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M A Pak

Publications and source records attributed to M A Pak.

15 recordsLinked to original sources
PubMed1994

Inhibition of adenosine kinase increases endogenous adenosine and depresses neuronal activity in hippocampal slices.

Endogenous adenosine in the extracellular space inhibits neuronal activity. The roles of adenosine kinase, S-adenosylhomocysteine-hydrolase and adenosine deaminase activities in the regulation of the adenosine levels were investigated in rat hippocampal slices. Iodotubercidin, an inhibitor of adenosine kinase, added to the perfusion fluid at 5 microM increased the release of adenosine from the slices more than 2-fold. Iodotubercidin treatment caused inhibition of population spike discharges and hyperpolarization of pyramidal cells, mimicking the effects of exogenously applied adenosine. Adenosine dialdehyde, an inhibitor of S-adenosylhomocysteine hydrolase, and erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), an inhibitor of adenosine deaminase had little or no effect on the parameters tested. The action of iodotubercidin was greater during deaminase inhibition. The A1-receptor antagonist DPCPX had actions opposite to those of adenosine and blocked the electrophysiological effects of exogenous adenosine and of iodotubercidin. Thus adenosine kinase activity is a significant factor in the regulation of adenosine levels in the hippocampus.

Adenosine↗
PubMed1991

[Contrast transfer function of the visual system].

Visually evoked potentials were used to determine the spatial contrast response function of the visual system and the visual acuity of the pigeon. The spatial contrast response describes the relationship between the contrast in a pattern of vertical stripes, whose luminance is a function of position, and the amplitude of the visually evoked response at various spatial frequencies for a given temporal frequency (pattern reversal frequency); it indicates how particular spatial frequencies are attenuated in the visual system. The visually evoked responses were recorded using monopolar stainless steel electrodes inserted into the stratum griseum superficiale of the optic tectum; the depth of penetration was determined on the basis of a stereotactic atlas. The stimulus patterns were generated on a video monitor placed 75 cm in front of the animal's eye perpendicular to the optic axis. The spatial contrast response function measured at 10% contrast and 0.5 Hz reversal frequency shows a peak at a spatial frequency of 0.5 c/deg, corresponding to 1 degree of visual angle, and decreases progressively at higher spatial frequencies. The high-frequency limit (cut-off frequency) for resolution of sinusoidal gratings, estimated from the contrast response function, is 15.5 c/deg, corresponding to a visual acuity of 1.9 min of arc.

Animals↗
PubMed1991

[The spatial frequency limits and the resolving power of the visual system of the pigeon].

The spatial contrast transfer function of the visual system of the pigeon was determined by recording from the optic tectum evoked potentials or extracellular unit activity in response to a pattern stimulus contrast transfer function, determined as a "response function", describes the relationship between the contrast in the pattern--which consisted of vertically oriented stripes of sinusoidally varying luminance--and the amplitude of the response at various spatial frequencies (c/deg). The transfer function yields an estimate of the high frequency limit, which in turn is a measure of visual resolving power. Action potentials were recorded extracellularly using glass microelectrodes; for evoked potentials, stainless steel electrodes were used. Recordings were made from the stratum griseum et fibrosum superficiale of the optic tectum. The highest spatial frequency detectable in a visual system is limited by various factors, including the diffraction of light at the pupil and the anatomical spacing of the photoreceptors. The pupil factor can be controlled in experiments in a suitable way. In this paper, the electrophysiologically determined high-frequency limit was compared with the theoretical resolution limit imposed by the photoreceptor mosaic. The experimental results show that the visual system of the pigeon has a high-frequency limit at a spatial frequency of 15.5 c/deg, corresponding to a visual acuity of 1.9 min of arc. The attempt to relate visual acuity in the pigeon to the anatomical spacing of the photoreceptors shows that the Nyquist frequency of the photoreceptor mosaic, the theoretical upper bound of the spatial resolution, agrees with measurement.

Animals↗
PubMed1990

Intrathecal injection of a specific enzyme in rats as a model for chemically induced spinal cord injury.

A spinal cord injury model is described using chymopapain as a neurotoxic agent in rats. The trauma was evaluated by neurophysiological and morphological methods. Electrically evoked compound action potentials were used to quantify the neurophysiological effects caused by the injection of chymopapain into the lumbar dural theca in rats. A branch of the sciatic nerve was stimulated with voltage impulses of constant amplitude (40 V) and duration (0.1 ms) at the right external malleolus. The responses were recorded at the dorsal root entry zone L1. We used different doses of the enzyme (1000 i.u./ml; 2500 i.u./ml; 5000 i.u./ml). A total amount of 0.1 ml was injected into the rats' lumbar spinal canal intrathecally (n = 18). The control rate (n = 8) were subjected to exactly the same stimulus and recording procedures but the test solution was a corresponding volume of isotonic saline. Two hours after injection the animals were examined clinically and then sacrificed for histology. The prolongation of the latency of the electrically evoked potentials caused by the enzyme was very clear at the doses of 2500 i.u. and 5000 i.u. being about 10-15% relative to the mean latency before administration of the drug. The histological evaluation showed hemorrhage, vascular damage and indirect signs of myelin edema of the lumbar nerve tissue. Our study indicates that chymopapain injections into the lumbar spinal canal can be used as a reproducible model for spinal cord injury research.

Animals↗
PubMed1990

[Organization of emergency medical services for the children in Tashkent].

The work of emergency medical care service in Tashkent is described, the experience gained in the provision of emergency medical care to children is presented. An information is given of the organization of emergency medical care at the prehospital level, the extent and the structure of children's emergency calls, the main regularities in seeking emergency calls, the main regularities in seeking emergency care by seasons and months of the year, by days and hours, the amount and the peculiarities of emergency care provided to children and also future trends in the development of emergency medical care for children in Tashkent.

Child↗
PubMed1989

The influence on spinal cord evoked potentials of chymopapain applied to the rat lumbar spine canal.

Electrically evoked compound action potentials were used to quantify the neurophysiologic abnormalities caused by application of chymopapain into the lumbar spinal canal in rats. A branch of the sciatic nerve was stimulated with voltage impulses of constant amplitude (40 V) and duration (0.1 msec) at the right external malleolus. The responses were recorded at the dorsal root entry zone L1. The authors used different doses of chymopapain (low dose = 100 IU, middle dose = 250 IU, and high dose = 500 IU). The control rats were subjected to exactly the same stimulus and recording procedures, but the test solution was a corresponding volume of isotonic saline. The prolongation of the latency of the electrically evoked potential due to chymopapain is very clear at the doses of 250 IU and 500 IU, being about 10 to 15% relative to the mean latency before application of the drug. This study indicates that locally applied chymopapain has a dose-dependent effect on nerve conduction properties.

Animals↗
PubMed1988

[Early functional and structural changes in the nerve tissue following contact with chymopapain].

Using magnetic resonance imaging, conventional histology, and spinal evoked potentials, the authors studied the short-term effects of chymopapain on nerve tissue in the lumbar vertebral canal region in rats. Chymopapain in different concentrations was injected into the subarachnoid space in 31 rats. As early as 20 minutes later latency changes in the evoked potentials were observed; the histologic examinations after two hours showed bleeding, swelling of the myelin, and vascular changes. Magnetic resonance imaging of the lumbar spine, performed after six hours, showed a markedly stronger signal, indicative of an intraspinal edema, as compared to the control group. The authors' studies are clear proof that changes in nerve tissue following contact with chymopapain occur considerably faster than hitherto assumed in the literature.

Animals↗
PubMed1988

[Long-term studies of the reproducibility of evoked spinal potentials from chronically implanted recording electrodes in the rat].

Over a period of 4 weeks we examined the reliability of spinal cord nerve potentials in the thoracic-lumbar region after stimulation of the hindpaw in 10 rats. Recording electrodes were chronically implanted over a period of 4 weeks into the lower thoracic and upper lumbar lamina by using an operation microscope. The action potentials were recorded at the dorsal root entry zone after stimulating the hindpaw. By comparing the data of the recording sessions we found a high reliability of the spinal cord evoked response over a long period. The experimental model is appropriate for the assessment of neurophysiological changes in the long term. Additionally we examined some basic characteristics of the evoked spinal response in this area.

Animals↗
PubMed1984

Ocular refraction and visual contrast sensitivity of the rabbit, determined by the VECP.

The ocular refraction of the rabbit was determined by inserting spherical lenses of between -3 and +4D and measuring the relative VECP amplitude in response to checkboard patterns of 21 min arc. The rabbit's eye was found to be almost emmetropic (+0. 5D ). The contrast sensitivity function of the rabbit's visual system was determined by recording the VECP in response to reversed gratings of different spatial frequencies. The contrast sensitivity was found to be highest at 0.35 c/deg with an upper cut-off frequency at 3 c/deg, corresponding to a grating acuity of 10 min arc.

Animals↗
PubMed1984

Modulation sensitivity and functional organization of the receptive field of ganglion cells in the pigeon (Columba livia).

Modulation sensitivity functions ( MSFs ) of retinal ganglion cells of the pigeon were determined by recording action potentials from their end branching in the optic tectum as response indicator. The inverse Fourier transformation of a MSF is taken to represent the receptive field sensitivity function approximating the receptive field profile of a ganglion cell.

Action Potentials↗
PubMed1982

Volume elasticity of the ocular avascular compartment in vivo and post mortem.

By considering the frequency dependence of the ocular volume elasticity it is possible to locate the static volume elasticity function of the avascular compartment of the eye in vivo. The procedure used involved measuring the dynamic volume elasticity function E = f(P,v), where E = volume elasticity, P = intraocular pressure, and v = frequency, in vivo and post mortem at a frequency higher than the apparent upper mechanical response frequency of the intraocular vascular bed. In addition, post mortem measurements were made of the volume elasticity function at a frequency which was as low as experimentally possible. For practical purposes the latter volume elasticity function may serve as an estimate of the static elasticity function of the avascular compartment in vivo. This is possible in all cases because at the high frequency level the dynamic volume elasticity functions measured in vivo and post mortem are identical.

Animals↗
PubMedDate not supplied

[Animal experiment studies of the diagnosis and therapy of chymopapain-induced spinal complications].

Short and long-term effects of chymopapain on nervous transmission were studied in rats. The drug was injected into the lumbar spinal canal. Action potentials evoked by electrical stimulation of the sciatic nerve were recorded at the dorsal root entry zone of the lumbar cord. One group of rats was pre-treated with dexamethasone. Chymopapain prolongs impulse latency compared to that of control rats treated with isotonic saline. The effect is dose dependent and can last for several weeks. Rats pre-treated with dexamethasone showed milder impairment of nervous transmission after chymopapain injection compared to the rats treated only with papain.

Animals↗
PubMedDate not supplied

[Characteristics of the action of nitroglycerin on the hemodynamics of patients with ischemic heart disease in relation to the content of SH groups of the blood serum and the tolerance for physical loading].

The authors examined 196 patients who had survived myocardial infarction and 21 persons without signs of myocardial ischemia. Nitroglycerin test with subsequent control of myocardial contractility, hemodynamics and blood serum content of SH-groups indicated that the level of SH-groups could serve as a prognostic criterion of efficacy of nitrates (inverse correlation).

Adult↗