Salt Restriction and Angiotensin-Converting Enzyme Inhibitor Improve the Responsiveness of the Small Artery in Salt-Sensitive Hypertension

Abstract

For salt-sensitive hypertension (SSH), salt restriction and angiotensin-converting enzyme (ACE) inhibitors are essential treatments, but their effect on the function of resistance arteries is unclear. Here, we present an intravital study to detect the effect of salt restriction and ACE inhibitors on the function of the mesenteric small artery (MSA) in SSH. Dahl salt-sensitive rats were randomized into the following groups: ACE inhibitor gavage, salt restriction, ACE inhibitor combined with salt restriction, and high-salt diet. After a 12-week intervention, the mesenteric vessels maintained their perfusion in vivo, and the changes in the diameter and blood perfusion of the MSAs to norepinephrine (NE) and acetylcholine (ACh) were detected. Switching from a high-salt diet to a low-salt diet (i.e., salt restriction) attenuated the vasoconstriction of the MSAs to NE and promoted the vasodilatation to ACh, while ACE inhibitor improved the vasodilatation more obviously. Pathologically, changes in local ACE, AT1R, and eNOS expression were involved in these processes induced by a high-salt diet. Our study suggests that salt restriction and ACE inhibitor treatment improve high salt intake-induced MSA dysfunction in SSH, and salt restriction is a feasible and effective treatment. Our findings may provide a scientific basis for the treatment of hypertension.

Keywords: Hypertension; angiotensin-converting enzyme (ACE) inhibitor; mesenteric small artery; salt intake; salt restriction; vascular reactivity.

Figure 1

The mean blood pressure (A) and heart rate (B) of Dahl-SS rats at baseline and at the end of the 6-week and 12-week dietary interventions. (C) Timeline of the protocol. HS: the group with high salt intake for 12 weeks, n=8; HLS: the group with high salt for the first 6 weeks, then low salt intake for the next 6 weeks, n=8; HB: the group with high salt intake plus Benazepril administered intragastrically for 12 weeks, n=9; HLB: the group with high salt for the first 6 weeks, then low salt intake for the next 6 weeks, and with Benazepril administration throughout 12 weeks, n=9.

Figure 2

NE-induced vasoconstriction and ACh-induced vasodilatation of MSAs. (A) NE-induced maximum percentage change of the MSA inner vasoconstriction. (B) Duration of NE-induced vasoconstriction, from the beginning of vasoconstriction to the end of artery diameter recovery. (C) ACh-induced maximum percentage change in MSA inner vasodilatation. (D) Duration of ACh-induced vasodilatation, from the beginning of ACh injection to the end of artery diameter recovery. The percentage change in vasoconstriction/vasodilatation was calculated as the percentage of MSA inner diameter changes after NE/ACh injection divided by the baseline inner diameter. NE and ACh were injected through a femoral vein (10 μg/kg). The second order branch of the mesenteric arteries was recorded by a high-speed camera attached to a microscope. The measurements of three MSA segments were averaged. HS: the group with high salt intake for 12 weeks, n=8; HLS: the group with high salt for the first 6 weeks, then low salt intake for the next 6 weeks, n=8; HB: the group with high salt intake plus Benazepril administered intragastrically for 12 weeks, n=9; HLB: the group with high salt for the first 6 weeks, then low salt intake for the next 6-week, and with Benazepril administration throughout 12 weeks, n=9. MSA: mesenteric small artery; NE: norepinephrine; ACh: acetylcholine. *: compared with HS group, P<0.05; **: compared with the HS group, P<0.01; #: compared with the HLB group, P<0.05; ##: compared with the HLB group, P<0.01; @: compared with the HB group, P<0.05.

Figure 3

Blood perfusion of the MSAs was recorded by full-field laser perfusion imaging. The MSAs of the proximal small intestine were recorded continuously for at least 10 minutes before and after injection of NE or ACh. (A) The average percentage changes in blood perfusion after NE injection. (B) The maximum percentage changes in blood perfusion after NE injection. (C) The average percentage increase in blood perfusion induced by ACh. (D) The maximum percentage increase in blood perfusion induced by ACh. The percentage changes within the MSAs were calculated as the average change in blood perfusion within a thirty-second interval after drug injection divided by baseline blood perfusion (i.e., thirty seconds of blood perfusion before NE/ACh administration). The maximum percentage change was calculated as the maximum blood perfusion induced by ACh (or minimum blood perfusion induced by NA) divided by the baseline blood perfusion. The measurements of three MSA segments were averaged. HS: the group with high salt intake for 12 weeks, n=8; HLS: the group with high salt for the first 6 weeks, then low salt intake for the next 6 weeks, n=8; HB: the group with high salt intake plus Benazepril administered intragastrically for 12 weeks, n=9; HLB: the group with high salt for the first 6 weeks, then low salt intake for the next 6 weeks, and with Benazepril administration throughout 12 weeks, n=9. MSA: mesenteric small artery; NE: norepinephrine; ACh: acetylcholine. *: compared with the HS group, P<0.05; **: compared with the HS group, P<0.01; #: compared with the HLB group, P<0.05.

Figure 4

Histological and immunohistochemical changes in the MSAs of Dahl-SS rats. The sectioned MSAs were stained with hematoxylin and eosin (HE), Masson's trichrome and immunohistochemistry (original magnification × 400, scale bar: 50 μm). For IHC, nuclei appeared brown in positive expression of MSA by immunohistochemistry assay. HS: the group with high salt intake for 12 weeks; HLS: the group with high salt for the first 6 weeks, then low salt intake for the next 6 weeks; HB: the group with high salt intake plus Benazepril administered intragastrically for 12 weeks; HLB: the group with high salt for the first 6 weeks, then low salt intake for the next 6 weeks, and with Benazepril administration throughout 12 weeks. MSA: mesenteric small artery; HE: hematoxylin and eosin; IHC: immunohistochemical staining; eNOS: endothelial nitric oxide synthase; ACE: angiotensin-converting enzyme; AT1R: angiotensin II type 1 receptor; AT2R: angiotensin II type 2 receptor. IMT/LD ratio: intima-media thickness/internal lumen diameter ratio; CD rate: collagen deposition rate. eNOS PE rate: positive expression rate of endothelial nitric oxide synthase; ACE PE rate: positive expression rate of angiotensin converting enzyme; AT1R PE rate: positive expression rate of angiotensin II type 1 receptor; AT2R PE rate: positive expression rate of angiotensin II type 2 receptor. *: compared with the HS group, P<0.05; **: compared with the HS group, P<0.01; #: compared with the HLB group, P<0.05; ##: compared with the HLB group, P<0.01; @: compared with the HB group, P<0.05; @@: compared with the HB group, P<0.01.