Histaminergic vasodilatation in the hindlimb of the dog

Histamine uptake and metabolism in the blood vessels of rats

In vitro radiolabeled histamine uptake and metabolism were investigated in abdominal aorta, iliac artery, mesenteric artery and hepatic portal vein of the rat. Histamine uptake was rapid and remained linear over the initial 10 min of the accumulation period. The uptake rate was temperature sensitive with marked reduction in rate at 0 degrees C. The accumulation at 0 degrees C represents nonspecific uptake. The uptake of histamine was observed to be related to the amount of tissue stores of histamine as well as extracellular sodium ion concentration. The only histamine metabolite detected in the vasculature examined was imidazoleacetic acid representing almost 80% of the chromatogram radioactivity. This observation indicates the primary metabolic pathway for histamine in rat vasculature involves the enzyme, diamine oxidase.

Neurogenic non-adrenergic cutaneous vasodilatation elicited by hypothalamic thermal stimulation in dogs

Hypothalamic heating in dogs with depleted peripheral adrenergic transmitter stores by means of chronic application of reserpine elicits increase of hindpaw blood flow. The vasodilator response is not affected by alpha- and beta-adrenergic or cholinergic blocking substances, nor by histamine-, or prostaglandin-antagonists. Dopamine and ergometrine produce cutaneous vasoconstriction which is antagonized or prevented by haloperidol. The hypothalamic vasodilator response is abolished after intra-arterial injection of haloperidol, after lumbar sympathetic chain section, or by high doses of hexamethonium. The results indicate neurogenic non-adrenergic control of skin blood flow in thermoregulation possibly acting through inhibition of dopaminergic vasoconstrictor mechanisms.

Evidence for dopaminergic vasodilator innervation of the canine paw pad

1 In chloralose-anaesthetized dogs pretreated with guanethidine and pancuronium, electrical stimulation (0.2 to 5 Hz) of the peripheral end of the cut tibial nerve caused a frequency-dependent increase in femoral blood flow which was restricted to the paw pads. 2 This neurogenic vasodilatation was not attenuated by atropine, mepyramine plus burimamide, indomethacin or propranolol. It was, however, attenuated in a dose-dependent manner by intra-arterial administration of the dopamine receptor antagonist, ergometrine (0.05 to 0.5 mg). 3 The effect of ergometrine could not be explained by non-specific effects on axonal conduction or transmission or by vasospasm of the blood vessels of the paw-pads. 4 In dogs with intact tibial nerves, a pharmacologically similar dilator response localized to the paw-pads could be elicited by electrical stimulation of loci in the ipsilateral diencephalon and midbrain. This response was not due to inhibition of adrenergic vasomotor tone and was abolished by systemic ganglion blockade or by tibial nerve section as well as by femoral arterial administration of ergometrine. 5 It is suggested that the vasculature of the canine paw pads is innervated by a population of autonomic axons which utilize dopamine or a related substance as a transmitter substance and activation of which causes vasodilation.

Autonomic neuroeffector junctions--reflex vasodilatation of the skin

A general model of the autonomic neuroeffector junction is proposed. In this model, emphasis is placed on the muscle effector bundle with electrotonic coupling between individual cells via gap junctions (or nexuses) and en passage release of transmitter from autonomic nerve varicosities. This release results in transmission to effector cells across junctional clefts ranging from about 20 nm in the vas deferens and iris to as much as 2000 nm in some large arteries. The ultrastructural identification of different autonomic nerve types is described. Current theories on the synthesis, storage, release, and inactivation of transmitter during cholinergic, adrenergic, and purinergic transmission are summarized. Some speculations are made about the possible involvement of purinergic nerves in the innervation of vessels and mast cells in the skin, and whether this involvement results in a functional link between ATP, histamine, bradykinin, and prostaglandin in cutaneous vasodilatation. Another possibility considered as the basis for this reflex is the release of substance P from sensory (pain) nerve collaterals in the skin.

Effects of guanethidine on histamine release during reflex vasodilatation in the dog

1 The effects of guanethidine pretreatment on the release of [14C]-histamine during the reflex vasodilatation induced in the atropinized gracilis muscle by rapid intravenous administration of noradrenaline, were studied in dogs. 2 After guanethidine treatment the haemodynamic reflex response was completely abolished and no appreciable modification of [14C]-histamine release from the gracilis muscle following intravenous noradrenaline was observed. 3 These results suggest the hypothesis that the withdrawal of the sympathetic discharge represents the mechanism of histamine release during the reflex vasodilatation. Therefore, guanethidine would suppress both the passive and the histaminergic component of the baroreceptor reflex through the abolition of the sympathetic tone.

Possible feed-back inhibition of noradrenaline release by purine compounds

The contractile responses to transmural stimulation of, and the overflow of tritium from the rat portal vein prelabelled with 3H-noradrenaline were studied. The contractile responses of the rat portal vein were sustained throughout the period of stimulation. The tension developed did not decline when two consecutive periods of stimulation were compared. In contrast, the tritium overflow decreased during the second period of stimulation. Preincubation with 3 micronM phenoxybenzamine during 30 min increased 3-fold the tritium overflow during stimulation. Phentolamine and phenoxybenzamine were nearly equipotent in reducing the vascular response to stimulation. In contrast, phentolamine was less potent than phenoxybenzamine in increasing the 3H-noradrenaline overflow elicited by stimulation. The results obtained with phentolamine are interpreted in terms of a different potency of phentolamine to produce blockade of prejunctional and postjunctional alpha-adrenoceptors in the rat portal vein. ATP inhibited by 70% the tritium overflow induced by stimulation. The potency of ATP in inhibiting the overflow increased when the prejunction alpha-adrenoceptors were blocked. The purine compounds ATP, ADP, AMP and adenosine were roughly equipotent in inhibiting stimulation-induced tritium overflow. The tritium released by stimulation decreased when uptake and metabolism of adenosine were inhibited. Under physiological conditions, a prejunctional purinergic inhibition of noradrenaline release might be involved in an endogenously mediated negative feed-back regulatory mechanism. It is possible that the purinergic inhibition of the noradrenaline liberation elicited by stimulation plays a physiological role in tissues with both purinergic and adrenergic innervation.

Cutaneous and muscular rasodilation in the canine hindlimb evoked by central stimulation

Using stereotaxic procedures, we electrically stimulated specific sites in the hypothalamus and midbrain of anesthetized dogs pretreated with guanethidine and atropine methonitrate. A tract in which stimulation caused noncholinergic dilator responses in the hindlimbs was identified. The course of this trace was different from that subserving cholinergic vasodilation in the hindlimb musculature. In a number of experiments we studied the proportional distribution of blood flow to leg and paw. Responses restricted to the paw were regarded as occurring mainly in cutaneous vessels; those restricted to the leg were regarded as occurring mainly in the skeletal muscle vessels. Some dilator responses in both beds were abolished by intra-arterial administration of antihistamines: other dilator responses were abolished by intra-arterial injections of dopamine antagonists. Centrally evoked dilation of leg and paw vessels by noncholinergic pathways suggests physiological roles for these fibers in the regulation of cardiovascular function.

Vasoconstrictor and pilomotor fibres in skin nerves to the cat's tail

Postganglionic neurones to the tail's skin of the cat were investigated with regard to their spontaneous activity, response characteristics to somatic stimuli and asphyxia, the conduction velocity of their axons, and the conduction velocity of the preganglionic axons converging on them. The cats were anaesthetized with chloralose, immobilized, and arteficially ventilated. With this regimen the postganglionic neurones were divided into two types: 1. Type 1 neurones are spontaneously active and exhibit reflexes upon somatic stimulation. During asphyxia they are mostly first depressed and then excited for about 2-3 min. Their axons conduct with 0.57 +/- 0.13 m/s (mean +/- SD). The pregaglionic axons converging on them conduct with 5.4 +/- 1.6 m/s. 2. Type 2 neurones are not spontaneously active and exhibit with few exceptions no reflexes on somatic stimuli. During asphyxia they are activated after 3-4 min, concomitantly with piloerection, when the activity in type 1 neurones is already decreasing. Their axons conduct with 0.84 +/- 0.14 m/s, the preganglionic axons converging on them conduct with 9.9 +/- 2.9 m/s. 3. From these characteristics it is concluded that type 1 neurones have vasomotor function and most type 2 neurones pilomotor function.

Activation and inhibition of muscle and cutaneous postganglionic neurones to hindlimb during hypothalamically induced vasoconstriction and atropine-sensitive vasodilation

1. Discharge patterns in postganglionic neurones to muscle and to hairy skin of the hindlimb of chloralose anaesthetized cats were investigated during electrical hypothalamic stimulation which induced either vasoconstriction or atropine sensitive vasodilation in the skeletal muscle. 2. Spontaneously active postganglionic neurones to muscle were activated both during hypothalamically induced vasoconstriction and active vasodilation. Stimulation of the hypothalamic vasodilator area induced mostly a sequence of activation-depression-activation in these neurones. Stimulation of cutaneous Group IV afferents elicited reflexes in these neurones; repetitive high frequency stimulation of large diameter afferents in the vago-depressor nerve produced depression of spontaneous activity followed by a post-inhibitory excitation. The characteristics of these neurones fit those that would be expected of vasoconstrictors. 3. Normally inactive postganglionic neurones to skeletal muscle could only be activated during hypothalamically induced atropine sensitive vasodilation. These neurones exhibit no reflexes on somatic stimulation. The axons of these neurones conduct faster than those of the spontaneously active postganglionic neurones. It is likely that they are cholinergic vasodilator neurones. 4. Most of the cutaneous postganglionic neurones to hairy skin were activated during stimulation of both the hypothalamic vasoconstrictor and the vasodilator areas. These neurones have the characteristics of cutaneous vasoconstrictor neurones. Part of the cutaneous not spontaneously active postganglionic neurones could neither be activated from the hypothalamus nor by somatic stimuli.

Visceromotor organization within descending spinal sympathetic pathways in the dog

The organization within the descending sympathetic pathways on the surface of the dorsolateral funiculus of the spinal cord was investigated in anesthetized dogs. Changes in heart rate and carotid, renal, and femoral resistances were calculated during electrical stimulation of the descending spinal sympathetic pathways; the location within the pathway of the maximum change in each of these parameters was then determined. Blood pressure increased at each stimulation point with the maximum increase at 3-3.5 mm ventrolateral to the dorsolateral sulcus (DLS). The maximum increases in carotid, renal, and femoral resistances occurred with stimulation at 0.5-1.5, 2, and 3 mm ventrolateral to the DLS, respectively. Maximum heart rate responses were elicited by stimulation 2 mm ventrolateral to the sulcus. These data demonstrate that the pathway is organized in a dorsal-to-ventral manner, with the most rostral parts of the body being represented most dorsally and the more caudal parts being represented more ventrally.

Dopamine-induced neurogenic vasodilatation in isolated perfused muscle preparation of the dog

Dopamine, injected into the lumbar aorta of the dog in doses which produce a reversible inhibition of synaptic transmission in the lumbar paravertebral ganglia (0.5-64x10-8 moles), produces a neurogenic vasodilatation in the isolated perfused hindleg or gracilis muscle. This was abolished by acute preganglionic decentralization and by administration into the perfused preparation of alpha-adrenoceptor blocking agents, but not of atropine or diphenhydramine. After decentralization, preganglionic electrical stimulation restored the dopamine-induced indirect vasodilatation. The neurogenic vasodilatation was also seen with intra-aortic injections of epinine (2-32x10-8 moles) and apomorphine (1.2-19.2x10-8 moles) and was preferentially blocked by haloperidol (0.26x10-6 moles). (-)-Noradrenaline, injected into the lumbar aorta in baroreceptor-denervated dogs, was found to be equipotent with dopamine in eliciting the neurogenic vasodilatation; this (-)-noradrenaline-induced effect was preferentially blocked by phentolamine (8x10-6 moles). The possibility that the neurogenic vasodilatation, which occurs upon intra-aortic injection of dopamine in the dog, is due to its gnaglionic-inhibitory effect is discussed.