The development of peripheral microvasculopathy with chronic metabolic disease in obese Zucker rats: a retrograde emergence?

Abstract

The study of peripheral vasculopathy with chronic metabolic disease is challenged by divergent contributions from spatial (the level of resolution or specific tissue being studied) and temporal origins (evolution of the developing impairments in time). Over many years of studying the development of skeletal muscle vasculopathy and its functional implications, we may be at the point of presenting an integrated conceptual model that addresses these challenges within the obese Zucker rat (OZR) model. At the early stages of metabolic disease, where systemic markers of elevated cardiovascular disease risk are present, the only evidence of vascular dysfunction is at postcapillary and collecting venules, where leukocyte adhesion/rolling is elevated with impaired venular endothelial function. As metabolic disease severity and duration increases, reduced microvessel density becomes evident as well as increased variability in microvascular hematocrit. Subsequently, hemodynamic impairments to distal arteriolar networks emerge, manifesting as increasing perfusion heterogeneity and impaired arteriolar reactivity. This retrograde "wave of dysfunction" continues, creating a condition wherein deficiencies to the distal arteriolar, capillary, and venular microcirculation stabilize and impairments to proximal arteriolar reactivity, wall mechanics, and perfusion distribution evolve. This proximal arteriolar dysfunction parallels increasing failure in fatigue resistance, hyperemic responses, and O₂ uptake within self-perfused skeletal muscle. Taken together, these results present a conceptual model for the retrograde development of peripheral vasculopathy with chronic metabolic disease and provide insight into the timing and targeting of interventional strategies to improve health outcomes.NEW & NOTEWORTHY Working from an established database spanning multiple scales and times, we studied progression of peripheral microvascular dysfunction in chronic metabolic disease. The data implicate the postcapillary venular endothelium as the initiating site for vasculopathy. Indicators of dysfunction, spanning network structures, hemodynamics, vascular reactivity, and perfusion progress in an insidious retrograde manner to present as functional impairments to muscle blood flow and performance much later. The silent vasculopathy progression may provide insight into clinical treatment challenges.

Keywords: microcirculation; microvascular systems; peripheral vascular disease; rat models of metabolic syndrome; skeletal muscle fatigue.

Figure 1.

Schematic representation of the flow of experiments, ages of the animals used, the major divisions in vascular structure/function markers, and the higher resolution parameters that were collected for each marker group. See text for additional details. Figure created using Biorender and published with permission. CVD, cardiovascular disease; MAP, mean arterial pressure; NO, nitric oxide; ROS, reactive oxygen species.

Figure 2.

Data describing the changes in estimated aggregate CVD risk for LZR and OZR from 7–20 wk of age. Data are presented as means ± SE; n = 10–13 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. CVD, cardiovascular disease; LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 3.

Data describing the changes in vascular metabolite markers for LZR and OZR from 7–20 wk of age. Results are summarized for PGI₂ (A), TxA₂ (B), and NO (C) bioavailability. Data are presented as means ± SE; n = 8–11 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. LZR, lean Zucker rat; NO, nitric oxide; OZR, obese Zucker rat; PGI₂, prostacyclin; TxA₂, thromboxane A₂.

Figure 4.

Data describing the changes in venular function markers for LZR and OZR from 7–20 wk of age. Results are summarized for adherent leukocytes (A), rolling leukocytes (B), and for the upper bound of acetylcholine-induced dilation for postcapillary venules (C) and collecting venules (D). Data are presented as means ± SE; n = 5–8 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 5.

Data describing the changes in microvascular density markers for LZR and OZR from 7–20 wk of age. Results are summarized for venular (A), net (B), and arteriolar (C) microvessel density within the gastrocnemius muscle. Data are presented as means ± SE; n = 10–12 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 6.

Data describing the changes in hemodynamic markers for LZR and OZR from 7–20 wk of age. Results are summarized for capillary tube hematocrit (A), mean microvascular hematocrit (B), and for the perfusion distribution (γ) at distal (3A–4A bifurcations; C) and proximal (1A–2A bifurcations; D). Data are presented as means ± SE; n = 6–9 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 7.

Data describing the changes in arteriolar reactivity markers for LZR and OZR from 7–20 wk of age. Results are summarized for the upper bound of the acetylcholine-induced dilation of distal (in situ; A) and proximal (ex vivo; B) resistance arterioles of skeletal muscle. Data are presented as means ± SE; n = 12–14 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 8.

Data describing the changes in vascular wall mechanics markers for LZR and OZR from 7–20 wk of age. Results are summarized for the slope (β) coefficient from the circumferential stress vs. strain relation for proximal resistance arterioles of LZR and OZR. Data are presented as means ± SE; n = 12–14 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 9.

Data describing the changes in functional outcomes markers for LZR and OZR from 7–20 wk of age. Results are summarized for muscle fatigue (A), active hyperemia (B), and muscle oxygen uptake (V̇o₂; C) at 3 min of 5-Hz isometric twitch muscle contractions. Data are presented as means ± SE; n = 5–8 for both LZR and OZR. *P < 0.05 vs. LZR. See text for details. LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 10.

Data describing the temporal development of specific indices within the 8 groups of markers of vasculopathy in the present study. Data are presented for systemic markers (A), metabolite and venular markers (B), capillary and hemodynamics markers (C), mechanics and arteriolar markers (D), and functional markers (E). Data are means ± SE, presented as percentage of the maximum dysfunction at each time point, defined as the difference between LZR and OZR at 20-wk time point. See text for details. LZR, lean Zucker rat; OZR, obese Zucker rat.

Figure 11.

Schematic representation of blood-tissue exchange (BTEX) units in skeletal muscle. Under normal conditions (top), the progressive deoxygenation of erythrocytes passing through capillaries is sufficient to supply the mitochondria within the radius of the conceptual Krogh cylinder in skeletal muscle. With chronic metabolic disease (bottom), the myriad impairments to the microcirculation create a condition where the combination of altered reactivity, hemodynamics, and microvessel rarefaction impair oxygen penetrance into the enlarged Krogh cylinder (presented in blue). This creates increasing numbers of mitochondria that are in hypoxic environments and can serve as a limiting factor for skeletal muscle function. R_t represents tissue cylinder radius, CD represents capillary density. Figure created using Biorender and published with permission.

Figure 12.

A summary schematic integration of the results of the present article. Moving from the younger, healthier state, the obese Zucker rat steadily moved from initial states of elevated CVD risk, altered vasoactive metabolite production and venular dysfunction to a loss in microvessel density and altered perfusion hemodynamics with worsening CVD risk. This is followed by impairments to arteriolar reactivity that are tightly coupled with attenuated functional outcome, i.e., poor muscle performance, decreased fatigue resistance, and blood flow. Finally, a remodeling of the arteriolar wall develops with the emergence of hypertension and serves to exacerbate the impairment to the preexisting functional outcomes. CVD, cardiovascular risk.