Blockade of noradrenaline-induced constrictions by yohimbine and prazosin differs between consecutive segments of cutaneous arteries in guinea-pig ears

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; βAR agonist) dilated 1A, 2A and 3A near-maximally with similar potency and efficacy; these dilatations were inhibited by 10(-7) m propranolol (βAR 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.

Prazosin modulates rapid eye movement sleep deprivation-induced changes in body temperature in rats

Prolonged rapid eye movement sleep deprivation (REMSD) causes hypothermia and death; however, the effect of deprivation within 24 h and its mechanism(s) of action were unknown. Based on existing reports we argued that REMSD should, at least initially, induce hyperthermia and the death upon prolonged deprivation could be due to persistent hypothermia. We proposed that noradrenaline (NA), which modulates body temperature and is increased upon REMSD, may be involved in REMSD- associated body temperature changes. Adult male Wistar rats were REM sleep deprived for 6-9 days by the classical flower pot method; suitable free moving, large platform and recovery controls were carried out. The rectal temperature (Trec) was recorded every minute for 1 h, or once daily, or before and after i.p. injection of prazosin, an alpha-1 adrenergic antagonist. The Trec was indeed elevated within 24 h of REMSD which decreased steadily, despite continuation of deprivation. Prazosin injection into the deprived rats reduced the Trec within 30 min, and the duration of effect was comparable to its pharmacological half life. The findings have been explained on the basis of REMSD-induced elevated NA level, which has opposite actions on the peripheral and the central nervous systems. We propose that REMSD-associated immediate increase in Trec is due to increased Na-K ATPase as well as metabolic activities and peripheral vasoconstriction. However, upon prolonged deprivation, probably the persistent effect of NA on the central thermoregulatory sites induced sustained hypothermia, which if remained uncontrolled, results in death. Thus, our findings suggest that peripheral prazosin injection in REMSD would not bring the body temperature to normal, rather might become counterproductive.

Cotransmission from sympathetic vasoconstrictor neurons to small cutaneous arteries in vivo

This study has characterized constrictions of small cutaneous arteries in the guinea pig ear in response to electrical stimulation of the cervical sympathetic nerve (SNS) in vivo. Video microscopy and on-line image analysis were used to examine diameter changes of ear arteries (80-140 micrometers resting diameter) in anesthetized guinea pigs. Trains of 50-300 impulses, but not single pulses or short trains, produced frequency-dependent (2-20 Hz) constrictions. The purinoceptor antagonist suramin (30 microM) greatly reduced constrictions produced by exogenous ATP but did not affect constrictions produced by SNS at 10 Hz or exogenous norepinephrine. The alpha(2)-adrenoceptor antagonist yohimbine (1 microM) enhanced the peak amplitude of sympathetic constrictions at lower stimulation frequencies (1-5 Hz). The amplitude of constrictions to SNS at 10 Hz was reduced, and the latency of constrictions was increased by the alpha(1)-adrenoceptor antagonist prazosin (1 microM). Constrictions to SNS at 10 Hz remaining after prazosin treatment were reduced in amplitude by dihydroergotamine (2 microM) and were attenuated further by the neuropeptide Y Y(1)-receptor antagonist 1229U91 (0.3 microM). Thus norepinephrine and neuropeptide Y act as cotransmitters to mediate sympathetic constriction of small ear arteries at higher stimulation frequencies (10 Hz), but ATP does not seem to contribute directly to these constrictions.

Regional differences in sympathetic neurotransmission to cutaneous arteries in the guinea-pig isolated ear

The effects of sympathetic nerve stimulation on different cutaneous arteries were examined in arteries isolated from guinea-pig ears, by measuring membrane potential changes in smooth muscle cells in response to electrical field stimulation. Resting membrane potential (RMP) was similar in proximal (main ear artery) and distal (3rd or 4th branch order) cutaneous arteries (mean -71 mV). Single stimuli evoked excitatory junction potentials (EJPs) in all arteries. The EJPs in proximal arteries were twice the amplitude, and the time constant of EJP decay was almost half the value, compared with distal cutaneous arteries. EJP amplitude was reduced by > 90% by suramin (30 microM) or alpha,beta,methylene-ATP (alpha,beta,m-ATP)(1 microM) in all proximal, and most distal arteries. Residual responses in distal arteries were resistant to tetrodotoxin. The N-type calcium channel blocker, omega-conotoxin GVIA (30 nM), reduced EJP amplitude by 70-100% in both proximal and distal arteries. Successive EJPs evoked by trains of stimuli at 1 to 5 Hz were depressed in amplitude in proximal arteries, but showed facilitation in distal arteries. EJP depression in proximal arteries was reversed to facilitation by the alpha2-adrenoceptor antagonist, yohimbine (30 nM). Trains of stimuli delivered at 10-20 Hz produced summation of EJPs and active membrane responses in 30% of proximal arteries. Active responses were never detected in distal arteries. Slow depolarizations following the EJPs were detected in most arteries after trains of stimuli, and were abolished by prazosin (0.3 microM) or omega-conotoxin GVIA (30 nM). The density of the perivascular plexus of axons innervating proximal arteries, demonstrated with catecholamine fluorescence histochemistry, was twice that in distal cutaneous arteries. These regional differences in sympathetic neurotransmission suggest that cutaneous vasoconstriction in response to thermoregulatory stimuli, which occurs predominantly in distal cutaneous segments, is likely to be qualitatively different from cutaneous vasoconstriction of proximal arteries in response to other physiological stimuli.

Selective innervation of different target tissues in guinea-pig cranial exocrine glands by sub-populations of parasympathetic and sympathetic neurons

This study has used multiple-labelling immunohistochemistry and quantitative analysis to examine the projections of subpopulations of parasympathetic and sympathetic neurons to different vascular and secretory structures in five cranial exocrine glands of guinea-pigs. Multiple subpopulations of parasympathetic axons, identified by immunoreactivity (IR) for various combinations of peptides, innervated arteries, arterioles, ducts and acini in sublingual, submandibular, parotid, lacrimal and zygomatic glands, although axons were absent from ducts in the parotid gland. Most parasympathetic axons contained IR for vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), with or without enkephalin (Enk). The proportion of parasympathetic axons that contained Enk-IR varied greatly between target tissues and glands: Enk-IR was more common in axons supplying secretory ducts, acini and arterioles than in axons innervating more proximal arteries; Enk-IR was less common in axons supplying the lacrimal gland than axons supplying the submandibular, lacrimal and zygomatic glands. Sympathetic axons with IR for tyrosine hydroxylase (TH) innervated arterial vessels in all glands, but innervated secretory structures only in the salivary glands. Sympathetic axons supplying proximal arterial segments often contained NPY-IR and sometimes also contained IR for dynorphin. Dynorphin-IR was more common in axons in the parotid, lacrimal and zygomatic glands than in the sublingual and submandibular glands. In contrast, axons supplying arterioles, ducts and acini lacked peptide IR. These results indicate that neuronal pathways regulating proximal arteries in cranial exocrine glands are different from the neuronal pathways regulating arterioles and acini, and may be different from neurons projecting to proximal secretory ducts. Furthermore, the peptides enkephalin, NPY and dynorphin are likely to make variable contributions to autonomic neurotransmission in different arterial segments and in different cranial exocrine glands.