Ageing impairs endothelium-dependent vasodilatation and alters redox signalling in diaphragm arterioles from male and female Fischer-344 rats

Abstract

Diaphragm hyperaemia and regional blood flow distribution are impaired with ageing, potentially consequent to altered vascular structure and/or diminished vasomotor function. Evidence from locomotory skeletal muscle suggests that age-related diaphragm vasomotor dysfunction may be related to a blunted endothelium-mediated vasodilatation, decreased nitric oxide (NO) bioavailability and/or augmented reactive oxygen species (ROS) generation. We hypothesized that, in the medial costal diaphragm with old age, there would be fewer feed arteries (FAs) and impaired vasomotor function, via endothelium-specific mechanisms, in first-order (1A) arterioles. In young (Y) and old (O) Fischer-344 rats, the number of medial costal diaphragm FAs was quantified. 1A arterioles (117–220 µm) were isolated, cannulated and pressurized via hydrostatic reservoirs. Thereafter endothelium-dependent (via ACh) vasodilatory responses were assessed. In a separate set of arterioles, ACh-mediated dilatation was assessed before and after treatment with the superoxide dismutase mimetic Tempol (100 µm) and Tempol plus the hydrogen peroxide (H₂O₂) scavenger catalase (100 U/ml). The average number of medial costal FAs was lower in the rat diaphragm with old age (p = 0.001). Endothelium- and nitric oxide synthase (NOS)-dependent vasodilatation was 21% lower in medial costal 1A arterioles from O rats (p < 0.001). Tempol decreased ACh-mediated vasodilatation of medial costal 1A arterioles from Y and O rats but did not eliminate age-related differences. Tempol plus catalase further decreased ACh-mediated vasodilatation in O but not Y vessels. In the medial costal diaphragm vasculature, ageing is associated with (1) arterial rarefaction, (2) impaired endothelium-dependent vasodilatation via NOS- and ROS-dependent mechanisms and (3) increased reliance on ROS-mediated vasodilatation.

Key points

• Old age blunts the hyperaemic response and alters regional blood flow distribution in the diaphragm. The effect of ageing on vascular structure and function in respiratory skeletal muscle is unknown.
• In young and old Fischer-344 rats of both sexes, we quantified the number of feed arteries (FAs) and assessed the vasoreactivity of first-order (1A) arterioles in the medial costal diaphragm.
• The number of medial costal diaphragm FAs was lower in old rats. In 1A arterioles endothelium-dependent vasodilatation was blunted, and reactive oxygen species (ROS)-mediated vasodilatory signalling was greater in old rats.
• We found no evidence of sex differences in diaphragm macrovascular structure, endothelial function or ROS-mediated signalling in young or old rats.
• Our findings in the diaphragm vasculature with ageing provide a mechanistic basis for the age-related deficits in diaphragm blood flow capacity.
• Therapeutic interventions targeting the diaphragm vasculature to improve perfusion and oxygen delivery may reduce the burden of age-related diaphragm dysfunction.