Biosynthesis and post-translational processing of the precursor to brain-derived neurotrophic factor

We examined the biosynthesis and post-translational processing of the brain-derived neurotrophic factor precursor (pro-BDNF) in cells infected with a pro-BDNF-encoding vaccinia virus. Metabolic labeling, immunoprecipitation, and SDS-polyacrylamide gel electrophoresis reveal that pro-BDNF is generated as a 32-kDa precursor that is N-glycosylated and glycosulfated on a site, within the pro-domain. Some pro-BDNF is released extracellularly and is biologically active as demonstrated by its ability to mediate TrkB phosphorylation. The precursor undergoes N-terminal cleavage within the trans-Golgi network and/or immature secretory vesicles to generate mature BDNF (14 kDa). Small amounts of a 28-kDa protein that is immunoprecipitated with BDNF antibodies is also evident. This protein is generated in the endoplasmic reticulum through N-terminal cleavage of pro-BDNF at the Arg-Gly-Leu-Thr(57)- downward arrow-Ser-Leu site. Cleavage is abolished when Arg(54) is changed to Ala (R54A) by in vitro mutagenesis. Blocking generation of 28-kDa BDNF has no effect on the level of mature BDNF and blocking generation of mature BDNF with alpha(1)-PDX, an inhibitor of furin-like enzymes, does not lead to accumulation of the 28-kDa form. These data suggest that 28-kDa pro-BDNF is not an obligatory intermediate in the formation of the 14-kDa form in the constitutive secretory pathway.

Relative validity of contextual and discrete cues

Contextual conditioning during relative validity training was explored in 3 experiments that used an appetitive Pavlovian conditioning preparation with rats. Magazine entries were the conditioned response. In Experiment 1, true-discrimination (TD: AX+, BX-) training generated weaker conditioning of X than did pseudodiscrimination (PD: AX+/-, BX+/-) training. The context showed a similar relative validity effect. Also, both PD training and simple partial reinforcement (X+/-) reduced contextual conditioning more than did unsignaled food, a demonstration of relative validity using partial reinforcement. Experiments 2 and 3 used within-subject and between-subjects designs, respectively, and showed that relative validity was determined by the summation of differences in conditioning to both the common element (X) and the context. Our results are consistent with an attentional model or with a computational comparator model but not with the Rescorla-Wagner (R. A. Rescorla & A. R. Wagner, 1972) model.

Relative validity effects with either one or two more valid cues in Pavlovian and instrumental conditioning

Two experiments investigated the relative validity effect with either 1 or 2 continuously reinforced cues in Wistar rats using appetitive Pavlovian and instrumental preparations. Discrimination training involved 3 compound cues containing a common element (1AX: 1BX: 2CX). In the first true-discrimination group (TD-1), CX was followed by food, but AX and BX were not. In the second true-discrimination group (TD-2), AX and BX but not CX were followed by food. In the third, pseudodiscrimination group (PD), food followed 50% of each compound. Compared with the PD group, there were lower levels of responding to X in Groups TD-1 and TD-2, which did not differ. That is, both TD treatments showed equivalent relative validity effects. There was evidence for a relative validity effect on the context. The Rescorla-Wagner model incorrectly predicts a smaller relative validity effect after the TD-2 than the TD-1 treatment. Comparator theory predicts these results.

A dilution immunoassay to measure myosin regulatory light chain phosphorylation

We examined the quantitation of myosin regulatory light chain phosphorylation (MRLCP) by Western blot and found both offset and saturation errors. The desirable characteristics of an MRLCP assay are that the dynamic range be 60- to 100-fold and that the detection threshold be known and preferably very small relative to total MRLC concentration. No technique examined provided all these characteristics. However, accurate measurements can be obtained by including serial dilutions of the sample to provide a fractional calibration scale in terms of the dephosphorylated light chain and by using interpolation of the phosphorylated band signal intensity to provide values for the relative phosphorylation ratio. We found that this method offers several advantages over methods that rely on signal ratios from single samples: The dilution ratio method is less subject to errors from differences in protein load, it offers estimates of the error in the individual measurement, and has some redundancy that increases the likelihood of obtaining a valid measurement despite gel or membrane artifacts.

Neurotrophin-3 sorts to the constitutive secretory pathway of hippocampal neurons and is diverted to the regulated secretory pathway by coexpression with brain-derived neurotrophic factor

Hippocampal neurons release nerve growth factor (NGF) through the constitutive secretory pathway, thus allowing the protein to be continuously available for promoting nerve cell survival. In contrast, hippocampal neurons use the regulated secretory pathway to process brain-derived neurotrophic factor (BDNF), which alters synaptic activity when released acutely from dense-core vesicles. Thus, understanding how neurons sort and deliver neurotrophins may provide clues to their functions in brain. In this study, we monitored the processing and delivery of neurotrophin-3 (NT-3). Pulse-chase studies, immunocytochemistry, and secretagogue-induced release experiments were performed on cultured hippocampal neurons and AtT-20 cells infected with vaccinia viruses encoding the NT-3 precursor (pro-NT-3). Results show that most newly synthesized NT-3 is released through the constitutive secretory pathway as a result of furin-mediated endoproteolytic cleavage of pro-NT-3 in the trans-Golgi network. Pro-NT-3 can also be diverted into the regulated secretory pathway when cells are treated with alpha1-PDX, a selective inhibitor of furin-like enzymes, or when pro-NT-3 expression is increased by transient transfection methods. In cells coinfected with viruses coding for pro-NT-3 and pro-BDNF, NT-3 is sorted into the regulated pathway, stored in secretory granules, and released in response to extracellular cues together with BDNF, apparently as a result of heterodimerization, as suggested by coimmunoprecipitation data. Taken together, these data show that sorting of the NT-3 precursor can occur in both the constitutive and regulated secretory pathways, which is consistent with NT-3 having both survival-promoting and synapse-altering functions.

Asymptotic judgment of cause in a relative validity paradigm

We report three experiments in which we tested asymptotic and dynamic predictions of the Rescorla-Wagner (R-W) model and the asymptotic predictions of Cheng's probabilistic contrast model (PCM) concerning judgments of causality when there are two possible causal candidates. We used a paradigm in which the presence of a causal candidate that is highly correlated with an effect influences judgments of a second, moderately correlated or uncorrelated cause. In Experiment 1, which involved a moderate outcome density, judgments of a moderately positive cause were attenuated when it was paired with either a perfect positive or perfect negative cause. This attenuation was robust over a large set of trials but was greater when the strong predictor was positive. In Experiment 2, in which there was a low overall density of outcomes, judgments of a moderately correlated positive cause were elevated when this cause was paired with a perfect negative causal candidate. This elevation was also quite robust over a large set of trials. In Experiment 3, estimates of the strength of a causal candidate that was uncorrelated with the outcome were reduced when it was paired with a perfect cause. The predictions of three theoretical models of causal judgments are considered. Both the R-W model and Cheng's PCM accounted for some but not all aspects of the data. Pearce's model of stimulus generalization accounts for a greater proportion of the data.

Evidence that brain-derived neurotrophic factor from presynaptic nerve terminals regulates the phenotype of calbindin-containing neurons in the lateral septum

Brain-derived neurotrophic factor (BDNF) is transported anterogradely in neurons of the CNS and can be released by activity-dependent mechanisms to regulate synaptic plasticity. However, few neural networks have been identified in which the production, transport, and effects of BDNF on postsynaptic neurons can be analyzed in detail. In this study, we have identified such a network. BDNF has been colocalized by immunocytochemistry with tyrosine hydroxylase (TH) in nerve fibers and nerve terminals within the lateral septum of rats. BDNF-containing nerve fibers terminate on a population of calbindin-containing neurons in lateral septum that contain TrkB, the high-affinity receptor for BDNF. Overexpression of BDNF in noradrenergic neurons increased levels of calbindin in septum, as well as in whole-brain lysates. Septal levels of calbindin and BDNF partially decreased after unilateral lesions of the medial forebrain bundle (MFB), induced with 6-hydroxydopamine, a treatment that abolished TH staining. These data suggest that BDNF is anterogradely transported within the MFB in catecholaminergic neurons arising from brainstem nuclei. To determine whether BDNF affects the production of calbindin in lateral septal neurons directly, we tested the effects of BDNF on cultures of septal neurons from embryonic day 16-17 rats. BDNF promoted the expression of calbindin, as well as the arborization of calbindin-containing neurons, but BDNF had no effect on cell division or survival. Together, these results suggest that BDNF, anterogradely transported in catecholaminergic neurons, regulates calbindin expression within the lateral septum.

Activity-dependent activation of TrkB neurotrophin receptors in the adult CNS

In this paper we have investigated the hypothesis that neural activity causes rapid activation of TrkB neurotrophin receptors in the adult mammalian CNS. These studies demonstrate that kainic acid-induced seizures led to a rapid and transient activation of TrkB receptors in the cortex. Subcellular fractionation demonstrated that these activated Trk receptors were preferentially enriched in the synaptosomal membrane fraction that also contained postsynaptic glutamate receptors. The fast activation of synaptic TrkB receptors could be duplicated in isolated cortical synaptosomes with KCl, presumably as a consequence of depolarization-induced BDNF release. Importantly, TrkB activation was also observed following pharmacological activation of brain-stem noradrenergic neurons, which synthesize and anterogradely transport BDNF; treatment with yohimbine led to activation of cortical TrkB receptors within 30 min. Pharmacological blockade of the postsynaptic alpha1-adrenergic receptors with prazosin only partially inhibited this effect, suggesting that the TrkB activation was partially due to a direct effect on postsynaptic cortical neurons. Together, these data support the hypothesis that activity causes release of BDNF from presynaptic terminals, resulting in a rapid activation of postsynaptic TrkB receptors. This activity-dependent TrkB activation could play a major role in morphological growth and remodelling in both the developing and mature nervous systems.

Inhibition of Ca2+-dependent contraction in swine carotid artery by myosin kinase inhibitors

Experiments were designed to examine the efficacy of the MLCK inhibitors wortmannin and ML-9 in intact smooth muscle to determine whether contractile agonists can induce a Ca(2+) and myosin light chain phosphorylation-independent contraction. Both wortmannin and ML-9 reduced active stress in a dose-dependent manner. Both inhibitors interfered with Ca2+ mobilization in either the K(+)-depolarized or agonist activated swine carotid media at concentrations greater than 10 microM. Wortmannin reduced MRLC phosphorylation and stress to resting levels in stimulated tissues while Ca2+ remained above resting levels. There was no evidence for Ca2+ and MRLC phosphorylation-independent stress generation in swine arterial smooth muscle.

Differential sorting of nerve growth factor and brain-derived neurotrophic factor in hippocampal neurons

Nerve growth factor (NGF) is released through the constitutive secretory pathway from cells in peripheral tissues and nerves where it can act as a target-derived survival factor. In contrast, brain-derived neurotrophic factor (BDNF) appears to be processed in the regulated secretory pathway of brain neurons and secreted in an activity-dependent manner to play a role in synaptic plasticity. To determine whether sorting differences are intrinsic to the neurotrophins or reflect differences between cell types, we compared NGF and BDNF processing in cultured hippocampal neurons using a Vaccinia virus expression system. Three independent criteria (retention or release from cells after pulse-chase labeling, depolarization-dependent release, and immunocytochemical localization) suggest that the bulk of newly synthesized NGF is sorted into the constitutive pathway, whereas BDNF is primarily sorted into the regulated secretory pathway. Similar results occurred with AtT 20 cells, including those transfected with cDNAs encoding neurotrophin precursor-green fluorescent protein fusions. The NGF precursor, but not the BDNF precursor, is efficiently cleaved by the endoprotease furin in the trans-Golgi network (TGN). Blocking furin activity in AtT 20 cells with alpha1-PDX as well as increasing the expression of NGF precursor partially directed NGF into the regulated secretory pathway. Therefore, neurotrophins can be sorted into either the constitutive or regulated secretory pathways, and sorting may be regulated by the efficiency of furin cleavage in the TGN. This mechanism may explain how neuron-generated neurotrophins can act both as survival factors and as neuropeptides.

Mammalian subtilisin/kexin isozyme SKI-1: A widely expressed proprotein convertase with a unique cleavage specificity and cellular localization

Using reverse transcriptase-PCR and degenerate oligonucleotides derived from the active-site residues of subtilisin/kexin-like serine proteinases, we have identified a highly conserved and phylogenetically ancestral human, rat, and mouse type I membrane-bound proteinase called subtilisin/kexin-isozyme-1 (SKI-1). Computer databank searches reveal that human SKI-1 was cloned previously but with no identified function. In situ hybridization demonstrates that SKI-1 mRNA is present in most tissues and cells. Cleavage specificity studies show that SKI-1 generates a 28-kDa product from the 32-kDa brain-derived neurotrophic factor precursor, cleaving at an RGLT downward arrowSL bond. In the endoplasmic reticulum of either LoVo or HK293 cells, proSKI-1 is processed into two membrane-bound forms of SKI-1 (120 and 106 kDa) differing by the nature of their N-glycosylation. Late along the secretory pathway some of the membrane-bound enzyme is shed into the medium as a 98-kDa form. Immunocytochemical analysis of stably transfected HK293 cells shows that SKI-1 is present in the Golgi apparatus and within small punctate structures reminiscent of endosomes. In vitro studies suggest that SKI-1 is a Ca2+-dependent serine proteinase exhibiting a wide pH optimum for cleavage of pro-brain-derived neurotrophic factor.

Inhibitory mechanisms for cross-bridge cycling: the nitric oxide-cGMP signal transduction pathway in smooth muscle relaxation

Relaxation follows sequestration of Ca2+ mobilized by an excitatory stimulus in striated muscle. Removal of excitatory stimuli also relaxes smooth muscle in vitro after reductions in the myoplasmic [Ca2+] and dephosphorylation of the myosin regulatory light chains. However, there are several experimental procedures that produce relaxation in the presence of excitatory stimuli and elevated Ca(2+)-dependent cross-bridge phosphorylation. Of potential widespread physiological importance are treatments that increase myoplasmic [cGMP] owing to the ubiquity of nitric oxide (NO) as a signalling molecule for endothelial-mediated vasodilation and inhibitory nerves in most types of smooth muscle. Several mechanisms are implicated in the NO-cGMP mediated relaxation. Most studies support reductions in myoplasmic Ca2+. However, there is evidence that increases in cGMP also lower the Ca(2+)-sensitivity of cross-bridge phosphorylation. This would contribute to a decline in force through actions on the myosin light chain kinase/phosphatase system. In addition, changes in the dependence of force on phosphorylation are observed in tissues partially relaxed by treatments that elevate cGMP. This demonstrates that either the attachment and cycling of phosphorylated cross-bridges is impaired or blocked, or that the formation of dephosphorylated, force-generating cross-bridges ('latch-bridges') is reduced. Protein kinase G-catalysed phosphorylation of either a thin filament protein that blocks attachment of cross-bridges or a protein that inhibits myosin light chain phosphatase may explain the NO-induced relaxation with elevated cross-bridge phosphorylation.

Minor role of a Ca2+-depleted sarcoplasmic reticulum in heterologous desensitization of smooth muscle to K+

Exposure of porcine carotid artery smooth muscle (PCASM) to histamine was followed by a large reduction in the rate of force generation in response to 40 mM KCl. This was shown to be a manifestation of slow attainment of a steady-state myoplasmic Ca2+ concentration ([Ca2+]i). We hypothesized that if net transsarcolemmal Ca2+ flux into the depolarized PCASM cells is the same before and after a desensitizing histamine treatment, then the transient attenuation of the increase in [Ca2+]i may be due to accelerated uptake of Ca2+ by a partially depleted sarcoplasmic reticulum (SR) acting as a Ca2+ sink or superficial buffer barrier. We tested this hypothesis by eliciting responses of "desensitized PCASM" to 40 mM KCl in the presence of cyclopiazonic acid (CPA), an SR Ca2+-ATPase inhibitor. Contractions of CPA-treated tissues were attenuated less than those of tissues not treated with CPA, but they were not abolished. CPA-insensitive mechanism(s) dominated the desensitization. We conclude that histamine pretreatment reduced net transsarcolemmal Ca2+ flux into PCASM in response to 40 mM KCl.

cGMP mediates corpus cavernosum smooth muscle relaxation with altered cross-bridge function

We tested the prevailing paradigm that relaxation of corpus cavernosum smooth muscle (CCSM) and penile erection depends upon nitric oxide-induced elevation of myoplasmic cGMP and reduced Ca2+-dependent myosin regulatory light chain phosphorylation levels. This hypothesis invokes a reversal of normal activation pathways. Upon stimulation with 250 microM phenylephrine, phosphorylation of the 20 kD myosin regulatory light chains of rabbit or human CCSM increased approximately 4-fold coincident with contraction. Removal of the agonist was followed by a slow reduction in cross-bridge phosphorylation and force to basal levels. The NO donor, sodium nitroprusside elicited a dose-dependent increase in tissue [cGMP] associated with a rapid relaxation in the continued presence of phenylephrine, although cross-bridge phosphorylation remained significantly elevated. Thus the NO-cGMP inhibitory pathway in CCSM is not simply a reversal of excitatory signal transduction mechanisms. An unidentified mechanism contributes to relaxation by decreasing the rate of cross-bridge recruitment through phosphorylation.

Sildenafil, a type-5 CGMP phosphodiesterase inhibitor, specifically amplifies endogenous cGMP-dependent relaxation in rabbit corpus cavernosum smooth muscle in vitro

PURPOSE

The primary mechanism for relaxation of corpus cavernosum smooth muscle (CCSM) and penile erection depends upon nitric oxide (NO)-induced elevation of myoplasmic cyclic guanosine monophosphate (cGMP). Agents that enhance the NO-cGMP signal transduction pathway may prove beneficial in treating erectile dysfunction. Sildenafil, a selective type-5 cGMP phosphodiesterase inhibitor, was investigated to determine the specific mechanism(s) involved in the therapeutic use of this compound to treat impotence.

MATERIALS AND METHODS

Isolated strips of rabbit corpus cavernosum were stimulated isometrically with phenylephrine. Graded relaxations were induced using various concentrations of sodium nitroprusside (SNP) alone and in combination with sildenafil. At fixed times, the tissues were rapidly frozen and processed for myosin light chain (MLC) phosphorylation using isoelectric focusing with Western blot analysis, and cGMP content using radioimmunoassay techniques.

RESULTS

Sildenafil alone reduced spontaneous tone in unstimulated CCSM, but had little effect on phenylephrine-induced isometric tension in the absence of a NO donor (SNP). Sildenafil sensitized the tissue to SNP for relaxation, but the relationship between relaxation and [cGMP] was unchanged by sildenafil. Relaxation from peak isometric force was correlated with [cGMP] but not MLC phosphorylation.

CONCLUSIONS

Sildenafil relaxes CCSM by amplifying the effects of the normal, endogenous cGMP dependent relaxation mechanisms.

Lesions of the hippocampus or fornix do not interfere with the relative validity effect on a discrete stimulus or the context

Two experiments with rats used an appetitively motivated instrumental discrimination procedure to examine whether damage to the hippocampal system would interfere with the relative validity effect in which a partially reinforced stimulus trained in compound acquires a weak conditioned response when concurrently trained CSs are perfect predictors of the presence or absence of the US [A.R. Wagner, F.A. Logan, K. Haberlandt, T. Price, Stimulus selection in animal discrimination learning, J. Exp. Psychol. 76 (1968) 171-180]. The true discrimination (TD) group received training with two compound cues containing a common element. One compound was reinforced and another never reinforced (AX+, BX-). Following TD training the common element (X) failed to acquire a strong conditioned response in comparison to a control, pseudo discrimination (PD) group (AX+/-, BX+/-), in which both compound cues were reinforced 50% of the time. Although (X) was reinforced on a partial reinforcement schedule, (50%) in both groups, conditioning of X was affected by the reinforcement schedules of the accompanying elements (A and B). Neither radio-frequency lesions of the fornix-fimbria (Experiment 1) nor neurotoxic lesions of the hippocampus (Experiment 2) interfered with this effect on X or on the same effect found during the inter trial interval (ITI). Rates of ITI responding were higher in the PD groups than in the TD groups suggesting stronger contextual conditioning following PD training. Furthermore, disruptions to the hippocampal system in Experiment 2 resulted in increased rates of lever pressing for food. These results do not support the notion that the hippocampus is critically involved in selective associations.

Neurons promote the translocation of peripheral myelin protein 22 into myelin

Schwann cells express low levels of myelin proteins in the absence of neurons. When Schwann cells and neurons are cultured together the production of myelin proteins is elevated, and myelin is formed. For peripheral myelin protein 22 (PMP22), the exact amount of protein produced is critical, because peripheral neuropathies result from its underexpression or overexpression. In this study we examined the effect of neurons on Schwann cell PMP22 production in culture and in peripheral nerve using metabolic labeling and pulse-chase studies as well as immunocytochemistry. Most of the newly synthesized PMP22 in Schwann cells is rapidly degraded in the endoplasmic reticulum. Only a small proportion of the total PMP22 acquires complex glycosylation and accumulates in the Golgi compartment. This material is translocated to the Schwann cell membrane in detectable amounts only when axonal contact and myelination occur. Myelination does not, however, alter the rapid turnover of PMP22 in Schwann cells. PMP22 may therefore be a unique myelin protein in that axonal contact promotes its insertion into the Schwann cell membrane and myelin without altering its rapid turnover rate within the cell.

Reciprocal path-scattering effects for a ground-based, monostatic laser radar tracking a space target through turbulence

A phenomenological model is developed for the strength and spatial width of the coherent intensity peak of backscatter produced by reciprocal path scattering through atmospheric turbulence. The model is applied to a ground-based, monostatic laser radar tracking a space target under the condition of optical atmospheric turbulence saturation.

Actin isoform expression, cellular heterogeneity, and contractile function in smooth muscle

Smooth muscles express four isoforms of actin: two smooth muscle specific and two cytoplasmic isoforms typically associated with the cytoskeleton of nonmuscle cells. The relative amounts of each isoform expressed and the total actin content vary with smooth muscle type, with development, in cell culture, pathologically, and potentially between cells within tissues. Our objective was to determine whether actin isoforms contribute to contractile diversity. Functional diversity may be the result of differences in the kinetics of the cross-bridge interaction with thin filaments consisting of different actin isoforms or of the fraction of cross-bridges developing force in series or in parallel resulting from thin filaments of different lengths. Our hypothesis was that functionally significant differences in actin isoform properties (i.e., myosin interactions or properties affecting thin filament lengths) would require isoform segregation into distinct populations of thin filaments within cellular domains (e.g., cytoskeletal and contractile) or in phenotypically different cells. We tested this hypothesis by determining the smooth muscle alpha- and gamma-actin and cytoplasmic beta-actin isoform composition of native thin filaments isolated from swine stomach using isoform-specific antibodies linked to colloidal gold beads with protein A (the cytoplasmic lambda-isoactin content was below the detection limit). The lengths of individual thin filaments were also estimated from electron micrographs. A statistically uniform population of thin filaments was observed consisting of randomly copolymerized isoactins in each filament with the same isoform proportions as the tissue. The average thin filament length was 1.35 +/- 0.06 (SEM) microns. These results, together with other studies, suggest that actin isoforms are functionally equivalent. The data imply that the high stress-generating and shortening capacities of smooth muscles are not primarily due to long thin filament to thick filament length ratios compared with striated muscles.

Heterologous desensitization of smooth muscle to K+ depolarization: retarded stimulus-[Ca2+]i coupling

To understand the phenomenon of postreceptor heterologous desensitization, we exposed porcine carotid media to 40 mM KCl physiological saline solution both before and after intervening treatment with histamine. Increasing histamine concentration or duration of exposure or decreasing the interval between histamine exposure and KCl progressively slowed the contractile responses to K+ depolarization. A delay in initiation and a slower rate of rise of KCl-induced stress in histamine-pretreated muscle were preceded by a slower rate of rise of aequorin-estimated myoplasmic Ca2+ concentration ([Ca2+]i), myosin regulatory light chain (MRLC) phosphorylation, and tissue stiffness, with no detectable change in the Ca2+ sensitivity of MRLC phosphorylation. This heterologous desensitization was not a diminished steady-state force but instead a profound slowing of contraction rates. This slowing was a manifestation of retardation of the rate at which [Ca2+]i rises to the level appropriate for the stimulus. The lack of rapid initial [Ca2+]i and cross-bridge phosphorylation transients as a consequence of histamine pretreatment resulted in very slow cross-bridge cycling rates and rates of force development (latch).

Prohormone convertases in mouse submandibular gland: co-localization of furin and nerve growth factor

Nerve growth factor (NGF) in mouse submandibular glands (SGs) is generated from a 35-kD precursor by proteolytic enzymes that have yet to be identified. Prohormone convertases (PCs) cleave the NGF precursor in vitro, and in this study we questioned whether PCs could process salivary NGF in vivo. mRNA coding for PC2 (but not PC1) was detected on Northern blots of SG mRNA and also by in situ hybridization within parasympathetic neurons of intralobular ganglia. Northern blot and in situ hybridization analyses also detect mRNA coding for furin. In SGs of male mice, furin mRNA levels are high at birth and remain high throughout development. In glands from female mice, levels decline during postnatal development and are lower in adults than in newborns. Immunocytochemistry detects furin immunoreactivity in pro-acinar and ductal cells of glands from newborn and pubescent mice. In glands of adults, furin immunoreactivity is detectable in acinar cells but highest levels are present in NGF-containing granular convoluted tubule cells. These data, taken together with those from previous studies, suggest that furin is a candidate processing enzyme for NGF in mouse submandibular glands.

Energetic cost of activation processes during contraction of swine arterial smooth muscle

1. The objective of this study was to partition the increase in ATP consumption during contraction of swine carotid arterial smooth muscle estimated from suprabasal oxygen consumption (suprabasal JO2) and lactate release (Jlactate) into a component associated with cross-bridge cycling (JX) and one reflecting activation (JA). 2. Two experimental approaches-varying length under constant activation, and varying activation at a long length (1.8 times the optimal length for force development (Lo)) where force generation is minimal-revealed a linear dependence of JO2 and activation energy (JA) on cross-bridge phosphorylation. Protocols inducing a large increase in myosin regulatory light chain (MRLC) phosphorylation at 1.8 Lo resulted in significant elevations of JO2 and marked reductions in the economy of force maintenance. Our evidence suggests that this is primarily due to the increased cost of cross-bridge phosphorylation. 3. The extrapolated estimate of JA during maximal K(+)-induced depolarization made by varying length was 16%, while at 1.8 Lo it was 33% of the suprabasal JO2 at Lo. Calculated activation energies ranged from 17 to 45% of the suprabasal JO2 at Lo and from 72 to 87% of the suprabasal JO2 at 1.8 Lo under stimulation conditions that varied steady-state MRLC phosphorylation from 15 to 50%. 4. The results suggest that the kinetics of cross-bridge phosphorylation-dephosphorylation can rival those of cross-bridge cycling during isometric contractions in swine arterial smooth muscle.