SEX-DIFFERENCES IN THE PARASYMPATHETIC NERVE CONTROL OF RAT-HEART

1. As it has been shown that oestrogen enhances the cholinergic muscar inic activity in the central nervous system, we studied sex difference s in the responses to parasympathetic nervous stimulation in the rat h eart using in vivo and in vitro preparations. 2. In in situ perfused, innervated hearts, stimulation of bilateral vagus nerves (15 Hz with 1 mu mol/L physostigmine) inhibited sympathetic nerve stimulation (5 Hz ) induced noradrenaline release to a greater extent in female than in male rats (54+/-5 vs 72+/-5% of control). Similarly vagus nerve stimul ation at 1-20 Hz reduced heart rate (HR) more in females than males, a nd this sex difference became more marked in the presence of physostig mine. The chronotropic effect of vagal stimulation was attenuated afte r ovariectomy but potentiated after castration when compared with sham -operated controls. In contrast, the muscarinic agonist methacholine r educed neural NA release and HR equally well in both sexes. 3. In anae sthetized rats, reduction in HR and mean arterial pressure by vagus ne rve stimulation (1-20 Hz) was more pronounced in females than in males after inhibition of acetylcholinesterase with physostigmine. 4. The r esults indicate that activation of parasympathetic nerve leads to grea ter presynaptic and postsynaptic effects in female than in male rat he arts, presumably due to a higher level of acetylcholine release follow ing nerve activation.