Local inhibition of nitric oxide and prostaglandins independently reduces forearm exercise hyperaemia in humans

Abstract

We tested the hypothesis that inhibition of synthesis of either nitric oxide (NO) or vasodilating prostaglandins (PGs) would not alter exercise hyperaemia significantly, but combined inhibition would synergistically reduce the hyperaemia. Fourteen subjects performed 20 min of moderate rhythmic forearm exercise (10% maximal voluntary contraction). Forearm blood flow (FBF) was measured by Doppler ultrasound. Saline or study drugs were infused (2 ml x min(-1)) into the forearm via a brachial artery catheter to locally inhibit synthesis of NO and PGs during steady state exercise (N(G)-nitro-L-arginine methyl ester (L-NAME), 25 mg over 5 min to inhibit NO synthase (NOS); and ketorolac, 3 mg over 5 min to inhibit cyclooxygenase (COX)). After achieving steady state exercise over 5 min (control), L-NAME was infused for 5 min, followed by 2 min saline, then by a 5 min infusion of ketorolac, and finally by 3 min of saline (n= 7). Drug order was reversed in seven additional subjects, such that single inhibition of NOS or COX was followed by combined inhibition. FBF during exercise decreased to 83 +/- 2% of control exercise (100%) with NOS inhibition, followed by a transient decrease to 68 +/- 2% of control during COX inhibition. However, FBF returned to levels similar to those achieved during NOS inhibition within 2 min (80 +/- 3% of control) and remained stable through the final 3 min of exercise. When COX inhibition was performed first, FBF decreased transiently to 88 +/- 4% of control (P < 0.01), and returned to control saline levels by the end of ketorolac infusion. Addition of L-NAME reduced FBF to 83 +/- 3% of control, and it remained stable through to the end of exercise. Regardless of drug order, FBF was approximately 80% of steady state control exercise (P < 0.01) during the last 30 s of exercise. We conclude that (1). NO provides a significant, consistent contribution to hyperaemia, (2). PGs contribute modestly and transiently, suggesting a redundant signal compensates for the loss of vasodilating PGs, and (3). NO and PG signals appear to contribute independently to forearm exercise hyperaemia.

Figure 1. Experiment timeline

A, 30 min after catheterization, baseline measurements were taken for 2 min with saline infusion. Subjects performed rhythmic forearm handgrip exercise for 20 min, with either saline or drug continuously infused (2 ml min⁻¹). At 5 min of exercise, l-NAME was infused for 5 min, followed by 2 min saline, then 5 min ketorolac, then 3 min saline, and finally 5 min recovery with saline. The drug order was reversed in 7 of 14 subjects. Arrows indicate approximate times that 30 s average data were collected for analysis, or the approximate time that nadir FBF were taken within l-NAME or ketorolac infusion; the timing of nadir varied between subjects (see Results). B, in 8 of the 14 subjects, l-NAME (2.5 mg ml⁻¹) and ketorolac (300 μg ml⁻¹) were infused at 1 ml min⁻¹ each for 20 min after the first 20 min exercise bout. After this 30 min rest period, subjects performed 5 min forearm exercise at the same workload with concurrent infusion of l-NAME and ketorolac (both 1 ml min⁻¹). FBF was measured for 2 min at rest, during exercise, and at 5 min recovery.

Figure 2. Effects of NOS followed by COX inhibition

A, an individual FBF tracing. Vertical lines indicate start and end of exercise. Open bars with boxes indicate infusion of l-NAME followed by ketorolac. B, mean absolute FBF data (+s.e.m.). l-NAME reduced FBF below control levels, although the subsequent decrease in FBF with ketorolac was not significantly lower than with l-NAME (P = 0.11). C, mean normalized FBF data (+s.e.m.). FBF data are normalized such that no FBF is defined as 0%, and steady state exercise FBF is defined as 100%. After reaching steady state exercise, l-NAME reduced FBF by ∼20%, and ketorolac transiently reduced FBF by 32%, returning to l-NAME levels by the end of ketorolac infusion. ‡Value is different from baseline (P < 0.05); *P < 0.05 compared to control exercise; #value is different than all other time points (P < 0.05).

Figure 3. Effects of COX followed by NOS inhibition

A, an individual FBF tracing. Vertical lines indicate start and end of exercise. Open bars with boxes indicate infusion of ketorolac followed by l-NAME. B, mean absolute FBF data (+s.e.m.). The decrease in FBF with ketorolac was not significantly lower than control with l-NAME (P > 0.05), but l-NAME reduced FBF by ∼20%. C, mean normalized FBF data (+s.e.m.) FBF data are normalized such that no FBF is defined as 0%, and steady state exercise FBF is defined as 100%. After reaching steady state exercise, ketorolac transiently reduced FBF by ∼12%, returning to control levels by the end of ketorolac infusion. l-NAME infusion reduced FBF by ∼20%, which remained stable through 20 min exercise. *Value is different (P < 0.05) from control exercise, ketorolac end and saline.

Figure 4. Effect of blocking NOS and COX before exercise onset

From the 20 min exercise bout, resting and 5 min steady state (saline, control exercise) data were compared to resting and 5 min steady state (DB, double blockade) data from the 5 min exercise bout. A, the mean FBF levels (+s.e.m.) at rest were less in DB, but the exercise FBF was similar (P > 0.2), as was the change from baseline (P > 0.37). B, as DB caused modest increases in blood pressure, the FVC data (+s.e.m.) are also shown. The FVC response to exercise was similar in control and DB conditions (P > 0.6), as was the increase in FVC above baseline (P > 0.8). *Value is different from control in Rest condition (P < 0.05).