Comparative Efficacy of Angiotensin II Type 1 Receptor Blockers Against Ventilator-Induced Diaphragm Dysfunction in Rats

Abstract

Mechanical ventilation (MV) is a life-saving intervention for many critically ill patients. Unfortunately, prolonged MV results in the rapid development of inspiratory muscle weakness due to diaphragmatic atrophy and contractile dysfunction (termed ventilator-induced diaphragm dysfunction (VIDD)). Although VIDD is a major risk factor for problems in weaning patients from MV, a standard therapy to prevent VIDD does not exist. However, emerging evidence suggests that pharmacological blockade of angiotensin II type 1 receptors (AT1Rs) protects against VIDD. Nonetheless, the essential characteristics of AT1R blockers (ARBs) required to protect against VIDD remain unclear. To determine the traits of ARBs that are vital for protection against VIDD, we compared the efficacy of two clinically relevant ARBs, irbesartan and olmesartan; these ARBs differ in molecular structure and effects on AT1Rs. Specifically, olmesartan blocks both angiotensin II (AngII) binding and mechanical activation of AT1Rs, whereas irbesartan prevents only AngII binding to AT1Rs. Using a well-established preclinical model of prolonged MV, we tested the hypothesis that compared with irbesartan, olmesartan provides greater protection against VIDD. Our results reveal that irbesartan does not protect against VIDD whereas olmesartan defends against both MV-induced diaphragmatic atrophy and contractile dysfunction. These findings support the hypothesis that olmesartan is superior to irbesartan in protecting against VIDD and are consistent with the concept that blockade of mechanical activation of AT1Rs is a required property of ARBs to shield against VIDD. These important findings provide a foundation for future clinical trials to evaluate ARBs as a therapy to protect against VIDD.

Figure 1

Olmesartan prevents diaphragm contractile dysfunction and fiber atrophy induced by prolonged mechanical ventilation. (a) Diaphragm specific force production as a function of the stimulation frequency (i.e., force‐frequency curve) measuredin vitroin costal diaphragm strips following 12 hours of mechanical ventilation (MV) or spontaneous breathing (SB). Values are means ± SD. *SB significantly different (P < 0.05) from MV. #Mechanical ventilation with olmesartan (MVO) significantly different from MV. †MVO significantly different (P < 0.05) from SB. (b) Diaphragm muscle fiber cross‐sectional area in: (1) type I fibers, (2) type IIa fibers, and (3) type IIb/x fibers. Values are mean ± SD. *Significantly different (P < 0.05) from SB. #Significantly different from MV. CSA, cross‐sectional area; MVI, mechanical ventilation with irbesartan.

Figure 2

Olmesartan prevents mitochondrial dysfunction, oxidative stress, and proteolysis in VIDD. (a) Plasma angiotensin 1–7 levels at completion of experiment. Values are means ± SD. *Significantly different (P < 0.05) from spontaneous breathing (SB) and mechanical ventilation (MV). (b) Mitochondrial respiratory control ratio (state 3/state4) measured using permeabilized diaphragm muscle fibers. Values are means ± SD. *Significantly different (P < 0.05) from SB. #Significantly different from MV. †Significantly different (P < 0.05) from mechanical ventilation with irbesartan (MVI). (c) The relative abundance of 4‐hydroxynonenal (4‐HNE)‐modified proteins (index of lipid peroxidation) in the diaphragm as determined by Western blot and normalized to α‐tubulin. Values are means ± SD. *Significantly different (P < 0.05) from SB. #Significantly different from MV. †Significantly different (P < 0.05) from MVI. (d) The ratio of phosphorylated (active) pSTAT to total STAT in the diaphragm. Values are means ± SD. *Significantly different (P < 0.05) from SB. #Significantly different from MV. †Significantly different (P < 0.05) from MVI. (e) Calpain‐mediated cleavage of alpha II spectrin releases a specific breakdown product at 145 kDa that can be quantified by western analysis; abundance of product is surrogate biomarker of calpain activity. Values are means ± SD. *Significantly different (P < 0.05) from SB. †Significantly different (P < 0.05) from MVI. (f) Caspase‐3 mediated cleavage of alpha II spectrin releases a specific breakdown product at 120kDa that can be quantified by western analysis; abundance of product is surrogate biomarker of calpain activity. Values are means ± SD. *Significantly different (P < 0.05) from SB. †Significantly different (P < 0.05) from MVI. MVO, mechanical ventilation with olmesartan.