Responsiveness of the Terminal Vascular Bed in Fast and Slow Skeletal Muscles to <i>α</i>-Adrenergic Stimulation

Abstract

In the light of known differences in bio-chemical, histological, and biophysical characteristics of fast and slow muscle, it was thought feasible to investigate possible differences in vascular properties. The present study has shown that vascular reactivity to α-adrenergic stimulation by both topical application of catecholamines (epinephrine and norepinephrine) and reflex activation of sympathetic nerves, is graded in both fast and slow muscles from precapillary sphincters which are the most sensitive, to small and then larger arterioles. In addition, the vessels of fast muscle are more sensitive to stimulation by those agents than are the vessels of slow muscle. The vascular bed in fast muscle is characterized by vasomotion (intermittency of flow rate and pattern) and many momentarily inactive capillaries (no moving red cells) in the resting state, whereas the flow in slow muscle capillaries is much more steady and uniform.

Limb oxygenation during the cold pressor test in spinal cord-injured humans

“…In vivo animal studies have shown that vasoconstrictor responses to sympathetic nerve activation or a-adrenoceptor stimulation are larger in quiescent fast-twitch muscles than in quiescent slow-twitch muscles [6,7,12]. The distribution of slow-twitch fibers/ fast-twitch fibers in able-bodied humans, calculated on the basis of data from various studies cited by Saltin et al [18], is approximately 50/50 in both BB and MG muscles.…”

Ontogenic Development of Vascular Reactivity in Fast and Slow Muscles of the Neonatal Rat Hindlimb

“…The adult pattern, described in a previous report [6], encompasses a range of sensitivities for arterioles in fast muscle which is several orders of magnitude greater than comparable vessels in slow muscle. In adult biceps femoris and medial gastrocnemius the smallest vessels (prccapillary) respond to threshold constrictor doses of 2x l()_5/tg/ml of norepinephirine, whereas small arterioles ( < 20 ¡nm dia meter and larger arterioles (20-40 t/m diameter) needed 2 x 10"4 and 2 x l 0 -*/fg/ml, respectively, to produce minimal effects.…”

“…The new-born rat has a mean arterial pressure of only about 14 mm Hg as compared with an adult pressure of 130 mm Hg. The various cardiovascular phenomena which are associated with one or the other type of muscle which have been studied so far, such as the high capillary density and high blood flow in slow muscle [3] and the greater flow response to exercise and adrenergic stimulation in the vasculature of fast muscle [4,5,6], may be only characteristic of the adult muscles. In the present experiments the relatively insensitive vascular smooth muscle cell in fast and slow muscle of the neonatal rat is another indication of the immaturity of the vasculature at birth.…”

“…On the other hand, fast, white muscles are usually phasically activated and they contain high concentrations of glycolytic enzymes: their anaerobic nature is further indicated by the low resting blood flow and sparse capillary supply. Recent investigations have indicated that the vascular smooth muscle cells in slow muscle are much less responsive to catecholamines and direct and reflex adrenergic sympathetic stimuli than are those of fast muscle [4,5,6). The capillary bed in resting white muscle is much more influenced by local vasomotor control (vasomotion) than that of red muscle which tends to have a steady, unvarying pattern of blood flow through all capillaries of the network [6].…”

“…Enzyme profiles and contraction speeds of these muscles differen tiated during the first two weeks after hatching but the differentiation of the capillary network and the occurrence of a high rate of blood flow in slow muscle began about the l()th day and was complete by the 28th day of life. In the light of these studies and the fact that a difference in the responses of the capillary beds of fast and slow muscles to catecholeamines had already been found [6], I decided to study the development of vas cular reactivity in neonatal rats, using the same techniques which I had used for study in adult animals.…”