In situ spectroscopic characterization of a solution-phase X-type ligand exchange at colloidal lead sulphide quantum dot surfaces.

We employed quantitative NMR spectroscopy and spectrophotometric absorbance titration to study a quantum dot X-type ligand exchange reaction. We find that the exchange is highly cooperative, where at low extents of exchange the change in free energy of the reaction, ΔGXC, is ∼11 kJ mol-1 while at higher extents of exchange ΔGXC saturates to ∼-4 kJ mol-1. A modified Fowler binding isotherm is developed to describe the reaction.

Oxidatively stable nanoporous silicon photocathodes with enhanced onset voltage for photoelectrochemical proton reduction.

Stable and high-performance nanoporous "black silicon" photoelectrodes with electrolessly deposited Pt nanoparticle (NP) catalysts are made with two metal-assisted etching steps. Doubly etched samples exhibit an ∼300 mV positive shift in photocurrent onset for photoelectrochemical proton reduction compared to oxide-free planar Si with identical catalysts. We find that the photocurrent onset voltage of black Si photocathodes prepared from single-crystal planar Si wafers by an Ag-assisted etching process increases in oxidative environments (e.g., aqueous electrolyte) owing to a positive flat-band potential shift caused by surface oxidation. However, within 24 h, the surface oxide layer becomes a kinetic barrier to interfacial charge transfer that inhibits proton reduction. To mitigate this issue, we developed a novel second Pt-assisted etch process that buries the Pt NPs deep into the nanoporous Si surface. This second etch shifts the onset voltage positively, from +0.25 V to +0.4 V versus reversible hydrogen electrode, and reduces the charge-transfer resistance with no performance decrease seen for at least two months. PEC performance was stable owing to Pt NP catalysts that were buried deeply in the photoelectrode by the second etch, below a thick surface layer comprised primarily of amorphous SiO2 along with some degree of remaining crystalline Si as observed by scanning and transmission electron micrographs. Electrochemical impedance studies reveal that the second etch leads to a considerably smaller interfacial charge-transfer resistance than samples without the additional etch, suggesting that burying the Pt NPs improves the interfacial contact to the crystalline silicon surface.

Fatigue in high- versus low-force voluntary and evoked contractions.

Low-frequency fatigue (LFF) is defined as a greater loss of force that occurs in during low versus high frequencies of stimulation. In order to determine which types of fatigue protocols are most likely to induce LFF, ten individuals participated in four different fatigue experiments which induced similar reductions in maximal force output as following: (1) 2-min intermittent high-frequency stimulation (40 Hz), (2) 4-min intermittent low-frequency stimulation (20 Hz), (3) sustained 100% maximal voluntary contraction (MVC), and (4) low-force voluntary contractions (20% MVC). Short (5s) trains of 10, 20, 30, 40, and 80 Hz were used to determine the force-versus-frequency relationships before and after the fatigue tasks. LFF was higher following the low-force voluntary contractions compared to the high-force voluntary and evoked contractions. The degree of LFF during the low-force voluntary contractions was most highly correlated to the duration of the fatigue task and to a lesser extent, to the decrease in maximal force output and the force-time integrals during the fatigue task.

Passive membrane properties and inward calcium current of human uterine smooth muscle cells.

Human myometrium was obtained at the time of cesarean delivery by means of excisional biopsy from the upper margin of the uterine incision through the lower uterine segment. Isolated smooth muscle cells were obtained by collagenase treatment. With the whole-cell patch clamp technique, voltage-clamp and current-clamp experiments were performed. Regenerative action potentials were seen in 3 mmol/L calcium bathing solution. Voltage-clamp experiments demonstrated the presence of transient inward currents and large outward currents. Inward currents were isolated with the use of tetraethylammonium ion as a blocking agent of outward current. In 3 mmol/L calcium inward current began at -60 mV and maximum inward current occurred at -40 mV. Maximum inward current density during the rising phase of the action potential was 1.0 microA/cm2 in 3 mmol/L calcium and was unaffected by reduction of sodium concentration in the bathing solution.

Morphologic and electrophysiologic characterization of isolated pregnant human myometrial cells.

Myometrium was obtained from pregnant volunteers by biopsy of the upper margin of the uterine incision at the time of cesarean section. A multistep enzymatic digestion with collagenase, trypsin, protease, and deoxyribonuclease yielded viable cells capable of contraction. Primary monolayer culture was carried out in the presence of human pregnant serum. Electron microscopic examination of freshly isolated and cultured cells revealed an ultrastructure indicative of smooth muscle cells. Intracellular microelectrode studies were performed on freshly isolated cells. Passive membrane properties were: resting membrane potential, -49.4 mV; specific membrane resistance, 6.06 kohms-cm2; specific membrane capacitance, 1.57 microfarad per square centimeter. Outward-going rectification was observed in response to depolarizing current pulses. Regenerative action potentials were not observed; however, transient voltage responses were elicited after depolarizing, but not hyperpolarizing, current pulses. These studies characterize a human tissue preparation that is applicable to electrophysiologic investigation of the control of uterine function.

The effects of aminophylline and nifedipine on contractility of isolated pregnant human myometrium.

To characterize the effects of aminophylline and nifedipine on pregnant human myometrium, in vitro contractility studies were performed on myometrial strips obtained at cesarean section. The strips were stimulated with oxytocin (800 mU/L) to simulate labor and then were exposed to increasing concentrations of aminophylline (40, 100, and 400 mumol/L) or nifedipine (5, 10, and 20 micrograms/L). Both drugs produced a dose-related decrease in contraction strength, as measured by the time-integrated force of contraction. Aminophylline lengthened the period of contraction in a manner that was not dose dependent. Low-dose nifedipine (5 micrograms/L) increased the period of contraction, but higher doses had no effect on frequency. Both drugs produced a net reduction in the effectiveness of labor, as measured by the average force (time-integrated force divided by period). These results indicate that both aminophylline and nifedipine may be clinically useful tocolytic agents.

Ultrastructure and morphometry of the stomach muscle of Amphiuma tridactylum.

An ultrastructural and stereological examination was performed on stomach smooth muscle of the salamander Amphiuma. This tissue has very large cells, ranging up to 12 X 1500 micron when relaxed. The extracellular space is 31% of the tissue volume, and the tissue contains 84.6% water. These values are similar to those of other amphibian and mammalian gastrointestinal smooth muscle. The cells possess the usual smooth muscle organelles. Thick, thin and intermediate filaments are present, along with membrane-associated and cytoplasmic dense regions. There is a well-developed sarcoplasmic reticulum and many microtubules. Caveolae are found in rows along the cellular surface; the caveolae increase the cellular surface area by about 70%. The ratio mean volume: surface area of the cells is 1.26 micron. This tissue appears to be typical of gastrointestinal smooth muscle, with the exception of the very large size of the cells.

Hyperpolarization in mouse parathyroid cells by low calcium.

Intracellular microelectrodes were used to record the resting membrane potential of mouse parathyroid cells in vitro. The mean value of the membrane potential in 2.5 mM calcium was -20 mV. Exposure to low-calcium solutions (1.5 mM) caused rapid hyperpolarization to a mean value of -50 mV. The relationship between extracellular calcium and the membrane potential was sigmoid, and the transition between high and low intracellular potentials occurred between 1.5 mM and 2.25 mM calcium. Magnesium, manganese, and lanthanum reversed the low-calcium hyperpolarization. In 1.5 mM calcium solutions, the relationship between external potassium (greater than 10 mM) and the membrane potential was 46 mV per 10-fold change in extracellular potassium. In 2.5 mM calcium solutions, the resting membrane potential was independent of the external potassium concentration. It is concluded that the resting membrane potential of mouse parathyroid cells is highly sensitive to the extracellular concentration of calcium and calcium-like ions. With the low-calcium secretory stimulus, hyperpolarization is accompanied by the development of strong dependence of the resting membrane potential on extracellular potassium levels.

Intercellular communication in the rat anterior pituitary gland. An in vivo and in vitro study.

The concept of "stimulus-secretion coupling" suggested by Douglas and co-workers to explain the events related to monamine discharge by the adrenal medulla (5, 7) may be applied to other endocrine tissues, such as adrenal cortex (36), pancreatic islets (4), and magnocellular hypothalamic neurons (6), which exhibit a similar ion-dependent process of hormone elaboration. In addition, they share another feature, that of joining neighbor cells via membrane junctions (12, 26, and Fletcher, unpublished observation). Given this, and the reports that hormone secretion by the pars distalis also involves a secretagogue-induced decrease in membrane bioelectric potential accompanied by a rise in cellular [Ca++] (27, 34, 41), it was appropriate to test the possibility that cells of the anterior pituitary gland are united by junctions.

Studies on calcium and sodium in uterine smooth muscle excitation under current-clamp and voltage-clamp conditions.

The objective of these studies was to define the roles of calcium and sodium in uterine smooth muscle excitation. The double sucrose-gap technique was used for current-clamp and voltage-clamp experiments. It was shown that neither sodium nor calcium alone is capable of supporting excitation in estrogen-dominated uterine smooth muscle. Calcium dependence was explained in part by increased membrane "leakage" current in calcium-free solution and calcium control of the voltage dependence of the early transient conductance. High concentrations of TTX did not affect the magnitude of the peak transient current while La(+++), Mn(++), and Co(++) greatly reduced or abolished it and decreased the steady-state current. From these and other data it was concluded that the regenerative mechanism in uterine smooth muscle has the functional characteristics of a single transient conductance channel whose activation requires the presence of both sodium and calcium. Insensitivity to TTX indicates that the molecular structure of the channel is unlike that in certain sodium-dependent systems, while the effects of La(+++), Mn(++), Co(++), and Ca(++) reveal a similar dependence of conductances on extracellular polyvalent cations.

Voltage-clamp studies on uterine smooth muscle.

These studies have developed and tested an experimental approach to the study of membrane ionic conductance mechanisms in strips of uterine smooth muscle. The experimental and theoretical basis for applying the double sucrose-gap technique is described along with the limitations of this system. Nonpropagating membrane action potentials were produced in response to depolarizing current pulses under current-clamp conditions. The stepwise change of membrane potential under voltage-clamp conditions resulted in a family of ionic currents with voltage- and time-dependent characteristics. In sodium-free solution the peak transient current decreased and its equilibrium potential shifted along the voltage axis toward a more negative internal potential. These studies indicate a sodium-dependent, regenerative excitation mechanism.

Basis of tetrodotoxin's selectivity in blockage of squid axons.

The blockage of nerve activity by tetrodotoxin is unusually potent and specific. Our experiments were designed to distinguish whether its specificity of action was based on the identification of ions, the direction of cation flow, or differences in the early transient and late steady conductance pathways. Alkali cations were substituted for sodium in the sea water, bathing an "artificial node" in a voltage-clamped squid axon. When tetrodotoxin was added to the artificial sea waters at a concentration of 100 to 150 mM, it was found to always block the flow of cations through the early transient channel, both inward and outward, but it never blocked the flow of ions using the late steady pathway. We conclude that the selectivity of tetrodotoxin is based on some difference in these two channels.

Comparison of tetrodotoxin and procaine in internally perfused squid giant axons.

Squid giant axons were internally perfused with tetrodotoxin and procaine, and excitability and electrical properties were studied by means of current-clamp and sucrose-gap voltage-clamp methods. Internally perfused tetrodotoxin was virtually without effect on the resting potential, the action potential, the early transient membrane ionic current, and the late steady-state membrane ionic current even at very high concentrations (1,000-10,000 nM) for a long period of time (up to 36 min). Externally applied tetrodotoxin at a concentration of 100 nM blocked the action potential and the early transient current in 2-3 min. Internally perfused procaine at concentrations of 1-10 mM reversibly depressed or blocked the action potential with an accompanying hyperpolarization of 2-4 mv, and inhibited both the early transient and late steady-state currents to the same extent. The time to peak early transient current was increased. The present results and the insolubility of tetrodotoxin in lipids have led to the conclusion that the gate controlling the flow of sodium ions through channels is located on the outer surface of the nerve membrane.

Tetrodotoxin does not block excitation from inside the nerve membrane.

Tetrodotoxin does not block the action potential or membrane sodium current when internally perfused through the giant axon of a squid at much higher concentrations than those required for blocking by external application. It is suggested that the gate for the sodium channel is located on the exterior surface of the axon, because tetrodotoxin is not lipid soluble.