Protective role of sympathetic nerve activity to exercising skeletal muscle in the regulation of capillary pressure and fluid filtration.
This study describes the integrated sympathetic/metabolic control of capillary pressure (Pc) and filtration in cat skeletal muscle as studied during graded exercise and superimposed graded (2, 6 and 16 Hz) vasoconstrictor nerve excitation. The applied technique permitted simultaneous analysis of the underlying changes of resistance in the whole vascular bed (RT) and in its large-bore arterial resistance vessels (greater than 25 microns), small arterioles (less than 25 microns) and veins. Graded exercise per se caused graded increases in capillary pressure, which at heavy work exceeded the resting control value by 12.2 mmHg, in turn leading to marked loss of plasma fluid by filtration. Sympathetic nerve stimulation was much more efficient in lowering capillary pressure during exercise than at rest, in spite of an exercise-induced marked attenuation of the vasoconstrictor response (RT). The sympathetically evoked capillary pressure fall per unit resistance increase was larger the greater the degree of exercise vasodilation, implying a highly nonlinear relation between capillary pressure and RT and also between the more direct determinant of capillary pressure the post- to precapillary resistance ratio, and RT. Strenuous exercise in vivo is known to be associated with a markedly increased reflex sympathetic discharge to exercising muscle which has been a puzzling feature in view of its untoward restriction of the exercise hyperaemia response. To the extent the present results are representative for this in vivo situation, they suggest that sympathetic discharge to exercising muscle, in spite of some flow restricting effect, might serve a highly beneficial function, causing effective protection against excessive work-induced rise of capillary pressure and harmful plasma fluid loss into the extravascular space of working muscle.
Maspers
M
M
Department of Physiology and Biophysics, University of Lund, Sweden.
Ekelund
U
U
Björnberg
J
J
Mellander
S
S
D000818:Animals; D001794:Blood Pressure / Q000502:physiology; D002196:Capillaries / Q000378:metabolism / Q000502:physiology; D002415:Cats; D009132:Muscles / Q000098:blood supply / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology*; D010949:Plasma / Q000378:metabolism; D013564:Sympathetic Nervous System / Q000502:physiology*; D014655:Vascular Resistance / Q000502:physiology