Transient excitatory responses to sustained stimulation of intramural nerves in isolated bovine lymphatic vessels

Spontaneous isometric contractions were measured in 2 cm segments of bovine mesenteric lymphatics. Field stimulation at 4 Hz was applied for a 20 min period. This caused a threefold increase in contraction frequency initially but the response faded rapidly so that, within 6 min of the beginning of stimulation, contraction frequency had returned to a value which was not significantly different from control. In contrast to the effect on frequency 3H efflux from vessels pre-loaded with [3H]noradrenaline was maintained at a value significantly higher than control for the entire stimulation period. The fade in response to exogenous noradrenaline was similar to that seen with field stimulation but with a slightly slower time course. Vessels desensitized with noradrenaline showed a greatly diminished response to field stimulation but responded normally to other agonists. Increasing the extracellular KCl concentration to 18 mM also caused a threefold increase in contraction frequency but, unlike the response to noradrenaline and field stimulation, this was maintained for the 20 min period during which [K+]o was raised. The results suggest that the adrenoceptors on lymphatic smooth muscle exhibit very rapid homologous desensitization.