Rhythmic contractile activity of the in vivo rabbit aorta

The cellular element of the aorta is largely smooth muscle; yet this organ has long been regarded as a passive elastic tube, without regard for the possible function of its smooth muscle cells. The isolated rabbit aorta became popular as a convenient smooth muscle preparation, and much has been learned about its cellular physiology, but unfortunately the functional role of this tissue was lost on its excision. Like many mammalian arterial preparations, the isolated aorta either fails to show spontaneous electrical or mechanical activity, or shows activity of a much slower frequency than the rhythmic activity of the heart, thus obscuring any possible relationship with the pulsatile activity of the heart. However, contractions are elicited from the isolated aorta on stimulation with various neural and hormonal agents. In effect, the aortic smooth muscle has been classified as multiunit, which suggests that physiological activation may be neurogenic. Thus, removal of the muscle from the body may result in disruption of the neural connections which function in normal activation of contraction. We show here that when recordings of aortic tension are made in vivo, rhythmic contractions are observed. Evidence is presented which indicates that the contractions are neurogenic in origin an exhibit a precise phasing pattern with the pulse wave.