Sodium activation kinetics of red blood cell Na+/Li+ countertransport in diabetes: methodology and controversy.

Although many studies report an elevated Vmax of red blood cell Na/Li countertransport (CTT) activity in patients with insulin-dependent diabetes mellitus (IDDM) complicated by renal disease, divergent reports exist. This article reviews the technical issues and selection criteria that fuel this controversy. In addition, new studies from this laboratory indicate that insulin in vitro and in the nonfasted state modulate CTT activity and may contribute to the discrepant findings. Incubation of red blood cells from fasted controls with physiologic concentrations of insulin induced a twofold increase in the Km for external Na+. Similarly, Na+ activation kinetics of Li+ efflux showed saturation between 50 and 150 mM Na+ in fasted controls whereas saturation, postprandially, occurred between 100 and 150 mM Na+ as a result of an increase in Km. To clarify the role of prandial status on the measurement of Na+/Li+ CTT activity in diabetes, Na+ activation kinetics were investigated in 34 nonfasting patients with IDDM. Li+ efflux was fully saturated between 80 and 150 mM Na+ in the normoalbuminuric subjects (N = 22), whereas saturation occurred between 150 and 280 mM Na+ in the patients with diabetic nephropathy (N = 14). Patients with nephropathy have higher values of Km for Na+ than do the patients free of renal complications (86 +/- 9.5 versus 41.3 +/- 3.4 mM Na+, respectively; P < 0.000012). The higher Km prevented complete saturation of Li+ efflux at 150 mM extracellular Na+ concentration and contributed to the underestimation of Vmax at 150 mM Na+ selectively in persons with renal complications.(ABSTRACT TRUNCATED AT 250 WORDS)