1. An association has been described between increased sodium/hydrogen (Na+/H+) exchange rates in various cells and microalbuminuria in type 1 diabetic patients. However, no data are available on the Na+/H+ exchange rate in type 2 diabetes and its association with urinary albumin excretion rates.

2. We have estimated platelet sodium-activated proton efflux (Na+/H+ exchange rate), based on a fluorimetric method, in 43 type 2 diabetic patients, of whom 29 were normoalbuminuric and 14 microalbuminuric, and in 10 non-diabetic control subjects. The factors measured were: buffering power, Km for external Na+ and Vmax. of the exchange rate.

3. There were no differences in Km and Vmax. for the Na+/H+ exchange between the subject groups. However, the 14 patients with microalbuminuria showed a significantly lower buffering capacity [17.2 (4.6) mmol l−1 pH unit−1] [mean (SD)] compared with non-diabetic control subjects [21.1 (1.9) mmol l−1 pH unit−1] (P = 0.020).

4. Among the 43 diabetic patients, 16 were hypertensive. These patients had similar characteristics of Na+/H+ exchange to the 27 normotensive diabetic patients and the control subjects.

5. There was no correlation between exchange rate variables of type 2 diabetic patients and fasting concentrations of insulin or albumin excretion rate.

6. We conclude that the platelets of microalbuminuric diabetic patients manifest a significantly lower buffering capacity. This lower buffering capacity may be due to abnormalities of other ion transport systems or to abnormalities in intermediary metabolism.

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