1. Increased erythrocyte sodium-lithium countertransport activity has been reported to be associated with nephropathy in type 1 diabetes and linked to a family history of essential hypertension.
2. This study aimed to determine the mechanism of increased sodium-lithium countertransport activity. Sodium-lithium countertransport kinetics were measured in uncomplicated and hyperlipidaemic type 1 diabetic patients.
3. In the nine out of 31 uncomplicated type 1 diabetic patients who had high sodium-lithium countertransport activity, the sodium affinity (Km) was normal but the maximum velocity (Vmax.) was increased.
4. Hyperlipidaemia, when present in diabetic patients, was associated with increased sodium-lithium countertransport activity, but could not explain the high activity in uncomplicated type 1 diabetic patients in whom plasma lipid concentrations were normal.
5. Sodium-lithium countertransport activity is increased in type 1 diabetes by a mechanism different to that in essential hypertension, where the mechanism is a low Km (increased sodium affinity). Hence familial hypertension cannot explain the raised sodium-lithium countertransport activity in type 1 diabetes.