1. Whether an alteration in cell membrane cation transport after exposure to insulin and angiotensin II (two important growth promoters that have been shown to be involved in the pathogenesis of atherosclerosis and hypertension) is present in cells from non-insulin-dependent diabetes patients with microalbuminuria, a known risk factor for cardiovascular and renal disease, is unknown. We therefore examined intracellular pH and calcium changes after acute exposure to insulin and angiotensin II in cultured skin fibroblasts from eight non-insulin-dependent diabetes patients with and eight others without microalbuminuria and from a group of seven matched, normal control subjects.
2. Cultured fibroblasts were loaded with 2′,7′-bis (2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester or fura 2-acetoxymethyl ester for continuous monitoring of intracellular pH and free calcium concentrations respectively.
3. In quiescent growth-arrested cells, both intracellular pH and free calcium concentrations were similar in the three groups of subjects. Acutely, insulin induced a gradual alkalinization in all groups of patients. The pH increase was significantly greater in non-insulin-dependent diabetes mellitus patients with microalbuminuria (ΔpH +0.24 ± 0.04 pH units) than in normoalbuminuric patients with non-insulin-dependent diabetes mellitus (0.08 ± 0.02; P < 0.01) and normal control subjects (0.05 ± 0.01; P < 0.01). Although the alkalinizing effect of angiotensin II was smaller than that obtained by insulin, intracellular pH increase after angiotensin addition was more pronounced in non-insulin-dependent diabetes mellitus patients with microalbuminuria (ΔpH ± 0.14 ± 0.04 pH units) than in those without (0.08 ± 0.02; P < 0.01) and in normal control subjects (0.02 ± 0.02; P < 0.01). That the increase in intracellular pH was mediated by the sodium—hydrogen antiport was demonstrated by its dependence on the presence of sodium in the medium and its inhibition by amiloride. Whereas insulin addition did not evoke any significant increase in intracellular free calcium levels in fibroblasts from the three groups studied, angiotensin II evoked a fast and transient rise in intracellular free calcium that was higher in fibroblasts from microalbuminuric patients with non-insulin-dependent diabetes mellitus than in cells from normoalbuminuric patients with non-insulin-dependent diabetes mellitus and control subjects. In the whole population of patients with non-insulin-dependent diabetes mellitus, the increase in intracellular pH after exposure to angiotensin II was positively correlated with intracellular free calcium increase (r = 0.53; P < 0.05), suggesting a possible role of intracellular free calcium levels in the activation of the sodium—hydrogen antiport.
4. In conclusion, we have described an association between increased agonist-induced responsiveness of sodium—hydrogen antiport activity and the presence of microalbuminuria in patients with non-insulin-dependent diabetes mellitus. This increased responsiveness, persisting in cultured fibroblasts after several passages in vitro, suggests that in vitro phenotypic characteristics of fibroblasts are likely to be genetically determined and to be, at least in part, independent of the degree of metabolic control in vivo.