Regional heterogeneity of α-adrenoreceptor subtypes in arteriolar networks of mouse skeletal muscle.

Activation of vascular adrenoreceptors (ARs) governs the magnitude and distribution of muscle blood flow in accord with the distribution of AR subtypes. Functional studies in the rat cremaster muscle indicate that α1ARs predominate in proximal arterioles (first-order, 1A) while α2ARs predominate in distal arterioles (third-order, 3A). However, little is known of AR subtype distribution in arteriolar networks of locomotor skeletal muscles, particularly in the mouse. We tested the hypotheses that functional AR subtypes exhibit heterogeneity among branches of arteriolar networks in a locomotor muscle and that the nature of this heterogeneity can vary between muscles having diverse functions. In anaesthetized male C57BL/6J mice (3 months old), concentration-response curves (10(-9) m to 10(-5) m, 0.5 log increments) were evaluated in the gluteus maximus muscle superfused with physiological saline solution (35°C, pH 7.4; n ≥ 5 per group). Noradrenaline (NA, non-selective αAR agonist) constricted 1A, 2A and 3A with similar potency and efficacy. Phenylephrine (PE; α1AR agonist) evoked greater (P < 0.05) constriction in 3A (inhibited by 10(-8) m prazosin; α1AR antagonist) while UK 14304 (UK; α2AR agonist) evoked greater (P < 0.05) constriction in 1A (inhibited by 10(-7) m rauwolscine; α2AR antagonist). Isoproterenol (isoprenaline; ßAR agonist) dilated 1A, 2A and 3A near-maximally with similar potency and efficacy; these dilatations were inhibited by 10(-7) m propranolol (ßAR antagonist) which otherwise had no effect on responses to NA, PE, or UK. Complementary experiments in the mouse cremaster muscle revealed a pattern of αAR subtype distribution that, while distinct from the gluteus maximus muscle, was consistent with that reported for the rat cremaster muscle. We conclude that functional αAR subtype distribution in arteriolar networks of skeletal muscle varies with muscle function as well as vessel branch order.

Regional activation of rapid onset vasodilatation in mouse skeletal muscle: regulation through α-adrenoreceptors.

Exercise onset entails motor unit recruitment and the initiation of vasodilatation. Dilatation can ascend the arteriolar network to encompass proximal feed arteries but is opposed by sympathetic nerve activity, which promotes vasoconstriction and inhibits ascending vasodilatation through activating α-adrenoreceptors. Whereas contractile activity can antagonize sympathetic vasoconstriction, more subtle aspects of this interaction remain to be defined. We tested the hypothesis that constitutive activation of α-adrenoreceptors governs blood flow distribution within individual muscles. The mouse gluteus maximus muscle (GM) consists of Inferior and Superior regions. Each muscle region is supplied by its own motor nerve and feed artery with an anastomotic arteriole (resting diameter 25 microm) that spans both muscle regions. In anaesthetized male C57BL/6J mice (3-5 months old), the GM was exposed and superfused with physiological saline solution (35 degrees C; pH 7.4). Stimulating the inferior gluteal motor nerve (0.1 ms pulse, 100 Hz for 500 ms) evoked a brief tetanic contraction and produced rapid (<1 s) onset vasodilatation (ROV; diameter change, 10 +/- 1 µm) of the anastomotic arteriole along the active (Inferior) muscle region but not along the inactive (Superior) region (n = 8). In contrast, microiontophoresis of acetylcholine (1 µm micropipette tip, 1 µA, 500 ms) initiated dilatation that travelled along the anastomotic arteriole from the Inferior into the Superior muscle region (diameter change, 5 +/- 2 µm). Topical phentolamine (1 µm) had no effect on resting diameter but this inhibition of α-adrenoreceptors enabled ROV to spread along the anastomotic arteriole into the inactive muscle region (dilatation, 7 +/- 1 µm; P < 0.05), where remote dilatation to acetylcholine then doubled (P < 0.05). These findings indicate that constitutive activation of α-adrenoreceptors in skeletal muscle can restrict the spread of dilatation within microvascular resistance networks and thereby increase blood flow to active muscle regions.

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.

Relationship between circulating cortisol and testosterone: influence of physical exercise.

Human research has shown the administration of cortisol into the circulation at rest will result in reduced blood testosterone levels. Many researchers have used these results to imply that physical exercise induced cortisol increases would perhaps result in subsequent reductions in circulating testosterone levels. Our purpose was to examine this concept and determine what, if any, relationship exists between circulating cortisol (C) and testosterone (T) in men (n = 45, 26.3 3.8 yr) at rest and after exercise. Blood samples were collect at rest (10 hour post-prandial; denoted as 'Resting'; n = 45) and again on the same day at 1.0 hr into recovery from intensive exercise (denoted as 'Exercise Recovery'; n = 45). Approximately 48-96 hr after this initial (Trial # 1) blood collection protocol the subjects replicated the exact procedures again and provided a second Resting and Exercise Recovery set of blood samples (Trial # 2). Blood samples from Trial # 1and Trial # 2 were pooled (Resting, n = 90; Exercise Recovery, n = 90). The blood samples were analyzed by radioimmunoassay for C, total T (TT), and free T (fT). Pearson correlation coefficients for the Resting samples ([TT vs. C] r < +0.01; [fT vs. C] r = +0.06) were not significant (p > 0.05). For the Exercise Recovery samples ([TT vs. C] r = -0.53; [fT vs. C] r = +0.21) correlation coefficients were significant (p < 0.05). The findings indicate that exercise does allow the development of a significant negative relationship between C and TT. Interestingly, a significant positive relationship developed between C and fT following exercise; possibly due to an adrenal cortex contribution of fT or disassociation of fT from sex hormone binding globulin. The detected in vivo relationships between C and T, however, were associative and not causal in nature and were small to moderate at best in strength. Key PointsPharmacologically increased levels of cortisol have a significant negative effect on circulating testosteroneAfter certain types of physical exercise a negative statistical associative relationship exist between cortisol and total testosterone.