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Biomedical subjects

P B Bennett

Publications and source records attributed to P B Bennett.

At least 19 recordsLinked to original sources
PubMed1992

Functional expression of an inactivating potassium channel cloned from human heart.

Recently a putative K+ channel with homology to the Shaker family of potassium channels has been cloned from human ventricular myocardium. However, proof that the cDNA encodes a K+ channel requires appropriate translation and expression of a functional ion-selective channel. Therefore, expression of this putative human K+ channel DNA was attempted by cytoplasmic injections of in vitro transcribed cRNA into Xenopus laevis oocytes and screening by two-electrode voltage-clamp methods. This resulted in expression of voltage-gated channels that rapidly inactivated (time constant of inactivation, 47.6 +/- 3.6 msec; 0 mV; n = 10) and were at least 50 times more selective for K+ than Na+ (Na+/K+ permeability ratio of 0.02). The channels showed voltage-dependent activation (half-maximal voltage, -34 +/- 0.7 mV; n = 5), and 50% of the channels were inactivated within 2 seconds when the membrane potential was clamped near -60 mV (half-maximal voltage, -62 +/- 7 mV; n = 10). The expressed protein resulted in a K+ current that had many properties similar to the 4-aminopyridine-sensitive calcium-insensitive component of the cardiac transient outward current that is observed in native cardiac myocytes and thus may serve as one molecular substrate for this current.

Animals↗
PubMed1992

Scope and design of the GUSI international research program.

During the 1980s a large new simulated underwater saturation diving system was constructed at GKSS, Geesthacht, Germany. This German Underwater Simulator (GUSI) has performed over 18 trimix (helium, 5% nitrogen, oxygen) dives with 115 divers at depths to 600 m, including 9 to 450 m or deeper for a total of 2672 man-days of saturation and 994 days of welding and other work. From October 1989, an international research program was initiated to permit scientists from other countries to carry out physiologic and medical research during these working dives. The results of the first year's program from 3 dives, GUSI 14, 16, and 17, all to 450 m, are described in this special edition of Undersea Biomedical Research. This brief paper gives the scope, design, and objectives of the program, the dive profiles, and the scientists and projects involved.

Decompression Sickness↗
PubMed1992

Urinary vasopressin and aldosterone and plasma volume during a saturation dive to 450 m.

Urinary vasopressin (VP), aldosterone (ALDO), osmotic substances, sodium excretion, and plasma volume were assessed in 4 healthy male divers during 2 predive control days, 2 compression days, 6 days at 46 atm abs, and 26 days of decompression with stops at 37 and 27 atm abs. At pressure the ambient gas was trimix (0.5 atm abs O2:5% N2:remainder He). All urine was collected throughout the dive. Samples were divided into daytime (0700-1900) and nighttime (1900-0700). Indocyanine green dye dilution was used to determine plasma volume at predive 1, 46, and 24 atm abs. In agreement with previous dives at 31 atm abs, there was a decrease in VP excretion during compression lasting until return to 1 atm abs (P less than 0.05). Also similar to the shallower dives at 31 atm abs, the normal diurnal pattern of VP excretion, daytime higher than nighttime (P less than 0.05), disappeared at pressure. Urine osmolality showed alterations compatible with responses to VP. In contrast to previous studies at 31 atm abs, but in agreement with a previous study at 49.5 atm abs, there was no sustained increase in urinary ALDO excretion and only a transient natriuresis during the compression phase, followed by a reduced sodium excretion. In confirmation of earlier conclusions from indirect evidence, direct measurements of plasma volume indicated a reduction of about 20% (P less than 0.05) at 46 atm abs which remained reduced after decompression to 24 atm abs.

Adult↗
PubMed1992

Platelet count in deep saturation diving.

Platelet counts were measured in 10 divers during the course of 4 experimental deep dives (450 and 600 m) and different anticoagulants were tested. The use of sodium citrate as an anticoagulant was associated with artifactual thrombocytopenia, whereas ethylenediaminetetraacetic acid proved to be satisfactory. During one of the dives (450 m) the use of fluorocarbon to remove excess dissolved inert gas before decompression of the samples was systematically tested. Mean platelet count decreased from 272,600 +/- 29,400 mm-3 (mean +/- SD) on Day 7 (450 m) to 177,700 +/- 26,400 mm-3 on Day 12 (360 m). Platelet count had recovered to 209,900 +/- 20,700 mm-3 at the time of surfacing. Mean hematocrit (expressed in percent) increased from 44.7 +/- 2.2 predive to 59.4 +/- 2.0 on Day 7 (450 m) to 40.9 +/- 2.8 at the time of surfacing. These changes were statistically significant (P less than 0.05). Platelet counts on samples that had been degassed with fluorocarbon were not different from samples that had been decompressed without degassing.

Citrates↗
PubMed1992

Renal responses during a dry saturation dive to 450 msw.

Four subjects were compressed to a simulated depth of 450 msw (46 bar) for 37 days in the main research chamber of the German underwater simulator diving facility at the GKSS Research Center, Geesthacht. The ambient gas was trimix. Urine was collected at 0700, 1300, and 1900 h each day for analysis of Na+, K+, volume, osmolality, and creatinine. Urine, antidiuretic hormone (ADH), and aldosterone were analyzed separately. Daily fluid, Na+, and K+ intake were analyzed throughout the dive. The aim of the investigation was to confirm the existence of a diuresis and natriuresis which had been observed in earlier saturation dives to 31 atm abs using He-O2. A significant diuresis was observed during compression despite a decrease in fluid intake. After compression the diuresis decreased somewhat but remained significantly above precompression control levels during the entire hyperbaric exposure. No significant change in fluid intake was observed. Daily Na+ and K+ excretion increased significantly during compression, which was accompanied by a significant increase in nocturnal excretion of Na+ and K+. Daily intake of Na+ and K+ decreased throughout the dive. Daily urine ADH decreased immediately upon compression and was associated with a parallel decrease in urine osmolality. In contrast, urinary aldosterone excretion exhibited no change during the dive despite the increase in Na+ and K+ excretion and decrease in Na+ intake.

Aldosterone↗
PubMed1991

Single inward rectifier potassium channels in guinea pig ventricular myocytes. Effects of quinidine.

The effects of quinidine on single inward rectifier K channels were investigated in cell-attached patches with 4.5 mM pipette potassium concentrations. Under these conditions, the single-channel slope conductance of the predominant conductance level of the inward rectifier channels was 3.9 +/- 0.3 pS at membrane potentials between -75 and -150 mV. Quinidine reversibly decreased the likelihood of channel opening to the main conductance level without reducing the single-channel conductance, and also reduced the probability of channel opening to subconducting levels. Quinidine had no significant effects on the channel open times, and the inhibition of channel opening was only slightly voltage dependent over the range of membrane potentials investigated. Quinidine induced a complete cessation of channel openings for brief periods (up to 2 min), suggesting that quinidine promoted occupancy of a state from which opening was less likely. Occasional long periods (up to an hour) with an absence of channel activity were also observed but quinidine did not appear to promote this behavior. The data suggest that quinidine decreases the ability of the channel to enter both main and subconducting states. By binding to a particular closed conformation of the channel, quinidine could reduce the likelihood of channel opening. The main features of these observations could be accounted for using the three-state kinetic model proposed by Sakmann, B. and G. Trube (1984b. J. Physiol. [Lond.]. 347:659-683.) with quinidine binding to the middle closed state.

Animals↗
PubMed1991

Voltage- and use-dependent modulation of calcium channel current in guinea pig ventricular cells by amiodarone and des-oxo-amiodarone.

Amiodarone is an effective antiarrhythmic drug handicapped by serious side effects. The mechanism of its antiarrhythmic activity is not known but is presumed to involve inhibition of current flowing through ion channels. Des-oxo-amiodarone, a close structural analogue of amiodarone, was synthesized based on the hypothesis that the toxic and therapeutic properties reside in different parts of the molecule and that chemical modification could result in a less toxic agent that yet preserved amiodarone's antiarrhythmic efficacy. We compared the effects of amiodarone and des-oxo-amiodarone on Ca current in enzymatically dispersed guinea pig ventricular myocytes using the whole-cell patch-clamp method. Amiodarone caused both a tonic and a phasic (use-dependent) reduction of the Ca current. The relationship between membrane potential and the availability for channel opening upon depolarization (inactivation curve) was shifted toward more negative membrane potentials by amiodarone (delta - 10.6 +/- 2.2 mV, n = 7). The use-dependent reduction of the Ca current was also dependent on the frequency of the voltage clamp steps (0.5 Hz, 40.2 +/- 7.9%; 1.0 Hz, 50.0 +/- 6.7%). Dex-oxo-amiodarone had a dual effect on the Ca current: After maintaining the membrane potential for several seconds at negative membrane potentials (less than -45 mV), the Ca current was increased by des-oxo-amiodarone. Des-oxo-amiodarone also shifted the Ca channel inactivation curve to more negative membrane potentials up to 16 mV. Consequently, Ca current could be increased or decreased depending on the experimental conditions. Enhancement of Ca current by des-oxo-amiodarone was transient and was supplanted entirely by the antagonistic effects of the drug after approximately 5 min. The antagonistic effects of des-oxo-amiodarone on Ca current were also use- and frequency-dependent.

Action Potentials↗
PubMed1991

Suppression of time-dependent outward current in guinea pig ventricular myocytes. Actions of quinidine and amiodarone.

Prolongation of cardiac action potentials may mediate some of the arrhythmia-suppressing and arrhythmia-aggravating actions of antiarrhythmic agents. In this study, suppression of time-dependent outward current by quinidine and amiodarone was assessed in guinea pig ventricular myocytes. The net time-dependent outward current contained at least two components: a slowly activating, La(3+)-resistant delayed rectifier current (IK) and a rapidly activating, La(3+)-sensitive current. Quinidine block of total time-dependent outward current during clamp steps to positive potentials was relieved as a function of time, whereas that induced by amiodarone was enhanced. In contrast, at negative potentials, suppression of current, whereas amiodarone reduced IK but not the La(3+)-sensitive current, suggesting that differential block of the two components of time-dependent current underlies the distinct effects of the two agents. In contrast to these disparate effects on total time-dependent outward current, steady-state reduction of IK by both drugs increased at positive voltages and saturated at approximately +40 mV; the voltage dependence of block by quinidine (17% per decade, +10 to +30 mV) was steeper than that by amiodarone (5% per decade, +10 to +20 mV). Block by quinidine was time dependent at negative potentials: on stepping from +50 to -30 mV, block initially increased very rapidly, and subsequent deactivation of IK was slowed. This effect was not seen with amiodarone. At -80 mV, quinidine block was relieved with a time constant of 40 +/- 15 msec (n = 4, twin-pulse protocol). The effects of quinidine on IK were compatible with neither a purely voltage-dependent model of quinidine binding nor a model incorporating both voltage- and state-dependent binding of quinidine to delayed rectifier channels having only one open state. The voltage- and time-dependent features of quinidine block were well described by a model in which quinidine has greater affinity for one of two open states of the channel. We conclude that the effects of quinidine and amiodarone on time-dependent outward current reflects block of multiple channels. Quinidine block of IK was far more voltage dependent than that produced by amiodarone, suggesting the drugs act by different mechanisms.

Action Potentials↗
PubMed1990

Voltage clamp of the cardiac sodium current at 37 degrees C in physiologic solutions.

The cardiac sodium current was studied in guinea pig ventricular myocytes using the cell-attached patch voltage clamp at 37 degrees C in the presence of 145 mM external sodium concentration. When using large patch pipettes (access resistance, 1-2 M omega), the capacity current transient duration was typically 70 microseconds for voltage clamp steps up to 150 mV. At 37 degrees C the maximum inward sodium current peaked in approximately 200 microseconds after the onset of a clamp step and at this strong depolarization, less than 10% of the sodium current developed during the capacity transient. The sodium current developed smoothly and the descending limb of the current-voltage relationship usually spanned a range of 40 mV. Moreover, currents reduced by inactivation of sodium channels could be scaled to superimpose on the maximum current. Current tails elicited by deactivation followed a monoexponential time course that was very similar for currents of different sizes. Data obtained over a range of temperatures (15 degrees-35 degrees C) showed that the steady-state inactivation and conductance-voltage curves were shifted to more negative voltages at lower temperatures. These results demonstrate the feasibility of investigating the sodium current of mammalian cardiac cells at 37 degrees C in normal physiological solutions.

Animals↗
PubMed1990

Time-dependent outward current in guinea pig ventricular myocytes. Gating kinetics of the delayed rectifier.

Several conflicting models have been used to characterize the gating behavior of the cardiac delayed rectifier. In this study, whole-cell delayed rectifier currents were measured in voltage-clamped guinea pig ventricular myocytes, and a minimal model which reproduced the observed kinetic behavior was identified. First, whole-cell potassium currents between -10 and +70 mV were recorded using external solutions designed to eliminate Na and Ca currents and two components of time-dependent outward current were found. One component was a La3(+)-sensitive current which inactivated and resembled the transient outward current described in other cell types; single-channel observations confirmed the presence of a transient outward current in these guinea pig ventricular cells (gamma = 9.9 pS, [K]o = 4.5 mM). Analysis of envelopes of tail amplitudes demonstrated that this component was absent in solutions containing 30-100 microM La3+. The remaining time-dependent current, IK, activated with a sigmoidal time course that was well-characterized by three time constants. Nonlinear least-squares fits of a four-state Markovian chain model (closed - closed - closed - open) to IK activation were therefore compared to other models previously used to characterize IK gating: n2 and n4 Hodgkin-Huxley models and a Markovian chain model with only two closed states. In each case the four-state model was significantly better (P less than 0.05). The failure of the Hodgkin-Huxley models to adequately describe the macroscopic current indicates that identical and independent gating particles should not be assumed for this K channel. The voltage-dependent terms describing the rate constants for the four-state model were then derived using a global fitting approach for IK data obtained over a wide range of potentials (-80 to +70 mV). The fit was significantly improved by including a term representing the membrane dipole forces (P less than 0.01). The resulting rate constants predicted long single-channel openings (greater than 1 s) at voltages greater than 0 mV. In cell-attached patches, single delayed rectifier channels which had a mean chord conductance of 5.4 pS at +60 mV ([K]o = 4.5 mM) were recorded for brief periods. These channels exhibited behavior predicted by the four-state model: long openings and latency distributions with delayed peaks. These results suggest that the cardiac delayed rectifier undergoes at least two major transitions between closed states before opening upon depolarization.

Action Potentials↗
PubMed1979

Can regular solution theory be applied to lipid bilayer membranes?

Direct measurement of the partition coefficient of n-hexane into phosphatidylcholine and phosphatidylcholine-cholesterol bilayers showed that (a) isotropic liquids are not good models for lipid bilayers and (b), Regular Solution Theory cannot, in general, be applied to lipid bilayer membranes at temperatures above their phase transition. Theoretical and experimental evidence given.

Cholesterol↗
PubMed1979

Spinal cord seizures elicited by high pressures of helium.

Rats with complete spinal transections were compressed in helium-oxygen to 120 bars. Tremors and increased EMG activity in limbs rostral as well as caudal to the lesions were observed beginning at 30 bars. Spinal seizures occurred at 95 bars, similar to cortical seizure thresholds of intact rats. Denervated limbs remained flaccid throughout the dives. No rostro-caudal progression of symptoms was evident in normal animals, but fluctuation of symptoms with increasing pressure was frequently observed. These findings are consistent with the hypothesis that the effects of pressure on aggregates of neurons exceed those on isolated components.

Animals↗
PubMed1979

Effect of interaction of volatile anesthetics and high hydrostatic pressure on central neurons.

Autoactive neurons in the central nervous system of Helix and Aplysia were studied after exposure to several volatile anesthetics and under compression by mineral oil. Voltage clamp studies reveal that halothane will eliminate the slow inward current that underlies oscillatory activity in burster neurons, while high pressure shifts the negative resistance region of the current without causing its elimination. Simultaneous application of the anesthetic and pressure results in the loss of inward current over a time course similar to that of halothane application alone. It is concluded that in this system, pressure and anesthetics are not acting antagonistically at the site in the membrane that controls slow wave conductances.

Anesthetics↗
PubMed1979

Effects of aspirin and dipyridamole on platelet function, hematology, and blood chemistry of saturation divers.

Twenty-four young male divers were assigned randomly to 4 treatment groups: Group I received aspirin (325 mg) three times daily; II received dipyridamole (75 mg) three times daily; III received both drug regimens; and IV received matching placebo. Double-blind procedures were followed. Treatment began 24 h prior to a 48-h saturation dive (inclusive of 17 h decompression) at a simulated depth of 18.3 m and continued throughout and for 3 days after the dive. A post-dive reduction in circulating platelet count was observed in all groups, except the group that received aspirin only. Platelet survival was shortened in all treatment groups. Five cases of Type I decompression sickness occurred and were treated by recompression, two in the aspirin plus dipyridamole group, two in the dipyridamole group, and one in the placebo group. Blood chemistry and hematology profiles showed that divers with decompression sickness had more elevated GOT, GPT, CPK, cholesterol and triglyceride levels, and greater reductions in platelet count, Platelet Factor 4 and Thrombin Clotting Time than most other subjects. Subjects receiving either aspirin or aspirin plus dipyridamole had fewer changes in these parameters. Failure of aspirin to potentiate, or add to, dipyridamole may be due to other actions of aspirin such as inhibition of prostacyclin synthesis. Further studies of the role of antiplatelet drugs in decompression sickness are warranted.

Adult↗
PubMed1979

Effect of helium/nitrogen/oxygen mixtures on HPNS convulsion threshold in euthermic rats.

This study compares the effects of various so-called trimix mixtures (10, 20, and 40% N2 in He-O2) on the convulsion threshold pressure (Pc) and EEG activity in 60 adult male Wistar rats with chronically implanted electrodes with those in 20 rats in He-O2 only. Restrained animals were individually compressed with trimix mixtures at 80 or 160 atm per hour to a simulated depth of 138 ATA; colonic temperature was maintained at normal levels. Pc was defined as the initial occurrence of overt sustained generalized tonic-clonic seizures, accompanied by typical "spike and wave" patterns in all EEG leads. As in man, in rats trimix increased the depth of the onset of HPNS tremors and myoclonic jerks in all six groups of rats. However, the Pc of the trimix groups was no different from the Pc of the helium-oxygen group (113 ATA), and at 40% N2, rats showed EEG seizures but no overt convulsions. These results are discussed in relation to those of other studies showing the extension of Pc in mice and monkeys attained by adding narcotics to heliox; the paper also considers the relevance of method of compression, addition of nitrogen, core temperature, and species differences, as well as the need for EEG measurements and direct observation of overt convulsions as indicators of an effective antagonism of HPNS.

Animals↗
PubMed1978

Nitrogen-oxygen saturation therapy in serious cases of compressed-air decompression sickness.

Decompression sickness and arterial air embolism which follow exposure to raised environmental pressures of compressed air are usually adequately treated by accepted recompression procedures of relatively short durations. With serious cases, however, conventional treatment may not allow sufficient time at depth for the complete resolution of manifestations because of the need to avoid pulmonary oxygen toxicity which is associated with a prolonged period of breathing compressed air. Treatment by nitrogen-oxygen saturation at a pressure equivalent of 30 m (100 ft) sea water is proposed. Based upon the success of three refractory cases treated by this procedure, recommendation are made for the conversion of standard compressed-air chambers into an emergency saturation mode for therapy.

Adult↗
PubMed1978

Naloxone fails to antagonize the righting response in rats anesthetized with halothane.

The righting responses of 75 Wistar rats during exposure to halothane, 0.5 per cent, 10 min after subcutaneous injection of naloxone, 10 or 20 mg/kg, or saline solution were examined. There was no difference in recoveries of the righting responses among the three groups. It is concluded that naloxone does not antagonize the loss of righting response produced by halothane anesthesia in the rat.

Anesthesia, Inhalation↗