Relative location of alpha- and beta-adrenoceptors to sites of release of sympathetic transmitter in the rabbit facial vein

Histological studies of the dorsal nasal, angularis oculi, and facial veins of sheep (Ovis aries)

Selective brain cooling (SBC) requires vasoactivity in the superficial veins of the face of the animal. This vasoactivity is possible because of an adequate amount of smooth muscle in the tunica media of each of these superficial vessels, enabling it to act as a "muscle sphincter". In this study, the angularis oculi, dorsal nasal, distal, and proximal parts of the facial veins in sheep were examined histologically to describe an anatomical basis for SBC. Measurements of the tunica media thickness, the lumen diameter, and the ratio of these measurements showed that the relative tunica media thicknesses in the angularis oculi vein and the dorsal nasal vein are statistically smaller (P < 0.001) than in the distal or the proximal parts of the facial vein. In the angularis oculi, dorsal nasal, and distal part of the facial vein, the tunicae mediae were composed of five to seven circularly arranged smooth muscle layers, suggesting their ability to vasoconstrict. The proximal part of the facial vein possesses both circularly and longitudinally arranged smooth muscle layers. The circular smooth muscle layers suggest a vasoconstrictory function, whereas the longitudinal smooth muscle layers suggest a vasoconstrictory function in this part of the facial vein. Both the dorsal nasal and the proximal part of the facial vein, but not the angularis oculi or the distal part of the facial vein, possess endothelial valves near their confluences with other veins. It was concluded from this study that the angularis oculi and the distal part of the facial vein vasoconstrict, whereas the proximal part of the facial vein vasodilates, enabling the necessary changes in blood flow in SBC.

Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels

Earlier findings suggested that the physiological firing rate in sympathetic nerves did not exceed 10 Hz. Later recordings have revealed that this is a net value: impulses are usually not continuous but occur in bursts of high frequencies separated by quiet periods. The effects of continuous and burst-like neurogenic activation in various isolated blood vessels were compared. In the first part of the study changes in vascular tone were registered. Electrical field stimulatory parameters were chosen to give tetrodotoxin-blockable, neurogenic responses. From dose-response experiments in rat caudal artery a net frequency of 6 Hz was chosen, for bursts usually designed as 30 Hz during 0.2 s followed by a quiet period of 0.8 s. In the rabbit ear artery the neurogenic contraction was enhanced by a mean of near 50% during stimulation with bursts. Now appeared a minor phentolamine-resistant portion, which was not due to release of NPY, 5-HT, histamine or ATP. Also in the rat caudal artery and monkey pial artery a significant enhancement of contraction was seen during burst simulation, whereas in the rabbit facial vein no significant difference in dilatation through beta-receptor activation was obtained. In vessels that do not normally respond with purely neurogenic contractions/dilatations during continuous stimulation, like pial arteries from the rat and rabbit, not even bursts revealed a neurogenic response. In a second series of experiments the influence of continuous and burst-like nerve activation on the release of [14C]noradrenaline was studied in monkey, rabbit and rat pial arteries, rat caudal artery and rabbit central ear artery and facial vein.(ABSTRACT TRUNCATED AT 250 WORDS)

The human adrenergic neurovascular mechanism

Although evidence is limited and derived from many studies with many different objectives on a variety of blood vessels under varying experimental conditions, it appears that the human adrenergic vascular neuroeffector mechanism is qualitatively similar to that defined in studies on animals. The characteristics of the alpha-adrenoceptor based upon pA2 measurements and other pharmacological characteristics show it to be similar to adrenoreceptors defined in animals. The sensitivity of vascular smooth muscle to norepinephrine is of the same order throughout the human vasculature and is similar to that seen in most animal vessels. In most blood vessels the alpha-adrenoreceptor is the dominant type. By contrast the beta-adrenoceptor component is usually small with a high threshold except in the coronary artery and facial vein. Sympathetic frequency-response relationships of both arteries and veins are similar to those from animal studies. The human alpha-adrenoceptor is coupled both to sequestered calcium sites and slow calcium channels, and these two mechanisms are responsible for the biphasic response of human blood vessels to norepinephrine. The slow calcium channel but not the sequestered calcium system is sensitive to calcium antagonists. There is evidence that the human adrenergic mechanism is altered in disease.