[Endothelial cell transplantation and growth behavior of the human corneal endothelium]

BACKGROUND

The human corneal endothelium has a limited proliferative capacity in vivo. Until now it has only been possible to replace damaged endothelium by transplantation of a donor cornea. After establishing methods for the isolation and in vitro cultivation of human corneal endothelial cells, transplantation of these cells my be an alternative therapeutic option.

MATERIALS AND METHODS

In this review methods for the in vitro cultivation of human corneal endothelial cells and their transplantation on the Descemet membrane of donor corneas are described.

RESULTS

In vitro proliferation of human adult corneal endothelial cells was achieved by the development of defined cell culture conditions, including supplementation of culture medium with specified growth factors and substances. Dependent on the culture conditions, as well as independent of them, in vitro cultured endothelial cells showed phenotypic changes and different proliferative behavior. Thus, molecular biological examinations revealed a different expression pattern of growth factor receptors in fibroblast-like endothelial cells (dedifferentiated) compared to typical endothelial cells (differentiated). Moreover, the proliferative capacity of the cells differed, dependent on their corneal location. Cells isolated from the peripheral part of donor corneas have a higher proliferative capacity than cells obtained from the central part. The propagation of corneal endothelial cells in vitro offered the possibility of their transplantation on donor corneas in an in vitro model. After transplantation, these cells formed a monolayer whose morphology and cell density depended on the differentiation of the cells. DNA synthesis was predominantly detectable in cells of the corneal periphery.

CONCLUSIONS

Our findings are the basis of the following hypothesis: the periphery of the cornea represents a regenerative zone of the corneal endothelium. The fact that early after transplantation corneal endothelial cells form a monolayer on the natural extracellular matrix (ECM), which shows contact inhibition, suggests that inhibitory factors are released by the Descemet membrane that influence the proliferation of the cells. Further studies on the regulation of the proliferation and differentiation of human corneal endothelial cells in vitro and after transplantation might offer the possibility to establish a selective procedure for the treatment of corneal endothelial cell loss in the near future.

Objective evaluation of regional left ventricular wall motion during dobutamine stress echocardiographic studies using segmental analysis of color kinesis images

OBJECTIVES

To test the feasibility of objective and automated evaluation of echocardiographic stress tests, we studied the ability of segmental analysis of color kinesis (CK) images to detect dobutamine-induced wall motion abnormalities and compared this technique with inexperienced reviewers of conventional gray-scale images.

BACKGROUND

Conventional interpretation of stress echocardiographic studies is subjective and experience dependent.

METHODS

CK images were obtained in 89 of 104 consecutive patients undergoing clinical dobutamine stress studies and were analyzed using custom software to calculate regional fractional area change in 22 segments in four standard views. Each patient's data obtained at rest was used as a control for automated detection of dobutamine-induced wall motion abnormalities. Independently, studies were reviewed without CK overlays by two inexperienced readers who classified each segment's response to dobutamine. A consensus reading of two experienced reviewers was used as the gold standard for comparisons. In a subgroup of 16 patients, these consensus readings and CK detection of wall motion abnormalities were compared with coronary angiography.

RESULTS

The consensus reading detected ischemic response to dobutamine in 43 of 1958 segments in 23 of 89 patients. Automated detection of stress-induced wall motion abnormalities correlated more closely with the standard technique than the inexperienced reviewers (sensitivity 0.76 vs. 0.55, specificity 0.98 vs. 0.94 and accuracy 0.97 vs. 0.92). When compared with coronary angiography in a subgroup of patients, analysis of CK images differentiated between normal and abnormal wall motion more accurately than expert readers of gray-scale images (accuracy of 0.93 vs. 0.82).

CONCLUSIONS

Analysis of CK images allows fast, objective and automated evaluation of regional wall motion, sensitive enough for clinical dobutamine stress data and more accurate than inexperienced readers. This method may result in a valuable adjunct to conventional visual interpretation of dobutamine stress echocardiography.

Quantitative assessment of regional right ventricular function with color kinesis

We used color kinesis, a recent echocardiographic technique that provides regional information on the magnitude and timing of endocardial wall motion, to quantitatively assess regional right ventricular (RV) systolic and diastolic properties in 76 subjects who were divided into five groups, as follows: normal (n = 20), heart failure (n = 15), pressure/volume overload (n = 14), pressure overload (n = 12), and RV hypertrophy (n = 15). Quantitative segmental analysis of color kinesis images was used to obtain regional fractional area change (RFAC), which was displayed in the form of stacked histograms to determine patterns of endocardial wall motion. Time curves of integrated RFAC were used to objectively identify asynchrony of diastolic endocardial motion. When compared with normal subjects, patients with pressure overload or heart failure exhibited significantly decreased endocardial motion along the RV free wall. In the presence of mixed pressure/volume overload, the markedly increased ventricular septal motion compensated for decreased RV free wall motion. Diastolic endocardial wall motion was delayed in 17 of 72 segments (24%) in patients with RV pressure overload, and in 31 of 90 segments (34%) in patients with RV hypertrophy. Asynchrony of diastolic endocardial wall motion was greater in the latter group than in normal subjects (16% versus 10%: p < 0.01). Segmental analysis of color kinesis images allows quantitative assessment of regional RV systolic and diastolic properties.

Use of harmonic imaging without echocardiographic contrast to improve two-dimensional image quality

The aim of this study was to determine whether harmonic imaging (HI) improves endocardial visualization during 2-dimensional echocardiography without echocardiographic contrast. HI differs from fundamental imaging (FI) by transmitting ultrasound at one frequency and receiving at twice the transmitted frequency. This technique has been used in conjunction with contrast echocardiography to enhance myocardial contrast visualization. HI and FI were sequentially performed in 20 patients. Images were digitally stored and subsequently reviewed by 2 observers for the quality of endocardial visualization. In addition, acoustic quantification was performed in both FI and HI modes and endocardial tracking qualitatively judged. HI was compared with FI during dobutamine stress echocardiography in 17 patients who were imaged at baseline and peak stress. Overall, the harmonic images had less clutter and better myocardial blood contrast. Individual segments were better visualized with HI in 30% to 73% of cases. The acoustic quantification endocardial tracking was rated better with HI in 67% of short-axis views and in 58% of apical 4-chamber views. During dobutamine stress testing the overall number of interpretable segments improved from 64% for FI to 84% with HI. Many segments traditionally difficult to image were improved with HI. HI without the use of contrast agents improved endocardial visualization during routine 2-dimensional echocardiography. This improved endocardial visualization led to better endocardial tracking with acoustic quantification and to more segments being clinically interpretable during dobutamine stress testing.

Assessment of small-diameter aortic mechanical prostheses: physiological relevance of the Doppler gradient, utility of flow augmentation, and limitations of orifice area estimation

BACKGROUND

Noninvasive assessment of functionally stenotic small-diameter aortic mechanical prostheses is complicated by theoretical constraints relating to the hemodynamic relevance of Doppler-derived transprosthetic gradients. To establish the utility of Doppler echocardiography for evaluation of these valves, 20-mm Medtronic Hall and 19-mm St Jude prostheses were studied in vitro and in vivo.

METHODS AND RESULTS

Relations between the orifice transprosthetic gradient (equivalent to Doppler), the downstream gradient in the zone of recovered pressure (equivalent to catheter), and fluid mechanical energy losses were examined in vitro. Pressure-flow relations across the 2 prostheses were evaluated by Doppler echocardiography in vivo. For both types of prosthesis in vitro, the orifice was higher than the downstream gradient (P<0.001), and fluid mechanical energy losses were as strongly correlated with orifice as with downstream pressure gradients (r2=0.99 for both). Orifice and downstream gradients were higher and fluid mechanical energy losses were larger for the St Jude than the Medtronic Hall valve (all P<0.001). Whereas estimated effective orifice areas for the 2 valves in vivo were not significantly different, model-independent dynamic analysis of pressure-flow relations revealed higher gradients for the St Jude than the Medtronic Hall valve at a given flow rate (P<0.05).

CONCLUSIONS

Even in the presence of significant pressure recovery, the Doppler-derived gradient across small-diameter aortic mechanical prostheses does have hemodynamic relevance insofar as it reflects myocardial energy expenditure. Small differences in function between stenotic aortic mechanical prostheses, undetectable by conventional orifice area estimations, can be identified by dynamic Doppler echocardiographic analysis of pressure-flow relations.

Different characteristics of endothelial cells from central and peripheral human cornea in primary culture and after subculture

Several methods for isolation and cultivation of human corneal endothelial cells have been described during the last few decades. In contrast to the situation in vivo, the cultured cells show mitogenic activity but often lose their typical morphological appearance. In this paper, we describe a technique to isolate and cultivate morphologically unchanged endothelium from the human cornea. This method revealed different characteristics of endothelial cells according to their position within the human cornea. Endothelial cells isolated from the central part have a morphology similar to that of cells in vivo (i.e., they are densely packed and show no mitogenic activity). In contrast, endothelial cells derived from the peripheral part of the cornea are characterized by mitogenic activity but their cell-to-cell attachment seems to be less tight than in vivo. The significance of these two different endothelial cell types for wound healing in the human cornea is discussed.

Comparative study on the effects of different growth factors on migration of bovine corneal endothelial cells during wound healing

PURPOSE

In a comparative study we investigated the effects of epidermal growth factor, acidic and basic fibroblast growth factor, transforming growth factor-beta1, and vascular endothelial growth factor on proliferation and migration of cultured bovine corneal endothelial cells during wound healing.

METHODS

Cell proliferation was determined by incorporation of 5-bromo-2'-deoxy-uridine and by cell counting. To investigate cell migration, we established a method to produce identically shaped cell-free areas in monolayers of cultured cells and documented the wound-healing process.

RESULTS

Concerning the five tested growth factors, only epidermal growth factor as well as basic and acidic fibroblast growth factor stimulated DNA synthesis, cell proliferation, and migration during wound healing. Vascular endothelial growth factor stimulated migration during wound healing without influencing DNA synthesis or cell proliferation.

CONCLUSION

Whereas epidermal growth factor and the basic and acidic forms of fibroblast growth factor enhance proliferation as well as migration during wound healing, vascular endothelial growth factor seems to be a stimulatory agent specific for corneal endothelial cell migration.

Differences in proliferation and migration of corneal endothelial cells [correction of epithelial cells] after cell transplantation in vitro

For regular function the human cornea requires an intact endothelial cell layer with a sufficiently high cell density. One approach used to compensate endothelial cell loss is transplantation of cultured corneal endothelial cells. Using a previously described transplantation protocol, we observed topographic differences after transplantation of cultured human corneal endothelial cells to recipient corneas previously denuded of their own endothelium. The results presented in this paper suggest different interactions of the transplanted endothelial cells with the central or the peripheral part of the corneal matrix, respectively. Furthermore, cells isolated from the center of a human cornea differ from those isolated from the periphery in terms of their mitogenic capacity. The significance of these observations for corneal endothelial cell transplantation is discussed.

[Effect of differentiation on expression of genes for growth factors and growth factor receptors in human corneal endothelial cells]

In earlier publications we described a method for the isolation and long-term cultivation of human corneal endothelial cells. In several cultures we observed dedifferentiation of these cells, shown by loss of their normal polygonal shape and assumption of a fibroblastlike morphology. To reveal the role of growth factors for this dedifferentiation we analyzed the expression of genes encoding growth factors and growth factor receptors by differentiated and dedifferentiated human corneal endothelial cells. By means of northern blot analysis we demonstrated expression of the genes encoding basic fibroblast growth factor (bFGF), transforming growth factor beta (TGF beta 1), vascular endothelial growth factor (VEGF), FGF receptor-1 (flg-type) and epidermal growth factor receptor (EGF receptor) by both differentiated and dedifferentiated human corneal endothelial cells. In addition, expression of VEGF was stimulated by supplementation of growth medium by bFGF. In contrast, expression of the gene encoding flt-1, a receptor for VEGF, was only observed in dedifferentiated and not in differentiated human corneal endothelial cells. Despite this expression of flt-1, the dedifferentiated cells showed no mitogenic response to VEGF. The role of growth factors for dedifferentiation of human corneal endothelial cells is discussed.

Echocardiographic quantification of regional left ventricular wall motion with color kinesis

BACKGROUND

Color kinesis is a new technology for the echocardiographic assessment of left ventricular wall motion based on acoustic quantification. This technique automatically detects endocardial motion in real time by using integrated backscatter data to identify pixel transitions from blood to tissue during systole on a frame-by-frame basis. In this study, we evaluated the feasibility and accuracy of quantitative segmental analysis of color kinesis images to provide objective evaluation of regional systolic endocardial motion.

METHODS AND RESULTS

Two-dimensional echocardiograms were obtained in the short-axis and apical four-chamber views in 20 normal subjects and 40 patients with regional wall motion abnormalities. End-systolic color overlays superimposed on the gray scale images were obtained with color kinesis to color encode left ventricular endocardial motion throughout systole on a frame-by-frame basis. These color-encoded images were divided into segments by use of custom software. In each segment, pixels of different colors were counted and displayed as stacked histograms reflecting the magnitude and timing of regional endocardial excursion. In normal subjects, histograms were found to be highly consistent and reproducible. The patterns of contraction obtained in normal subjects were used as a reference for the objective automated interpretation of regional wall motion abnormalities, defined as deviations from this pattern. The variability in the echocardiographic interpretation of wall motion between two experienced readers was similar to the diagnostic variability between the consensus of the two readers and the automated interpretation.

CONCLUSIONS

Color kinesis is a promising new tool that may be used clinically to improve the qualitative and quantitative evaluation of spatial and temporal aspects of global and regional wall motion. In this initial study, segmental analysis of color kinesis images provided accurate, automated, and quantitative diagnosis of regional wall motion abnormalities.

Influence of vascular endothelial growth factor on bovine corneal endothelial cells in a wound-healing model

In this study we determined the influence of vascular endothelial growth factor (VEGF) on bovine corneal endothelial cell proliferation and wound healing. Proliferation was determined by measurement of DNA replication as well as by counting of the number of cells present after a defined growth period. In a wound-healing model, reproducible cell-free areas were created within monolayers of cultured bovine corneal endothelial cells and the migration of the cells into these areas was analyzed. The DNA replication and cell proliferation of bovine corneal endothelial cells were not influenced by VEGF. In contrast, in the wound-healing model, VEGF supplementation at concentrations of 1 and 10 ng/ml increased the cell density of the wounded area by 20% and 50%, respectively, as compared with the cell density of wounds left untreated by VEGF. Furthermore, no increase in DNA replication was found in cells involved in wound healing. Our results demonstrate that healing of bovine corneal endothelial cell layers after wounding is predominantly performed by cell migration rather than by proliferation. This migration can be stimulated by the addition of exogenous VEGF.

Expression of genes coding growth factors and growth factor receptors in differentiated and dedifferentiated human corneal endothelial cells

Growth and differentiation of human corneal endothelial cells are shown to be influenced by growth factors and by the extracellular matrix. Under standard culture conditions, these cells tend to dedifferentiate to a fibroblast-like phenotype. The aim of this study was to examine the expression of genes encoding growth factors and growth factor receptors in differentiated and dedifferentiated human corneal endothelial cells and in keratocytes (stromal fibroblasts) by means of Northern blot analysis. Expression of genes coding basic fibroblast growth factor (FGF), FGF receptor-1 (flg-type), epidermal growth factor receptor, and transforming growth factor-beta 1 in keratocytes and human corneal endothelial cells could be confirmed. In contrast, significant expression of the gene encoding acidic FGF was not detected. However, expression of the genes encoding vascular endothelial growth factor (VEGF) and the high-affinity tyrosine kinase receptor for VEGF (flt-1) in human corneal endothelial cells could be demonstrated for the first time. Moreover, exogenous basic FGF seemed to have a positive influence on the VEGF gene expression level. The flt-1 gene was expressed in dedifferentiated, but not in differentiated human corneal endothelial cells. The results indicate a possible role of VEGF in the differentiation of human corneal endothelial cells.

Noninvasive method for determination of arterial compliance using Doppler echocardiography and subclavian pulse tracings. Validation and clinical application of a physiological model of the circulation

BACKGROUND

The Poiseuillian model of the arterial system currently applied in clinical physiology does not explain how arterial pressure is maintained during diastole after cessation of pulsatile aortic inflow. Arterial pressure-flow relations can be more accurately described by models that incorporate arterial viscoelastic properties such as arterial compliance. Continuous pressure and flow measurements are needed to evaluate these properties. Since the techniques used to date to acquire such data have been invasive, physiological models of the circulation that incorporate these properties have not been widely applied in the clinical setting. The purpose of this study was (1) to validate noninvasive methods for continuous measurement of central arterial pressure and flow and (2) to determine normal reference values for arterial compliance using physiological models of the circulation applied to the noninvasively acquired pressure and flow data.

METHODS AND RESULTS

Simultaneously acquired invasive and noninvasive aortic pressures (30 patients), flows (8 patients), and arterial mechanical properties (8 patients) were compared. Pressure was measured by high-fidelity catheter aortic micromanometer (invasive) and calibrated subclavian pulse tracing (noninvasive). Aortic inflow was determined from thermodilution-calibrated electromagnetic flow velocity data (invasive) and echo-Doppler data (noninvasive). Arterial compliance was determined for two- and three-element windkessel models of the circulation using the area method and an iterative procedure, respectively. Once validated, the noninvasive methodology was used to determine normal compliance values for a reference population of 70 subjects (age range, 20 to 81 years) with normal 24-hour ambulatory blood pressures and without Doppler-echocardiographic evidence for structural heart disease. The limits of agreement between invasive and noninvasive pressure data, compared at 10% intervals during ejection and nonejection, were narrow over a wide range of pressures, with no significant differences between methods. Invasive and noninvasive instantaneous aortic inflow values differed slightly but significantly at the start of ejection (P < .05), but during the latter 90% of ejection, values for the two methods were similar, with narrow limits of agreement. Total vascular resistance and arterial compliance values derived from invasive and noninvasive data were similar. Arterial compliance values for the normal population using the two-element model (C2E) ranged from 0.74 to 2.44 cm3/mm Hg (mean, 1.57 +/- 0.38 cm3/mm Hg), with a beat-to-beat variability of 5.2 +/- 3.9%. C2E decreased with increasing age (r = -.73, P < .001) and tended to be higher in men (1.67 +/- 0.41 cm3/mm Hg) than in women (1.51 +/- 0.35 cm3/mm Hg, P = .07). Compliance values for the three-element model (C3E) were predictably smaller than for the two-element model (mean, 1.23 +/- 0.30; range, 0.59 to 2.16 cm3/mm Hg, P < .001 versus C2E) but correlated with C2E values (r = .81, P < .001) and were also inversely related to age (r = -.56, P < .001). Ridge regression and principal component analyses both showed the compliance value to be a composite function whose variation could be best predicted by consideration of simultaneous values for five major hemodynamic determinants: heart rate, mean flow, mean aortic pressure, minimal diastolic pressure, and end-systolic pressure. Multivariate analysis revealed age and sex to be independent predictors of compliance (P < .01 for both). There were no differences in compliance between black and white subjects.

CONCLUSIONS

Noninvasive methods can be used to acquire the hemodynamic data necessary for clinical application of physiological models of the circulation that incorporate arterial viscoelastic properties such as arterial compliance. The strong inverse linear relation between model-based compliance estimates and age mandates incorporation of this demographic parameter in

Ultrasonic backscatter system for automated on-line endocardial boundary detection: evaluation by ultrafast computed tomography

OBJECTIVES

The purpose of this study was to evaluate the accuracy of the recently developed echocardiographic on-line endocardial border detection system using ultrafast computed tomography, an independent and proved tomographic imaging modality.

BACKGROUND

The automated system for on-line endocardial border detection identifies the blood-tissue interface by acoustic quantification of the ultrasonic backscatter signal.

METHODS

Eighteen subjects were screened by conventional echocardiography and acoustic quantification. Ten of these, with high quality echocardiographic images, were also examined by ultrafast computed tomography. Comparable image planes at the midpapillary level were analyzed. Measurements of left ventricular cavity area were compared at end-diastole and end-systole and time course analyses of cavity area during the cardiac cycle were performed.

RESULTS

There was good correlation between values for left ventricular end-diastolic area (r = 0.99), end-systolic area (r = 0.93) and fractional area change (r = 0.91) using the two methods. The on-line backscatter system underestimated end-diastolic area (p < 0.001), but the negative bias was small (-1.6 cm2) and the 95% confidence intervals were narrow (-3.6 cm2 to +0.4 cm2). In contrast, the backscatter system overestimated end-systolic area (p < 0.02); the positive bias for this variable was also small (+2.6 cm2) but the confidence intervals were relatively wide (+7.9 to -2.8 cm2). The negative bias of backscatter values for cavity area was fairly constant during diastole and early systole (range -5% to -10%), but during the second half of systole, backscatter values increased progressively relative to computed tomographic values. Real time values for fractional area change measured by the backscatter system were 13% smaller than those determined by ultrafast computed tomography (p < 0.001), with wide confidence intervals (+3% to -30%). Absolute peak rates of area change during systole and diastole were lower by 39% (p < 0.001) and 41% (p < 0.01), respectively, using the on-line ultrasonic backscatter system. Time course analyses revealed the errors to be consistent with cardiac cycle-dependent alterations in gain sensitivity of the ultrasonic backscatter system.

CONCLUSIONS

The ultrasonic backscatter system is associated with cyclic cavity area measurement errors that need to be addressed if its early promise for on-line assessment of ventricular function is to be fulfilled. Incorporation of an electrocardiographically triggered time-varying gain control may improve accuracy for on-line analysis of ventricular performance.

Further Studies on the Induction of Nitrate Reductase by Arginine in the Filamentous Cyanobacterium Oscillatoria chalybea

In an earlier publication we reported on the role of arginine for the development of nitrate reductase activity in cells of the filamentous cyanobacterium Oscillatoria chalybea (Bednarz and Schmid, Z. Naturforsch. 46c, 591 (1991)). In the present paper we present further evidence that arginine is the natural inducer for nitrate reductase activity. Thus, we show that the induction is regulated by transcription, probably related to the apoprotein or the molybdenumcofactor. We also examined the influence of arginine on nitrate reductase activity in the filamentous cyanobacterium Anabaena PCC7120. In contrast to Oscillatoria chalybea this cyanobacterium forms heterocysts and shows nitrogen fixation activity. Like in Oscillatoria chalybea nitrate reductase activity in Anabaena PCC7120 is stimulated in the presence of arginine as the sole nitrogen source. However, this stimulation is limited to an early growth stage. Subsequently, nitrogen fixation activity appears and nitrate reductase activity decreases.

Diurnal blood pressure variation and cardiac mass in American blacks and whites and South African blacks

American blacks exhibit higher nocturnal blood pressure than American whites with similar daytime pressure. To determine whether this represents a true racial difference, as opposed to a consequence of different environmental factors, we measured ambulatory blood pressure, cardiac left ventricular mass, and urinary electrolyte excretion in 22 South African blacks (15 women, aged 36 +/- 12 years) and age, sex, and daytime mean pressure-matched American blacks and whites. While all three groups exhibited similar daytime blood pressure, American blacks displayed significantly higher nighttime mean blood pressure. Both African blacks and American whites experienced the same fall in nighttime blood pressure. Left ventricular mass index was highest in American blacks and lowest in South African blacks. Urine sodium excretion was similar in all groups, but both black populations excreted significantly less potassium than American whites. The data suggest that the differences in diurnal blood pressure rhythm between American blacks and whites do not represent a true racial difference, but more likely are environmental in origin. Furthermore, since both black populations had similar cation excretion rates, yet differed in blood pressure pattern and cardiac mass, divergence in dietary sodium or potassium intake cannot explain the ethnic group differences in the United States.

Induction of Nitrate Reductase Activity by Arginine in the Filamentous Cyanobacterium Oscillatoria chalybea

Abstract In Oscillatoria chalybea, as in other cyanobacteria, nitrate reduction depends directly on photosynthetic activity. Hence, nitrate reduction occurs only in the light and appears inhibited when photosynthesis is inhibited by the herbicide DCMU . Growth of Oscillatoria cells is possible on a variety of exogenous nitrogen sources in the medium and appears largely independent on the type of nitrogen source. However, if citrulline is the exogenous nitrogen source or if no exogenous nitrogen source is given in the medium, growth appears almost fully inhibited. Nitrate reductase activity measured in French-press particles of nitrate-grown cells is depend­ent on the age of the culture with maximum nitrate reductase activity being reached on the 5th day. Thereafter activity decreases steeply to less than 20% of the maximal activity within 10 days. Besides the growth stage it is the type of exogenous nitrogen source used in the medium which is important for the development of nitrate reductase activity. It appears that in the presence of nitrate, nitrite and arginine, nitrate reductase activity is induced whereas in the presence of ammonia or amino acids like alanine nitrate reductase activity is not induced, as already reported in the literature. Nitrate reductase is also induced if arginine and ammonia are simultaneously offered as exogenous nitrogen source. Arginine metabolism in Oscillatoria cells is characterized by the fact that thylakoid preparations of Oscillatoria catalyze the trans­ formation of arginine to give ornithine and ammonium. The arginine-metabolizing enzyme differs from the usual arginine-induced arginase. The enzyme seems to be constitutive, not manganese-dependent, exhibiting an approximately 5 times higher substrate affinity to arginine than the known arginase. In the present paper we propose that in Oscillatoria it is arginine which induces the synthesis of nitrate reductase.