Effect of norepinephrine on contractility of isolated mesenteric lymphatics

Norepinephrine increases the spontaneous contraction frequency of isolated bovine lymphatic vessels but, paradoxically, reduces contraction force. In this investigation contractions were elicited by electrical stimulation, applied as field stimulation or across a sucrose gap. Evoked contractions were not reduced in force by concentrations of norepinephrine that have previously been shown to dramatically reduce contractility in spontaneously active vessels. However, at higher concentrations a beta-inhibitory component of the norepinephrine effect was often observed. Simultaneous recordings of electrical and mechanical activity from spontaneously active vessels showed that each spontaneous contraction was initiated by a single action potential. Norepinephrine significantly disrupted electrical activity causing increased action potential frequency, reduction in action potential amplitude, and frequent failure of full action potential development. We conclude that norepinephrine depresses the force of spontaneous contractions by disruption of spontaneous pacemaking rather than by any direct effect on vessel contractility.