Growth hormone exerts acute vascular effects independent of systemic or muscle insulin-like growth factor I

Abstract

Context: Endothelial dysfunction is common in patients with GH deficiency who are at increased risk for premature cardiovascular death. GH regulates vascular tone and reactivity in humans.

Objective: Our objective was to explore the mechanisms underlying the GH's acute vascular effects. DESIGN AND STUDY SETTING: There were 10 healthy, lean and young, volunteers studied after an overnight fast. GH was infused systemically for 6 h at 0.06 microg/kg.min. Biopsy of the vastus lateralis muscle was done in seven subjects before and after GH infusion. Human aortic endothelial cells (HAECs) were incubated with GH in vitro.

Results: GH infusion increased plasma GH to 32.9 +/- 1.5 ng/ml and forearm blood flow by 66% (P < 0.001). GH infusion did not significantly change plasma IGF-I concentrations, muscle IGF-I mRNA expression, and muscle Akt phosphorylation, suggesting a lack of IGF-I action in muscle. Because it was reported that GH exerts an acute vascular effect via a nitric oxide (NO)-dependent mechanism, we performed additional in vitro experiments using HAECs. HAECs express abundant GH receptors. Incubating HAECs with GH at 30 ng/ml for 3 or 6 h did not alter endothelial NO synthase (eNOS) protein content but time dependently increased the phosphorylation and activity of eNOS, thus demonstrating a direct effect of GH on endothelial cells.

Conclusions: GH exerts an acute vascular effect independent of both systemic and local IGF-I production, and this effect is likely via direct action on GH receptors and eNOS in the vascular endothelium.

Figure 1

Study protocol.

Figure 2

Effect of GH infusion on forearm blood flow in healthy humans. GH infusion at 0.06 μg/kg·min significanty increased forearm blood flow (P < 0.001, ANOVA). Compared with basal: *, P < 0.0003; **, P = 0.0001.

Figure 3

Effect of GH infusion on muscle IGF-I mRNA expression and Akt phosphorylation in healthy humans. A, Muscle IGF-I mRNA expression (n = 7; P = 0.535). B, Representative Akt gel from two subjects. C, Quantitative Akt phosphorylation (n = 5; P = 0.72).

Figure 4

HAECs express abundant GH receptor (GHR). Left two lanes, HAECs containing 500 μg proteins were immunoprecipitated (IP) and immunoblotted (IB) using anti-GHR antibody. Right lane, Rat liver proteins (50 μg) were used as a positive control.

Figure 5

Effect of GH on eNOS phosphorylation and activity in cultured HAECs. A, eNOS phosphorylation (n = 4 for each group). Compared with respective controls: *, P < 0.005; **, P < 0.004. B, eNOS activity (n = 3 for each group). Compared with respective control: *, P < 0.01. C, Effect of GH on Akt and AMPK phosphorylation. Representative gel of two independent experiments. p, Phosphorylated; T, total.