Acute lysyl oxidase inhibition alters microvascular function in normotensive but not hypertensive men and women

Abstract

The lysyl oxidase (LOX) family of enzymes regulates collagen cross-linking. LOX is upregulated in hypertension, increasing vascular stiffness. In vivo human research is sparse, as long-term LOX inhibition in animals causes vascular instability. Our aim was to evaluate the effects of LOX inhibition on cutaneous microvascular function to determine whether LOX function was upregulated in hypertensive humans. Four intradermal microdialysis fibers were placed in the forearm of 10 young [age: 24 ± 1 yr, mean arterial pressure (MAP): 87 ± 2 mmHg], 10 normotensive (age: 50 ± 2 yr, MAP: 84 ± 1 mmHg), and 10 hypertensive (age: 53 ± 2 yr, MAP: 112 ± 2 mmHg) subjects. Two sites were perfused with 10 mM β-aminopropionitrile (BAPN) to inhibit LOX. The remaining two sites were perfused with lactated Ringer solution (control). A norepinephrine dose response (10^-12-10^-2 M) was performed to examine receptor-mediated vasoconstrictor function. A sodium nitroprusside dose response (10^-8-10^-1.3 M) was performed to examine vascular smooth muscle vasodilator function. Red blood cell flux was measured via laser-Doppler flowmetry and normalized to cutaneous vascular conductance (flux/MAP). LogEC₅₀ values were calculated to determine changes in vasosensitivity. Skin tissue samples were analyzed for both extracellular matrix-bound and soluble LOX. LOX inhibition augmented vasoconstrictor sensitivity in young (control: -6.0 and BAPN: -7.1, P = 0.03) and normotensive (control: -4.8 and BAPN: -7.0, P = 0.01) but not hypertensive (control: -6.0 and BAPN: -6.1, P = 0.79) men and women. Relative to young subjects, extracellular matrix-bound LOX expression was higher in hypertensive subjects (young: 100 ± 8 and hypertensive: 162 ± 8, P = 0.002). These results suggest that upregulated LOX may contribute to the vascular stiffness and microvascular dysfunction characteristic in hypertension. NEW & NOTEWORTHY Matrix-bound lysyl oxidase (LOX) and LOX-like 2 expression are upregulated in the microvasculature of hypertensive men and women. Microvascular responsiveness to exogenous stimuli is altered with localized LOX inhibition in healthy men and women but not hypertensive adults. The LOX family differentially affects microvascular function in hypertensive and normotensive men and women.

Keywords: hypertension; lysyl oxidase; microvascular.

Fig. 1.

A: representative Western blots of soluble-bound lysyl oxidase (LOX)/LOX-like 2 (LOXL2) in two normotensive (N) subjects and two hypertensive (H) subjects. B and C: soluble LOX (B) and LOXL2 (C) expression in young (n = 4), middle-aged normotensive (n = 4), and essential hypertensive (n = 8) subjects. Expression was normalized to GAPDH. Statistics were by one-way ANOVA. *P < 0.05 compared with young subjects.

Fig. 2.

A: representative Western blots of extracellular matrix-bound lysyl oxidase (LOX) (LOX)/LOX-like 2 (LOXL2) in two normotensive (N) subjects and two hypertensive (H) subjects. B−D: extracellular matrix-bound LOX (B and C) and LOXL2 (D and E) expression in young (n = 4), normotensive (n = 5), and hypertensive subjects (n = 9). Data were normalized as fold changes from the average young or middle-aged normotensive subject analyzed in each set of Western blots. Statistics were by unpaired t-test. *P < 0.05, difference in expression between young/normotensive and hypertensive subjects.

Fig. 3.

Pharmacological curve modeling for the sodium nitroprusside (SNP) dose response with and without lysyl oxidase (LOX) inhibition in young (A), middle-aged normotensive (B), and essential hypertensive (C) groups. LOX inhibition did not alter logEC₅₀ in any group. There was a main effect of LOX inhibition on SNP-mediated vasodilation in the middle-aged normotensive group. Statistics were by repeated-measures ANOVA. *P < 0.05. CVC, cutaneous vascular conductance.

Fig. 4.

Pharmacological curve modeling of norepinephrine (NE)-mediated vasoconstriction with and without lysyl oxidase (LOX) inhibition in young (A), middle-aged normotensive (B), and essential hypertensive (C) groups. Data are expressed as percent changes from baseline (%CVC_baseline). Sensitivity to NE was augmented in the young and middle-aged normotensive groups but not the essential hypertensive group. There was a main effect of LOX inhibition on NE-mediated vasoconstriction in the young and middle-aged normotensive groups. Statistics were by repeated-measures ANOVA. *P < 0.05.

Fig. 5.

Skin blood flow data in the time course experiment performed in a subset of subjects from the young group (n = 5). Lysyl oxidase inhibition with β-aminopropionitrile (BAPN) did not alter sodium nitroprusside (SNP)-mediated vasodilation (10⁻⁴ M SNP) or norepinephrine (NE)-mediated vasoconstriction (10⁻⁷ M NE) at any time point relative to the BAPN-free baseline. Statistics were by one-way ANOVA. Data are expressed as percent changes from baseline (%CVC_baseline).