KIR channel activation contributes to onset and steady-state exercise hyperemia in humans

Abstract

We tested the hypothesis that activation of inwardly rectifying potassium (KIR) channels and Na(+)-K(+)-ATPase, two pathways that lead to hyperpolarization of vascular cells, contributes to both the onset and steady-state hyperemic response to exercise. We also determined whether after inhibiting these pathways nitric oxide (NO) and prostaglandins (PGs) are involved in the hyperemic response. Forearm blood flow (FBF; Doppler ultrasound) was determined during rhythmic handgrip exercise at 10% maximal voluntary contraction for 5 min in the following conditions: control [saline; trial 1 (T1)]; with combined inhibition of KIR channels and Na(+)-K(+)-ATPase alone [via barium chloride (BaCl2) and ouabain, respectively; trial 2 (T2)]; and with additional combined nitric oxide synthase (N(G)-monomethyl-l-arginine) and cyclooxygenase inhibition [ketorolac; trial 3 (T3)]. In T2, the total hyperemic responses were attenuated ~50% from control (P < 0.05) at exercise onset, and there was minimal further effect in T3 (protocol 1; n = 11). In protocol 2 (n = 8), steady-state FBF was significantly reduced during T2 vs. T1 (133 ± 15 vs. 167 ± 17 ml/min; Δ from control: -20 ± 3%; P < 0.05) and further reduced during T3 (120 ± 15 ml/min; -29 ± 3%; P < 0.05 vs. T2). In protocol 3 (n = 8), BaCl2 alone reduced FBF during onset (~50%) and steady-state exercise (~30%) as observed in protocols 1 and 2, respectively, and addition of ouabain had no further impact. Our data implicate activation of KIR channels as a novel contributing pathway to exercise hyperemia in humans.

Keywords: blood flow; contraction; skeletal muscle; vasodilation.

Fig. 1.

Experimental timeline. Three trials were performed in all protocols and trial 1 (T1) always consisted of saline infusion as a control condition. In protocols 1 (A) and 2 (B), trial 2 (T2) consisted of the combined infusion of barium chloride [BaCl₂; inwardly rectifying potassium (K_IR) channel inhibitor] and ouabain (Na⁺-K⁺-ATPase inhibitor) and trial 3 (T3) consisted of the combined infusion of BaCl₂, ouabain, N^G-monomethyl-l-arginine (l-NMMA; to inhibit nitric oxide synthesis) and ketorolac (to inhibit prostaglandin synthesis). During the loading period of drug infusions at rest, 3 single contractions were performed to facilitate drug delivery. In protocol 1, to avoid previously observed fluctuations in baseline hemodynamics, drugs were stopped before resting measures were made and exercise commenced. Measurements were made at rest, at the onset of 10% maximal voluntary contraction (MVC) rhythmic handgrip exercise, and throughout the 5-min bout of exercise. In protocol 2, drugs were continuously coinfused throughout rest and the entire handgrip exercise trial. C: in protocol 3, drugs were also continuously infused and to determine the independent effect of K_IR channel inhibition in trial 2, BaCl₂ alone was infused and in trial 3, combined BaCl₂ and ouabain were administered.

Fig. 2.

Protocol 1: effect of inhibition of hyperpolarizing and endothelium-dependent vasodilator mechanisms on the onset of exercise hyperemia. A: forearm blood flow (FBF) across the 3 experimental conditions is shown at rest and during 10% MVC rhythmic handgrip exercise. Data are in 3-s bins corresponding to a contraction:relaxation cycle during exercise. B: statistical analysis was performed on 15-s averages of blood flow data (5 3-s bins). BaCl₂ + ouabain significantly reduced the onset of exercise hyperemia, and l-NMMA + ketorolac had an additional effect within the first 15 s of exercise. *P < 0.05 vs. control; †P < 0.05 vs. BaCl₂ + ouabain; n = 11 subjects.

Fig. 3.

Protocol 2: effect of inhibition of hyperpolarizing and endothelium-dependent vasodilator mechanisms on steady-state exercise hyperemia. In protocol 2, with continuous infusion of the inhibitors, a significant reduction in FBF (A) and forearm vascular conductance (FVC; B) was observed with combined BaCl₂ + ouabain infusion. Here, the addition of l-NMMA + ketorolac to BaCl₂ + ouabain further reduced both steady-state FBF and FVC. RHG Ex, rhythmic handgrip exercise. *P < 0.05 vs. control; †P < 0.05 vs. BaCl₂ + ouabain; n = 8 subjects.

Fig. 4.

Protocol 3: effect of independent inhibition of K_IR channels on exercise hyperemia. A: FBF across the 3 experimental conditions is shown at rest and during 10% MVC rhythmic handgrip exercise. Data are in 3-s bins corresponding to a contraction:relaxation cycle during exercise. B: statistical analysis was performed on 15-s averages of blood flow data (5 3-s bins). In all inhibited conditions, exercise hyperemia was lower from 30 to 300 s. *P < 0.05 vs. BaCl₂; vs. BaCl₂ + ouabain. After 15 s, there was no statistically significant reduction with infusion of BaCl₂ + ouabain. †P < 0.05 vs. BaCl₂; n = 8 subjects.