Spatial heterogeneity in skeletal muscle microvascular blood flow distribution is increased in the metabolic syndrome

Abstract

Previous studies have demonstrated that the metabolic syndrome is associated with impaired skeletal muscle arteriolar function, although integrating observations into a conceptual framework for impaired perfusion in peripheral vascular disease (PVD) has been limited. This study builds on previous work to evaluate in situ arteriolar hemodynamics in cremaster muscle of obese Zucker rats (OZR) to integrate existing knowledge into a greater understanding of impaired skeletal muscle perfusion. In OZR cremaster muscle, perfusion distribution at microvascular bifurcations (γ) was consistently more heterogeneous than in controls. However, while consistent, the underlying mechanistic contributors were spatially divergent as altered adrenergic constriction was the major contributor to altered γ at proximal microvascular bifurcations, with a steady decay with distance, while endothelial dysfunction was a stronger contributor in distal bifurcations with no discernible role proximally. Using measured values of γ, we found that simulations predict that successive alterations to γ in OZR caused more heterogeneous perfusion distribution in distal arterioles than in controls, an effect that could only be rectified by combined adrenoreceptor blockade and improvements to endothelial dysfunction. Intravascular (125)I-labeled albumin tracer washout from in situ gastrocnemius muscle of OZR provided independent support for these observations, indicating increased perfusion heterogeneity that was corrected only by combined adrenoreceptor blockade and improved endothelial function. These results suggest that a defining element of PVD in the metabolic syndrome may be an altered γ at microvascular bifurcations, that its contributors are heterogeneous and spatially distinct, and that interventions to rectify this negative outcome must take a new conceptual framework into account.

Fig. 1.

Schematic representation of the in situ cremasteric arteriolar bifurcation used for assessing “parent” and “daughter” arteriolar mechanical and hemodynamic/perfusion responses to pharmacological challenge. Open arrows represent parent or daughter arteriolar diameter in response to a specific condition; solid arrows represent parent or daughter arteriolar erythrocyte velocity in response to a specific challenge. These data are utilized to determine both arteriolar flow volume and perfusion heterogeneity at bifurcations (γ). Please see text for additional detail.

Fig. 2.

Microvascular perfusion distribution (γ) at bifurcations spanning 1A (parent) and 2A (daughter) arterioles within in situ cremaster muscle. Data are presented as means ± SE for lean Zucker rats (LZR) under control conditions and in obese Zucker rats (OZR) under control conditions and following treatment with the adrenoreceptor antagonist phentolamine (P), the antioxidant TEMPOL (T), the PGH₂/TxA₂ receptor blocker SQ-29548 (SQ), the nitric oxide synthase inhibitor l-NAME (LN), or combinations of these agents. A: data describing the magnitude of γ at the 1A-2A bifurcation under the specific experimental conditions. B: % recovery (to the level determined in LZR) in γ at that bifurcation as a result of the imposed pharmacological challenge. Please see text for additional detail. A, *P < 0.05 vs. LZR; †P < 0.05 vs. OZR. B, *P < 0.05 vs. no change.

Fig. 3.

Microvascular perfusion distribution (γ) at bifurcations spanning 2A (parent) and 3A (daughter) arterioles within in situ cremaster muscle. Data are presented as means ± SE for LZR under control conditions and in OZR under control conditions and following treatment with the adrenoreceptor antagonist phentolamine (P), the antioxidant TEMPOL (T), the PGH₂/TxA₂ receptor blocker SQ-29548 (SQ), the nitric oxide synthase inhibitor l-NAME (LN), or combinations of these agents. A: data describing the magnitude of γ at the 2A-3A bifurcation under the specific experimental conditions. B: % recovery (to the level determined in LZR) in γ at that bifurcation as a result of the imposed pharmacological challenge. Please see text for additional detail. A, *P < 0.05 vs. LZR; †P < 0.05 vs. OZR. B, *P < 0.05 vs. no change.

Fig. 4.

Microvascular perfusion distribution (γ) at bifurcations spanning 3A (parent) and 4A (daughter) arterioles within in situ cremaster muscle. Data are presented as means ± SE for LZR under control conditions and in OZR under control conditions and following treatment with the adrenoreceptor antagonist phentolamine (P), the antioxidant TEMPOL (T), the PGH₂/TxA₂ receptor blocker SQ-29548 (SQ), the nitric oxide synthase inhibitor l-NAME (LN), or combinations of these agents. A: data describing the magnitude of γ at the 3A-4A bifurcation under the specific experimental conditions. B: % recovery (to the level determined in LZR) in γ at that bifurcation as a result of the imposed pharmacological challenge. Please see text for additional detail. A, *P < 0.05 vs. LZR; †P < 0.05 vs. OZR. B, *P < 0.05 vs. no change.

Fig. 5.

Microvascular perfusion distribution (γ) at bifurcations spanning 4A (parent) and 5A (daughter) arterioles within in situ cremaster muscle. Data are presented as means ± SE for LZR under control conditions and in OZR under control conditions and following treatment with the adrenoreceptor antagonist phentolamine (P), the antioxidant TEMPOL (T), the PGH₂/TxA₂ receptor blocker SQ-29548 (SQ), the nitric oxide synthase inhibitor l-NAME (LN), or combinations of these agents. A: data describing the magnitude of γ at the 4A-5A bifurcation under the specific experimental conditions. B: % recovery (to the level determined in LZR) in γ at that bifurcation as a result of the imposed pharmacological challenge. Please see text for additional detail. A, *P < 0.05 vs. LZR; †P < 0.05 vs. OZR. B, *P < 0.05 vs. no change.

Fig. 6.

Predicted perfusion distributions in the distal microcirculation of skeletal muscle of LZR and OZR under the conditions of the present study. Frequency distributions are calculated on the basis of an eight bifurcation network using the microvascular perfusion distribution coefficients (γ) determined in the in situ cremaster muscle presented in Figs. 2–5. Individual panels present the distribution of perfusion across 256 (2⁸) parallel arterioles under each experimental condition as a result of the simulation of a dichotomous branching network. Please see text for additional detail.

Fig. 7.

Data describing the washout of ¹²⁵I-labeled albumin from the in situ gastrocnemius muscle of LZR and OZR under the conditions of the present study. Data are presented as means ± SE for LZR (n = 10) under control conditions and OZR under control conditions (total n = 28) and following intravenous treatment with the adrenoreceptor antagonist phentolamine (P; n = 10), the antioxidant TEMPOL (T; n = 9), the PGH₂/TxA₂ receptor blocker SQ-29548 (SQ; n = 9), or for combinations of these agents (n = 8–10 for each).

Fig. 8.

Data (presented as means ± SE) describing the four moments of the washout of ¹²⁵I-labeled albumin from the in situ gastrocnemius muscle of LZR and OZR under the conditions of the present study. Data are shown for the mean transit time of the washout (A), the relative dispersion (RD) of the washout (B), the distribution skewness (C), and kurtosis (D). Please see text for details. *P < 0.05 vs. LZR. †P < 0.05 vs. OZR.