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

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 (HO) 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.