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Rete mirabile of goat: In vitro effects of adrenergic stimulation
Brain Research, 289 (1983) 281-284 Elsevier
281
Rete Mirabile of Goat: In Vitro Effects of Adrenergic Stimulation GODOFREDO DII~GUEZ, M. VICTORIA CONDE, BERNARDINO GOMEZ, JOSI~ R. IGLESIAS, JESI]S MAR~N and SALVADOR LLUCH* Departamentos de Fisiologia y Farrnacolog(a, Facultad de Medicina, Universidad Aut6noma, Arzobispo Morcillo 1, Madrid 34, and (J. R. I.) Departamento de Neuropatologta, Centro Ram6n y Cajal, Madrid (Spain) (Accepted April 26th, 1983) Key words: retial arteries - - cerebral arteries - - sympathetic stimulation - - cerebrovascular resistance
The carotid rete of the goat, a network of small arteries, is placed in the pathway of the main arteries which supply the brain. This structure lies within the cavernous sinus on each side of the pituitary. The presence of a carotid rete in many mammals has led to frequent speculations about its physiological function. The present study was designed to examine comparatively the responsiveness of goat retial and cerebral arteries to direct or indirect adrenergic stimulation. The contractile response of isolated retial arteries (150-500/zm in external diameter) to norepinephrine (10-8-10--4M), field electrical stimulation (2-16 c/s), and tyramine (10-6-10-3 M) was negligible. In contrast, cerebral arteries of 300-500 gm in external diameter exhibited dose- or frequency-dependent contractions qualitatively similar to those previously described in larger cerebral arteries. The norepinephrine content in the walls of retial arteries was about 13% of that measured in cerebral arteries. These results suggest that the role of the carotid rete in the regulation of resistance to blood flow during adrenergic stimulation is negligible or nonexistent. INTRODUCTION B l o o d is carried to the brain of the goat by the internal maxillary artery which divides to form the carotid rete (rete mirabile caroticum), a n e t w o r k of freely anastomosing arteries lying within the cavernous sinus on each side of the pituitaryl,8 (Fig. 1). The presence of a carotid rete in m a n y m a m m a l s has led to much speculation a b o u t its physiological functions. E x p e r i m e n t s carried out in various animal species have established that the carotid rete functions as a heat exchanger to p r e v e n t the overheating of the brain3,4.11. H o w e v e r , there is little e x p e r i m e n t a l evidence to support the h e m o d y n a m i c significance of the carotid rete in regulating c e r e b r a l arterial pressure or flow2,15. Previous studies in anesthetized goats have indicated that the rete vasculature is m o r e sensitive to injections of n o r e p i n e p h r i n e and isoproterenol into the lingual artery (proximal to the rete) than cerebral vessels 9. These e x p e r i m e n t s in vivo have serious limitations since the rete is i n t e r p o s e d in series between the carotid vessels and the circle of Willis and, therefore, the o b s e r v e d responses on the * To whom correspondence should be addressed. 0006-8993/83/$03.00 (~) 1983 Elsevier Science Publishers B.V.
rete vasculature m a y have been the result of the concomitant effects on cerebral vessels. To lessen this p r o b l e m , the present study was designed to examine comparatively the responsiveness of isolated retial and cerebral arteries to adrenergic stimulation. Norepinephrine (1-norepinephrine h y d r o c h l o r i d e , Sigma) was used to investigate the response of the vascular effector system, whereas field electrical stimulation and tyramine (tyramine hydrochloride, Sigma) were applied to test the availability of the n e u r o t r a n s m i t t e r p r e s e n t in the vessels wall. Both electrical stimulation and t y r a m i n e exert their constrictor effects on brain vessels through the release of n o r e p i n e p h r i n e from the adrenergic nerve endingsT.13. MATERIALS AND METHODS Fifteen female goats, weighing 32-53 kg, were anesthetized with i.v. injections of 2 % sodium thiopental and killed by injecting 15 ml of s a t u r a t e d solution of potassium chloride. T h e brain and the carotid rete were i m m e d i a t e l y r e m o v e d and retial arteries
282
AC~M C~EOBu
Fig. 1. Left: schematic representation of the carotid rete and its afferent and efferent arteries. EC, external carotid artery; IM, internal maxillary artery; RA, ramus anastomoticus; D, dental artery; Bu, buccinator artery; EO, ophthalmic and ethmoidat arteries: IC. internal carotid artery; MC, middle cerebral artery; AC, anterior cerebral artery; PC, posterior communicating artery; B, basilar artery; CW, circle of Willis. Middle: photomicrograph of retial arteries using dissecting microscope. Right: light photomicrograph of a section of the carotid rete (hematoxylin-eosin). and branches of the middle cerebral artery were dissected free and cut into cylindrical segments 2 mm in length. Two groups of retial arteries were made depending on their outside diameter: group Ra, 150-250/~m, and group Rb, 300-500/~m. Cerebral arteries ranged from 300 to 500/~m in external diameter. All measurements were made using an ocular micrometer within a Wild M8 zoom microscope. Two stainless-steel pins, 50/~m in diameter, were introduced through the arterial lumen. Each segment was prepared for isometric recording in a 6 ml bath containing modified Krebs-Henseleit solution 7. The solution was equilibrated with 95% oxygen and 5% carbon dioxide to give a p H of 7.3-7.4. Temperature was held at 37 °C. A resting tension of 300 mg was applied to the tissue and the segments were allowed to equilibrate for 60-90 min before any drug was added. Drugs were dissolved in physiological saline solution containing 0.01% ascorbid acid and d o s e response curves for norepinephrine (10-8-10-4 M) and tyramine (10-6-10- 3 M) were determined in a cumulative manner. Field electrical stimulation of the arterial segment was possible by means of two platinum electrodes mounted on each side of the preparation 7. Trains of 300 biphasic square-wave pulses of 0.2 ms duration at 2, 4, 8 and 16 Hz were applied in r a n d o m sequence at supramaximal intensity (about 80 V) using a Grass SD5 stimulator. The arteries of the circle of Willis and the carotid
rete from 6 goats were taken for determination of norepinephrine tissue concentration according to the radioenzymatic method of Henry et al. ~2. The results were expressed as #g/g of wet tissue. The carotid retina from another 6 goats were fixed in 10% formalin. Each specimen was embedded in paraffin and sectioned in cranio-caudal direction at 5 y m . Sections were stained with hematoxylin-eosin, cresyl violet, and van Gieson's trichromic to demonstrate general morphology and connective tissue. Data are expressed as the means _+ S.E.M. Statistical analysis was done by means of Student's t-test, considering as significant a probability value of less than 0.05. RESULTS Contractile response curves to norepinephrine, electrical stimulation and tyramine from retial and cerebral arteries are illustrated in Fig. 2. Retial arteries exhibited a very low sensitivity to direct or indirect adrenergic activation. Retial arteries of group A (150-250 ~m) were practically unresponsive to norepinephrine, electrical stimulation and tyramine. Electrical stimulation of retial arteries of group B (300-500 ktm) did not elicit any measurable contractile effects. These arteries, however, were more sensitive to norepinephrine and tyramine than those of group A. The maximal contractile effects of norepinephrine on retial arteries of group B were only 20% of those obtained in cerebral arteries, and the con-
283
600-
CA
CA E 400 Z 0
Z ILl t---
200
Rb Ra
0 -8
-7
-6
-5
Ra
~.~,.,~CA
.
.
.
.
_Rb ~ Ra
-4
N E (Log M)
Ty (Log M)
ES ( H z )
Fig. 2. Contractile response curves to norepinephrine (NE), tyramine (Ty) and electrical stimulation (ES) of retial (Ra, 150-250/tm in external diameter, and Rb, 300-500/~m) and middle cerebral arteries (CA, 300-500 ~m) obtained from 6 goats. centration of this amine producing half-maximum contraction (ECs0) was about 32 times higher in retial than in cerebral arteries. The maximal responses to tyramine were approximately 23% of the responses observed in cerebral arteries whereas the ECs0 value was about 8 times higher in retial arteries. In cerebral arteries the content of norepinephrine was 2.1 + 0.6/~g/g (mean _+ S . E . M . ) o f tissue whereas retial arteries contained 0.28 + 0.06/ag/g. DISCUSSION It is not surprising that the carotid rete, intimately associated with the supply of blood to the brain, has intrigued investigators for a long time. The difficulty of carrying out physiological experimentations in the carotid rete of the goat is mainly due to the deep location of this structure and to the surgical inaccessibility of particular retial vessels for exposure and cannulation. Therefore, we used a technique where the responsiveness of isolated retial arteries of different calibers could be evaluated in vitro and compared to the responses observed in branches from the middle cerebral artery. The present results indicate that retial arteries may have a low population of receptor sites for norepinephrine; they also suggest that norepinephrine has a low affinity for its receptors in these particular arteries. Both tyramine and sympathetic stimulation exert their effects on cerebral vessels mainly through the displacement of norepinephrine from nerve terminals 7.13. Accordingly, the concentration response
curves for tyramine and the effects of nerve stimulation showed the same pattern as was seen with the response to no~'epinephrine. In contrast to the paucity of responses of retial arteries, cerebral arteries of 300-500 p m in external diameter exhibited dose- or frequency-dependent contractions qualitatively similar to those previously described in larger cerebral arteries 7. Two morphological features of arteries have been regarded as determinant factors in their response to adrenergic stimulation, namely the density of innervation and the wall thickness to lumen ratio 5.10. These factors appear to correlate with the maximum constrictor responses observed in various vessels. With regard to the possible presence of adrenergic nerve terminals in the carotid fete of the goat, a recent study using scanning and electron microscopy suggests that this might be the case 6. However, the low content of norepinephrine measured in these vessels together with the lack of responses to sympathetic stimulation or tyramine may reflect a sparse adrenergic innervation. In contrast, pial vessels, with a high catecholamine content in their walls, contracted in response to tyramine or sympathetic stimulation. Histological examination of the carotid rete revealed that most of the component vessels have an internal diameter ranging from 80 to 150 pm, a muscle layer width from 90 to 100 p m and a wall thickness to lumen ratio ranging from 1:1 to 2:1. The adventitial coat is rich in collagen fibers and its outer surface is covered by a single layer of endothelium which forms
284 the lining of the cavernous sinus. These features are
means of vasomotor reflexes or hormonal influences.
essentially similar to those previously seen in the
Our present experiments suggest that the role of the
sheep 14. O u r results in vitro indicate that the respon-
carotid rete in the regulation of resistance to blood
siveness of retial arteries is not coupled with the rela-
flow during adrenergic stimulation is negligible or nonexistent. In contrast, it is apparent that cerebral
tively thick wall observed in these vessels. However, a well-developed vessel wall is consistent with the
blood vessels rather than retial vessels are the site of
considerations have been d e m o n s t r a t e d in other ani-
controlled resistance in situations in which direct adrenergic activation or release of the neurotransmitter takes place.
mal species with a well-developed rete, such as the giraffe 16, and are in agreement with the possible signifi-
ACKNOWLEDGEMENTS
wide changes in pressure that these arteries may support u n d e r normal conditions9. These h e m o d y n a m i c
cance of the rete in regulating cerebral arterial pressure during postural changes. However, it is premature to accept the action of the rete mirabile in markedly modifying the cerebral circulation by
This study was supported by Grants from the Comisi6n Asesora de Investigaci6n and Ministerio de Sanidad.
REFERENCES 1 Andersson, B. and Jewell, P. A., The distribution of carotid and vertebral blood in the brain and spinal cord of the goat, J. exp. Physiol., 41 (1956)462-474. 2 Ask-Upmark, E., The carotid sinus and the cerebral circulation. An anatomical, experimental and clinical investigation, including some observations on rete mirabile caroticum, Acta psychiat, neurol., Suppl. 5-7 (1935) 1-374. 3 Baker, M. A., Chapman, L. W. and Nathanson, M., Control of brain temperature in dogs: effects of tracheostomy, Respirat. Physiol., 22 (1974) 325-333. 4 Baker, M. A. and Hayward. J. N., The influence of the nasal mucosa and the carotid rete upon hypothalamic temperature in sheep, J. Physiol. (Lond.), 198 ,(1968) 561-579. 5 Bevan, J. A. and Purdy, R. E., Variations in adrenergic innervation and contractile responses of the rabbit saphenous artery, Circulat. Res., 32 (1973) 746--751. 6 Burns, E. M., Braverman, B., Kruckeberg, T. W., Gaetano, P. K., Dobben, G. D. and Shulman, M., The rete mirabile: an SEM/TEM study. In Abstracts Society for Neurosciences, Vol. 7, llth Annual Meeting, Los Angeles, CA, Oct. 18-23, 1981, p. 118. 7 Conde, M. V., Marin, J., Salaices, M., Marco, E. J., G6mez, B. and Lluch, S., Adrenergic vasoconstriction of the goat middle cerebral artery, Amer. J. Physiol., 235 (1978) H131-H135. 8 Daniel, P. M., Dawes, J. D. K. and Pritchard, M. M. L.,
9 10 11 12
13 14 15
16
Studies of the carotid rete and its associated arteries, Phil, Trans. Roy. Soc. London B, 237 (1953) 173-208. Edelman, N. H., Epstein, P., Cherniack, N. S. and Fishman, A. P., Control of cerebral blood flow inthe goat; role of the carotid rete, Amer. J. Physiol., 223 (1972) 615-619. Gillespie, J. S. and Rae, R. M., Constrictor and compliance responses of some arteries to nerve or drug stimulation, J. Physiol. (Lond.), 223 (1972) 109-130. Hayward, J. N. and Baker, M. A., The role of the cerebral arterial blood in the regulation of brain temperature in the monkey, Amer. J. Physiol., 215 (1968) 389--403. Henry, D. P., Starman, B. J., Johnson, D. G. and Williams, R. H., A sensitive radioenzymatic assay for norepinephrine in tissues and plasma, Life Sci., 16 (1975) 375-384. Lluch, S., G6mez, B., Alborch, E. and Urquilla, P. R., Adrenergic mechanisms in cerebral circulation of the goat, Amer. J. Physiol., 228 (1975) 985-989. McGrath, F., Observations on the intracranial carotid rete and the hypophysis in the mature female pig and sheep, J. Anat., 124 (1977) 689-699. Nagel, E. L., Morgane, P. J., McFarland, W. L. and Galliano, R. E., Rete mirabile of dolphin: its pressure-damping effect on cerebral circulation, Science, 161 (1968) 898-899. Van Citters, R. L., Franklin, D. L., Vatner, S. F., Patrick, T. and Warren, J. V., Cerebral hemodynamics in the giraffe, Trans. Assos. Amer. Physiol., 82 (293-304).
281
Rete Mirabile of Goat: In Vitro Effects of Adrenergic Stimulation GODOFREDO DII~GUEZ, M. VICTORIA CONDE, BERNARDINO GOMEZ, JOSI~ R. IGLESIAS, JESI]S MAR~N and SALVADOR LLUCH* Departamentos de Fisiologia y Farrnacolog(a, Facultad de Medicina, Universidad Aut6noma, Arzobispo Morcillo 1, Madrid 34, and (J. R. I.) Departamento de Neuropatologta, Centro Ram6n y Cajal, Madrid (Spain) (Accepted April 26th, 1983) Key words: retial arteries - - cerebral arteries - - sympathetic stimulation - - cerebrovascular resistance
The carotid rete of the goat, a network of small arteries, is placed in the pathway of the main arteries which supply the brain. This structure lies within the cavernous sinus on each side of the pituitary. The presence of a carotid rete in many mammals has led to frequent speculations about its physiological function. The present study was designed to examine comparatively the responsiveness of goat retial and cerebral arteries to direct or indirect adrenergic stimulation. The contractile response of isolated retial arteries (150-500/zm in external diameter) to norepinephrine (10-8-10--4M), field electrical stimulation (2-16 c/s), and tyramine (10-6-10-3 M) was negligible. In contrast, cerebral arteries of 300-500 gm in external diameter exhibited dose- or frequency-dependent contractions qualitatively similar to those previously described in larger cerebral arteries. The norepinephrine content in the walls of retial arteries was about 13% of that measured in cerebral arteries. These results suggest that the role of the carotid rete in the regulation of resistance to blood flow during adrenergic stimulation is negligible or nonexistent. INTRODUCTION B l o o d is carried to the brain of the goat by the internal maxillary artery which divides to form the carotid rete (rete mirabile caroticum), a n e t w o r k of freely anastomosing arteries lying within the cavernous sinus on each side of the pituitaryl,8 (Fig. 1). The presence of a carotid rete in m a n y m a m m a l s has led to much speculation a b o u t its physiological functions. E x p e r i m e n t s carried out in various animal species have established that the carotid rete functions as a heat exchanger to p r e v e n t the overheating of the brain3,4.11. H o w e v e r , there is little e x p e r i m e n t a l evidence to support the h e m o d y n a m i c significance of the carotid rete in regulating c e r e b r a l arterial pressure or flow2,15. Previous studies in anesthetized goats have indicated that the rete vasculature is m o r e sensitive to injections of n o r e p i n e p h r i n e and isoproterenol into the lingual artery (proximal to the rete) than cerebral vessels 9. These e x p e r i m e n t s in vivo have serious limitations since the rete is i n t e r p o s e d in series between the carotid vessels and the circle of Willis and, therefore, the o b s e r v e d responses on the * To whom correspondence should be addressed. 0006-8993/83/$03.00 (~) 1983 Elsevier Science Publishers B.V.
rete vasculature m a y have been the result of the concomitant effects on cerebral vessels. To lessen this p r o b l e m , the present study was designed to examine comparatively the responsiveness of isolated retial and cerebral arteries to adrenergic stimulation. Norepinephrine (1-norepinephrine h y d r o c h l o r i d e , Sigma) was used to investigate the response of the vascular effector system, whereas field electrical stimulation and tyramine (tyramine hydrochloride, Sigma) were applied to test the availability of the n e u r o t r a n s m i t t e r p r e s e n t in the vessels wall. Both electrical stimulation and t y r a m i n e exert their constrictor effects on brain vessels through the release of n o r e p i n e p h r i n e from the adrenergic nerve endingsT.13. MATERIALS AND METHODS Fifteen female goats, weighing 32-53 kg, were anesthetized with i.v. injections of 2 % sodium thiopental and killed by injecting 15 ml of s a t u r a t e d solution of potassium chloride. T h e brain and the carotid rete were i m m e d i a t e l y r e m o v e d and retial arteries
282
AC~M C~EOBu
Fig. 1. Left: schematic representation of the carotid rete and its afferent and efferent arteries. EC, external carotid artery; IM, internal maxillary artery; RA, ramus anastomoticus; D, dental artery; Bu, buccinator artery; EO, ophthalmic and ethmoidat arteries: IC. internal carotid artery; MC, middle cerebral artery; AC, anterior cerebral artery; PC, posterior communicating artery; B, basilar artery; CW, circle of Willis. Middle: photomicrograph of retial arteries using dissecting microscope. Right: light photomicrograph of a section of the carotid rete (hematoxylin-eosin). and branches of the middle cerebral artery were dissected free and cut into cylindrical segments 2 mm in length. Two groups of retial arteries were made depending on their outside diameter: group Ra, 150-250/~m, and group Rb, 300-500/~m. Cerebral arteries ranged from 300 to 500/~m in external diameter. All measurements were made using an ocular micrometer within a Wild M8 zoom microscope. Two stainless-steel pins, 50/~m in diameter, were introduced through the arterial lumen. Each segment was prepared for isometric recording in a 6 ml bath containing modified Krebs-Henseleit solution 7. The solution was equilibrated with 95% oxygen and 5% carbon dioxide to give a p H of 7.3-7.4. Temperature was held at 37 °C. A resting tension of 300 mg was applied to the tissue and the segments were allowed to equilibrate for 60-90 min before any drug was added. Drugs were dissolved in physiological saline solution containing 0.01% ascorbid acid and d o s e response curves for norepinephrine (10-8-10-4 M) and tyramine (10-6-10- 3 M) were determined in a cumulative manner. Field electrical stimulation of the arterial segment was possible by means of two platinum electrodes mounted on each side of the preparation 7. Trains of 300 biphasic square-wave pulses of 0.2 ms duration at 2, 4, 8 and 16 Hz were applied in r a n d o m sequence at supramaximal intensity (about 80 V) using a Grass SD5 stimulator. The arteries of the circle of Willis and the carotid
rete from 6 goats were taken for determination of norepinephrine tissue concentration according to the radioenzymatic method of Henry et al. ~2. The results were expressed as #g/g of wet tissue. The carotid retina from another 6 goats were fixed in 10% formalin. Each specimen was embedded in paraffin and sectioned in cranio-caudal direction at 5 y m . Sections were stained with hematoxylin-eosin, cresyl violet, and van Gieson's trichromic to demonstrate general morphology and connective tissue. Data are expressed as the means _+ S.E.M. Statistical analysis was done by means of Student's t-test, considering as significant a probability value of less than 0.05. RESULTS Contractile response curves to norepinephrine, electrical stimulation and tyramine from retial and cerebral arteries are illustrated in Fig. 2. Retial arteries exhibited a very low sensitivity to direct or indirect adrenergic activation. Retial arteries of group A (150-250 ~m) were practically unresponsive to norepinephrine, electrical stimulation and tyramine. Electrical stimulation of retial arteries of group B (300-500 ktm) did not elicit any measurable contractile effects. These arteries, however, were more sensitive to norepinephrine and tyramine than those of group A. The maximal contractile effects of norepinephrine on retial arteries of group B were only 20% of those obtained in cerebral arteries, and the con-
283
600-
CA
CA E 400 Z 0
Z ILl t---
200
Rb Ra
0 -8
-7
-6
-5
Ra
~.~,.,~CA
.
.
.
.
_Rb ~ Ra
-4
N E (Log M)
Ty (Log M)
ES ( H z )
Fig. 2. Contractile response curves to norepinephrine (NE), tyramine (Ty) and electrical stimulation (ES) of retial (Ra, 150-250/tm in external diameter, and Rb, 300-500/~m) and middle cerebral arteries (CA, 300-500 ~m) obtained from 6 goats. centration of this amine producing half-maximum contraction (ECs0) was about 32 times higher in retial than in cerebral arteries. The maximal responses to tyramine were approximately 23% of the responses observed in cerebral arteries whereas the ECs0 value was about 8 times higher in retial arteries. In cerebral arteries the content of norepinephrine was 2.1 + 0.6/~g/g (mean _+ S . E . M . ) o f tissue whereas retial arteries contained 0.28 + 0.06/ag/g. DISCUSSION It is not surprising that the carotid rete, intimately associated with the supply of blood to the brain, has intrigued investigators for a long time. The difficulty of carrying out physiological experimentations in the carotid rete of the goat is mainly due to the deep location of this structure and to the surgical inaccessibility of particular retial vessels for exposure and cannulation. Therefore, we used a technique where the responsiveness of isolated retial arteries of different calibers could be evaluated in vitro and compared to the responses observed in branches from the middle cerebral artery. The present results indicate that retial arteries may have a low population of receptor sites for norepinephrine; they also suggest that norepinephrine has a low affinity for its receptors in these particular arteries. Both tyramine and sympathetic stimulation exert their effects on cerebral vessels mainly through the displacement of norepinephrine from nerve terminals 7.13. Accordingly, the concentration response
curves for tyramine and the effects of nerve stimulation showed the same pattern as was seen with the response to no~'epinephrine. In contrast to the paucity of responses of retial arteries, cerebral arteries of 300-500 p m in external diameter exhibited dose- or frequency-dependent contractions qualitatively similar to those previously described in larger cerebral arteries 7. Two morphological features of arteries have been regarded as determinant factors in their response to adrenergic stimulation, namely the density of innervation and the wall thickness to lumen ratio 5.10. These factors appear to correlate with the maximum constrictor responses observed in various vessels. With regard to the possible presence of adrenergic nerve terminals in the carotid fete of the goat, a recent study using scanning and electron microscopy suggests that this might be the case 6. However, the low content of norepinephrine measured in these vessels together with the lack of responses to sympathetic stimulation or tyramine may reflect a sparse adrenergic innervation. In contrast, pial vessels, with a high catecholamine content in their walls, contracted in response to tyramine or sympathetic stimulation. Histological examination of the carotid rete revealed that most of the component vessels have an internal diameter ranging from 80 to 150 pm, a muscle layer width from 90 to 100 p m and a wall thickness to lumen ratio ranging from 1:1 to 2:1. The adventitial coat is rich in collagen fibers and its outer surface is covered by a single layer of endothelium which forms
284 the lining of the cavernous sinus. These features are
means of vasomotor reflexes or hormonal influences.
essentially similar to those previously seen in the
Our present experiments suggest that the role of the
sheep 14. O u r results in vitro indicate that the respon-
carotid rete in the regulation of resistance to blood
siveness of retial arteries is not coupled with the rela-
flow during adrenergic stimulation is negligible or nonexistent. In contrast, it is apparent that cerebral
tively thick wall observed in these vessels. However, a well-developed vessel wall is consistent with the
blood vessels rather than retial vessels are the site of
considerations have been d e m o n s t r a t e d in other ani-
controlled resistance in situations in which direct adrenergic activation or release of the neurotransmitter takes place.
mal species with a well-developed rete, such as the giraffe 16, and are in agreement with the possible signifi-
ACKNOWLEDGEMENTS
wide changes in pressure that these arteries may support u n d e r normal conditions9. These h e m o d y n a m i c
cance of the rete in regulating cerebral arterial pressure during postural changes. However, it is premature to accept the action of the rete mirabile in markedly modifying the cerebral circulation by
This study was supported by Grants from the Comisi6n Asesora de Investigaci6n and Ministerio de Sanidad.
REFERENCES 1 Andersson, B. and Jewell, P. A., The distribution of carotid and vertebral blood in the brain and spinal cord of the goat, J. exp. Physiol., 41 (1956)462-474. 2 Ask-Upmark, E., The carotid sinus and the cerebral circulation. An anatomical, experimental and clinical investigation, including some observations on rete mirabile caroticum, Acta psychiat, neurol., Suppl. 5-7 (1935) 1-374. 3 Baker, M. A., Chapman, L. W. and Nathanson, M., Control of brain temperature in dogs: effects of tracheostomy, Respirat. Physiol., 22 (1974) 325-333. 4 Baker, M. A. and Hayward. J. N., The influence of the nasal mucosa and the carotid rete upon hypothalamic temperature in sheep, J. Physiol. (Lond.), 198 ,(1968) 561-579. 5 Bevan, J. A. and Purdy, R. E., Variations in adrenergic innervation and contractile responses of the rabbit saphenous artery, Circulat. Res., 32 (1973) 746--751. 6 Burns, E. M., Braverman, B., Kruckeberg, T. W., Gaetano, P. K., Dobben, G. D. and Shulman, M., The rete mirabile: an SEM/TEM study. In Abstracts Society for Neurosciences, Vol. 7, llth Annual Meeting, Los Angeles, CA, Oct. 18-23, 1981, p. 118. 7 Conde, M. V., Marin, J., Salaices, M., Marco, E. J., G6mez, B. and Lluch, S., Adrenergic vasoconstriction of the goat middle cerebral artery, Amer. J. Physiol., 235 (1978) H131-H135. 8 Daniel, P. M., Dawes, J. D. K. and Pritchard, M. M. L.,
9 10 11 12
13 14 15
16
Studies of the carotid rete and its associated arteries, Phil, Trans. Roy. Soc. London B, 237 (1953) 173-208. Edelman, N. H., Epstein, P., Cherniack, N. S. and Fishman, A. P., Control of cerebral blood flow inthe goat; role of the carotid rete, Amer. J. Physiol., 223 (1972) 615-619. Gillespie, J. S. and Rae, R. M., Constrictor and compliance responses of some arteries to nerve or drug stimulation, J. Physiol. (Lond.), 223 (1972) 109-130. Hayward, J. N. and Baker, M. A., The role of the cerebral arterial blood in the regulation of brain temperature in the monkey, Amer. J. Physiol., 215 (1968) 389--403. Henry, D. P., Starman, B. J., Johnson, D. G. and Williams, R. H., A sensitive radioenzymatic assay for norepinephrine in tissues and plasma, Life Sci., 16 (1975) 375-384. Lluch, S., G6mez, B., Alborch, E. and Urquilla, P. R., Adrenergic mechanisms in cerebral circulation of the goat, Amer. J. Physiol., 228 (1975) 985-989. McGrath, F., Observations on the intracranial carotid rete and the hypophysis in the mature female pig and sheep, J. Anat., 124 (1977) 689-699. Nagel, E. L., Morgane, P. J., McFarland, W. L. and Galliano, R. E., Rete mirabile of dolphin: its pressure-damping effect on cerebral circulation, Science, 161 (1968) 898-899. Van Citters, R. L., Franklin, D. L., Vatner, S. F., Patrick, T. and Warren, J. V., Cerebral hemodynamics in the giraffe, Trans. Assos. Amer. Physiol., 82 (293-304).