Inhibitors of ACE/kininase II enhance insulin sensitivity, an action that is mediated in part by bradykinin (BK). We investigated whether insulin interacts with the BK receptor signaling to modulate the inositol 1,4,5-trisphosphate (IP3) response to BK in L8 rat skeletal myoblasts. Stimulation of the cultures with BK (10 nmol/l) for 15 s increased IP3 from a basal level of 75.2 +/- 7.6 to 200.2 +/- 15.7 pmol/mg protein. Treatment of the cultures with 1, 2, and 20 nmol/l of insulin for 90 min before adding BK increased IP3 formation by the same BK dose to 328.2 +/- 19, 434.5 +/- 18, and 460.8 +/-21.3 pmol/mg protein, respectively. When wortmannin was administered to inhibit phosphatidylinositol (PI) 3-kinases at lower concentration (1 nmol/l), it increased IP3 formation stimulated by BK only when insulin was present. At a higher concentration (100 nmol/l), wortmannin significantly enhanced BK-induced IP3 formation in the absence of insulin. Genistein and tyrphostin A-23, tyrosine kinase inhibitors, completely reversed the elevated IP3 formation by BK and insulin. The IP3 response to 10 nmol/l BK was 223.3 +/- 11.8 pmol/mg protein in the absence of insulin and 402.2 +/- 12.0 pmol/mg protein in the presence of 2 nmol/l insulin. However, when exposing the cultures to 1 nmol/l genistein or tyrphostin A-23, the IP3 response to BK in the presence of insulin decreased to 211.8 +/- 46.7 and 187.7 +/- 19.9 pmol/mg protein. Tyrphostin A-1, the inactive analog, was ineffective. Exposing the cells to 1 micromol/ 3,4,5-trimethoxybenzoic acid 8-[diethylamino]octyl ester, an intracellular Ca2+ antagonist, did not change the potentiation by insulin. But, exposing them to 0.1 micromol/l n-[6-aminohexyl]-5-chloro-1-naphthalene-sulfonamide, a calmodulin antagonist, resulted in enhanced IP3 response to BK alone to 292.2 +/- 18.5 pmol/mg protein and to BK in the presence of 1, 2, and 20 nmol/l insulin to 488 +/- 22.2, 625.5 +/- 11.6, and 665.2 +/- 15.9 pmol/mg protein, respectively. In conclusion, insulin potentiates BK-induced IP3 production in L8 rat skeletal myoblasts, and this action of insulin involves a tyrosine kinase. Inhibition of PI 3-kinases potentiated BK-induced IP3 formation in the presence of insulin. Calmodulin blocked the action of insulin. These results support a modulatory effect of insulin on the BK signaling system via a tyrosine kinase in L8 rat skeletal myoblasts that results in increased IP3 formation. Because BK release from skeletal muscle increases during contractions, this action of insulin is likely to play a role in the modulation of the excitation-contraction coupling process of the skeletal muscle.

This content is only available via PDF.