The effects of changes of extramural, 'intrathoracic', pressure on aortic arch baroreceptors.

1. The isolated aortic arch was perfused by a method enabling the mean pressure, pulse pressure and pulse frequency to be varied independently. The preparation was also subjected to phasic and non-phasic changes of extramural pressure.2. The aortic arch baroreceptor impulse activity in single or few-fibre preparations was increased by raising the intra-aortic pressure and by applying a negative extramural pressure at constant intra-aortic pressure.3. Curves relating impulse frequency and negative extramural pressure were similar to those relating impulse frequency to intra-aortic pressure. The effective stimulus to the aortic arch baroreceptors is the transmural pressure resulting from the algebraic difference of the intra-aortic and extramural pressures.4. Rhythmical alterations in extramural pressure caused phasic changes in baroreceptor impulse activity. As the pressure became more negative, the impulse frequency increased and other baroreceptors were recruited.5. During pulsatile perfusion of the aortic arch the maximum impulse activity occurred when the negative phase of extramural pressure coincided with the systolic phase of the perfusion pressure.6. These findings are discussed in relation to the effects of changes of intrathoracic pressure on aortic baroreceptor activity in vivo