Regulation of capillary hemodynamics by KATP channels in resting skeletal muscle

Abstract

ATP-sensitive K⁺ channels (K_ATP ) have been implicated in the regulation of resting vascular smooth muscle membrane potential and tone. However, whether K_ATP channels modulate skeletal muscle microvascular hemodynamics at the capillary level (the primary site for blood-myocyte O₂ exchange) remains unknown. We tested the hypothesis that K_ATP channel inhibition would reduce the proportion of capillaries supporting continuous red blood cell (RBC) flow and impair RBC hemodynamics and distribution in perfused capillaries within resting skeletal muscle. RBC flux (f_RBC ), velocity (V_RBC ), and capillary tube hematocrit (Hct_cap ) were assessed via intravital microscopy of the rat spinotrapezius muscle (n = 6) under control (CON) and glibenclamide (GLI; K_ATP channel antagonist; 10 µM) superfusion conditions. There were no differences in mean arterial pressure (CON:120 ± 5, GLI:124 ± 5 mmHg; p > 0.05) or heart rate (CON:322 ± 32, GLI:337 ± 33 beats/min; p > 0.05) between conditions. The %RBC-flowing capillaries were not altered between conditions (CON:87 ± 2, GLI:85 ± 1%; p > 0.05). In RBC-perfused capillaries, GLI reduced f_RBC (CON:20.1 ± 1.8, GLI:14.6 ± 1.3 cells/s; p < 0.05) and V_RBC (CON:240 ± 17, GLI:182 ± 17 µm/s; p < 0.05) but not Hct_cap (CON:0.26 ± 0.01, GLI:0.26 ± 0.01; p > 0.05). The absence of GLI effects on the %RBC-flowing capillaries and Hct_cap indicates preserved muscle O₂ diffusing capacity (DO₂ m). In contrast, GLI lowered both f_RBC and V_RBC thus impairing perfusive microvascular O₂ transport (Q̇m) and lengthening RBC capillary transit times, respectively. Given the interdependence between diffusive and perfusive O₂ conductances (i.e., %O₂ extraction∝DO₂ m/Q̇m), such GLI alterations are expected to elevate muscle %O₂ extraction to sustain a given metabolic rate. These results support that K_ATP channels regulate capillary hemodynamics and, therefore, microvascular gas exchange in resting skeletal muscle.

Keywords: ATP-sensitive K+ channel; blood flow; intravital microscopy; microcirculation; red blood cell.

FIGURE 1

Percentage of capillaries supporting red blood cell (RBC) flow during control (CON; n = 6) and K_ATP channel inhibition (glibenclamide, GLI; n = 6) conditions. Dashed lines represent individual muscle data

FIGURE 2

Capillary red blood cell flux (top panel) and velocity (bottom panel) during control (CON; n = 6) and K_ATP channel inhibition (glibenclamide, GLI; n = 6) conditions. Dashed lines represent individual muscle data. *p < 0.05 vs. CON

FIGURE 3

Relative frequency histograms of capillary red blood cell flux (top panel) and velocity (bottom panel) during control (CON; n = 6) and K_ATP channel inhibition (glibenclamide, GLI; n = 6) conditions. Arrows show mean values. *p < 0.05 vs. CON

FIGURE 4

Individual and mean capillary relationships (top and bottom panels, respectively) between red blood cell flux and velocity during control (CON) and K_ATP channel inhibition (glibenclamide, GLI) conditions. Each data point represents a single capillary in the top panel (n = 30) and the average value for all capillaries within a single muscle (n = 6) in the bottom panel