Muscle contraction, but not insulin, increases microvascular blood volume in the presence of free fatty acid-induced insulin resistance

Abstract

Objective: Insulin and contraction each increase muscle microvascular blood volume (MBV) and glucose uptake. Inhibiting nitric oxide synthase blocks insulin's but not contraction's effects. We examined whether contraction could augment the MBV increase seen with physiologic hyperinsulinemia and whether free fatty acid (FFA)-induced insulin resistance differentially affects contraction- versus insulin-mediated increases in MBV.

Research design and methods: Rats were fasted overnight. Plasma FFAs were increased by intralipid/heparin infusion (3 h), insulin was increased with a euglycemic clamp (3 mU x min(-1) x kg(-1)), and hindlimb muscle contraction was electrically stimulated. Muscle MBV was measured using contrast-enhanced ultrasound. Insulin transport into muscle was measured using (125)I-insulin. BQ-123 (0.4 mg/h) was used to block the endothelin-1 (ET-1) receptor A.

Results: Superimposing contraction on physiologic hyperinsulinemia increased MBV within 10 min by 37 and 67% for 0.1 or 1 Hz, respectively (P < 0.01). FFA elevation alone did not affect MBV, whereas 0.1 Hz stimulation doubled MBV (P < 0.05) and increased muscle insulin uptake (P < 0.05) despite high FFA. Physiologic hyperinsulinemia during FFA elevation paradoxically decreased MBV (P < 0.05). This MBV decrease was reversed by either 0.1 Hz contraction or ET-1 receptor A antagonism, and the combination raised MBV above basal.

Conclusions: Contraction recruits microvasculature beyond that seen with physiologic hyperinsulinemia by a distinct mechanism that is not blocked by FFA-induced vascular insulin resistance. The paradoxical MBV decline seen with insulin plus FFA may result from differential inhibition of insulin-stimulated nitric oxide-dependent vasodilation relative to ET-1 vasoconstriction. Our results implicate ET-1 as a potential mediator of FFA-induced vascular insulin resistance.

FIG. 1.

Lipid and insulin infusion protocols. Time points for microbubble infusion and data acquisition are denoted by (*) for contrast- enhanced ultrasound. Blood samples (20 μl) were taken at t = 0, 30, 60, 120, and 180 min.

FIG. 2.

Combined effect of insulin and contraction on MBV. Protocol 1 results (n = 7): Effect of insulin (3 mU · kg⁻¹ · min⁻¹ × 90 min) and isometric hindlimb contraction (0.1, 1.0, 2.0 Hz; 10 min/bout) on MBV. *P < 0.05 vs. baseline (white bar); **P < 0.05 vs. baseline, insulin 60 min (light gray); #P < 0.05 vs. baseline, insulin 60 min and 0.1 Hz (dark gray); one-way repeated-measures ANOVA.

FIG. 3.

Effect of contraction on AMPKα phosphorylation and glucose uptake. A: Effect of isometric contraction on fasting basal muscle AMPKα phosphorylation (Thr¹⁷²). *P < 0.05 vs. 0.1 Hz (10 min), 0.1 Hz (45 min); one-way ANOVA (n = 5). B: Protocol 2 results: Combined effect of insulin and brief, low-frequency contraction on muscle glucose utilization (n = 6). A ³H-2DG bolus (50 μCi) was given right before the onset of contraction and circulated a total of 10 min. *P < 0.05 vs. basal conditions, #P < 0.05 vs. insulin + rest, paired t test.

FIG. 4.

Effect of FFAs and contraction on MBV and FBF. A: Protocol 3 results: The effect of FFA-provoked insulin resistance on MBV after 1 h, 3 h, and 3 h + 10 min 0.1 Hz isometric hindlimb muscle contraction (n = 6). *P < 0.05 vs. FFAs 1 h, FFAs 3 h; one-way repeated-measures ANOVA. B: Effects of treatment on FBF; protocols 3–5 (n = 4 per group). *P < 0.05; one-way repeated-measures ANOVA (vs. baseline).

FIG. 5.

Effect of FFAs on contraction-mediated muscle ¹²⁵I-insulin uptake. An intravenous 1.5 μCi bolus of ¹²⁵I-insulin was given 5 min after starting low-frequency (0.1 Hz) isometric contraction (protocol 2). Intact insulin in wet tissue was determined using trichloroaetic acid precipitation and then normalized to dry tissue weight (n = 7). *P < 0.05.

FIG. 6.

Effect of FFA microvascular responses to insulin and contraction. Intralipid/heparin (FFAs) and insulin were infused as described in experimental protocol 4. In the presence of elevated FFAs, insulin had a paradoxical effect to decrease MBV vs. baseline. Brief low-frequency contraction (0.1 Hz, 10 min) restored MBV to baseline (n = 9). *P < 0.05 vs. all other treatment groups; one-way repeated-measures ANOVA.

FIG. 7.

Combined ET_A antagonism and FFA elevation on the microvascular response to insulin and contraction. The ET_A receptor anagonist, BQ-123 (0.4 mg/h), was infused with intralipid/heparin (FFAs) as described in protocol 5. The physiologic euglycemic-hyperinsulinemic clamp (3 mU · kg⁻¹ · min⁻¹ × 90 min) started 1 h after FFA/BQ infusion. Contraction (0.1 Hz) was superimposed over the last 10 min. MBV was measured at the indicated time points (n = 6). *P < 0.05 vs. all other groups; one-way repeated-measures ANOVA.