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T Stevens

Publications and source records attributed to T Stevens.

At least 19 recordsLinked to original sources
PubMed1998

Strain-associated differences in hypoxic chemosensitivity of the carotid body in rats.

Studies in humans indicate genetic effects on the ventilatory response to hypoxia, but the site of these effects is unknown. The present study explores the question of whether there are genetically directed effects on the intrinsic hypoxic chemosensitivity of the carotid body. The approach was to study these responses in two inbred rat strains [spontaneously hypertensive rats (SHR) and Fischer 344 (F-344)] and to measure in vivo carotid chemosensitivity as the change in carotid sinus nerve (CSN) activity during progressive, isocapnic hypoxia and the isolated, in vitro responses of excised superfused carotid bodies, loaded with the fluorimetric indicator fura 2, measured as the cytosolic calcium response to moderate hypoxia (PO2 = 55 mmHg). CSN responses in F-344 rats (n = 12) were uniformly low, with a shape parameter A of 13.8 +/- 6.59 (SE), whereas responses in SHR (n = 15) were sevenfold higher (108 +/- 24.1; P < 0.002) and showed greater variation. In vitro, intracellular calcium responses of superfused carotid bodies estimated from the fluorimetric ratio (340/380 nm) showed a greater peak increase during hypoxia in carotid bodies from SHR (140 +/- 4.7%) than from F-344 rats (114 6.0%; P < 0.01). Our results indicate strain-related differences in hypoxic chemosensitivity that are intrinsic to the carotid body and that could mediate genetic effects on ventilatory responsiveness to hypoxia.

Animals↗
PubMed1998

Pulmonary microvascular and macrovascular endothelial cells: differential regulation of Ca2+ and permeability.

Cytosolic Ca2+ concentration ([Ca2+]i) plays an important role in control of pulmonary vascular endothelial cell (ECs) barrier function. In this study, we investigated whether thapsigargin- and ionomycin-induced changes in cytosolic Ca2+ induce permeability in rat pulmonary microvascular (RPMV) versus macrovascular (RPA) ECs. In Transwell cultures, RPMVECs formed a tighter, more restrictive barrier than RPAECs to 12,000-, 72,000-, and 150,000-molecular-weight FITC-labeled dextrans. Thapsigargin (1 microM) produced higher [Ca2+]i levels in RPAECs than in RPMVECs and increased permeability in RPAEC but not in RPMVEC monolayers. Due to the attenuated [Ca2+]i response in RPMVECs, we investigated whether reduced activation of store-operated Ca2+ entry was responsible for the insensitivity to thapsigargin. Addition of the drug in media containing 100 nM extracellular Ca2+ followed by readdition media with 2 mM extracellular Ca2+ increased RPMVEC [Ca2+]i to a level higher than that in RPAECs. Under these conditions, RPMVEC permeability was not increased, suggesting that [Ca2+]i in RPMVECs does not initiate barrier disruption. Also, ionomycin (1.4 microM) did not alter RPMVEC permeability, but the protein phosphatase inhibitor calyculin A (100 nM) induced permeability in RPMVECs. These data indicate that, whereas increased [Ca2+]i promotes permeability in RPAECs, it is not sufficient in RPMVECs, which show an apparent uncoupling of [Ca2+]i signaling pathways or dominant Ca(2+)-independent mechanisms from controlling cellular gap formation and permeability.

Animals↗
PubMed1998

Signal transduction and regulation of lung endothelial cell permeability. Interaction between calcium and cAMP.

Pulmonary endothelium forms a semiselective barrier that regulates fluid balance and leukocyte trafficking. During the course of lung inflammation, neurohumoral mediators and oxidants act on endothelial cells to induce intercellular gaps permissive for transudation of proteinaceous fluid from blood into the interstitium. Intracellular signals activated by neurohumoral mediators and oxidants that evoke intercellular gap formation are incompletely understood. Cytosolic Ca2+ concentration ([Ca2+]i) and cAMP are two signals that importantly dictate cell-cell apposition. Although increased [Ca2+]i promotes disruption of the macrovascular endothelial cell barrier, increased cAMP enhances endothelial barrier function. Furthermore, during the course of inflammation, elevated endothelial cell [Ca2+]i decreases cAMP to facilitate intercellular gap formation. Given the significance of both [Ca2+]i and cAMP in mediating cell-cell apposition, this review addresses potential sites of cross talk between these two intracellular signaling pathways. Emerging data also indicate that endothelial cells derived from different vascular sites within the pulmonary circulation exhibit distinct sensitivities to permeability-inducing stimuli; that is, elevated [Ca2+]i promotes macrovascular but not microvascular barrier disruption. Thus this review also considers the roles of [Ca2+]i and cAMP in mediating site-specific alterations in endothelial permeability.

Animals↗
PubMed1998

Store-operated calcium entry promotes shape change in pulmonary endothelial cells expressing Trp1.

Activation of Ca2+ entry is known to produce endothelial cell shape change, leading to increased permeability, leukocyte migration, and initiation of angiogenesis in conduit-vessel endothelial cells. The mode of Ca2+ entry regulating cell shape is unknown. We hypothesized that activation of store-operated Ca2+ channels (SOCs) is sufficient to promote cell shape change necessary for these processes. SOC activation in rat pulmonary arterial endothelial cells increased free cytosolic Ca2+ that was dependent on a membrane current having a net inward component of 5.45 +/- 0.90 pA/pF at -80 mV. Changes in endothelial cell shape accompanied SOC activation and were dependent on Ca2+ entry-induced reconfiguration of peripheral (cortical) filamentous actin (F-actin). Because the identity of pulmonary endothelial SOCs is unknown, but mammalian homologues of the Drosophila melanogaster transient receptor potential (trp) gene have been proposed to form Ca2+ entry channels in nonexcitable cells, we performed RT-PCR using Trp oligonucleotide primers in both rat and human pulmonary arterial endothelial cells. Both cell types were found to express Trp1, but neither expressed Trp3 nor Trp6. Our study indicates that 1) Ca2+ entry in pulmonary endothelial cells through SOCs produces cell shape change that is dependent on site-specific rearrangement of the microfilamentous cytoskeleton and 2) Trp1 may be a component of pulmonary endothelial SOCs.

Amino Acid Sequence↗
PubMed1997

Do individuals with Williams syndrome have bizarre semantics? Evidence for lexical organization using an on-line task.

Williams syndrome, a neurodevelopmental disorder, has attracted a great deal of debate concerning the purported intactness of language in the face of other serious cognitive deficits. As more in-depth studies of specific aspects of WS language have emerged, the notion of a preserved language module has been seriously challenged. Although WS vocabulary scores are often impressive, several investigators have claimed the WS semantics are aberrant. All studies hitherto have been based on off-line experiments which necessarily involve metalinguistic processes. This clearly affects the performance of individuals with cognitive deficits. We report here an on-line study probing the semantic structure of the WS lexicon, using a task-semantic priming-which minimises metalinguistic demands. We show that WS subjects display the same taxonomic/category and thematic/functional priming effects as normal controls. The results are discussed in terms of the differences between receptive and expressive language, as well as the fact that although semantic memory and the automatic access to semantic information for individual words is normal in WS, the integration of semantic information into sentence comprehension may be abnormal. The importance of online tasks to highlight such differences is stressed.

Adolescent↗
PubMed1997

Effects of single nightly injections of growth hormone-releasing hormone (GHRH 1-29) in healthy elderly men.

Age-related reductions in growth hormone (GH) and insulin-like growth factor-I (IGF-I) may contribute to decreased muscle mass and strength in older persons. The relationship of this phenomenon to skeletal muscle bioenergetics has not been reported. We sought to determine whether administration of GH-releasing hormone (GHRH) would sustain increases in GH and IGF-I and improve skeletal muscle function and selected measures of body composition and metabolism. We measured GH secretion, muscle strength, muscle histology, and muscle energy metabolism by phosphorus nuclear magnetic resonance spectroscopy (31P-NMRS), body composition, and endocrine-metabolic functions before and after 6 weeks of treatment. Eleven healthy, ambulatory, non-obese men aged 64 to 76 years with low baseline IGF-I levels were treated at home as outpatients by nightly subcutaneous self-injections of 2 mg GHRH for 6 weeks. We measured GH levels in blood samples obtained every 20 minutes from 8:00 PM to 8:00 AM; AM serum levels of IGF-I, IGF binding protein-3 (IGFBP-3), and GH binding protein (GHBP); muscle strength; muscle histology; the normalized phosphocreatine abundance, PCr/[PCr + Pi], and intracellular pH in forearm muscle by NMRS during both sustained and ramped exercise; body composition by dual-energy x-ray absorptiometry (DEXA); lipid levels; and glucose, insulin, and GH levels during an oral glucose tolerance test (OGTT). GHRH treatment increased mean nocturnal GH release (P < .02), the area under the GH peak ([AUPGH] P < .006), and GH peak amplitude (P < .05), with no change in GH pulse frequency or in levels of IGF-I, IGFBP-3, or GHBP Two of six measures of muscle strength, upright row (P < .02) and shoulder press (P < .04), and a test of muscle endurance, abdominal crunch (P < .03), improved. GHRH treatment did not alter exercise-mediated changes in PCr/[PCr + Pi] or intracellular pH, but decreased or abolished significant relationships between changes in PCr/[PCr + Pi] or pH and indices of muscle strength. GHRH treatment did not change weight, body mass index, waist to hip ratio, DEXA measures of muscle and fat, muscle histology, glucose, insulin, or GH responses to OGTT, or lipids. No significant adverse effects were observed. These data suggest that single nightly doses of GHRH are less effective than multiple daily doses of GHRH in eliciting GH- and/or IGF-I-mediated effects. GHRH treatment may increase muscle strength, and it alters baseline relationships between muscle strength and muscle bioenergetics in a manner consistent with a reduced need for anaerobic metabolism during exercise. Thus, an optimized regimen of GHRH administration might attenuate some of the effects of aging on skeletal muscle function in older persons.

Aged↗
PubMed1997

Word learning in a special population: do individuals with Williams syndrome obey lexical constraints?

Williams syndrome (WS), a rare neurodevelopmental disorder, is of special interest to developmental psycholinguists because of its uneven linguistico-cognitive profile of abilities and deficits. One proficiency manifest in WS adolescents and adults is an unusually large vocabulary despite serious deficits in other domains. In this paper, rather than focus on vocabulary size, we explore the processes underlying vocabulary acquisition, i.e. how new words are learned. A WS group was compared to groups of normal MA-matched controls in the range 3-9 years in four different experiments testing for constraints on word learning. We show that in construing the meaning of new words, normal children at all ages display fast mapping and abide by the constraints tested: mutual exclusivity, whole object and taxonomic. By contrast, while the WS group showed fast mapping and the mutual exclusivity constraint, they did not abide by the whole object or taxonomic constraints. This suggests that measuring only the size of WS vocabulary can distort conclusions about the normalcy of WS language. Our study shows that despite equivalent behaviour (i.e. vocabulary test age), the processes underlying how vocabulary is acquired in WS follow a somewhat different path from that of normal children and that the atypically developing brain is not necessarily a window on normal development.

Age Factors↗
PubMed1997

Inhaled nitric oxide pretreatment but not posttreatment attenuates ischemia-reperfusion-induced pulmonary microvascular leak.

BACKGROUND: Ischemia-reperfusion (I/R) pulmonary edema probably reflects a leukocyte-dependent, oxidant-mediated mechanism. Nitric oxide (NO) attenuates leukocyte-endothelial cell interactions and I/R-induced microvascular leak. Cyclic adenosine monophosphate (cAMP) agonists reverse and prevent I/R-induced microvascular leak, but reversal by inhaled NO (INO) has not been tested. In addition, the role of soluble guanylyl cyclase (sGC) activation in the NO protection effect is unknown. METHODS: Rat lungs perfused with salt solution were grouped as either I/R, I/R with INO (10 or 50 ppm) on reperfusion, or time control. Capillary filtration coefficients (Kfc) were estimated 25 min before ischemia (baseline) and after 30 and 75 min of reperfusion. Perfusate cell counts and lung homogenate myeloperoxidase activity were determined in selected groups. Additional groups were treated with either INO (50 ppm) or isoproterenol (ISO-10 microM) after 30 min of reperfusion. Guanylyl cyclase was inhibited with 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ-15 microM), and Kfc was estimated at baseline and after 30 min of reperfusion. RESULTS: (1) Inhaled NO attenuated I/R-induced increases in Kfc. (2) Cell counts were similar at baseline. After 75 min of reperfusion, lung neutrophil retention (myeloperoxidase activity) and decreased perfusate neutrophil counts were similar in all groups. (3) In contrast to ISO, INO did not reverse microvascular leak. (4) 8-bromoguanosine 3',5'-cyclic monophosphate (8-br-cGMP) prevented I/R-induced microvascular leak in ODQ-treated lungs, but INO was no longer effective. CONCLUSIONS: Inhaled NO attenuates I/R-induced pulmonary microvascular leak, which requires sGC activation and may involve a mechanism independent of inhibition of leukocyte-endothelial cell interactions. In addition, INO is ineffective in reversing I/R-induced microvascular leak.

Administration, Inhalation↗
PubMed1997

Cytosolic Ca2+ and adenylyl cyclase responses in phenotypically distinct pulmonary endothelial cells.

Pulmonary microvascular endothelium forms a tighter barrier than does pulmonary artery endothelium; the mechanism of this important phenotypic difference is uncertain. We examined two regulators of endothelial permeability, cytosolic Ca2+ concentration ([Ca2+]i) and adenosine 3',5'-cyclic monophosphate (cAMP), in microvascular (PMVEC) and pulmonary conduit artery (PAEC) endothelium. Both resting and stimulated [Ca2+]i were lower in PMVEC compared with PAEC (resting [Ca2+]i, 94 +/- 7 vs. 123 +/- 8 nM; ATP-stimulated peak, 1.04 +/- 0.14 vs. 1.98 +/- 0.13 microM). Sustained Ca2+ transients in response to either ATP or thapsigargin were reduced in PMVEC compared with PAEC (ATP, 199 +/- 22 vs. 411 +/- 43 nM; thapsigargin, 195 +/- 13 vs. 527 +/- 65 nM), suggesting reduced Ca2+ influx in PMVEC. Reduced Ca2+ influx in PMVEC was confirmed by Mn2+ quenching and patch-clamp experiments. mRNA for Ca(2+)-inhibitable and protein kinase C-stimulated adenylyl cyclases was detected in both cell types. Whereas ATP caused a [Ca2+]i-mediated decrease in cAMP in PAEC, ATP caused a protein kinase C-mediated increase in cAMP in PMVEC. We conclude that PMVEC express a unique phenotype that favors enhanced barrier function through attenuated Ca2+ influx and preservation of cAMP content.

Adenosine Triphosphate↗
PubMed1997

Thrombin inhibits myosin light chain dephosphorylation in endothelial cells.

Histamine and thrombin increase myosin light-chain kinase-mediated phosphorylation of myosin light chain (MLC) in human umbilical vein endothelial cells (HUVEC). The increase in MLC phosphorylation caused by thrombin persists longer (330 min) than the increase caused by histamine (<5 min), although both increase cell calcium similarly. We hypothesized that some of the longer duration of the increase in MLC phosphorylation caused by thrombin was because of inhibition of myosin dephosphorylation by thrombin. Calyculin A, an inhibitor of type 1 and 2A protein phosphatases, caused a time-dependent increase in MLC phosphorylation in unstimulated HUVEC. As thrombin-stimulated phosphorylation approached its peak at 15 min, calyculin A caused progressively less of an increase in MLC phosphorylation in thrombin-stimulated HUVEC, and no increase at the peak of thrombin stimulation. In HUVEC in which cell calcium was maintained at 600 nM, thrombin increased MLC phosphorylation above the level caused by increased calcium alone at a time coinciding with the peak of thrombin stimulation. However, when phosphatase activity was already inhibited with calyculin A, thrombin did not further increase MLC phosphorylation in cells in which calcium was maintained at 600 nM calcium. Thrombin increases MLC phosphorylation in HUVEC not only by increasing cell calcium but also by inhibiting calyculin A-sensitive dephosphorylation of MLC.

Calcium↗
PubMed1997

Ca(2+)-inhibitable adenylyl cyclase and pulmonary microvascular permeability.

Intracellular mechanisms responsible for endothelial cell disruption are unknown, although either elevated cytosolic Ca2+ ([Ca2+]i) or decreased adenosine 3',5'-cyclic monophosphate (cAMP) promotes permeability. Recent identification that Ca(2+)-inhibitable adenylyl cyclase establishes an inverse relationship between [Ca2+]i and cAMP in macrovascular endothelial cells provided a possible mechanism of development of permeability. However, these data utilized an in vitro model; lacking was evidence supporting 1) expression of Ca(2+)-inhibitable adenylyl cyclase in pulmonary microvascular endothelium and 2) Ca2+ inhibition of adenylyl cyclase and cAMP content as a paradigm for inflammatory mediator-induced permeability in the intact circulation. We therefore addressed these issues in microvascular endothelial cells derived from rat lung and in an isolated perfused rat lung preparation. Results demonstrate expression of a Ca(2+)-inhibitable adenylyl cyclase in microvascular endothelial cells. Furthermore, data suggest that Ca2+ inhibition of adenylyl cyclase is necessary for development of microvascular permeability in the intact circulation. We conclude Ca2+ inhibition of cAMP represents a critical step in genesis of microvascular permeability in the intact pulmonary circulation.

Adenylyl Cyclase Inhibitors↗
PubMed1995

Ca(2+)-inhibitable adenylyl cyclase modulates pulmonary artery endothelial cell cAMP content and barrier function.

Maintenance by the endothelium of a semi-permeable barrier is critically important in the exchange of oxygen and carbon dioxide in the lung. Intracellular free Ca2+ ([Ca2+]i) and cAMP are principal determinants of endothelial cell barrier function through their mutually opposing actions on endothelial retraction. However, details of the mechanisms of this antagonism are lacking. The recent discovery that certain adenylyl cyclases (EC 4.6.1.1) could be acutely inhibited by Ca2+ in the intracellular concentration range provided one possible mechanism whereby elevated [Ca2+]i could decrease cAMP content. This possibility was explored in pulmonary artery endothelial cells. The results indicate that a type VI Ca(2+)-inhibitable adenylyl cyclase exists in pulmonary artery endothelial cells and is modulated by physiological changes in [Ca2+]i. Furthermore, the results suggest the inverse relationship between [Ca2+]i and cAMP that is established by Ca(2+)-inhibitable adenylyl cyclase plays a critical role in modulating pulmonary artery endothelial cell permeability. These data provide evidence that susceptibility to inhibition of adenylyl cyclase by Ca2+ can be exploited in modulating a central physiological process.

Adenylyl Cyclases↗
PubMed1995

Effect of a standardized rewarming protocol and acetaminophen on core temperature after coronary artery bypass.

BACKGROUND: Cardiac surgical patients who require hypothermic cardiopulmonary bypass experience hypothermia, normothermia, and hyperthermia during the early postoperative period. Research-based rewarming protocols are needed to manage temperature variations. OBJECTIVE: To describe the effect of a standardized rewarming protocol and acetaminophen on the following outcome variables: core temperature, peak core temperature, rewarming time, and hyperthermia. METHODS: Patients (N = 60) were rewarmed using a standardized rewarming protocol. Electric heating blankets were used for subjects with core temperatures less than 36 degrees C on admission to the intensive care unit; other subjects were covered with cotton bath blankets. Subjects were also assigned to one of three acetaminophen groups (650 mg at 38.1 degrees C, 650 mg at 37 degrees C, 1300 mg at 37 degrees C). RESULTS: Using the protocol, subjects warmed to normothermia in 3.6 to 6 hours. The 16-hour core temperature thermal curves of heating blanket versus cotton bath blanket subjects differed significantly; thermal curves of the acetaminophen groups were similar. Peak core temperature was significantly lower in heating blanket subjects and unaffected by acetaminophen group. The onset of hyperthermia was not significantly affected by the method of rewarming (electric heating blanket versus cotton blankets) or acetaminophen group. Rewarming time was significantly longer for electric heating blanket subjects. CONCLUSIONS: Our results indicate that mildly hypothermic subjects rewarmed with electric heating blankets during the early postoperative period have lower peak core temperatures and longer rewarming times than those rewarmed with cotton bath blankets. Acetaminophen administration at normothermia does not significantly affect peak core temperature or the onset of hyperthermia.

Acetaminophen↗
PubMed1994

Autoregulation of endothelin-1 secretion by cultured human keratinocytes via the endothelin B receptor.

We investigated endothelin-1 (ET-1) receptor expression on normal human keratinocytes (HK). We show that HK express the ETB receptor isoform and respond to ET-1 with a 2.7-fold increase in intracellular free calcium. HK did not respond to ET-1 with increased proliferation; however, 30 nM ET-1 caused a 51.7% decrease in ET-1 accumulation in HK-conditioned medium. We propose that HK ET-1 receptors function in autocrine regulation of ET-1 secretion.

Cell Division↗