or formocresol. Some canals in each group were filed. Animals were sacrificed after 6 months. Bacterial evaluations were done before placing medications, one week later and six months after that. The number of bacteria were reduced in all treatment groups. Root growth almost near completion was observed in more teeth treated with tetracycline than in the formocresol group.

Systematic comparison of the effects of hippocampal and fornix-fimbria lesions on acquisition of three configural discriminations

The effects of lesions to the hippocampal system on acquisition of three different configural tasks by rats were tested. Lesions of either the hippocampus (kainic acid/colchicine) or fornix-fimbria (radiofrequency current) were made before training. After recovery from surgery, rats were trained to discriminate between simple and compound-configural cues that signaled the availability or nonavailability of food when a bar was pressed. When positive cues were present, one food pellet could be earned by pressing a lever after a variable time had elapsed. The trial terminated on food delivery (variable interval 15 s). This procedure eliminates some possible alternative explanations of the results of previous experiments on configural learning. Hippocampal lesions increased rates of responding and retarded acquisition of a negative patterning task (A+, B+, AB-); using a ratio measure of discrimination performance these lesions had a milder retarding effect on a biconditional discrimination (AX+, AY-, BY+, BX-), and they had no effect on a conditional context discrimination (X: A+, B-; Y: A-, B+). Fornix-fimbria lesions did not affect acquisition of any of these tasks but increased rates of responding. The results suggest that several task parameters determine the involvement of the hippocampus in configural learning; however, all tasks tested can also be learned to some extent in the absence of an intact hippocampal system, presumably by other learning/memory systems that remain intact following surgery. The lack of effect of fornix-fimbria lesions on any of these tasks suggests that retrohippocampal connections with other brain areas may mediate hippocampal contributions to the learning of some configural tasks. An analysis of these results and of experiments on spatial learning situations suggests that involvement of the hippocampus is a function of the degree to which correct performance depends on a knowledge of relationships among cues in a situation.

Cariostatic effect of aspartame in rats

This study examined the effects of aspartame (APM) on caries and the Streptococcus sobrinus population. Six groups of rats (20 in each group) colonized with S. sobrinus were fed varying amounts of sucrose (SU) and APM in their diets as follows: 1-0.15% APM, 2-0.30% APM, 3-30% SU, 4-30% SU + 0.15% APM, 5-50% SU and 6-50% + 0.15% APM. Ten from each group were sacrificed at 6 weeks and 10 at 12 weeks. S. sobrinus populations at both intervals were negligible in groups 1 and 2 and had no differences between groups 3, 4, 5 and 6. No caries were found in groups 1 and 2 Animals fed SU plus APM had significantly lower caries than animals fed the same amounts of SU (p = 0.001-0.0002). We conclude that APM is noncariogenic and anticariogenic.

Cellular processing of the nerve growth factor precursor by the mammalian pro-protein convertases

In order to define the enzymes responsible for the maturation of the precursor of nerve growth factor (proNGF), its biosynthesis and intracellular processing by the pro-protein convertases furin, PC1, PC2, PACE4, PC5 and the PC5 isoform PC5/6-B were analysed using the vaccinia virus expression system in cells containing a regulated and/or a constitutive secretory pathway. Results demonstrate that in both cell types furin, and to a lesser extent PACE4 and PC5/6-B, are the best candidate proNGF convertases. Furthermore, two processed NGF forms of 16.5 and 13.5 kDa were evident in constitutively secreting cell lines such as LoVo and BSC40 cells, whereas only the 13.5 kDa form was observed in AtT20 cells, which contain secretory granules. Both forms display the same N-terminal sequence as mature NGF, and were also produced following site-directed mutagenesis of the C-terminal Arg-Arg sequence of NGF into Ala-Ala, suggesting that the difference between them is not at the C-terminus. Co-expression of proNGF with furin and either chromogranin B or secretogranin II (but not chromogranin A) in BSC40 cells eliminated the 16.5 kDa form. Data also show that N-glycosylation of the pro-segment of proNGF and trimming of the oligosaccharide chains are necessary for the exit of this precursor from the endoplasmic reticulum and its eventual processing and secretion. Sulphate labelling experiments demonstrated that proNGF is processed into mature NGF following the arrival of the precursor in the trans-Golgi network. This comparative study shows that the three candidate mammalian subtilisin/kexin-like convertases identified process proNGF into NGF and that the nature of the final processed products is dependent on the intracellular environment.

Cellular processing of the neurotrophin precursors of NT3 and BDNF by the mammalian proprotein convertases

In order to define the enzymes responsible for the maturation of the precursors of brain-derived neurotrophic factor (proBDNF) and neurotrophin-3 (proNT3), we have analysed their biosynthesis and intracellular processing by the proprotein convertases furin, PC1, PC2, PACE4, PC5 and its isoform PC5/6-B. In these studies, we utilized a vaccinia virus expression system in either BSC40 or the furin activity-deficient LoVo cells. Results demonstrated that in both cells furin and, to a lesser extent, PACE4 and PC5/6-B effectively process proBDNF and proNT3. Furthermore, we have determined that human proNT3 is sulfated, suggesting that processing of proNT3 occurs following the arrival of the precursor to the Trans Golgi Network.

Pulmonary mucormycosis

Mucormycosis is an opportunistic, angioinvasive fungal infection characteristically affecting individuals with diabetes mellitus, chronic renal failure, and hematologic malignancies. In most cases it is a rapidly progressive infection with an 80% overall mortality. Radiographic manifestations are usually nonspecific focal consolidation or masses. The air crescent sign is a rare manifestation of angioinvasive fungi and indicates either aspergillosis or mucormycosis.

Dependence of force on length at constant cross-bridge phosphorylation in the swine carotid media

1. The dependence of force (F) on length (L) in smooth muscle remains uncertain since (i) it is influenced by changes in activation (myosin light chain phosphorylation), (ii) no anatomical reference length for the contractile unit is available, (iii) the length at which optimum force is generated (L(o)) exhibits a broad, flat optimum, and (iv) the presence of an extensive connective tissue network makes it difficult to stretch tissues without damage. 2. A swine carotid medial ring preparation prepared by removal of the adventitia and endothelium could be stretched to 1.8 L(o) without decreasing active force generation on return to shorter lengths. 3. A highly reproducible mechanically defined reference length, L(o), was obtained by fitting force-length data between 0.3 and 1.6 L(o) with a third-order polynomial where L = L(o) when dF/dL = 0. 4. Activation as assessed by myosin regulatory light chain (MRLC) phosphorylation increased with length in 100 microM histamine-stimulated tissues from 0.6 to 1.8 L(o). 5. Activation was constant in K(+)-depolarized and field-stimulated tissues from 1.0 to 1.8 L(o) allowing determination of the descending limb of the force-length relation to be assessed independently of activation. 6. The slope of the descending limb of the force-length relation was linear except at very long lengths, which often produced tissue damage. The slope was not statistically different from that estimated for sarcomeres in vertebrate skeletal muscle. 7. The medial ring preparation and the procedures used to define the reference length provide advantages for the measurement of length-dependent variables.

Genistein, a tyrosine kinase inhibitor, reduces Ca2+ mobilization in swine carotid media

DiSalvo and colleagues (Biochem. Biophys. Res. Commun. 190: 968-974, 1993) found that tyrosine kinase inhibitors reduced force at constant Ca2+ concentrations in permeabilized mesenteric arterioles. These data suggest that tyrosine kinase activation could regulate Ca2+ sensitivity in intact vascular smooth muscle. We tested this hypothesis by examining the effects of the tyrosine kinase inhibitor genistein on intracellular Ca2+ concentration ([Ca2+]i), myosin regulatory light chain (MRLC) phosphorylation, and isometric stress in intact swine carotid media tissues. Pretreatment with 30 microM genistein attenuated histamine-induced increases in [Ca2+]i (estimated using the photoprotein aequorin), MRLC phosphorylation, and stress. The genistein-dependent decrease in [Ca2+]i quantitatively accounted for the decrease in MRLC phosphorylation and stress. There was no measurable change in the Ca2+ dependence of MRLC phosphorylation or the dependence of force on MRLC phosphorylation. Genistein inhibited contractions independently of the source of activator Ca2+. These data suggest that tyrosine kinase(s) may influence force development in the intact swine carotid media by altering [Ca2+]i rather than modulating the Ca2+ sensitivity of MRLC phosphorylation.

Technical note: detection and quantification of supplemental fungal beta-glucanase activity in animal feed

Selected hydrolytic enzymes are added to animal feeds in order to degrade specific antinutritional factors and(or) to increase availability of certain components of feedstuffs to the animal. A method is described that allows detection and quantification of beta-glucanase activity in complex feedstuffs. The method is based on radial diffusion of an enzyme-containing feed extract through an agar gel in which lichenan substrate (a relatively inexpensive glucan of mixed beta 1-->4 and beta 1-->3 linkages) has been dissolved. A linear relationship between the diameter of the zone of substrate hydrolyzed and the log of enzyme activity present was observed. The assay described is technically straightforward and requires no specialized equipment. At typical commercial inclusion levels (1 kg/t), the activity of a supplemental beta-glucanase, added to feed in a commercial mill was determined by averaging several measurements, with a precision of +/- 4%, variation between individual readings of +/- 11.3% (SD), and recovery of 109%. By using high-concentration feed extracts, the method was sensitive enough to detect background and(or) supplemental beta-glucanase activities as low as .05 kg/t supplement equivalent. This method allows consumers, producers, and regulatory authorities to measure the activity of beta-glucanase in feed at commercial inclusion levels and, hence, study the effects of processes such as pelleting and extrusion on such supplements.

Dependence of ATP consumption on cross-bridge phosphorylation in swine carotid smooth muscle

1. Ca(2+)-dependent phosphorylation of the myosin regulatory light chain (MRLC) initiates cross-bridge cycling and contraction in smooth muscle. A four-state cross-bridge model, in which Ca(2+)-dependent phosphorylation is the only proposed regulatory mechanism, can predict the mechanical output of the swine carotid media. Our aims were to determine whether ATP consumption rates and the economy of force maintenance are regulated functions of MRLC phosphorylation as predicted by the model. 2. Steady-state force and oxygen consumption were measured in medial rings of swine carotid arteries activated with depolarizing solutions and agents capable of maintaining a wide range of steady-state myoplasmic Ca2+ and MRLC phosphorylation levels. 3. Suprabasal ATP consumption increased almost linearly with MRLC phosphorylation and exhibited a hyperbolic increase with active stress, as predicted. 4. The economy of stress maintenance fell with increases in suprabasal phosphorylation. 5. In absolute terms the energetic cost of covalent regulation by cross-bridge phosphorylation was small, although it may be a significant fraction of the ATP consumption associated with contraction.

Cyclic nucleotide dependent relaxation in vascular smooth muscle

Although not without controversy, the mechanisms inducing contraction of vascular smooth muscle are relatively well defined. There is a stimulus-induced increase in myoplasmic [Ca2+] with activation of myosin light chain kinase by the Ca(2+)-calmodulin complex, phosphorylation of the 20-kDa regulatory light chain of myosin, with subsequent cross-bridge cycling and force development. Ca(2+)-dependent phosphorylation of the myosin regulatory light chain appears to be the primary mechanism responsible for regulating stress in vascular smooth muscle. The relationship between myoplasmic [Ca2+] and myosin phosphorylation (i.e., the calcium sensitivity of phosphorylation) is regulated. It is higher with agonist stimulation than in tissues depolarized with high potassium solutions or after skinning procedures. The relationship between myosin phosphorylation and stress appears to be invariant with physiologic stimulation. This suggests that cross-bridge phosphorylation normally determines contraction. The mechanisms of relaxation are less well defined. In the most simple scheme, reduction of myoplasmic [Ca2+] with a fall in myosin light chain kinase activity would suffice to account for dephosphorylation of the regulatory light chain and relaxation. However, other mechanisms have been implicated in cyclic nucleotide dependent relaxation in vascular and other smooth muscle tissues. The current hypotheses of the mechanism of cyclic nucleotide dependent relaxation in vascular smooth muscle are reviewed.

Regulation of tissue-specific expression of alternative peripheral myelin protein-22 (PMP22) gene transcripts by two promoters

Mutations affecting the peripheral myelin protein-22 (PMP22) gene have been shown to be associated with inherited peripheral neuropathies. To provide the molecular basis for the analysis of such mutations, we have cloned and characterized the human PMP22 gene. It spans approximately 40 kilobases and contains four coding exons. Detailed analysis of its 5'-flanking region suggested the presence of two alternatively transcribed, but untranslated exons. Mapping of separate PMP22 mRNA transcription initiation sites to each of these exons indicates that PMP22 expression is regulated by two alternatively used promoters. In support of this hypothesis, both putative promoter sequences demonstrated the ability to drive expression of reporter genes in transfection experiments. Furthermore, the structures of the 5'-portions of the PMP22 genes appear to be identical in rat and human, supporting the biological significance of the observed arrangement of regulatory regions. The relative expression of the alternative PMP22 transcripts is tissue-specific, and high levels of the exon 1A-containing transcript are tightly coupled to myelin formation. In contrast, exon 1B-containing transcripts are predominant in non-neural tissues and in growth-arrested primary fibroblasts. Interestingly, although a strong upregulation of PMP22 mRNA was observed in cultured Schwann cells in the presence of the adenylate cyclase activator forskolin under various culture conditions, the regulation of the different PMP22 mRNA species did not mimic the regulation that occurs during myelin formation in vivo. The observed regulation of the PMP22 gene by a complex molecular mechanism is consistent with the proposed dual role of PMP22 in neural and non-neural tissue.

Energetics of crossbridge phosphorylation and contraction in vascular smooth muscle

Ca(2+)-dependent crossbridge phosphorylation is the primary mechanism governing crossbridge cycling in smooth muscle. A four-state crossbridge model in which phosphorylation is the only proposed regulatory mechanism was successful in predicting the mechanical properties of the swine carotid media including latch (sustained force with reduced crossbridge cycling). This model also predicts that the ATP consumption of crossbridge phosphorylation is approximately equal to that of crossbridge cycling and that ATP consumption will rise hyperbolically with increases in steady-state force. This review shows these predictions to be consistent with the available energetics data for the carotid media. The absolute energetic cost of covalent regulation is modest and less than the energy savings associated with latch. However, covalent regulation should reduce the total mechanical efficiency of smooth muscle relative to striated muscle.

What is special about smooth muscle? The significance of covalent crossbridge regulation

Knowledge of the molecular mechanisms involved in chemomechanical transduction and the regulatory elements determining contraction and relaxation are largely derived from cross-striated muscle. However, it has become clear that vertebrate smooth muscles have features lacking in striated muscle. These derive from regulated crossbridge cycling rates observed as variable shortening velocities and ATP consumption rates. The special properties of smooth muscle are associated with differences in the physiological roles of muscle in the walls of hollow organs and reflect a common molecular motor governed by different regulatory mechanisms. A remarkably simple scheme involving Ca(2+)-dependent phosphorylation of crossbridges in smooth muscle can predict much of the mechanical and energetics behavior characterizing the muscle of hollow organs. Nevertheless, many unresolved issues are identified that are the focus of current research efforts.

Immunocytochemical localization of sodium channels in an insect central nervous system using a site-directed antibody

Antibodies to channel proteins and specific peptide sequences have been previously used to localize voltage-activated sodium channels in the rat brain. Here we describe the first localization of sodium channels in an insect nervous system using a site-directed antibody. The mesothoracic ganglion of the cockroach was stained with an antibody to the highly conserved SP19 sequence. Antibody labelling was visualized by light microscopy using the avidin/biotin method on wax sections, and transmission electron microscopy of immunogold-labelled thin sections. Central ganglia of insects contain clearly separated regions of cell bodies, synaptic neuropil, axon tracts, and nerves. Antibody staining by light microscopy was limited to neurons, and was intense in axons throughout the ganglion and nerves. Staining was also strong in the cytoplasm, but not the nuclei, of many neuronal cell bodies. Neuropil regions were relatively lightly labelled. These findings can be correlated with the known electrophysiology of the ganglion. Electron microscopy detected sodium channels in areas surrounding axons, probably including axon membranes and enveloping glial cell membranes. Axonal mitochondria were also heavily labelled, suggesting a sodium channel transport function for these organelles.

Immunological relationships of NGF, BDNF, and NT-3: recognition and functional inhibition by antibodies to NGF

Polyclonal antibodies raised against mouse 2.5S NGF (mNGF) and against synthetic peptides made from hydrophilic portions of mNGF have been used to compare the immunological properties of mNGF, human recombinant brain-derived neurotrophic factor (hrBDNF), and human recombinant neurotrophin-3 (hrNT-3). Affinity-isolated antibodies raised against intact mNGF reacted with all three neurotrophins when tested by ELISA and totally or partially blocked the bioactivities of the proteins in survival assays of embryonic chicken sensory and sympathetic neurons. On Western blots, mNGF antibodies reacted with all three neurotrophins but less well with hrBDNF and hrNT-3 than with mNGF. Antibodies to hydrophilic peptides within NGF (amino acids 23-35, 59-67, 69-79, and 91-100) showed partial reactivity with some but not all of the neurotrophins when tested by ELISA and on Western blots. The peptide antibodies were also selectively effective in reducing the survival-promoting activity of the neurotrophins on sensory neurons. Results show that mNGF, hrBDNF, and hrNT-3 are immunologically related proteins and that mNGF antibodies react also with other members of the neurotrophin family.

Expression of myosin regulatory light-chain isoforms and regulation of phosphorylation in smooth muscle

Our objectives were to 1) determine how growth state and cell density affect the expression of the smooth muscle (SM) and nonmuscle (NM) isoforms of the 20-kDa myosin regulatory light chains (MLC20) in cultured rat aortic smooth muscle cells (SMC) and 2) to determine whether angiotensin II stimulates differential phosphorylation of SM and NM MLC20 isoforms in an effort to assess whether the SM and NM isoforms may subserve different cellular functions. The results demonstrated that changes in the SM MLC20 isoform content were inversely correlated with cell growth but independent of cell density. MLC20 phosphorylation levels were 20.8 +/- 2.9 and 19.4 +/- 3.7% for SM and NM isoforms, respectively, in unstimulated, substrate-attached SMC. Angiotensin II transiently elevated phosphorylation levels of both the SM and NM MLC20 isoforms to 60-70%. No differences in either the magnitude or the kinetics of phosphorylation were observed for the SM vs. NM isoforms. Forskolin, 3-isobutyl-1-methylxanthine, or isoproterenol treatment led to parallel dephosphorylation of the SM- and NM-specific isoforms followed by depolymerization of stress fibers and cell arborization. The studies provide evidence that growth arrest of cultured SMC enhances expression of cell-specific/-selective proteins characteristic of differentiated SM. However, there was no evidence for differential phosphorylation changes of SM and NM MLC20 isoforms in response to activating or relaxing agents as expected if these isoforms subserve different cellular functions.

Models of the mechanism for crossbridge attachment in smooth muscle

The mechanism responsible for formation of attached, dephosphorylated crossbridges (latchbridges) in smooth muscle is controversial. Myosin light chain phosphorylation may be obligatory for crossbridge attachment; if this were the case, latchbridges would arise solely by dephosphorylation of attached, phosphorylated crossbridges. Alternatively, the presence of attached crossbridges could induce cooperative activation by allowing dephosphorylated crossbridges to attach to the thin filament. We evaluated whether four-state models based on dephosphorylation and/or cooperativity-regulated attachment could quantitatively predict smooth muscle contractile behaviour. Five quantitative models for transitions between crossbridge states were developed. Mechanisms for latchbridge formation included: (1) dephosphorylation, (2) cooperativity-regulated attachment dependent only on attached, phosphorylated crossbridges, (3) cooperativity-regulated attachment dependent on all attached crossbridges, (4) dephosphorylation and cooperativity-regulated attachment dependent only on attached, phosphorylated crossbridges, and (5) dephosphorylation and cooperativity-regulated attachment dependent on all attached crossbridges. All five models approximated the time course of contraction and the dependence of steady-state stress on myosin phosphorylation in the swine carotid artery. In the two models that had cooperative attachment regulated by all attached crossbridges, small increases in the rate constant for cooperativity-regulated attachment resulted in positive feedback and irreversible contraction. We suggest that a number of four-state crossbridge models can predict contractile behaviour in arterial smooth muscle. Potentially, latchbridges could be formed by both dephosphorylation and cooperativity-regulated attachment. If cooperativity-regulated latchbridge attachment does exist in smooth muscle, we suggest that it should be dependent only on the number of phosphorylated crossbridges rather than all attached crossbridges.

Detection and processing of peripheral myelin protein PMP22 in cultured Schwann cells

Peripheral myelin protein, 22 kDa (PMP22), is a myelin molecule associated with Schwann cells in peripheral nerves (Snipes, G. J., Suter, U., Welcher, A. A., and Shooter, E. M. (1992) J. Cell Biol. 117, 225-238). Mutations affecting the PMP22 gene have been implicated in the trembler mutation in mice (Suter, U., Welcher, A. A., Ozcelik, T., Snipes, G. J., Kosaras, B., Francke, U., Billings-Gagliardi, S., Sidman, R. L., and Shooter, E. M. (1992) Nature 356, 241-244; Suter, U., Moskow, J. J., Welcher, A. A., Snipes, G. J., Kosaras, B., Sidman, R. L., Buchberg, A. M., and Shooter, E. M. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 4382-4386) and Charcot-Marie-Tooth Disease in humans (Patel, P. I., Roa, B. B., Welcher, A. A., Schoener-Scott, R., Trask, B. J., Pentao, L., Snipes, G. J., Garcia, C. A., Francke, U., Shooter, E. M., Lupski, J. R., and Suter, U. (1992) Nature genet. 1, 159-165). In this report, we have studied PMP22 production in cultured rat Schwann cells. Schwann cells contain a 1.8-kilobase mRNA transcript coding for PMP22, and its production is up-regulated in vitro by forskolin. Metabolic labeling combined with immunoprecipitation methods using antibodies raised against synthetic peptides of PMP22 reveal that Schwann cells generate the protein from an 18-kDa precursor form which is post-translationally modified by N-linked glycosylation. A second molecule (molecular mass, 48 kDa) that reacted with PMP22 antibodies was also detected in Schwann cells but is not related chemically to PMP22 as determined by pulse-chase labeling. Metabolic labeling of rat sciatic nerve and Western blot analyses of purified rat sciatic nerve myelin reveal that deglycosylation of PMP22 gives rise to an 18-kDa protein similar in size to that in Schwann cells. These results indicate that cultured Schwann cells may provide a good model in which to investigate the production and function of PMP22 and to establish the cellular basis for the protein's involvement in inherited peripheral neuropathies.

Nerve growth factor and epidermal growth factor in mouse submandibular glands: identical diurnal changes and rates of secretagogue-induced synthesis

The granular convoluted tubule cells of the mouse submandibular gland contain high concentrations of nerve growth factor (NGF) and epidermal growth factor (EGF). Levels of the two growth factors increase in parallel during development and after stimulation by thyroid hormone, androgens, and glucocorticoids, suggesting that similar controls regulate the production of both proteins. At the same time, however, the granular convoluted tubules hypertrophy, with resulting increases in glandular size and protein content. In this study, we have monitored changes in salivary concentrations of NGF and EGF in the absence of detectable changes in submandibular size or protein concentration. Both growth factors exhibited identical diurnal changes, peaking between 1200-2000 h. In addition, after secretagogue-stimulated release of the proteins into saliva, reaccumulation began after 2 h, with normal levels of both molecules attained 6 h posttreatment. These data indicate that submandibular NGF and EGF can be controlled by similar cellular or molecular mechanisms independent of the regulation of generalized growth responses.

Nitrovasodilators relax arterial smooth muscle by decreasing [Ca2+]i and uncoupling stress from myosin phosphorylation

Elevations in guanosine 3',5'-cyclic monophosphate concentration ([cGMP]) are proposed to induce arterial smooth muscle relaxation by either 1) decreasing myoplasmic [Ca2+] ([Ca2+]i), 2) decreasing the [Ca2+]i sensitivity of phosphorylation, or 3) uncoupling force from myosin phosphorylation. We evaluated the importance of each of these mechanisms by measuring changes in [cGMP], aequorin- and fura-2-estimated [Ca2+]i, myosin light chain phosphorylation, and stress in histamine-stimulated swine carotid arteries. In tissues submaximally stimulated with 3 microM histamine, nitroprusside (NP) induced a proportional decrease in myoplasmic [Ca2+] and myosin phosphorylation, suggesting that the relaxation was at least partially induced by decreases in [Ca2+]i without a change in the [Ca2+]i sensitivity of phosphorylation. In tissues maximally stimulated with 10 microM histamine, NP and nitroglycerin produced significant relaxations that were not associated with significant sustained reductions in [Ca2+]i or myosin phosphorylation. With both submaximal and maximal histamine stimulation, nitrovasodilators produced more substantial relaxation than that expected from the nitrovasodilator-induced reduction in myosin phosphorylation. These results suggest that nitrovasodilators relax histamine-stimulated swine arterial smooth muscle by at least two mechanisms: 1) reducing [Ca2+]i, an effect observed in submaximally stimulated tissues, and 2) uncoupling of stress from myosin phosphorylation.