Do cardiac neurons play a role in the intrinsic control of heart rate in the rat?

Three experiments were performed to see whether cardiac neurons contribute to the intrinsic control of heart rate in right atria of adult rats. The intrinsic heart rate response (IRR) was examined by raising right atrial pressure from 2 to 8 mmHg for 3 min. In isolated preparations of the right atrium, the IRR was not significantly altered by the addition of either 1 mM atropine (n = 6; control +19 ± 3 min−1; atropine +18 ± 3 min−1 (mean ± S.E.M.)) or 1 mM propranolol (n = 5; control +22 ± 4 min−1; propranolol +21 ± 3 min−1). Tetrodotoxin (0.5 µM) had no effect on the IRR (n = 6; control +37 ± 5 min−1; tetrodotoxin 38 ± 5 min−1). In another experiment, 2-day-old rat pups were injected with capsaicin (50 mg kg−1; treated) or with vehicle (control). There was no difference in the IRR of right atrial preparations taken from control and treated animals after they reached adulthood (control (n = 7) and treated (n = 8): +30 ± 4 and +32± 4 min−1). The influence of right atrial pressure on the efficacy of vagal stimulation was examined. The rate response to vagal stimulation was reduced similarly in control and treated preparations when pressure was elevated from 2 to 4 mmHg (control and treated: −34 ± 5% and −33 ± 3%). The effectiveness of the capsaicin treatment was confirmed by the depletion of substance P-immunoreactive nerve fibres in cardiac tissues. Together, these results strongly suggest that cardiac neurons are not involved in intrinsic heart rate control.