Adrenergic Excitatory Influences on Initiation and Conduction of Electrical Activity in the Rat Portal Vein

The longitudinal smooth muscle of the rat portal vein shows synchronized spontaneous contractions. Both initiation and propagation of this activity are of myogenic nature. Under the influence of noradrenaline (NA), or postganglionic nerve stimulation, the contraction frequency is known to increase markedly. In order to analyse pacemaker function and synchronization of activity in the isolated portal vein, both during spontaneous activity and during NA induced excitation, a method was designed which allowed simultaneous recording of the mechanical contractions and of the electrical activity from three separate areas in the preparation. Spontaneous, intermittent bursts of electrical spike activity spread over the preparation at a velocity of 5 to 30 mm/sec. Each propagated burst was associated with a mechanical response. Evidence is presented indicating that the individual spike potentials within a burst were propagated. Administration of exogenous NA markedly increased the frequency of burst discharge from an active pacemaker area, so that the preparation became synchronously driven at a higher rate, eventuelly leading to a maintained tetanus. In addition, propagated activity from “ectopic” pacemaker areas appeared, which caused irregular contractile responses. Qualitatively similar activity patterns were obtained by tyramine administration. The pronounced synchronization, and the increase in frequency of burst discharge elicited by such release of endogenous NA, suggest that myogenic propagation plays a most important role in the effectiveness of nervous control of this type of vascular smooth muscle.