Letters to the Editor

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The following is an abstract of the article discussed in the subsequent letter:

Marshall, Sally M., Klavs W. Hansen, Ruth Østerby, Jan Frystyk, Hans Ørskov, and Allan Flyvbjerg. Effects of heparin on renal morphology and albuminuria in experimental diabetes. Am. J. Physiol. 271 (Endocrinol. Metab. 34): E326-E332, 1996.---Female nondiabetic and streptozotocin diabetic Wistar rats received 200 units heparin two times daily by subcutaneous injection for 6 mo. Mesangial volume fraction was reduced in heparin-treated control (CH) compared with untreated control (C) animals (CH 0.18 ± 0.02 vs. C 0.24 ± 0.02, P < 0.05), but other histological parameters were similar. In the heparin-treated diabetic (DH) group, wet kidney weight was increased compared with the untreated diabetic (D) group (DH 1,156 ± 39 vs. D 1,050 ± 30 mg, P < 0.05), as were absolute, but not fractional, glomerular volume (P < 0.05) and capillary volume (P < 0.05). Basement membrane thickness (DH 193 ± 3 vs. D 231 ± 9 nm, P < 0.01) and mesangial/glomerular volume fraction (P < 0.001) were decreased. Urinary albumin excretion was increased in the heparin-treated control animals compared with control animals [CH 980 (range 150-4,446) vs. C 221 (range 86-654) µg/24 h, P < 0.001] and in the heparin-treated diabetic animals compared with the diabetic animals [DH 12,785 (range 4,495-29,520) vs. D 899 (range 450-1,335) µg/24 h, P < 0.001]. Thus the possibly deleterious increases in glomerular capillary volume and albumin excretion may negate the beneficial effects of heparin on mesangial and basement membrane structures.

    LETTER
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Heparin and Diabetic Nephropathy

To the Editor: The paper by Marshall et al. (3) is an interesting contribution to what is known about the effect of heparin and derivatives on nephropathies. Besides confirming effects on the glomerular basement membrane thickness and on the expansion of mesangial matrix (1), it shows that unfractionated commercial heparin also affects glomerular capillary volume and glomerular and overall kidney size. The latter changes may be harmful to kidney function, as occurs with increased albuminuria observed in this study in groups treated with heparin. The effect on albuminuria is puzzling, because previous investigations by us and others (see review in Ref. 1) demonstrate the reduction of albuminuria with heparin treatment in diabetic animals. We wish to raise two points.

The first concerns the hypothesis of Marshall et al. (3) that an increase in the available glomerular filtration surface would explain the higher albuminuria in heparin-treated animals compared with control rats. In their heparin-treated control group, albuminuria is higher than in untreated control animals, despite there being no change in capillary glomerular volume and only a 10% higher capillary diameter (an insignificant difference) observed. This means that the filtration surface does not differ from or is even lower1 than that of controls. It is thus difficult to reconcile the fivefold increase in albuminuria in this group with the pathophysiological explanation given by Marshall et al. In forming an alternative, speculative hypothesis, it should be noted that albumin undergoes proximal tubule reabsorption. This reabsorption is also charge dependent: if heparin appears in the glomerular ultrafiltrate2 (2) (because of the high dosage used, three- to fourfold higher than the therapeutical anticoagulant dosage in humans), then its electronegative charge might bring about interference with the tubular handling of albumin, increasing albuminuria. Before concluding that heparin alters glomerular permeability to proteins, we should also recall that the hard and fast rule for evaluating glomerular permeability is still the clearance of neutral and anionic dextrans, whereas albumin urinary excretion is only an indirect indicator. We have previously demonstrated by the dextran clearance method that a chemically modified heparin prevented any disturbance in charge permselectivity of the glomerular basement membrane in experimental diabetic nephropathy (1).

Second, when we refer to heparin, we generally overlook its being a heterogeneous group of polysaccharides, with various values of sulfation, molecular weight, and different biological activities. Commercial heparins from different suppliers are similar for the anticoagulant activity but not for their many other activities. An example is antiproliferation, the best-investigated nonanticoagulation-related activity of heparin. Despite having the same anticoagulant activity, commercial heparins from different suppliers show a broad range of antimitogenicity, and some are even mitogenic (4). Furthermore, different heparin dosages may have different biological activity, so that the same heparin may be both antiproliferative and proliferative (4).

Because studies on the effect of heparin on experimental diabetic nephropathy differ in their drug formulations and dosages, contradictory results concerning albuminuria and kidney weight might be due to the different qualities and quantities of drugs in different protocols. For example, results of the study of Marshall et al. (3) differ from those of our 1994 experiment (1), which also showed no difference in kidney weight between heparin-treated and untreated diabetic and control rats.

In conclusion, we certainly agree with Marshall et al. (3) that further study is necessary before widespread clinical use of heparin in diabetic nephropathy. Improved knowledge of the structure-function relationship of heparin and glycosaminoglycans and a dose-response finding of these drugs might provide a unique opportunity to select new heparin derivatives with specific effects on diabetic nephropathy and possibly, most importantly, devoid of anticoagulant activity.

    FOOTNOTES

1  Roughly, if we assume that the capillary volume can be calculated by considering this volume as corresponding to a unique cylinder and that the capillary volumes in control (c) and heparin-treated control rats (ch) are equal, then Pi  r2c · Lc = Pi  r2ch · Lch (where r is the radius and L the length), so that, because rch is ~10% higher than rc, Lc should be 21% greater than Lch; thus the filtration surface, i.e., the cylinder surface (2 Pi r · L) in heparin-treated controls might be even lower than in control rats.

2  Unfractionated heparin is eliminated through two separate mechanisms. At lower doses, a saturable mechanism is the primary route, but at higher doses the drug is cleared by a nonsaturable mechanism (endothelial binding and cell internalization), and the renal route is the nonsaturable mechanism (2).

    REFERENCES
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1.   Gambaro, G., and B. Baggio. Glycosaminoglycans: a new paradigm in the prevention of proteinuria and progression of glomerular disease. Nephrol. Dial. Transplant. 11: 762-764, 1996[Medline].

2.   Kandrotas, R. J. Heparin pharmacokinetics and pharmacodynamics. Clin. Pharmacokinet. 22: 359-374, 1992[Medline].

3.   Marshall, S. M., K. W. Hansen, R. Østerby, J. Frystyk, H. Ørskov, and A. Flyvbjerg. Effects of heparin on renal morphology and albuminuria in experimental diabetes. Am. J. Physiol. 271 (Endocrinol. Metab. 34): E326-E332, 1996[Abstract/Free Full Text].

4.   Wright, T. C., J. J. Castellot, M. Petitou, J.-C. Lormeau, J. Choay, and M. J. Karnovsky. Structural determinants of heparin's growth inhibitory activity. J. Biol. Chem. 264: 1534-1542, 1989[Abstract/Free Full Text].

Giovanni Gambaro Bruno Baggio
Division of Nephrology
Institute of Internal Medicine
University Hospital
35 128 Padua, Italy

    REPLY
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To the Editor: We are pleased that Drs. Gambaro and Baggio recognize the potential importance of our work, even though it is a year since publication. They take issue with our hypothesis that the increased albumin excretion observed in heparin-treated control and diabetic animals may arise from an increase in the available glomerular filtration surface. They are correct in their assertions that glomerular capillary volume fraction and average capillary diameter were not significantly increased in the heparin-treated nondiabetic animals compared with their appropriate controls. We did not obtain a direct measure of filtration surface in this series, and therefore statements relating to its magnitude can only be tentative. Conclusions on filtration surface area cannot be drawn from capillary length, as Gambaro and Baggio have attempted in footnote 1, because the ratio of capillary-mesangial interface to filtration surface is unknown: the ratio is likely to have decreased, as did mesangial volume fraction in the heparin-treated nondiabetic and diabetic animals. Thus an increase in the available glomerular filtration surface remains a possible partial explanation for our findings. Such an effect obviously does not preclude an additional effect mediated by changes in tubular handling of albumin. We have no information on glomerular or tubular handling of substances other than albumin in our study.

Gambaro and Baggio also point to the differences in the physical properties and actions of different forms of heparin. We did discuss this briefly in our article but were constrained by issues of space. We would agree that different preparations have very different properties. However, several other publications in addition to our own have reported negative or neutral effects of heparin on albumin excretion. In type 1 diabetic patients with overt nephropathy, Tamsma et al. (3) found no benefit of low-molecular-weight heparin treatment compared with placebo. In an experimental model of mesangioproliferative glomerulonephritis, administration of nonanticoagulant heparin begun late (days 10-17) did not affect the course of the disease (1). In streptozotocin diabetes, low-molecular-weight heparin at a dose of 2 mg · kg body wt- 1 · day- 1 had no effect on albumin excretion rate despite normalizing renal levels of transforming growth factor-beta 1 mRNA (2). Thus several groups have questioned the beneficial effects of a variety of different heparin preparations.

In view of all of this conflicting evidence, we would wholeheartedly echo the conclusions of Gambaro and Baggio that much more work is needed on many fronts before heparin can be considered for routine clinical use in diabetic nephropathy.

    REFERENCES
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Letter
References

1.   Burg, M., T. Ostendorf, A. Mooney, K. M. Koch, and J. Floege. Treatment of experimental mesangioproliferative glomerulonephritis with nonanticoagulant heparin: therapeutic efficacy and safety. Lab. Invest. 76: 505-516, 1997[Medline].

2.  Oturai, P., B. Rolin, and H. Vissing. Effects of heparin on kidney TGF-1 mRNA in experimental diabetic nephropathy. Diabetes 46, Suppl. 1: 120A, 1997.

3.   Tamsma, J. T., F. J. van der Woulde, and H. H. Lemkes. Effect of sulphated glycosaminoglycans on albuminuria in patients with overt diabetic (type 1) nephropathy. Nephrol. Dial. Transplant. 11: 182-185, 1996[Abstract].

Sally M. Marshall
Klaus W. Hansen
Ruth Østerby
Jan Frystyk
Hans Ørskov
Allan Flyvbjerg
Institute of Experimental Clinical Research
University of Aarhus and Medical Department M
(Diabetes and Endocrinology)
University Hospital
DK 8000 Aarhus, Denmark


AJP Endocrinol Metab 274(1):E192-E193
0193-1849/98 $5.00 Copyright © 1998 the American Physiological Society