Autonomic responsiveness in stress-sensitive and stress-resistant pigs

The responsiveness of the autonomic nervous system was studied in twenty-six 140 day old thiopentone anaesthetized lean stress-sensitive Pietrain (P) and fatter stress-resistant Gloucester (G) pigs, using the Valsalva like manoeuvre (VLM), and the i.v. tyramine, noradrenaline (NA) and phenylephrine tests. The heart rate responses to the VLM and tyramine tests were greater in P. The pressor response to NA was lower in P, and the change in heart rate per unit change in blood pressure-during phenylephrine infusion was the same in the two breeds. The sympathetic nervous system in P was, therefore, more responsive than that in G, and this was attributable to a higher pre-adrenoreceptor responsiveness in P. The potentially greater sympathetic responsiveness in stress-sensitive pigs could account for their greater leanness, their stress-induced metabolic acidosis, and for their susceptibility to myocardial failure.