Serum N-acetyl-beta-D-glucosaminidase activity in a genetic rat model of non-insulin-dependent diabetes mellitus

Salt-resistant blood pressure and salt-sensitive renal autoregulation in chronic streptozotocin diabetes

Hyperfiltration occurs in early type 1 diabetes mellitus in both rats and humans. It results from afferent vasodilation and thus may impair stabilization of glomerular capillary pressure by autoregulation. It is inversely related to dietary salt intake, the "salt paradox." Restoration of normal glomerular filtration rate (GFR) involves increased preglomerular resistance, probably mediated by tubuloglomerular feedback (TGF). To begin to test whether the salt paradox has pathogenic significance, we compared intact vs. diabetic (streptozotocin) Long-Evans rats with normal and increased salt intake, 1 and approximately 3% by weight of food eaten, respectively. Weekly 24-h blood pressure records were acquired by telemetry before and during diabetes. Blood glucose was maintained at approximately 20 mmol/l by insulin implants. GFR was significantly elevated only in diabetic rats on normal salt intake, confirming diabetic hyperfiltration and the salt paradox. Renal blood flow dynamics show strong contributions to autoregulation by both TGF and the myogenic mechanism and were not impaired by diabetes or by increased salt intake. Separately, systolic pressure was not elevated in diabetic rats at any time during 12 wk with normal or high salt intake. Autoregulation was effective in all groups, and the diabetic-normal salt group showed significantly improved autoregulation at low perfusion pressures. Histological examination revealed very minor glomerulosclerosis and modest mesangial expansion, although neither was diagnostic of diabetes. Periodic acid-Schiff-positive droplets found in distal tubules and collecting duct segments were diagnostic of diabetic kidneys. Biologically significant effects attributable to increased salt intake were abrogation of hyperfiltration and of the left shift in autoregulation in diabetic rats.

Characterization of hyperinsulinemic recombinant inbred (RI) strains (SMXA-5 and SMXA-9) derived from normoinsulinemic SM/J and A/J mice

We discovered two mouse strains (SMXA-5 and SMXA-9) with hyperinsulinemia among the substrains and progenitor strains (SM/J and A/J) of the SMXA recombinant inbred (RI) strains, and characterized the two strains at 20 weeks of age. SMXA-5 (mean +/- S.E.M: 9.6 +/- 1.7 ng/ml) and SMXA-9 (7.7 +/- 1.3 ng/ml) males had higher serum immunoreactive insulin levels than SM/J (1.4 +/- 0.3 ng/ml) and A/J (1.1 +/- 0.1 ng/ml) males in the nonfasting condition. The hypoglycemic response to insulin at 30 min after injection was significantly less in SMXA-5 males than in SM/J mice. Glucose tolerance test revealed that the incidence of impaired glucose tolerant males was 58% (11/19) in SMXA-5 and 42% (10/24) in SMXA-9 strains, but none in SM/J and A/J strains. SMXA-5 (209 +/- 29 mg/dl) and SMXA-9 (235 +/- 31 mg/dl) had higher serum triglyceride levels than SM/J (126 +/- 14 mg/dl) and A/J (89 +/- 5 mg/dl) males in the nonfasting condition. Histologic examination revealed enlarged islets in the pancreas of hyperinsulinemic SMXA-5 male mice. Moreover, SMXA-5 and SMXA-9 mice exhibited mild obesity. SMXA-5 and SMXA-9 males were therefore characterized by hyperinsulinemia, impaired glucose tolerance, hypertriglyceridemia and mild obesity which resembled some of the phenotypes of human Syndrome X, although both progenitor strains were normal so far as we examined. Since the RI strains are a powerful tool to facilitate polygenic-trait analysis, SMXA-5 and SMXA-9 mice will be useful materials to investigate the genetic basis of complex diseases, and are possible new metabolic models in relation to hyperinsulinemia.

Serum N-acetyl-beta-D-gulucosaminidase activity increases in association with insulin resistance in patients with coronary artery disease

N-acetyl-beta-D-glucosaminidase (NAG) is released from lysosomes, but the clinical significance of its serum activity in the pathogenesis of coronary artery disease has not been well understood. We measured serum NAG activity by a colorimetric method in consecutive 168 patients suspected of having coronary artery disease who underwent diagnostic coronary angiography. In addition, we evaluated the relationship between the activity and severity of coronary artery disease, as well as various coronary risk factors. Serum NAG activity was higher in the multi-vessel disease group than in the no stenotic lesion group (9.2+/-2.3 vs. 7.8+/-1.8 U/l, P<0.01) and in the single-vessel disease group (vs. 8.2+/-2.2 U/l, P<0.05). In all patients, Gensini score was closely correlated with the serum NAG activity (r = 0.39, P<0.001). Multiple regression analysis showed that serum NAG activity was correlated with plasma insulin level (r = 0.49, P<0.01), but not correlated with other coronary risk factors. In 126 patients without apparent diabetes mellitus, serum NAG was also correlated with plasma insulin level (r = 0.37, P<0.01) and additionally with insulin resistanc determined by homeostasis model assessment (r = 0.47, P<0.01). Our results suggested that serum NAG activity correlates with the severity of coronary artery disease in relation to plasma insulin level and insulin resistance, and thus can be an indicator of coronary artery disease based upon abnormalities of glucose metabolism.