RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



949 



CORTEX 



Metabolic Products (?) 

 No Known blockade — 

 5 Receptor (dilator) 



Receptor (Constrictor) 

 No known blockade 



Circulating Vasoconstrictor 

 (Angiotensin) 



Epinephrine 



<X Receptor 



(Constrictor) 



Adrenergic Constrictor 

 Blockade (PhenoxyDenzamine) 



Levarterenol 

 (L Norepinephrine) 



Sympatholytic 

 (Guanethidine. bretyliumi 



"J Receptor (dilator) 



Cholinergic Blockode 

 (atropine) 



Acetylcholine 



Sympathetic 



15 



< 



Dilator 





Epinephrine 

 (isoproterenol) 



Adrenergic Dilator Blockade 

 (di-chloro- isoproterenol) 



Ganglionic Blockade 

 (tetroethylammonium) 



/9 Receptor (dilator) 



Sympathetic Constrictor 



Acetylcholine 



o-J 



-Acetylcholine 



fig. 17. Diagram of hypothetical receptor sites on arterioles in skeletal muscle. (See text for dis- 

 cussion.) [Modified from Green & Kepchar (45).] 



induced by lumbar sympathetic chain stimulation, 

 but has essentially no effect upon the dilator response 

 to epinephrine (45, 90a, 1 17). 



On the basis of detailed studies similar to the above, 

 Ahlquist (1) proposed that two adrenergic receptors 

 are present in blood vessels controlling the resistance 

 to flow in vascular beds (fig. 17). The first of these 

 has a constrictor (excitatory) action on vascular 

 smooth muscle; but because it has an inhibitory action 

 elsewhere (notably in the gut) it was not referred to 

 as an excitatory receptor but, noncommittally, as the 

 alpha receptor. It is blocked by phenoxybenzamine 

 and similar adrenergic blocking agents. The second 

 type of receptor causes relaxation of vascular smooth 

 muscle but because it has an excitatory effect on the 

 heart it was denoted as the beta receptor. It is blocked 

 by dichloroisoproterenol. The alpha receptor is 

 most sensitive to epinephrine, and least sensitive to 

 isoproterenol, whereas the beta receptor is most 

 sensitive to isoproterenol and least sensitive to 

 arterenol. 



We would prefer to limit the term alpha receptor 

 to those receptors which are located on blood vessels. 

 As such the alpha receptor would always be constrictor 

 and innervated by sympathetic nerve fibers. We 



would prefer also to limit the beta receptor to vascular 

 sites. As such it would always be inhibitory (vaso- 

 dilator) and to the best of our knowledge never 

 innervated. 



In addition to these two adrenergic receptors of 

 Ahlquist, other vascular receptors have been proposed 

 (45). A third group, the gamma dilator receptors, are 

 excited by acetylcholine and by stimulation of 

 sympathetic dilator fibers and blocked by cholinergic 

 blocking agents such as atropine (45). A fourth 

 possible group, the delta dilator receptors, may be 

 excited by metabolic products or low tissue 2 

 tension; they are probably the receptors which 

 participate in autoregulation and reactive hyperemia. 

 At present no agents are known which block the 

 delta receptors. Several more receptors are probably 

 present; those receptors, which respond to constrictor 

 agents such as angiotensin, vasopressin, and similar 

 polypeptides, are grouped together under the term 

 "epsilon constrictor receptors"; no agents are known 

 which can block these receptors. 



The potency of various agents in inducing con- 

 striction or dilation varies from one vascular bed to 

 another. Cerebral and cardiac vascular beds show 

 essentially no constrictor response to adrenergic 



