Nervous and Local Chemical Control of Pre‐Capillary Sphincters in Skeletal Muscle as Measured by Changes in Filtration Coefficient

Abstract

The influence of nervous and local chemical factors on the pre‐capillary sphincter section of the muscle vascular bed has been studied in terms of changes in the filtration coefficient (CFC). The results suggest that the vasoconstrictor fibres, beside their powerful influence on the resistance and capacitance vessels, affect also the pre‐capillary sphincters, which determine the number of capillaries open to flow. However, the influence exerted by the centrally controlled vasoconstrictor fibres on the pre‐capillary sphincters in the skeletal muscles is rapidly overcome by ‘vasodilator metabolites’, accumulating when flow is reduced or metabolism is increased. Such local metabolic factors exert an especially powerful control of the sphincter section as compared with the neurogenic influence. Thus CFC in the skeletal muscles tends to rise at any decrease of blood flow/ tissue metabolism ratio e. g. from ‘resting’ values around 0.015 up to 0.04—0.05, during muscular exercise; an increase also occurs when flow is reduced. Flow reduction following mechanical arterial obstruction induces a relatively greater CFC increase than when flow is reduced by the constrictor fibres, which unmasks their weak but still significant influence on the pre‐capillary sphincter section.

Evidence for the shift of blood to the nutritive circulation during reactive hyperaemia

“…All the discoloured tissues were carefully dissected away and weighed to give values of 61*3, 60-5, 60-7, and 59*4 g. On the assumption that in the usual case 60 g of tissue were perfused, the actual blood flow during the controlled period was about 7-8 ml./100 g of muscle tissue/min. This value agrees with other measurements of blood flow through the perfused muscle of an anaesthetized cat (Cobbold, Folkow, Kjellmer & Mellander, 1963 Fig. 4.…”

“…Parallel studies by Renkin & Rosell (1962a) on the isolated perfused limbs of dogs and cats indicated that the vasodilator fibres have little or no effect on the blood-to-tissue transport of 86Rb+ even though their activity reduces peripheral resistance. Profound vasodilatation of the arterioles and relatively negligible effect on pre-capillary sphincters following section of sympathetic vasoconstrictor fibres has been demonstrated by Renkin & Rosell (1962b) and Cobbold et al (1963). Although Rapaport, Saul, Hyman & Morton (1952) found no increase in clearance of 24Na+ and 1311-from muscle after lumbar sympathetic block in man, direct stimulation of the sympathetic chain in atropinized cats (Hyman et (Cobbold et al 1963).…”

The influence of the sympathetic nervous system on individual vessels of the microcirculation of skeletal muscle of the rat

“…Using these assumptions he came to the conclusion that sympathetic stimulation produced a strong and well maintained constriction of the arterial vessels and a weaker, but nevertheless substantial, constriction of all the venous vessels. Later, capillary filtration coefficients, calculated from the rate of change of tissue volume after a known rise in venous pressure, were used to follow the behaviour of the highly specialized pre-and post-capillary sections said to be involved in the fine regulation of capillary flow and pressure (Cobbold, Folkow, Kjellmer & Mellander, 1963). The results of these experiments led to the suggestion that terminal arterioles and precapillary sphincters were initially constricted by increased sympathetic activity but that in spite of maintained sympathetic stimulation they then relaxed, perhaps under the influence of vasodilator metabolites.…”

Effect of cigarette smoking on human precapillary sphincters

“…On the other hand, little attention has been paid to the possible effect of smoking on precapillary sphincters. If human precapillary sphincters are innervated by sympathetic vasoconstrictor nerves, as is the case in the cat and dog (Folkow & Mellander, 1960;Renkin & Rosell, 1962a, b;Cobbold, Folkow, Kjellmer & Mellander, 1963), then cigarette smoking may be supposed to induce the sphincter constriction. However, as previous studies showed that precapillary sphincters in human limbs are insensitive to sympathectomy (Kitchin, 1955) or sympathetic blockade (Rapaport, Saul, Hyman & Morton, 1952) suggesting lack of innervation, we cannot predict whether the actual constriction of sphincters occurs during smoking.…”

“…As a measure of the state of precapillary sphincter tonus, the capillary filtration coefficient (CFC) which is the product of the 'functional capillary surface area' and the filtration constant of capillary wall has been used (Cobbold et al, 1963;Landis & Pappenheimer, 1964 example, lessens the capillary filtration coefficient by decreasing the number of functioning capillaries. Capillary filtration coefficient of human limbs can be determined by the pressure plethysmograph (Krogh, Landis & Turner, 1932;Landis & Gibbon, 1933) and this was used in the present study to observe the effect of smoking on precapillary sphincters.…”