Exercise training increases cardiac vagal tone and decreases resting heart rate. The mechanisms by which these effects are brought about are not fully understood. The nitric oxide-cyclic guanosine monophosphate (NO-cGMP) pathway is implicated in the cholinergic regulation of heart rate (e.g. Herring & Paterson, 2000), and plays a role in adaptations taking place in the coronary vasculature (Sessa et al. 1994) and cardiac sympathetic innervation (Mohan et al. 2000) following training. We investigated whether male mice that had been selectively bred for increased wheel-running (Koteja et al. 1999) had increased heart rate responses to peripheral vagal stimulation after being allowed to spontaneously run for 20 weeks (+EX, n = 8) compared with selected mice without a wheel (-EX, n = 8). All data are expressed as means ± S.E.M.

Ventricle/body weight ratios were +EX: 6.60 ± 0.58 mg g^-1 vs. -EX: 3.97 ± 0.34 mg g^-1 (P < 0.001) at 20 weeks. The negative chronotropic responses to right vagal nerve stimulation (VNS, 3-5 Hz) and bath-application of the acetylcholine analogue, carbamylcholine (CCh, 0.01-100 µM) were measured in isolated atria taken from humanely killed +EX and -EX mice. In +EX atria, spontaneous beating rates were significantly lower than those in -EX (325 ± 16 vs. 379 ± 6 b.p.m., P < 0.01), and the heart rate (HR) responses to VNS were significantly greater in +EX atria (see Fig. 1, P < 0.01). However, there was no difference between the HR responses to CCh in +EX and -EX atria (IC₅₀ (+EX) = 6.78 ± 0.62 µM vs. IC₅₀ (-EX) = 6.70 ± 0.69 µM), suggesting that the differences observed during VNS resulted from a presynaptic adaptation. Inhibition of nNOS with vinyl-L-nio hydrochloride (L-VNIO, 100 µM) significantly attenuated the HR response to VNS in both +EX and -EX atria (P < 0.01), but it had a significantly greater effect in +EX atria (P < 0.05). Moreover, L-VNIO abolished the significant difference between the control HR responses to VNS in +EX and -EX atria (see Fig. 1), suggesting that this difference may have been related to an exercise-dependent up-regulation of nNOS in +EX atria. Inhibition of soluble guanylyl cyclase with 1H-[1,2,4]-Oxadiazolo[4,3-a]quinoxalin-1-one (10 µM), had a similar effect to L-VNIO in all atria. Enhancing the bioavailability of NO-cGMP with sodium nitroprusside (10 µM) or the cGMP analogue 8-Br-cGMP (0.5 mM) increased the HR responses to VNS, but not to CCh, to the same extent in both +EX and -EX atria.These results show that exercise training enhances the HR response to peripheral vagal nerve stimulation by a presynaptic NO-cGMP-dependent mechanism that probably facilitates cholinergic neurotransmitter release.

Figure 1. Decrease in HR with VNS at 5 Hz was enhanced in +EX compared with -EX. This difference was abolished after nNOS inhibition and was restored with 1 mM L-arginine. *P < 0.05, unpaired t test. † nNOS inhibition significantly attenuated vagal bradycardia in +EX and -EX (P < 0.01, control vs. L-VNIO vs. L-Arg, repeated measures ANOVA).

Herring, N. & Paterson, D.J. (2000). J. Mol. Cell Cardiol. 32 (10), 1795-1804.

Koteja, P., Garland, T., Sax, J.K., Swallow, J.G. & Carter, P.A. (1999). Anim. Behav. 58 (6), 1307-1318.

Mohan, R.M., Choate, J.K., Golding, S., Herring, N., Casadei, B. & Paterson, D.J. (2000). Cardiovasc Res. 47 (1), 90-98.

Sessa, W.C., Pritchard, K., Seyedi, N., Wang, J. & Hintze, T.H. (1994). Circ. Res. 74 (2), 349-353.