Cardiac pacing under hyperbaric conditions.

Temporary external pacemakers have been reported to fail under hyperbaric conditions. In this study we investigated cardiac pacing under hyperbaric conditions. Permanent hermetically sealed pacemakers were found to function well under hyperbaric conditions, while several models of temporary external pacemakers failed. The electrical characteristics of pacing leads did not change under hyperbaric conditions. External pacing under hyperbaric conditions may be accomplished safely by using a permanent pacemaker attached to the patient's temporary external leads.

Far-field potentials as a method of monitoring acute and chronic spinal cord injury.

Far-field potentials (FFPs) were studied in monkeys to determine the utility of such responses in evaluating acute and chronic spinal cord injury. Monkeys (Macaca fascicularis) were anesthetized with 70% N2O 30% O2, immobilized with pancuronium bromide, and maintained on a respirator. Spinal cord transections were made at level T3-T4 and included bilateral dorsal columns or anterolateral columns, right or left hemisection, or central cord lesions. Percutaneous stimulation of both posterior tibial nerves was performed at a frequency of 3 Hz using 3-4 mA for 0.3 msec duration. Potentials were recorded from chronically implanted epidural electrodes (right ear reference) through a 300-3,000 Hz band pass filter. Each far-field potential represents the average of 256 individual responses recorded for a duration of 25 msec before and after cord lesion for up to 8 weeks. Seven reproducible components in the far-field potential could be identified. Following dorsal column transection all components were reduced in amplitude. After anterolateral column transection only latencies were altered. Right or left hemisections caused both attenuation of component amplitudes and latency alteration. Central cord lesions resulted in no detectable amplitude or latency disturbances. Control records likewise showed no changes. No major alterations, following day 1 posttransection, were observed, attesting to the relative stability of far-field potentials over long periods of time. Far-field potentials therefore may be useful in monitoring spinal cord injury.

The effects of spinal cord injury on somatosensory evoked potentials produced by interactions between afferent pathways.

The purpose of the present study was to determine the utility of somatosensory evoked potentials (SEPs) produced by interaction between afferent pathways for spinal cord injury evaluation. Interaction was measured as alterations in SEP configuration compared to controls. Monkeys anesthetized with N2O were used. Stimulus intensities were sufficient to excite all nerve fibers. The conditioning stimulus (CS) was applied to the left peroneal nerve and test stimulus (TS) to the left radial nerve. CS-TS intervals were 100 msec in duration. SEPs were recorded from primary cortical receiving area for the forelimb. Different surgical lesions were made at spinal cord level T3-T4. Amplitudes of TS-produced-SEPs remained unchanged postlesion. CS-produced-SEPs were found to be dependent upon the integrity of anterolateral column pathways. Accentuated interaction was observed following dorsal column ablation. Interaction was slightly enhanced after left hemisection but diminished after right hemisection or central cord lesion. Interaction between spatially separate afferent inputs as measured by SEP alteration was determined to be a sensitive indicator of spinal cord injury.

The effects of low spinal injury on the latencies of cortical evoked responses from forelimb stimulation.

A previous report showed that following spinal cord transaction well below the C7 level in cats, later components of the cortical somatic evoked potential (SEP) (40 msec or longer) produced in response to median or radial nerve stimulation were abnormal (Katz et al., 1977). Evidence presented here shows that early components of the radial nerve SEP are also altered following spinal cord transection. Latencies of both early and late components were increased as higher functional transections of the spinal cord were made. This monotonic increase in latency of the primary components supports the hypothesis that the results may be due to increasing loss of excitatory input. Correlation of latency change with level of injury may serve as a useful diagnostic tool.

Cultured human proximal tubule cells as a model for aminoglycoside nephrotoxicity.

Despite numerous clinical and animal studies, the initial injury and pathogenesis of aminoglycoside nephrotoxicity remains unclear. To compliment and extend existing research avenues, a cell culture model system representative of the human proximal tubule (HPT) was tested to determine its applicability for use in studies assessing aminoglycoside-induced cellular toxicity. For this determination, the proximal tubule cell cultures were exposed to increasing concentrations of streptomycin and monitored for cell death and light and electron microscopic changes under both confluent (resting) and subconfluent (actively-dividing) culture conditions. Confluent cultures exposed to streptomycin were also assessed for possible alterations in transport activities by monitoring the electrical properties of the cells through Ussing chamber analysis. Both the confluent and subconfluent cultures demonstrated concentration-dependent toxicity to streptomycin. Ultrastructural analysis disclosed that both actively-dividing and stationary cultures contained "myeloid bodies" within the cytoplasm, consistent with those known to occur in vivo. In studies relating cell numbers to the dosage and time of exposure to streptomycin, the confluent cultures demonstrated and "insult-recovery" period at toxic, but sub-lethal, concentration, again correlating to the known in vivo experience with this class of antibiotics. The subconfluent cultures demonstrated increased resistance to the toxic effects of streptomycin, again mimicking the clinical experience with aminoglycoside toxicity. Chamber analysis, at a streptomycin dose well below the toxic level, indicated changes in the transport activities of these cultured cells. It is proposed that the use of cultured proximal tubule cells could be a useful model system to extend current research avenues assessing the mechanism of aminoglycoside nephrotoxicity.

Growth characteristics of cultured human proximal tubule cells exposed to aminoglycoside antibiotics.

It is well known that the nephrotoxic lesions that occur during aminoglycoside-induced nephrotoxicity are both dose- and time-dependent. It was the purpose of this study to determine if a cell culture model based on the human proximal tubule would exhibit similar dose- and time-dependent relationships when exposed to aminoglycosides of various nephrotoxic potential. For this determination, the human proximal tubule (HPT) cells were exposed to increasing concentrations of streptomycin, kanamycin, gentamicin, and neomycin and monitored for cell growth and toxicity over an 18-day period of exposure. Both actively-dividing and resting cells were assessed with regard to aminoglycoside exposure. At high levels of aminoglycoside exposure, linear regression analysis disclosed that the rank order of toxicity of the aminoglycosides to be: neomycin greater than kanamycin greater than gentamicin greater than streptomycin. Both actively-dividing and resting cultures of HPT cells displayed both dose- and time-dependency with regard to toxicity and the ability of the cells to regenerate in the continued presence of aminoglycoside exposure. This pattern of dose- and time-dependency was unique for each aminoglycoside and varied depending on the replicative state of the cells. With the exception of neomycin, clear evidence was obtained that toxicity and cell regeneration were occurring simultaneously throughout the time course of aminoglycoside exposure; the equilibrium between the two processes determining overall cell toxicity or regeneration. In addition, the HPT cells exposed to gentamicin displayed a unique pattern of toxicity and cell regeneration when compared to the other aminoglycosides tested, with gentamicin having an increased ability to stimulate cell proliferation. While the results obtained are in excellent agreement with that known from the clinical experience with the aminoglycosides, the dose- and time-dependency of the responses will require careful attention to growth state during employment in experimental protocols.

Effect of hyperbaric oxygen on 2-deoxy-D-glucose and L-glutamate transport in rat cortical synaptosomes.

Initial velocities of uptake of L-glutamic acid and 2-deoxy-D-glucose (2-Dg) have been measured in cortical synaptosomes from rats which had been exposed to oxygen at high pressure (OHP) and compared to similar measurements in normobaric controls. Exposure to OHP had no significant effect on glutamate uptake at any combination of sodium and glutamate used. In contrast, OHP reduced 2-Dg uptake by an average of 17.5%. Although Kt was little affected, OHP exposure reduced apparent maximal transport capacity by 15%. Since hyperbaria with normal pO2 had no significant effect on uptake, the effect of OHP is an oxygen effect, rather than a pressure effect. The effects of OHP on uptake do not parallel the effects of age; glutamate transport capacity was reduced in aged animals, while 2-Dg transport was unaffected.

Effects of spinal cord lesions on somatic evoked potentials altered by interactions between afferent inputs.

In cats anesthetized with alpha-chloralose, somatic evoked potentials (SEP) were recorded in response to electrical stimulation of surgically isolated peripheral nerves. Selected surgical lesions were made at T9-L1 spinal cord and were histologically verified. Two stimulus magnitudes were used to activate peripheral nerves, one only exciting the large fibers and another exciting the small fibers as well. Control SEPs were recorded in response to stimulation of both large and small fibers of the radial nerve. The later components (latencies greater than 40 msec) of this SEP were suppressed when evoked 100 msec after application of a conditioning stimulus (CS) to the large fibers of either peroneal nerve. Bilateral transection of the dorsal columns and spinocervical tracts eliminates these effects. Increasing the CS intensity to include small diameter fibers again resulted in reduction of the later components of the SEP. This interaction was largely eliminated if the transection was extended to include mid-lateral cord tracts. These results suggest that the SEP can be influenced by small fiber afferent activity conducted in mid-ventrolateral spinal cord in the absence of the dorsal columns and spinocervical tracts. Alterations in the forelimb-evoked SEP by a conditioning hindlimb stimulus is a sensitive indicator of spinal cord integrity. This method may be used to assess whether low spinal injury spares ventrolateral columns.

Effect of hyperbaric oxygen on GABA transport in rat brain synaptosomes.

Initial velocities of uptake of GABA have been measured in rat brain synaptosomes from animals which had been exposed to oxygen at high pressure (OHP) and compared to similar measurements in normobaric controls. For hypothalamus, no changes in GABA uptake occurred subsequent to exposure to OHP. For cortical synaptosomes, however, exposure to OHP resulted in a decreased velocity of GABA uptake at all combinations of [Na] and [GABA] used. The OHP data were found to fit the same transport model as found previously for control data. Thus, OHP exposure did not alter the basic mechanism by which sodium and GABA interact with the carrier in the process of transport. However, the constants which quantitate the model were changed by OHP exposure. As a consequence, the several kinetic parameters which are calculated from the model change in the OHP animals. These kinetic parameters are compared to similar calculations for both normobaric control animals and normobaric aged animals. Although the effects of OHP do not precisely parallel the effects of aging, the alterations in kinetic parameters are in several ways similar in the aged and OHP animals.

The effects of interaction between large and small diameter fiber systems on the somatosensory evoked potential.

The effect of interaction between large and small diameter fiber systems on the somatosensory evoked potential (SEP) was studied in anesthetized cats. Activation of large diameter fibers of the peroneal or radial nerves eliminates the late components of the SEP produced by stimulation of all fibers in the contralateral median or radial nerves. The inhibitory effects of a selective conditioning stimulus to the large diameter fibers of the peroneal nerve on the radial nerve evoked SEP was eliminated by bilateral transection of the dorsal column and spino-cervical tracts. However, interaction could still be obtained following transection when both large and small diameter fibers in the peroneal nerve were stimulated. The results of this study support the hypothesis that a correlation exists between activity in different fiber groups in afferent nerves, their conduction pathways through the cord, and the components of the cortical evoked potential.

Morphology and Epithelial Ion Transport of the Alkaline Gland in the Atlantic Stingray (Dasyatis sabina).

The alkaline glands are two fluid-filled sacs that lie on the ventral, posterior surface of each kidney in skates and rays. In this study, the morphology, transepithelial ion transport, fluid constituents, and histochemistry of the alkaline glands of the Atlantic stingray, Dasyatis sabina, were investigated. The duct from each gland joined the corresponding vas deferens and the resulting two common ducts emptied into the cloaca. Dark burgundy, aqueous fluid (pH 8.0-8.2) was secreted into the sacs by a simple columnar epithelium with extensive rough endoplasmic reticulum and large secondary lysosomes containing lipofuscin and membrane fragments. Zonulae occludentes were deep (~22 fibrils), reflecting an electrically tight epithelium (732 ohms/cm2). Carbonic anhydrase activity was localized histochemically within the intercellular spaces and less intensely in the mid-basal cytoplasm. In vitro electrophysiology showed that baseline shortcircuit current (Isc, 29.1 {mu}A/cm2) was reduced 67.0% after Cl- removal from the medium. Cl- removal also completely abolished luminal alkalinization (baseline 4.5 +/- 0.7 {mu}Eq of acid/cm2/h). Luminal exposure to the chloridebicarbonate exchange inhibitor, DIDS, reduced Isc by 38%. Simultaneous administration of DIDS and bumetanide (Na+/K+/Cl- cotransport inhibitor) to the serosal side of the tissue caused the Isc to decrease >100%. Serosal exposure to ouabain (Na-K, ATPase inhibitor) decreased Isc 48%, whereas amiloride (sodium ion channel blocker) and acetazolamide (carbonic anhydrase inhibitor) had no statistically significant effect on Isc or alkalinization rates. Taken together the results suggest the presence of apical epithelial bicarbonate exchangers that are chloride or sodium dependent, basal sodium and HCO3- transport, and an Isc that is not totally dependent on Na+-K+ ATPase.

Electrophysiology and ultrastructure of cultured human proximal tubule cells.

The present study was undertaken to determine if cultures of human proximal tubule cells would retain in vivo properties inherent to this segment in the intact nephron. Ussing chamber studies demonstrated that these cultured cells generated transepithelial potential differences of approximately -2.0 mV and resistances of 0.310 K omega.cm2, supporting the concept that the proximal tubule is a "leaky" epithelium. The electrical properties did not change when the cells were exposed to amiloride (10(-4) M), but did respond to acetazolamide (10(-4) M), consistent with responses known to occur in proximal tubules. Ultrastructural analysis of these cells demonstrated features indicative of proximal tubule cells. When grown on permeable supports, where apical and basolateral growth medium compartments were maintained separate from each other, the cells were noted to undergo increased differentiation with morphological evidence of cell polarity. Freeze fracture analysis demonstrated well-formed tight junction strands and segregation of unique numbers of intramembranous particles in apical, lateral, and basal membranes. The replicas also demonstrated the presence of aggregates though to represent gap junctions, structures which occur exclusively in the proximal segment of the human nephron. These observations provide evidence that this human cell culture model originates from the human proximal tubule and retains, in culture, many of the properties associated with proximal tubule cell function and structure in vivo.

Classification of somatic evoked potentials through maximum entropy spectral analysis.

Maximum entropy (ME) spectra are currently considered to be superior to spectra calculated by means of Fast Fourier Transforms (FFT) because the former offer higher resolution. However, in terms of classification of somatic evoked potentials (SEPs), FFT spectra are found to be better than ME spectra. An even better set of parameters for discriminating SEPs is the set of reflection coefficients derived in the process of calculating the ME spectra. They permit separation of SEPs obtained from monkeys with dorsal column lesion from SEPs obtained from normal monkeys, as well as distinguishing between SEPs obtained from large nerve fiber stimulation and SEPs from stimulation of all nerve fibers.

The effects of low spinal injury on somatosensory evoked potentials from forelimb stimulation.

Spinal cord transection below C6 in anesthetized cats results in an alteration in the configuration of the SEP following direct or percutaneous stimulation of the median or radial nerves. The most significant alterations were in components occurring at latencies from 40 to 60 msec. The results of these experiments support the general conclusion that the spinal cord and supraspinal structures act as a functional unit and that the SEP is not solely determined by input over segmental pathways In addition, the alteration in the SEP produced by median nerve stimulation following spinal cord injury below C6 may serve clinically as a monitor of events at the site of injury in cases where an SEP from lower limb stimulation is no longer obtainable.

Cadmium nephrotoxicity in human proximal tubule cell cultures.

Human proximal tubule kidney cells grown in a serum-free tissue culture medium were exposed to concentrations of CdCl2 in a range of 0.5 to 10 micrograms/ml. Cells were observed from 1 to 20 d upon initiation of cadmium in the culture fluid. Both confluent and subconfluent populations of cells were treated and evaluated for cytotoxicity. Both populations exhibited a concentration-dependent toxicity to ionic cadmium. For cells treated with 2.0 to 10 micrograms/ml Cd, the decreases in cell numbers were largely irreversible. However, cells treated with Cd in a range of 0.5 to 1.0 micrograms/ml exhibited a partial recovery of cell number and control morphology. In this range, recovery was more efficient in the subconfluent cultures. Fine structural alterations in Cd-treated tubule cells included condensation of nuclear chromatin, loss of microvilli structure, disorganization of lateral membrane interdigitation, as well as decreased uptake of aminoglycoside antibiotics as evidenced by decreased numbers of myeloid bodies in these cells. The results of this study imply that use of a human proximal tubule culture system has potential in discerning structural and functional effects of cadmium as well as other nephrotoxic metals and compounds on the human kidney.