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Biomedical subjects

U Stabell

Publications and source records attributed to U Stabell.

At least 19 recordsLinked to original sources
PubMed1999

Rod-cone color mixture: effect of size and exposure time.

Chromaticity coordinates of monochromatic lights were obtained 8 deg extrafoveally at a retinal illumination of 50 photopic td during the cone-plateau period and also with the subject in a dark-adapted state, while size and exposure time of the test field were varied. Unexpectedly, we found that for the dark-adapted condition the points representing the chromaticity matches of the spectral lights generally moved away from the achromatic point in the chromaticity diagram when size or exposure time of the test stimulus was increased. Furthermore, the chromaticity shift toward the achromatic point, obtained between the cone-plateau period and the dark-adapted state (i.e., with rod intrusion), tended to decrease with these two test parameters. In fact, when size and exposure time both were at the maximum level investigated (7 deg, 500 ms), there was no measurable shift in chromaticity with rod intrusion. Our results suggest that the cone system may become progressively more effective in suppressing the rod system as an effect of both size and exposure time.

Color Perception↗
PubMed1998

Chromatic rod-cone interaction during dark adaptation.

Chromatic rod-cone interaction in mesopic vision was investigated by measuring chromaticity coordinates of spectral lights 3 deg extrafoveally during long-term dark adaptation, under conditions where the eye was chromatically adapted. It was concluded that (a) the chromaticity shift obtained between the cone-plateau period and the dark-adapted state is due, at least under some conditions, to rod signals elicited by the test stimulus, (b) the chromaticity points of cone-mediated colors may change in different directions in the chromaticity diagram as an effect of the rod intrusion, (c) cone-cone and rod-cone color-mixture processing may, at least under some conditions, be different, and (d) chroma-related processes of rods and cones tend to suppress each other, with rods dominating at low and cones at high mesopic intensities.

Color↗
PubMed1996

Peripheral colour vision: effects of rod intrusion at different eccentricities.

Chromaticities of monochromatic lights from different parts of the spectrum were measured both during the cone-plateau period of the long-term dark-adaptation curve and in a completely dark-adapted state. The measurements were obtained at 3, 8, 30 and 65 deg in the temporal field of view and at 1, 2, 3 and 4 log units above the cone-plateau level. The results show that cone-mediated colours obtained during the cone-plateau period in general are desaturated when rod signals intrude during long-term dark adaptation. The desaturation effect of rods obtained at high mesopic illumination levels was found to increase when the test field was moved from 3 to 30 deg but to reduce markedly between 30 and 65 deg. Surprisingly, the desaturation was clearly observable even at a retinal illumination of 20,000 ph td. The desaturation effect of rods is explained by the suggestion that differences in ongoing activity rates of the different types of spectrally opponent cells become levelled out to some extent when light signals from rods intrude during dark adaptation.

Color Perception↗
PubMed1996

Long-term rod dark adaptation in man. Threshold measurements, rhodopsin regeneration and allosteric sensitivity regulation. An evaluation.

Recent evidence strongly suggests that the relationship between threshold elevation (T) and fraction of bleached rhodopsin (B), obtained during a major, middle period of long-term rod dark adaptation in man, is well described by a power function, i.e., T = k.Bn, where k is a multiplicative constant and n is the exponent. Due primarily to the low reliability of measurements of rhodopsin regeneration, however, the exponent n of the power function cannot, at present, be given an exact value. Available information indicates that the value of the exponent ranges between 2.4 and 4. Implications of this uncertainty are discussed within the framework of the allosteric, tetrameric model of rod dark adaptation. It is concluded that this model in its simplest form may only offer a first approximation of the real system implicated in the process.

Allosteric Regulation↗
PubMed1994

Mechanisms of chromatic rod vision in scotopic illumination.

After viewing a coloured patch for 30 sec, successive contrast colours were triggered by stimulating either rods or cones. The conditions were arranged so that the rod and cone stimuli matched both with respect to chromaticness and brightness in a chromatically neutral state of adaptation. The results showed that the contrast colours triggered by rods were strikingly similar to those triggered by cones. Yet, the scotopic contrast colours, as compared with the photopic ones, were generally found to be somewhat displaced toward blue. This displacement was attributed to the difference in test conditions. Thus, it was suggested that, although rods may excite all the different types of spectrally opponent cells, they generally tend to prefer the short-wave cells. Moreover, it was concluded that the scotopic successive contrast colours are triggered by rod signals feeding into the primary rod pathway and therefore must originate centrally to the receptor level.

Color Perception↗
PubMed1993

Rod-cone interaction in form detection.

Using a Wright colorimeter, absolute threshold, absolute form threshold and specific form threshold were measured during long-term dark adaptation in the extrafoveal retina. The specific form threshold was found to fall markedly at about the cone-rod break but thereafter rose steeply. Furthermore, during the rod phase of the dark adaptation the form percept of the small, slender rectangular test field changed qualitatively from a line or rectangle to a circular field at all mesopic intensities. The results indicate that light signals from rods may both facilitate and suppress cone-mediated information about form, and that the rod system may completely dominate the perception of form several log units above the absolute dark-adapted cone threshold when the eye is dark adapted.

Dark Adaptation↗
PubMed1992

Mechanisms of long-term dark adaptation.

It has previously been suggested that long-term dark adaptation is controlled by bleaching signals that regulate the activity of an allosteric, positively cooperative protein (Stabell et al., 1986a, b). Recent biochemical evidence strongly supports this assumption, indicating that the primary regulator of the light-sensitive channels in the plasma membrane of the outer segments of the photoreceptors is a homo-oligomeric, allosteric, positively cooperative protein. In this report, we discuss the possibility that signals from bleached photopigments may control the dark-adaptation process through the allosteric protein of the plasma membrane. It is suggested that the concentrations of the bleached photopigment and of the allosteric effector are reciprocal quantities.

Animals↗
PubMed1991

Dark adaptation of the long-wave cones at different eccentricities.

Using a Wright colorimeter the ordinary long-term, long-wave cone dark-adaptation curve was measured at 0, 2, 4, 7, 17, 25, 40 and 49 degrees nasally in the visual field. In opposition to previous findings, the results show that the dark-adaptation function of the long-wave cones changes markedly when the test field is moved outward from the rod-free fovea. It is suggested that the kinetics of the long-wave cone photopigment change with eccentricity. Also, at variance with previous findings, the present curves at all eccentricities may reasonably well be interpreted as consisting of three different sections; a first section where the threshold decreases rapidly, followed by a major, approximately linear section and a terminating section that converges asymptotically towards the final level of sensitivity. This finding suggests that the dark-adaptation process of the cone system, under the given experimental conditions, is based on three somewhat different processes.

Color Perception↗
PubMed1991

Recurrent hyperparathyroidism after total parathyroidectomy and autotransplantation in patients with long-term hemodialysis.

From 1986 to 1990, we performed total parathyroidectomy with simultaneous autotransplantation (TPX with AT) of parathyroid tissue into the forearm in 13 patients on chronic maintenance hemodialysis with severe secondary hyperparathyroidism. The indications for surgery included persistent hypercalcemia, hyperphosphatemia, roentgenological signs of renal osteopathy, soft tissue calcifications and severe pruritus. The parathyroidectomy was performed by an experienced surgeon who removed all four parathyroid glands and transplanted 10 mm3 of parathyroid tissue into the brachioradialis muscle. After an average postoperative period of 33.5 months, 10 of 13 patients developed recurrent clinical and biochemical hyperparathyroidism requiring reoperation. Seven of these patients underwent removal of the autograft. In 5 patients, however, the success was minimal. Four of these patients had a second autograft resection, but only transient improvement resulted. Our recurrence rate after TPX with AT is very high. The histological aspect of the resected autograft is similar to tumor-like growth and this may account for the results. A therapeutic alternative to TPX with AT appears to be necessary.

Calcitriol↗
PubMed1990

Rod suppression of cone-mediated information about colour and form during dark adaptation.

Following substantial bleaches, the specific form and hue thresholds were measured during dark adaptation with a test stimulus of 1 x 2 degrees at 40 degrees extrafoveally. The wavelength of the test field was varied between runs. The results show that both thresholds started to rise at about the cone-rod break of the dark-adaptation curve, irrespective of wavelength used in the test. Furthermore, the specific threshold for form was found to rise when a scotopic stimulus was superimposed on a photopic test flash. On the other hand, both thresholds remained at the cone-plateau level when the test flash was confined within the rod-free fovea. In order to explain the rise in the specific thresholds, it is suggested that signals from rods generated directly in response to the test stimulus may suppress both cone-mediated form and colour. It is also suggested that this type of rod-cone interaction represents a general characteristic involved in several kinds of visual information processing.

Attention↗
PubMed1990

Dark adaptation of foveal cones during the cone-plateau period.

Following substantial bleaching by "white" light, absolute threshold, relative spectral sensitivity and sensation of hue of monochromatic lights were measured at the central fovea during the cone-plateau period. The absolute-threshold level was found to increase and then decrease markedly, the relative spectral sensitivity remained invariant, while the sensation of hues of monochromatic lights from the long- and middle-wave regions of the spectrum changed toward hues of shorter wavelengths.

Dark Adaptation↗
PubMed1990

Erythropoiesis and erythrocyte age distribution in hemodialysis patients undergoing erythropoietin therapy.

Renal anemia is caused in part by a reduced life span of red blood cells (RBCs) and by reduced erythropoietin biosynthesis in the damaged kidney. The RBC age can be determined by density gradient centrifugation and estimation of cell-age-dependent enzyme activities, as aspartate aminotransferase. The RBC age distribution influences the median density (D50) of RBCs and the blood rheology in coherence with the hematocrit. In our study, the median density was determined by Percoll density gradient centrifugation in 18 healthy subjects (D50 = 1.0674 +/- 0.0016 g/ml) and in 14 hemodialysis patients (D50 = 1.0674 +/- 0.0016 g/ml in the course of recombinant human erythropoietin (rhEPO) therapy. During the first 4 weeks of therapy, a strong rejuvenation of RBCs was observed whereby the D50 reached a minimum after 2 weeks (D50 = 1.0655 +/- 0.0022 g/ml; p less than 0.05 vs. value before therapy) and a steady state after 4 weeks (D50 = 1.0658 +/- 0.0013 g/ml; p less than 0.1 vs. value before therapy). In 5 of the patients with elevated plasma parathyroid hormone (i-PTH) concentrations greater than 10 pmol/l, a significantly (p less than 0.05) reduced amount of younger RBCs (D50 = 1.0675 +/- 0.0016 g/ml) was observed in the first 2 weeks of rhEPO therapy as compared to patients with i-PTH less than 10 pmol/l (D50 = 1.0677 +/- 0.0019 g/ml). Thus, erythropoiesis in the early phase of rhEPO therapy is strongly influenced by elevated plasma i-PTH concentrations. Therefore, a gradual increase in rhEPO doses is preferable before therapy at elevated doses with an uncontrolled increase in RBC amount.

Adult↗
PubMed1989

Dark-adaptation mechanisms of the long-wave foveal cones.

The ordinary long-term rod and cone dark-adaptation curves have generally been assumed to follow a single exponential rate of recovery. However, in two previous papers on rod dark-adaptation (Stabell et al., 1986a, b), the recovery curve was found to consist of three different sections. The results of the present paper show the same type of recovery function with three different sections for the long-term dark-adaptation curve of the long-wave cone system. During the major, middle section log cone threshold, like log rod threshold, is linearly related to the logarithm of the concentration of bleached photopigment. Presupposing that the bleached cone photopigment acts as a ligand, the change in threshold level obtained during the middle section of the dark-adaptation curve is well described by the change in activity rate of an allosteric, postively cooperative enzyme built as a dimer.

Dark Adaptation↗
PubMed1989

Changes in the concentrations of hydroxyproline, glycine and serine in the plasma of haemodialysis patients undergoing erythropoietin therapy.

The concentrations of proline, hydroxyproline, glycine and serine were determined in the plasma of 39 haemodialysis patients and 18 healthy subjects, using liquid chromatography with fluorescence detection. Plasma concentrations of the N-terminal immunoreactive parathyrin were also measured. In haemodialysis patients, the plasma concentrations of glycine (p less than 0.01), hydroxyproline (p less than 0.05) and proline (p less than 0.10) were significantly increased, whereas the serine concentrations (p less than 0.01) were decreased, compared with those of the healthy controls. Haemodialysis patients showed greatly elevated plasma N-terminal immunoreactive parathyrin values (greater than 30 pmol/l), which showed a significant correlation with the hydroxyproline values (r = 0.79). Fourteen haemodialysis patients received erythropoietin therapy. In these patients, changes in the concentrations of plasma amino acids were observed up to one year after the beginning of therapy. In the course of the erythropoietin therapy, the plasma concentrations of glycine (p less than 0.05) and hydroxyproline (p less than 0.10) of the haemodialysis patients decreased, whereas the concentration of serine increased (p less than 0.05) to approximately normal values. The results indicate that erythropoietin therapy leads to a normalization of amino acid metabolism.

Adult↗
PubMed1985

[The pelvicaliceal system in the sonogram--distinction between physiological and pathological dilatation].

Dilatation of the pelvicalyceal system was documented in volunteers with healthy kidneys in whom renal excretation was increased by means of a variety of methods. As a rule, the proximal ureter is not visualised. In mild or moderate urinary obstruction, however, the ureter is almost always visible, and stasis is mostly unilateral. Although sonography is a method of good sensitivity, the excretory urogram remains an essential diagnostic tool in the examination of the pelvicalyceal and ureteral systems.

Adult↗
PubMed1984

Color-vision mechanisms of the extrafoveal retina.

Wavelength discrimination, spectral sensitivity as well as color-matching performance were measured at the fovea and at different eccentricities in the peripheral retina. The results show that the underlying mechanisms of color vision in the normal peripheral retina are different from those of the classic forms of congenital color blindness. On the other hand, a close correspondence was found between color-vision characteristics obtained in the extrafoveal retina and in patients with acquired color-vision defects due to diseases of the optic nerve, suggesting that the loss of color discrimination with eccentricity and during progression of these diseases has a common underlying basis.

Color Perception↗
PubMed1984

Dark-adaptation of the human rod system.

Following substantial bleaching, dark-adaptation thresholds of a complete rod monochromat and of a subject with normal colour vision were measured using a Wright colorimeter. When precautions were taken to ensure that the fixation point fell on the same retinal area during the threshold measurements as during the bleaching period, the dark-adaptation threshold curves of the rod monochromat followed exactly the same course as those of the normal subject subsequent to the cone-rod break of the long-term, normal dark-adaptation curve; irrespective of the intensity and the duration of the bleaching and the wavelength of the test stimulation. In contrast to the normal subject, however, the dark-adaptation curves of the rod monochromat showed no evidence of any cone function at photopic intensities. Furthermore, as opposed to previous measurements which show a simple linear relationship between fraction of bleached rhodopsin and log threshold, the present results show that there is a close linearity between log fraction of bleached rhodopsin and log threshold. This linear relationship is obtained despite varying extents of bleaching and subsequent dark-adaptation periods.

Adaptation, Ocular↗