Interaction between sympathetic nerve activation and muscle fibre contraction in resistance vessels of hamster retractor muscle

Abstract

The interaction between skeletal muscle contraction and sympathetic nerve activation (SNA) on blood flow during exercise has remained ambiguous due to indirect estimates of vasomotor control. In the hamster retractor muscle (n=54), interactions between three levels of SNA (approximately 3, 6 and 12 Hz) and of contractile activity (2.5, 10 and 20 % duty cycle) were studied in feed arteries (FA) and first- (1A), second- (2A), and third-order (3A) arterioles using intravital microscopy. During functional dilatation with rhythmic muscle contractions, sympathetic vasoconstriction was sustained in FA and 1A but impaired in 2A and 3A (P<0.05), where vessels 'escaped' from responding to SNA. To account for changes in baseline diameter and blood flow during contractions, vasodilatation was induced passively (2-3 levels) in resting muscles with papaverine or sodium nitroprusside. Compared to functional dilatation, the range of passive dilatation was similar in 3A and progressively greater in 2A, 1A and FA. With passive dilatation, SNA responses were sustained in 2A and increased with baseline diameter in 3A. Blood flow through FA (rest, approximately 20 nl s(-1)) increased approximately 5-fold during contractile activity and approximately 10-fold during passive dilatation. Absolute flow reductions (nl s(-1)) with SNA increased during contractile activity and during passive dilatation; relative flow reductions were impaired during functional dilatation (P<0.05) and remained constant during passive dilatation. Thus, SNA can restrict blood flow to exercising muscle by constricting FA and 1A while dilatation prevails in 2A and 3A. Such concerted interaction will promote oxygen extraction when blood flow is restricted to maintain arterial pressure.

Figure 1. Hamster retractor muscle preparation for intravital microscopy

A (top view), the muscle is reflected away from the anaesthetised hamster, immersed in a chamber (10 ml volume) integral to the acrylic platform, and secured in clamps connected to micrometer spindles at each end to control muscle length. Fresh bicarbonate-buffered physiological salt solution (PSS; pH 7.4, 35 °C) is introduced at the proximal end (10 ml min⁻¹) and aspirated at the distal end to maintain constant fluid level with superfusion along the muscle. B (side view), the PSS is introduced through ports positioned above and below the muscle to minimise unstirred layers. A load beam (LCL-1136, Omega; Stamford, CT, USA; resolution: ±0.1 g) is mounted in series with the muscle records tension. C (enlarged top view), platinum electrodes are positioned (within the chamber shown in A) along both sides of the muscle for activating skeletal muscle fibres through field stimulation. Perivascular sympathetic nerve activation is via a nerve cuff positioned around a FA. Two FA are illustrated, each giving rise to first-, second- and third-order arterioles (1A, 2A and 3A, respectively); associated veins, venules and capillaries omitted for clarity.

Figure 2. Sympathetic vasoconstriction in resting skeletal muscle

A, representative diameter recordings during intermediate SNA for 30 s (period indicated by dark horizontal bar) in FA, 1A, 2A and 3A. B, summary data. Vessel branch orders are indicated across horizontal axis, each with respective levels of SNA (L, low; I, intermediate; H, high). Diameter change (ΔD), %ΔD and diameter × time integrals (during SNA, DTI_s; total, DTI_t; see A) are indicated along respective vertical axes for each branch order. Values for n and diameter of vessels are given in the first paragraph of Results.

Figure 4. Differential effects of passive vasodilatation on sympathetic vasoconstriction in proximal and distal vessels

Representative diameter recordings in response to intermediate SNA. Baseline diameter (D_o) was increased (vertical arrow) with papaverine. A, in FA, constriction at the end of SNA was similar at rest (bottom trace) and during submaximum levels of dilatation yet diminished during maximum dilatation (top trace). B, in 3A, constriction at the end of SNA increased with D_o across levels of dilatation. Note difference in ordinate scales for FA and 3A.

Figure 3. Differential effects of muscle contractions on sympathetic vasoconstriction in FA and 3A

Representative traces for diameter (D_o, baseline; D_m, minimum during SNA; D_e, at end of SNA; notations on respective tracings apply to all) and tension development before, during and after intermediate SNA at rest and during rhythmic muscle contractions of 2.5, 10 and 20 % DC. A, constriction of FA in response to SNA during functional vasodilatation at 2.5 % DC (bottom record) was similar to that at rest and increased with DC (middle and top records). B, constriction of 3A in response to SNA during functional vasodilatation decreased as DC increased. During 10 % DC, vasoconstriction subsided during SNA (i.e. ‘escape’ occurred, with D_m < D_e). At 20 % DC, vasoconstriction during SNA was completely inhibited. Maximum diameters: FA, 105 μm; 3A, 22 μm. Note consistency of SNA responses at rest.

Figure 5. Sympathetic vasoconstriction during muscle contractions and during passive vasodilatation

Horizontal axes indicate baseline diameters from which SNA was initiated in respective branch orders. Vertical axes indicate magnitude of vasoconstriction (ΔD) at the end of SNA. A, muscle contractions. Baseline diameter recorded during rest (symbols at left in each panel); 2.5, 10 and 20 % DC increase to the right in each panel. Data for arterioles are from same vessels as in Fig. 2B. Data for FA are pooled for vessels in Figs 2 and 6. B, passive vasodilatation. Baseline diameter recorded during spontaneous tone (symbols at left in each panel) and progressive dilatation increases to the right for each branch order (n = 7 FA; n = 5 each for 1A, 2A and 3A). Error bars (s.e.m.) are representative; those not shown are omitted for clarity. * Enhanced response compared to response at initial resting diameter, P < 0.05. # Impaired response compared to response at initial resting diameter, P < 0.05.

Figure 6. Blood flow responses to SNA in feed arteries during muscle contractions and during passive vasodilatation

Horizontal axes indicate baseline blood flow from which SNA was initiated (Q_o). From each baseline, absolute changes in blood flow (ΔQ) at the end of SNA are indicated along the vertical axes of the left panels; relative changes in blood flow (%ΔQ) are indicated along the vertical axes of the right panels. Error bars are as in Fig. 5. A, muscle contractions. Summary data (n = 8) at rest (symbols at left in each panel) and during contractions (10 and 20 % DC increase to the right in each panel). Maximum FA diameter and blood flow (with 10 μM SNP): 90 ± 5 μm and 137 ± 22 nl s⁻¹, respectively. Vasomotor responses of these FA are shown in Fig. 5A. B, passive vasodilatation. Summary data (n = 5) from FA with spontaneous resting tone (symbols at left in each panel) and during 3 levels of dilatation (incrementing to maximum). Vasomotor responses of these FA are shown in Fig. 5B. The greater resting and maximum diameters (102 ± 3 μm) of FA in B compared to A account for, respectively, the greater resting and maximum Q_o values. *Enhanced response compared to response with spontaneous resting flow, P < 0.05. # Impaired response compared to response with spontaneous resting flow, P < 0.05.

Figure 7. Responses of V_rbc and WSR to SNA in feed arteries during muscle contractions and during passive vasodilatation

Horizontal axes indicate baseline values (V_o and WSR_o) from which SNA was initiated. Changes in V_rbc (ΔV_rbc) and WSR (ΔWSR) at the end of SNA are shown by the symbols below respective baseline values. Error bars are as in Fig. 5. A, muscle contractions. Summary data (from same vessels as Fig. 6A) at rest with spontaneous vasomotor tone (symbols at left in each panel) and during contractions (10 and 20 % DC increase to the right in each panel). Maximum V_rbc and WSR (with 10 μM SNP): 32 ± 3 mm s⁻¹ and 1842 ± 151 s⁻¹, respectively. B, passive vasodilatation. Summary data (from same vessels as in Fig. 6B) are from FA with spontaneous tone (symbols at left in each panel) and during 3 levels of dilatation (incrementing to maximum). *Enhanced response compared to response with spontaneous resting tone, P < 0.05.