Elevated intracellular Ca2+ affects Lii-Nao countertransport in human red blood cells

Increased red cell sodium-lithium countertransport and lymphocyte cytosolic calcium are separate phenotypes in patients with essential hypertension

Increased red blood cell sodium-lithium countertransport (SLC) activity and elevated intracellular calcium have been observed in hypertensive patients. The association of these ion transport abnormalities with each other and with another phenotype, insulin resistance, has been suggested. We investigated whether elevated SLC activity and increased lymphocyte cytosolic calcium (Ca(cyt)) occur in the same individuals and whether either is associated with hyperinsulinaemia. We measured SLC activity, lymphocyte Ca(cyt)and fasting insulin levels in hypertensive patients and normal subjects. Consistent with prior studies, SLC activity was significantly and positively correlated with fasting insulin levels (r = 0.45, P < 0.01). However, SLC activity and lymphocyte Ca(cyt) were significantly but inversely correlated (r = -0.42, P < 0.01) and lymphocyte Ca(cyt) was also inversely correlated with fasting insulin (r = -0.55, P < 0.001). When the study participants were instead separated into two groups based on fasting insulin levels, those above the median (15 microU/ml) had significantly higher SLC activity and significantly lower Ca(cyt). When separated by lymphocyte Ca(cyt) levels (above or below 120 nM) those patients with low lymphocyte Ca(cyt) had significantly higher SLC activity and significantly higher insulin levels. Multiple linear regression showed that fasting insulin was significantly predictive of SLC activity (P = 0.05) and Ca(cyt) (P < 0.01). Thus, elevated SLC activity and increased lymphocyte Ca(cyt) are separate and distinct ion transport phenotypes in hypertensive patients, linked through a relationship to hyperinsulinaemia that is direct with SLC activity and inverse with lymphocyte Ca(cyt).

Is increased erythrocyte sodium-lithium countertransport a useful marker for diabetic nephropathy?

Genetic predisposition to essential hypertension has been proposed as a risk factor for the development of diabetic nephropathy in type 1 (insulin-dependent) diabetes mellitus. An increased sodium-lithium countertransport activity (NaLiCT) has been suggested as a genetic marker for essential hypertension. We therefore evaluated NaLiCT in diabetic patients with (N = 39) or without (N = 23) diabetic nephropathy (DNP), patients with non-diabetic renal diseases (N = 42) and in healthy controls (N = 24). The NaLiCT was elevated in both diabetic patient groups compared to healthy controls (median 244; range 134 to 390 mumol.liter cells-1.hr-1), but was not different in patients with DNP (median 314; range 162 to 676), without DNP (median 325; range 189 to 627) and patients with non-diabetic renal disease (median 300; range 142 to 655). The genetic predisposition to DNP is illustrated by the fact that diabetic sibs of probands with DNP showed a higher occurrence of DNP than diabetic sibs of patients without DNP. We analyzed whether familial DNP clustered with an increased NaLiCT. The NaLiCT in sibs concordant for the presence of DNP (N = 10; median 307; range 217 to 428 mumol.liter cells-1.hr-1) was not significantly different from that in sibs concordant for absence of DNP (N = 15; median 279; range 189 to 442). We conclude that erythrocyte sodium-lithium countertransport activity cannot be used as a marker to identify patients at risk for the development of diabetic nephropathy.