Gastric tone modifies the responses to extrinsic neural stimuli in the anaesthetized ferret

To determine the relation between resting intragastric pressure and neuronally evoked motility responses, the stomach of the anaesthetized ferret was inflated with two volumes within the physiological range. Vagal evoked contractions decreased as the resting pressure was increased. The decrease affected responses to high, but not low, intensity stimulation of the vagus. Vagal evoked relaxation of the stomach increased as a linear function of resting pressure. Atropine lowered the resting pressure in vagotomized ferrets but did not alter the relation between vagal evoked relaxation and resting pressure. Intra-arterial acetylcholine evoked a contraction followed by a relaxation. The contraction was reduced at higher resting pressures but the relaxation increased. Hexamethonium had no effect on the contraction but substantially reduced the relaxation. Stimulation of the greater splanchnic nerve evoked two responses. The first, direct relaxation of the stomach (independent of cholinergic activity), bore the same relation to resting pressure as vagal evoked relaxation. The second, inhibition of vagal evoked contraction, had no relation to resting pressure. The degree of reduction in the amplitude of vagal evoked contraction by an immediately preceding vagal stimulation decreased at higher resting pressures. This was the opposite of vagal evoked relaxation. The capacity of stimulation of one cervical vagus to evoke a response as large as that from stimulation of two cervical vagi was shown to depend on the use of near-maximal levels of stimulation. At low levels of stimulation the responses to two vagi were additive, and at the lowest levels there was facilitation. It was concluded that the effect of resting pressure on the amplitude of evoked contractions was mediated mainly by smooth muscle, although modulation of the quantities of transmitter released from nerves could play a minor part. The practical implications of manipulating resting pressure by changes in volume were discussed. In particular, the manipulation of resting pressure as a method of differentiating the mechanisms of central nervous influence on gastric motility was compared to surgical and pharmacological nerve block.