Airway smooth muscle, tidal stretches, and dynamically determined contractile states

In the classic theory of airway lumen narrowing in asthma, active force in airway smooth muscle is presumed to be in static mechanical equilibrium with the external load against which the muscle has shortened. This theory is useful because it identifies the static equilibrium length toward which activated airway smooth muscle would tend if given enough time. The corresponding state toward which myosin-actin interactions would tend is called the latch state. But are the concepts of a static mechanical equilibrium and the latch state applicable in the setting of tidal loading, as occurs during breathing? To address this question, we have studied isolated, maximally contracted bovine tracheal smooth muscle subjected to tidal stretches imposed at 0.33 Hz. We measured the active force (F) and stiffness (E), which reflect numbers of actin-myosin interactions, and hysteresivity (eta) which reflects the rate of turnover of those interactions. When the amplitude of imposed tidal stretch (epsilon) was very small, 0.25% of muscle optimal length, the steady-state value of F approximated the isometric force, E was large, and eta was small. When epsilon was increased beyond 1%, however, F and E promptly decreased and eta promptly increased. The muscle could be maintained in these steady, dynamically determined contractile states for as long as the tidal stretches were sustained; when epsilon subsequently decreased back to 0.25%, F, E, and eta returned slowly toward their previous values. The provocative stretch amplitude required to cause active force or muscle stiffness to fall by half, or hysteresivity to double, was slightly greater than 2%. These observations are consistent with a direct effect of stretch upon bridge dynamics in which, with increasing tidal stretch amplitude, the number of actin-myosin interactions decreases and their rate of turnover increases. We conclude that the interactions of myosin with actin are at every instant tending toward those that would prevail in the isometric steady state, but tidal changes of muscle length cause an excess in the rate of detachment. These stretch-induced detachment events can come so fast compared with the rate of attachment that static equilibrium conditions are never attained. If so, then airway lumenal narrowing and the underlying contractile state would be governed by a dynamic mechanical process rather than by a mechanical equilibrium of static forces.     

Activation of signal transduction kinases by tamoxifen

PURPOSE: To study the signal transduction mechanisms of tamoxifen via the activation of MAPKs, JNK and ERK in order to understand its regulation of gene expression. METHODS: The effects of tamoxifen (TAM) on the activation of serine/threonine mitogen-activated protein kinase (MAPK, p42/ERK2) and the stress-activated protein kinases (p46 SAPK or c-Jun N-terminal kinase, JNK1) were evaluated using a human cervical epitheloid carcinoma HeLa cell line. RESULTS: TAM activated both JNK1 and ERK2 activities in a time- and dose-dependent manner in HeLa cells. The activation of JNK1 was enhanced when the cells were pretreated with prooxidant H2O2. CONCLUSIONS: These studies show that TAM activates the signal transduction kinases, JNK1 and ERK2, which may play important roles in the regulation of gene expression by TAM.     

Hardiness, coping, and burnout in the nursing workplace

This article reviews the potential of a number of drug classes to produce or aggravate hypertension, with particular emphasis on older patients. Although little information is available regarding the hypertensive effects of mineralocorticoids in the elderly, glucocorticoids usually induce an increase in blood pressure (BP) that is dosage-dependent. Nonsteroidal anti-inflammatory drugs occasionally increase BP, but this effect is not usually clinically relevant. The alleged hypertensive potential of oral antihyperglycaemic agents has not yet been substantiated. In contrast, some antidepressants may be particularly dangerous in terms of their hypertensive complications. Although hypertension may be induced by monoamine oxidase inhibitors (MAOIs) given as monotherapy, severe hypertension is more likely to occur when MAOIs are combined with other sympathomimetic drugs. Several other drugs may also increase the risk of hypertension in elderly patients, but reliable data are lacking for most of these agents. Elderly people who are being treated with drugs that may increase BP should be closely monitored. This is particularly true for drugs that act on the CNS.     

Haloperidol-based irreversible inhibitors of the HIV-1 and HIV-2 proteases

The proteases expressed by the HIV-1 and HIV-2 viruses process the polyproteins encoded by the viral genomes into the mature proteins required for virion replication and assembly. Eight analogs of haloperidol have been synthesized that cause time-dependent inactivation of the HIV-1 protease and, in six cases, HIV-2 protease. The IC50 values for the analogues are comparable to that of haloperidol itself. Enzyme inactivation is due to the presence of an epoxide in two of the analogues and carbonyl-conjugated double or triple bonds in the others. Irreversible inactivation is confirmed by the failure to recover activity when one of the inhibitors is removed from the medium. At pH 8.0, the agents inactivate the HIV-1 protease 4-80 times more rapidly than the HIV-2 protease. Faster inactivation of the HIV-1 protease is consistent with alkylation of cysteine residues because the HIV-1 protease has four such residues whereas the HIV-2 protease has none. Inactivation of the HIV-2 protease requires modification of non-cysteine residues. The similarities in the rates of inactivation of the HIV-2 protease by six agents that have intrinsically different reactivities toward nucleophiles suggest that the rate-limiting step in the inactivation process is not the alkylation reaction itself. At least five of the agents inhibit polyprotein processing in an ex vivo cell assay system, but they are also toxic to the cells.     

Clinical applications of fusion imaging in oncology

Recent developments in tumor imaging, made possible by advances in instrumentation and radiopharmaceuticals, has led to an increasing need for accurate anatomic correlation of single photon emission computed tomography (SPECT) and positron emission tomography (PET) images. Fusion imaging permits the functional strengths of SPECT and PET to be combined with the anatomic resolution of computed tomography (CT) and magnetic resonance imaging (MRI). Clinical applications of fusion imaging include the evaluation of brain tumors, lymphoma, hepatic lesions and monoclonal antibody studies. The continued development of these techniques will eventually allow fusion imaging to become a routine part of nuclear medicine practice.     

The interaction of the general anesthetic etomidate with the gamma-aminobutyric acid type A receptor is influenced by a single amino acid

The gamma-aminobutyric acid type A (GABAA) receptor is a transmitter-gated ion channel mediating the majority of fast inhibitory synaptic transmission within the brain. The receptor is a pentameric assembly of subunits drawn from multiple classes (alpha1-6, beta1-3, gamma1-3, delta1, and epsilon1). Positive allosteric modulation of GABAA receptor activity by general anesthetics represents one logical mechanism for central nervous system depression. The ability of the intravenous general anesthetic etomidate to modulate and activate GABAA receptors is uniquely dependent upon the beta subunit subtype present within the receptor. Receptors containing beta2- or beta3-, but not beta1 subunits, are highly sensitive to the agent. Here, chimeric beta1/beta2 subunits coexpressed in Xenopus laevis oocytes with human alpha6 and gamma2 subunits identified a region distal to the extracellular N-terminal domain as a determinant of the selectivity of etomidate. The mutation of an amino acid (Asn-289) present within the channel domain of the beta3 subunit to Ser (the homologous residue in beta1), strongly suppressed the GABA-modulatory and GABA-mimetic effects of etomidate. The replacement of the beta1 subunit Ser-290 by Asn produced the converse effect. When applied intracellularly to mouse L(tk-) cells stably expressing the alpha6beta3gamma2 subunit combination, etomidate was inert. Hence, the effects of a clinically utilized general anesthetic upon a physiologically relevant target protein are dramatically influenced by a single amino acid. Together with the lack of effect of intracellular etomidate, the data argue against a unitary, lipid-based theory of anesthesia.