Innervation pattern and neurogenic response of rabbit veins

The adrenergic neuroeffector mechanism has been assessed in 14 different veins of the rabbit chosen to represent vessels of different, function and regional location. The pattern, distribution and density of the adrenergic innervation were assessed by fluorescence histochemistry. The magnitude of the neurogenic contractile response of isolated segments was studied <i>in vitro</i> in relation to the maximum response to <i>1</i>-norepinephrine. The diameter and wall thickness of the veins was determined. The amount of adrenergic innervation determined histochemically was related to the size of the contractile response. Both fluorescence and response were absent in the superficial cervical, femoral, subcutaneous from back, brachial and deep circumflex iliac branch veins. The amount of adrenergic innervation increased in order in the renal, inferior vena cava, pulmonary, jugular, parietal branch of internal iliac, mesenteric, central ear, cephalic and small saphenous veins. In the latter two veins, innervation was not restricted to the adventitio-medial junction but entered the media. The pattern of innervation and contractile response may be related to the functional need for active venoconstriction in the various parts of the capacitance section of the vascular system.

Evidence for uneven alpha-receptor distribution in the rat portal vein

The smooth muscle of the rat portal vein which supports propagation becomes supersensitive to exogenous l -norepinephrine after elimination of the vessel's intramural neuronal uptake mechanism. Such supersensitivity is generally associated with intimate neuromuscular connections. Since the adrenergic vasomotor fibers in the portal vein do not enter the muscle tissue, it has been proposed that muscle cells close to the nerve terminals must determine the response to norepinephrine. With the aim of elucidating the mechanism behind the prejunctional supersensitivity, the total uptake of labeled norepinephrine in the portal vein was analyzed and its distribution within the vessel wall was evaluated directly using isotopically labeled frozen sections. The total uptake of the portal vein, related to its endogenous norepinephrine content, was comparable with that in other vascular and nonvascular tissues. The 3 H-uptake profile of the vessel wall showed that the neurogenic uptake was confined to the narrow adrenergic plexus between the longitudinal and the circular muscle layers. The neuronal uptake mechanism therefore could only influence the concentration of exogenous norepinephrine at muscle cells close to the nerve terminals. These results support the hypothesis that the prejunctional supersenstivity in the rat portal vein indicates that the alpha receptors are located on muscle cells in the vicinity of the adrenergic nerve terminals.

Comparison of adrenergic mechanisms in an elastic and a muscular artery of the rabbit

Elastic and muscular arteries are known to differ in function and in the magnitude of their response to vasoconstrictor influences. With the isolated thoracic aorta and ear artery of the rabbit as representative arteries, the morphological, physiological, and pharmacological correlates of these differences have been sought among their presynaptic adrenergic mechanisms. The adrenergic nerve plexus in the ear artery is wider and the nodes are denser than they are in the aorta. There is some evidence consistent with the hypothesis that the thicker the plexus, the greater the reuptake of released transmitter, the smaller the transmitter overflow, and the lower the efficiency of uptake of exogenous norepinephrine. Measurements of fractional release of tritiated norepinephrine suggest that qualitative differences in the adrenergic transmitter storage and release mechanisms may exist between the two vessels. Thus the considerable functional difference between the two vessels is in part, at any rate, a consequence of adrenergic mechanisms which differ both quantitatively and qualitatively from each other.

[3H]adenosine triphosphate: release during stimulation of enteric nerves

The isolated taenia coli of the guinea pig takes up tritiated adenosine, adenosine monophosphate, adenosine diphosphate, and adenosine triphosphate, in preference to tritiated inosine and adenine. After uptake, [(3)H]adenosine is converted and retained primarily as [(3)H]adenosine triphosphate. Tritium is released from taenia coli treated with [(3)H]adenosine upon activation of the nonadrenergic inhibitory nerves. These results are consistent with the previous evidence that adenosine triphosphate may be the transmitter from the nerves.

Distribution theory of resistance of neurogenic vasoconstriction to alpha-receptor blockade in the rabbit

The effects of α-receptor-blocking agents on the contractile responses of isolated rabbit arteries to sympathetic nerve stimulation and exogenous l -norepinephrine ( l -NE) were compared. In the pulmonary artery and aorta, yohimbine, phentolamine, and phenoxybenzamine blocked the response to nerve stimulation less than that to an equipotent dose of l -NE. This resistance of neurogenic response was independent of the frequency and number of stimuli and persisted after inhibition of the nerve l -NE uptake by cocaine. The neurogenically released transmitter l -NE probably forms a high concentration near the adventitia-media junction, whereas the exogenous NE is distributed evenly throughout the thickness of media. Thus higher concentrations of α-receptor-blocking agents would be needed to block the effect of neurogenic l -NE than to block that of exogenous l -NE. This explanation of the resistance was thought to be more appropriate to the large vessels tested than that based on neuroeffector proximity.