CIRCULATION IN SKELETAL MUSCLE 



[ 369 



10 



Minutes after exercise 



fig. 16. Results showing vasodilatation in normal and 

 sympathectomized muscle after exercise. Ordinate gives 

 increase in blood flow above normal level. [After Grant (113).] 



curare. Curare does not paralyze vasodilator nerve 

 endings so that the vasodilatation must have been 

 due to the contractile process. Muscles given atro- 

 pine, and then stimulated, vasodilate quite normally 

 in spite of cholinergic vasodilator nerve block (11, 

 106). Dogs are normally active after extirpation of 

 both sympathetic chains (53) and human beings with 

 sympathectomized limbs walk and cycle and take 

 all forms of normal exercise. Before lumbar sympa- 

 thectomy a policeman ran 380 yards in 65 and 61 

 sec; 99 days after sympathectomy he did it in 60 

 and 61.5 sec (H. Barcroft and J. S. Paddle, un- 

 published observation). Figure 16 shows that in a 

 sympathectomized forearm the blood flow rose 19 

 ml per 100 ml per min after clenching a bar hard for 1 

 min, in a normal forearm the blood flow rose only 

 18 ml (113). Such findings show that the sympathetic 

 vasodilators were not responsible for muscle vaso- 

 dilatation in any of these activities. 



However, there is no doubt that in exercise sympa- 

 thetic impulse discharge to skeletal muscular system 

 may alter. Blair el al. (41) recorded blood flow in 

 both forearms and harassed a subject to do his best 

 to exercise one of them in which voluntary movement 

 had been paralyzed by a curare-like substance. 

 The subject's strenuous efforts were accompanied 

 by vasodilatation in both his forearms. As this was 

 equal in the two sides, they concluded that the spe- 

 cific vasodilator fibers to a specific muscle group are 

 not activated during activation of the motor nerves 

 to the group in question. 



In other experiments the circulatory changes in 

 the forearm were recorded while subjects, who were 

 recumbent, performed bicycling exercises with their 

 legs. During these exercises arterial blood pressure 

 rose and forearm blood flow decreased so that vascu- 

 lar resistance in the forearm increased. This vaso- 

 constriction was still present when the cutaneous 

 nerves were blocked but it was absent after deep 

 nerve block. It was mediated by the sympathetic 

 fibers to muscle vessels. Since it was not affected by 

 previous atropinization of the forearm it must have 

 been due to activation of the vasoconstrictor fibers. 



There then is a paradox. The vasodilators were 

 activated when the subject tried hard to exercise his 

 paralyzed forearm, but it was the vasoconstrictors 

 that were activated during the bicycling experiments. 

 Can this be explained as follows? In the bicycling 

 experiment the vasoconstriction was probably a 

 manifestation of generalized vasoconstriction of the 

 resistance and capacitance vessels, involving the 

 splanchnic area too, and providing blood for the 

 large increase in output necessary to supply the active 

 legs. In this exercise the effect of activation of the 

 vasodilator fibers may have been overpowered by 

 much stronger activation of the constrictors. On the 

 other hand, when an emotionally stressed subject 

 begins exercise the combined actions of the vaso- 

 dilator nerves and the local factor would be expected 

 to cause more than usually rapid vasodilatation in 

 his active muscles. 



ACTION OF SYMPATHOMIMETIC SUBSTANCES 



Noradrenaline 



Given intra-arterially, in animals or man, nor- 

 adrenaline constricts muscle vessels in all effective 

 doses (27, 29, 55, 59, 94, 117, 178). Given intrave- 

 nously in animals its constrictor action may be over- 

 come by the rise in blood pressure; if this is prevented 

 (59) or obviated (55) the muscle vessels constrict. 

 In man, at the beginning of an intravenous noradren- 

 aline infusion, there may be a transient vasodilatation, 

 and after this the flow settles down at about the initial 

 rate for the rest of the infusion period; reflex vaso- 

 dilatation of sympathetic nervous origin usually 

 masks noradrenaline's direct constrictor action 

 (25, 178)- 



Adrenaline 



The literature contains numerous references to the 

 effect of adrenaline on the circulation in skeletal 



