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Biomedical subjects

R Horn

Publications and source records attributed to R Horn.

At least 145 records · Page 8Linked to original sources
PubMed1984

Statistical properties of single sodium channels.

Single channel currents were obtained from voltage-activated sodium channels in outside-out patches of tissue-cultured GH3 cells, a clonal line from rat pituitary gland. In membrane patches where the probability of overlapping openings was low, the open time histograms were well fit by a single exponential. Most analysis was done on a patch with exactly one channel. We found no evidence for multiple open states at -25 and -40 mV, since open times, burst durations, and autocorrelation functions were time independent. Amplitude histograms showed no evidence of multiple conductance levels. We fit the gating with 25 different time-homogeneous Markov chain models having up to five states, using a maximum likelihood procedure to estimate the rate constants. For selected models, this procedure yielded excellent predictions for open time, closed time, and first latency density functions, as well as the probability of the channel being open after a step depolarization, the burst duration distribution, autocorrelation, and the distribution of number of openings per record. The models were compared statistically using likelihood ratio tests and Akaike's information criterion. Acceptable models allowed inactivation from closed states, as well as from the open state. Among the models eliminated as unacceptable by this survey were the Hodgkin-Huxley model and any model requiring a channel to open before inactivating.

Animals↗
PubMed1984

Inactivation viewed through single sodium channels.

Recordings of the sodium current in tissue-cultured GH3 cells show that the rate of inactivation in whole cell and averaged single channel records is voltage dependent: tau h varied e-fold/approximately 26 mV. The source of this voltage dependence was investigated by examining the voltage dependence of individual rate constants, estimated by maximum likelihood analysis of single channel records, in a five-state kinetic model. The rate constant for inactivating from the open state, rather than closing, increased with depolarization, as did the probability that an open channel inactivates. The rate constant for closing from the open state had the opposite voltage dependence. Both rate constants contributed to the mean open time, which was not very voltage dependent. Both open time and burst duration were less than tau h for voltages up to -20 mV. The slowest time constant of activation, tau m, was measured from whole cell records, by fitting a single exponential either to tail currents or to activating currents in trypsin-treated cells, in which the inactivation was abolished. tau m was a bell-shaped function of voltage and had a voltage dependence similar to tau h at voltages more positive than -35 mV, but was smaller than tau h. At potentials more negative than about -10 mV, individual channels may open and close several times before inactivating. Therefore, averaged single channel records, which correspond with macroscopic current elicited by a depolarization, are best described by a convolution of the first latency density with the autocorrelation function rather than with 1 - (channel open time distribution). The voltage dependence of inactivation from the open state, in addition to that of the activation process, is a significant factor in determining the voltage dependence of macroscopic inactivation. Although the rates of activation and inactivation overlapped greatly, independent and coupled inactivation could not be statistically distinguished for two models examined. Although rates of activation affect the observed rate of inactivation at intermediate voltages, extrapolation of our estimates of rate constants suggests that at very depolarized voltages the activation process is so fast that it is an insignificant factor in the time course of inactivation. Prediction of gating currents shows that an inherently voltage-dependent inactivation process need not produce a conspicuous component in the gating current.

Animals↗
PubMed1984

Org. 6368: a nondepolarizing neuromuscular blocking agent with a novel effect on end-plate conductance.

The effects of the nondepolarizing agent pancuronium and three derivatives on end-plate currents (e.p.c.s), evoked by neural stimulation at the amphibian neuromuscular junction, were investigated using conventional voltage clamp techniques. All four agents depressed peak e.p.c. amplitude and also shortened the exponential time constant of the e.p.c. decay compared with control. The properties of one derivative, Org. 6368, were investigated in detail and revealed the drug to produce marked blockade of end-plate ionic conductance relative to receptor blockade. Analysis of the kinetic behavior of Org. 6368 revealed a novel mechanism of channel blockade and dissociation which is very different to that observed with some of the local anesthetics. The rate of association of Org. 6368 with the channel binding site, 1.92 X 10(7) M-1 sec-1 at 0 mV, had virtually no dependence on membrane voltage with an e-fold increase with a change of voltage of 2940 mV while the rate of dissociation, 26.4 sec-1 at 0 mV, exhibited a marked dependence on voltage with an e-fold increase with a change of voltage of only -39.5 mV. It is proposed that the rate of association of Org. 6368 is rate limited by diffusion while the rate of dissociation from the channel binding site is increased by hyperpolarization, due to the drug molecule being attracted into the membrane field before leaving the binding site.

Animals↗
PubMed1984

Suicide reconstruction by glue-lift of gunshot residue.

Based upon the recently developed glue-lift collection of gunshot residue particles for examination in the scanning electron microscope with energy dispersive X-rays, this laboratory has undertaken a research program to determine if reconstruction of gunshot deaths is feasible. Because undisturbed conditions of suicide victims may help in securing unambiguous results and high success rates, the program has been carried out to reconstruct suicides only. Data obtained from 13 firearms suicides and their laboratory reconstruction which involved primarily shotguns and handguns, indicate that reconstruction can be immensely useful to interpretation of the gunshot residue distributions on a suspect's or victim's hands. This reports outlines the basic experiments performed to relate the residue emission from the gun to the deposits found on the firing hand(s) and a description of the reconstruction technique that uses a target to simulate a human body. The technique can determine the specifics of a victim's hand position at the time of firing and which hand was used to pull the trigger. Because the technique is sensitive to the nature of the grasp of the firing hand and of the supporting hand, in the case of a shotgun, it has been possible in all cases to date, to determine which one of the potential eight hand positions existed at the time of shooting.

Adult↗
PubMed1983

Estimating kinetic constants from single channel data.

The process underlying the opening and closing of ionic channels in biological or artificial lipid membranes can be modeled kinetically as a time-homogeneous Markov chain. The elements of the chain are kinetic states that can be either open or closed. A maximum likelihood procedure is described for estimating the transition rates between these states from single channel data. The method has been implemented for linear kinetic schemes of fewer than six states, and is suitable for nonstationary data in which one or more independent channels are functioning simultaneously. It also provides standard errors for all estimates of rate constants and permits testing of smoothly parameterized subhypotheses of a general model. We have illustrated our approach by analysis of single channel data simulated on a computer and have described a procedure for analysis of experimental data.

Animals↗
PubMed1983

Effects of FSH, isoproterenol, and cyclic AMP on the production of lactate and pyruvate by cultured Sertoli cells.

We have examined the hormonal regulation of the secretion of lactate and pyruvate from cultured rat Sertoli cells. FSH and isoproterenol caused 3-6-fold stimulation of lactate and pyruvate secretion, whereas ovine LH, TSH, and prolactin were ineffective. Dibutyryl cyclic AMP (10(-4) M) stimulated the secretion of lactate and pyruvate to the same extent as FSH. Much lower stimulation was observed when Sertoli cells from 43-day old rats were exposed to FSH or isoproterenol. FSH increased lactate secretion in a concentration-dependent manner. The concentration of FSH (NIH-S14) causing half-maximal stimulation of lactate secretion (150 ng/ml) was similar to that causing 50% maximal stimulation of Sertoli cell adenylyl cyclase. Both FSH and isoproterenol caused a time-dependent increase in lactate levels in the incubation medium during the first 6-9 hr after the addition of hormones, after which levels were constant or decreased. Thus, the production of lactate and pyruvate by cultured Sertoli cells is stimulated both by FSH and isoproterenol and these effects are exerted via cyclic AMP.

Animals↗
PubMed1983

Cyclic nucleotides and catecholamines in rheumatoid arthritis.

A controlled cross over study on the effect of fasting on cyclic nucleotide and catecholamine excretion was carried out in 12 female RA patients. They were hospitalized during a control and fasting period. The fasting period started with 3 days of habituation to hospital conditions, followed by 7 days of total fasting. Thereafter followed 3 days of realimentation. The control period was in every respect identical with the fasting period, with the exception of food intake. All medication was stopped when the patients were admitted to the hospital. The urinary concentrations of cAMP, cGMP, E and NE were monitored daily. The clinical status of the subjects was evaluated every second day using Ritchie's clinical index. Clinical and laboratory measures of inflammatory activity remained unaltered during the control period, but improved significantly during fasting. Excretion of cAMP and cGMP was low during the control period when compared with published normal levels. The cAMP/cGMP ratio in urine was normal, however. The urinary levels of E and NE were normal in the control period, but increased significantly during fasting. Excretion of cAMP decreased during fasting, while urinary cGMP levels decreased initially, but rose to prefasting levels towards the end of the fast. The cAMP/cGMP ratio increased during the first days of the fast, with a maximum on days 2 and 3. This increment coincided in time with the greatest rate of improvement in clinical joint activity.

Adult↗
PubMed1982

Effect of N-bromoacetamide on single sodium channel currents in excised membrane patches.

We have studied the effect of N-bromoacetamide (NBA) on the behavior of single sodium channel currents in excised patches of rat myotube membrane at 10 degree C. Inward sodium currents were activated by voltage steps from holding potentials of about -100 mV to test potentials of -40 mV. The cytoplasmic-face solution was isotonic CsF. Application of NBA or pronase to the cytoplasmic face of the membrane irreversibly removed sodium channel inactivation, as determined by averaged single-channel records. Teh lifetime of the open channel at -40 mV was increased about 10-fold by NBA treatment without affecting the amplitude of single-channel currents. A binomial analysis was used both before and after treatment to determine the number of channels within the excised patch. NBA was shown to have little effect on activation kinetics, as determined by an examination of both the rising phase of averaged currents and measurements f the delay between the start of the pulse and the first channel opening. Our data support a kinetic model of sodium channel activation in which the rate constant leading back from the open state to the last closed state is slower than expected from a strict Hodgkin-Huxley model. The data also suggest that the normal open-channel lifetime is primarily determined by the inactivation process in the voltage range we have examined.

Acetamides↗
PubMed1981

The effect of tetramethylammonium on single sodium channel currents.

Voltage-dependent Na conductance of rat myotubes was studied by patch recordings of single-channels. The patches were excised from the cell with the patch electrode, and the cytoplasmic surface was bathed in either CsF or tetramethylammonium (TMA)-F. Inward currents were examined from -20 to -50 mV. In this range Cs and TMA both appeared to be nearly impermeant, but TMA blocked the channel in a voltage-dependent manner. A first-order blocking site was located a maximum of 89% of the way through the membrane field from the cytoplasmic surface.

Animals↗
PubMed1980

Asymmetry of the acetylcholine channel revealed by quaternary anesthetics.

Tissue-cultured rat myoballs were examined electrophysiologically with a suction pipette, which was used for voltage clamping and internal perfusion. The lidocaine derivative QX-314 caused a time- and membrane potentia-dependent block of acetylcholine-induced current only when applied from the extracellular membrane surface. The same compounds caused a use-dependent block of the sodium channel only from the intracellular membrane surface. These experiments demonstrate a fundamental asymmetry of the acetylcholine receptor-channel complex.

Acetylcholine↗
PubMed1980

Cyclic nucleotides in rheumatoid arthritis.

Cyclic 3',5'-adenosine monophosphate (cAMP) and cyclic 3',5' guanosine monophosphate (cGMP) may influence important regulatory mechanisms in the rheumatoid inflammatory process. It has been claimed that fasting improves the condition of the patient with rheumatoid arthritis (RA). The present study was designed to measure cAMP in plasma and urine cGMP in urine in medically untreated RA patients. 12 female patients were investigated in a cross-over study during a control and a fasting period. They received no other drugs than analgesics during these periods. Levels of plasma and urinary cAMP found during the control period were somewhat lower than previously reported. However, the ratio cAMP/cGMP in urine was 10 to 1 which is reported to be normal. Clinical and laboratory variables of inflammatory activity were significantly improved during the 7-day fasting period. The ratio of cAMP/cGMP in urine was significantly increased on days 2-4 and coincided in time with the maximum of clinical improvement. Cyclic AMP concentrations were lowered both in plasma and urine during fasting. This is in contrast to fasting in normal and obese subjects reported in previous studies.

Adult↗
PubMed1980

Single channel currents from excised patches of muscle membrane.

The currents through single acetylcholine-activated channels were measured on membrane fragments that had been torn from rat muscle myotubes with patch pipettes. The membrane fragments were sealed into the pipette by using the "gigohm-seal" technique of Neher, which also permitted voltage clamp of the membrane via the patch electrode. Membrane patches were excited by sudden withdrawal of the electrode from the cell. Substitution of fluoride for chloride ions in the bathing solution could prevent or reverse the tendency for the membrane at the electrode tip to seal over into a closed vesicle. The single membrane layer at the electrode tip could remain functional for up to 30 min. The apparent single channel conductance was minimally affected by excision. The current-voltage relationships for the single channel currents show that the inside (i.e., cytoplasmic surface) of the membrane fragment was exposed to the bathing solution. In symmetric Na solutions the current-voltage curve was nearly linear and reversed at approximately 0 mV. In other bathing solutions from 40 to 500 mM NaF, the observed zero current potential was close to that predicted by the Nernst equation. We present evidence that internal Na interacts with the channel, causing both saturation of outward current and block of inward current. At + 100 mV the apparent dissociation constant for internal Na was 138 mM.

Animals↗
PubMed1980

Acetylcholine-induced current in perfused rat myoballs.

Spherical "myoballs" were grown under tissue culture conditions from striated muscle of neonatal rat thighs. The myoballs were examined electrophysiologically with a suction pipette which was used to pass current and perfuse internally. A microelectrode was used to record membrane potential. Experiments were performed with approximately symmetrical (intracellular and extracellular) sodium aspartate solutions. The resting potential, acetylcholine (ACh) reversal potential, and sodium channel reversal potential were all approximately 0 mV. ACh-induced currents were examined by use of both voltage jumps and voltage ramps in the presence of iontophoretically applied agonist. The voltage-jump relaxations had a single exponential time-course. The time constant, tau, was exponentially related to membrane potential, increasing e-fold for 81 mV hyperpolarization. The equilibrium current-voltage relationship was also approximately exponential, from -120 to +81 mV, increasing e-fold for 104 mV hyperpolarization. The data are consistent with a first-order gating process in which the channel opening rate constant is slightly voltage dependent. The instantaneous current-voltage relationship was sublinear in the hyperpolarizing direction. Several models are discussed which can account for the nonlinearity. Evidence is presented that the "selectivity filter" for the ACh channel is located near the intracellular membrane surface.

Acetylcholine↗