Blunting of rapid onset vasodilatation and blood flow restriction in arterioles of exercising skeletal muscle with ageing in male mice.

Exercise capacity and skeletal muscle blood flow are diminished with ageing but little is known of underlying changes in microvascular haemodynamics. Further, it is not clear how the sympathetic nervous system affects the microcirculation of skeletal muscle with ageing or whether sex differences prevail in the regulation of arteriolar diameter in response to muscle contractions. In the gluteus maximus muscle of C57BL/6 mice, we tested the hypothesis that ageing would impair 'rapid onset vasodilatation' (ROV) in distributing arterioles (second-order, 2A) of old (20-month) males (OM) and females (OF) relative to young (3-month) males (YM) and females (YF). Neither resting (approximately 17 microm) nor maximum (approximately 30 microm) 2A diameters differed between groups. In response to single tetanic contractions at 100 Hz (duration, 100-1000 ms), ROV responses were blunted by half in OM relative to OF, YM or YF. With no effect in YM, blockade of alpha-adrenoreceptors with phentolamine (1 mum) restored ROV in OM. Topical noradrenaline (1 nM) blunted ROV in YM and YF to levels seen in OM and further suppressed ROV in OM (P < 0.05). To evaluate arteriolar blood flow, red blood cell velocity was measured in 2A of OM and YM; respective heart rates (353 +/- 22 vs. 378 +/- 15 beats min(1)) and carotid arterial blood pressures (76 +/- 3 vs. 76 +/- 1 mmHg) were not different. Blood flows at rest (0.6 +/- 0.1 vs. 1.6 +/- 0.2 nl s(1)) and during maximum dilatation (2.0 +/- 0.8 vs. 5.4 +/- 0.8 nl s(1)) with sodium nitroprusside (10 microM) were attenuated >60% (P < 0.05) in OM. Blood flow at peak ROV was blunted by 75-80% in OM vs. YM (P < 0.05). In response to 30 s of rhythmic contractions at 2, 4 and 8 Hz, progressive dilatations did not differ with age or sex. Nevertheless, resting and peak blood flows in YM were 2- to 3-fold greater (P < 0.05) than OM. We suggest that ageing blunts ROV and restricts blood flow to skeletal muscle of OM through subtle activation of alpha-adrenoreceptors in microvascular resistance networks.