Peter B. Bennett

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.

Anesthetic antagonism of the effects of high hydrostatic pressure on locomotory activity of the brine shrimp Artemia

The locomotory activity of small groups of brine shrimp (Artemia salina) was studied under conditions of high hydrostatic pressure, varying temperatures and exposure to several gaseous anesthetics. Both compression and exposure to anesthesia reduced the animals swimming activity, while temperature increased or decreased activity as it was raised or lowered from ambient. The effect of the anesthetics was less during periods of simultaneous exposure to high hydrostatic pressure. It is concluded that pressure antagonism of anesthesia is demonstrable in invertebrate organisms and may represent a fundamental interaction of these parameters in biological systems.

Effect of epidural lidocaine on spinal cord blood flow

The effect of epidural lidocaine on spinal cord blood flow (SCBF) was measured in mongrel dogs. Spinal evoked potentials were used to measure spinal cord conduction and enabled us to time the measurement of SCBF associated with maximum reduction in latency and amplitude of spinal evoked potentials produced by instillation of 5 ml of 2% lidocaine in the epidural space. We used the radioactive microsphere technique to quantitate SCBF in the cervical, upper thoracic, lower thoracic, and lumbar regions of the spinal cord. A narcotic/relaxant technique was used to eliminate SCBF changes associated with other anesthetic agents. Other parameters known to affect SCBF were kept constant and within normal physiologic values. As compared with saline solution controls, there were significant decreases in SCBF in all the four areas of spinal cord in animals given epidural lidocaine, most marked in the lumbar area.

Assessment of the interaction of hyperbaric N2, CO2, and O2 on psychomotor performance in divers

Diving narcosis results from the complex interaction of gases, activities, and environmental conditions. We hypothesized that these interactions could be separated into their component parts. Where previous studies have tested single cognitive tasks sequentially, we varied inspired partial pressures of CO2, N2, and O2 in immersed, exercising subjects while assessing multitasking performance with the Multi-Attribute Task Battery II (MATB-II) flight simulator. Cognitive performance was tested under 20 conditions of gas partial pressure and exercise in 42 male subjects meeting U.S. Navy age and fitness profiles. Inspired nitrogen (N2) and oxygen (O2) partial pressures were 0, 4.5, and 5.6 ATA and 0.21, 1.0, and 1.22 ATA, respectively, at rest and during 100-W immersed exercise with and without 0.075-ATA CO2 Linear regression modeled the association of gas partial pressure with task performance while controlling for exercise, hypercapnic ventilatory response, dive training, video game frequency, and age. Subjects served as their own controls. Impairment of memory, attention, and planning, but not motor tasks, was associated with N2 partial pressures >4.5 ATA. Sea level O2 at 0.925 ATA partially rescued motor and memory reaction time impaired by 0.075-ATA CO2; however, at hyperbaric pressures an unexpectedly strong interaction between CO2, N2, and exercise caused incapacitating narcosis with amnesia, which was augmented by O2 Perception of narcosis was not correlated with actual scores. The relative contributions of factors associated with diving narcosis will be useful to predict the effects of gas mixtures and exercise conditions on the cognitive performance of divers. The O2 effects are consistent with O2 narcosis or enhanced O2 toxicity.

Physiology of Diving

The significant subdivisions of lung or respiratory system volume are functional residual capacity, residual volume, and total lung capacity; these are shown in figure III-1. Functional residual capacity (FRC) is the volume of gas in the respiratory system at the end of a normal expiration. Residual volume (RV) is the gas volume in the system at maximal expiration, and total lung capacity (TLC) is the gas volume at maximal inspiration. Vital capacity (VC), the maximum amount of gas an individual can move, is the difference between TLC and RV. The determinants of these subdivisions of lung volume are shown for a young man in figure III-2, which plots the passive pressure-volume characteristics of the lung, the chest wall, and the total respiratory system.

Cardiovascular and endocrine responses to 90° tilt during a 35-day saturation dive to 46 and 37 ATA

INTRODUCTION Hyperbaria-induced diuresis is accompanied by decreased basal and stimulated release of arginine vasopressin (AVP) and decreased blood volume possibly contributing to the reported orthostatic intolerance. Since hyperosmolality is not a consistent finding, the explanation of blood volume reduction at hyperbaria must involve an osmotic component to the diuresis. Investigations of a possible involvement of atrial natriuretic peptide (ANP) to the hyperbaric diuresis have revealed mixed results. METHODS Urinary excretion of electrolytes, AVP, and aidosterone were measured in four male subjects studied at 1 atmosphere absolute (ATA) and at 46 and 37 ATA (0.5 atmospheres pressure O2: 5% N2: remainder He) during a 35-d saturation dive. Also, the supine and 90 degrees tilt-stimulated plasma levels of AVP, plasma renin activity (PRA), and aldosterone, and the suppressed responses of ANP and the cardiovascular responses to tilt were determined at these pressures. RESULTS Tilt-stimulated levels of PRA were increased two- to threefold and the AVP response was eliminated throughout hyperbaria, except in two episodes of tilt-induced syncope where AVP was elevated 10- to 20-fold. This pattern supports most previous reports. Contrary to some reports, both supine and tilt-suppressed levels of ANP were reduced by about 50% at all three tilt experiments conducted at hyperbaria compared to predive control values. DISCUSSION These results suggest an altered ANP response at pressures of 37 ATA or greater, which is consistent with an appropriate ANP response to blood volume reduction and further suggest that the hyperbaric diuresis is not dependent on increased ANP.

Phase changes induced by cyclopropane of phosphatidylserine bilayers in the presence and absence of calcium

Abstract The effects of calcium on the thermodynamic parameters of dipalmitoyl-phosphatidylserine (DPS) bilayers in the presence of 1 atm nitrogen and cyclopropane was investigated calorimetrically. It found that at pH 3.1, in the absence of calcium, cyclopropane produces two endothermic phase transitions, at temperatures 63.0° and 60.5°, as compared to the single endothermic transition that was found in the presence of nitrogen. The enthalpies of the cyclopropane transition were also reduced. In the presence of calcium and nitrogen the transition temperature was 66.5°. Upon substituting cyclopropane for nitrogen two endothermic transitions were observed. The main peak remained at 66.5° while the second peak melted at 58.0°. This lower peak melts at about the same temperature as the calcium free dispersion in the presence of cyclopropane. Thus, in the presence of an inert gas, the ionic environment of a lipid can moderate the effect a particular iner; gas may have. Moreover, the presence of an inert gas can cause calcium to apparently dissociate from the bilayer to form a separate calcium free phase.