KALLIKREIN-BINDING PROTEIN-LEVELS ARE REDUCED IN THE RETINAS OF STREPTOZOTOCIN-INDUCED DIABETIC RATS

Purpose. To determine the involvement of rat kallikrein-binding protei n (RKBP) in the development of diabetic retinopathy. Methods. Diabetes was induced by streprozotocin (STZ) (55 mg/kg body weight in 0.05 M c itrate buffer, pH 4.5) in male Sprague-Dawley rats (150 to 175 g, 6 we eks old) as confirmed by hyperglycemia and reduced body weight. Retina s were dissected from animals at 1, 2, and 4 months of diabetes. The f unctional activity of RKBP in retinal homogenates was determined by it s complex formation with tissue kallikrein. Immunoreactive RKBP levels were measured by enzyme-linked immunosorbent assay. The RKBP messenge r RNA (mRNA) levels in the retina were measured by Northern blot analy sis using the RKBP complementary DNA, probe. The activity of total Na,K+-ATPase was determined by a radioassay. Total protein concentration was determined by a protein assay. Results. The kallikrein-binding ac tivity was reduced in the retinas of STZ-diabetic rats at 1 (59%), 2 ( 50%), and 4 (35%) months of diabetes compared to those of age-matched control subjects. Levels of immunoreactive RKBP were significantly low er in the diabetic animals at each time point examined compared to tho se of: control subjects. At 1 and 2 months of diabetes, RKBP levels (n anogram/milligram protein) were decreased significantly to 6.9 +/- 0.7 (n = 8) and 10.6 +/- 1.0 (n = 8), respectively, compared to those of age-matched control subjects (14.1 +/- 0.7, n = 8, P < 0.001, and 14.1 +/- 1.2, n = 8, P < 0.01). At 4 months of diabetes, retinal RKBP leve ls were lower in both control and diabetic groups, bur: RKBP levels in diabetic groups were significantly lower (5.8 +/- 0.6, n = 8) than th ose of the age-matched control subjects (8.4 +/- 0.9, n = 8, P < 0.01) . Similarly, Northern blot analysis showed that RKBP mRNA levels were reduced in the retina of each group of STZ-diabetic rats, suggesting t hat the decrease in RKBP occurred at the level of transcription. Concl usions. The results show that STZ-induced diabetic rats have decreased retinal RKBP;moreover, this suggests that RKBP may contribute to diab etic retinopathy.