Urinary excretion of kappa light chains in patients with diabetes mellitus

The urinary excretion of kappa light chains, β₂-microglobulin and albumin was examined in patients with newly diagnosed and long-standing insulin-dependent (IDDM) and non-insulin-dependent (NIDDM) diabetes mellitus, and compared to age-matched control subjects. Patients with IDDM diagnosed within two months, presented with normal albumin excretion, whereas the concentrations of β₂-microglobulin and kappa light chain in urine were higher than in control subjects. The initiation of insulin therapy reduced, but did not completely normalize, the elevated rate of kappa light chain excretion. Patients with IDDM of long duration showed increased urine excretion of kappa light chains and albumin. In keeping with the findings in IDDM, patients with newly diagnosed NIDDM (within one year) showed increased urinary excretion of kappa light chains compared with control subjects. There was, however, no further increase in light chain excretion with longer duration of NIDDM. To study the effect of short-term hyperglycemia on urinary protein excretion, 12 normal subjects participated in a three-step hyperglycemic clamp study, during which their plasma glucose concentration was raised by +50, +125 and +300 mg/dl. The urine excretion of albumin and β₂-microglobulin rose progressively with each hyperglycemic clamp step, whereas that of kappa light chain excretion was unaffected by hyperglycemia. We conclude that increased urinary excretion of kappa light chain is a consistent finding in all types of diabetes mellitus, and can be observed even when the albumin excretion is normal. Since the serum concentration of kappa light chain is normal in diabetes, the increased urinary excretion of kappa light chains must be of renal origin. Although the underlying mechanism(s) responsible for the kappa light chainuria are unknown, it cannot be ascribed to the effect of acute hyperglycemia on glomerular and tubular function.