Background: Studies in experimental animals and human cells have demonstrated increased intracellular calcium (Ca(i2) signalling and Galphai signal transduction associated with hypertension. We have recently shown that angiotensin II-induced mobilization of Ca(i2) is enhanced in fibroblasts from hypertensive individuals in comparison with that in normotensive individuals and that it is blunted by insulin and pertussis toxin in insulin-sensitive, but not in insulin-resistant, patients. This suggests that G(i)-mediated signal transduction is reduced in insulin-resistant hypertension.
Objective: To investigate the expression and regulation of Galpha(i2) subunit in insulin-sensitive and insulin-resistant hypertensive individuals.
Methods: G protein alpha(i2) subunit mRNA was measured in cultured skin fibroblasts from patients with insulin-sensitive and insulin-resistant hypertension, by real-time reverse transcriptase polymerase chain reaction. We also investigated the effects of short-term exposure to fetal calf serum, angiotensin II and insulin, alone and in combination, on the expression of Galpha(i2) in vitro. Spectrofluorophotometric measurement of free Cai was performed in monolayers of 24 h serum-deprived cells in basal conditions and after exposure to angiotensin II, with and without pre-incubation with insulin.
Results: Expression of Galpha(i2) was significantly greater in fibroblasts from hypertensive individuals than in normotensive individuals and the increase was unrelated to age and body mass. The difference was largely accounted for by greater values in insulin-sensitive than in insulin-resistant hypertensive individuals. In fibroblasts from those with insulin-sensitive hypertension, angiotensin II and insulin were additive to fetal calf serum in increasing the expression of Galpha(i2). In these patients, insulin blunted the angiotensin-II induced Cai transient. In contrast, in those with insulin-resistant hypertension, Galpha(i2) was lower and unresponsive to angiotensin II and insulin. Finally, in fibroblasts from insulin-resistant patients, insulin was unable to reduce the angiotensin II-induced Cai peak.
Conclusions: A subnormal Galpha(i2)-mediated signal transduction may be involved in the pathogenesis of cellular insulin resistance in hypertension. This novel Galpha(i2)-mediated signal transduction associated with insulin sensitivity in fibroblasts may help to control excessive angiotensin II signalling.