Relaxation of bovine mesenteric lymphatics in response to transmural stimulation

The responses of isolated quiescent segments of bovine mesenteric lymphatics to 10-s trains of pulses (50 V for 0.3 ms at 2 Hz) administered transmurally have been studied using various drugs. Each train of pulses consistently produced a brief contraction that was completely blocked by tetrodotoxin, alpha-adrenoceptor antagonists, bretylium, and guanethidine. Cocaine potentiated the contractions. Tyramine caused an increase in the basal tone, some spontaneous contractions, and an inhibition of contraction. The contraction was sometimes followed by a relaxation that could be of short duration or long lasting. Both types of relaxation were abolished by tetrodotoxin. Propranolol reduced both types of relaxation, causing more suppression of the short type. Bretylium completely blocked the short type of relaxation and slightly reduced the long-lasting type. Hexamethonium and atropine had no effect on any of the responses. These findings suggest that the contraction resulting from transmural stimulation is due to the action of excitatory postganglionic sympathetic nerves. It is concluded that both types of relaxation occur mainly in response to beta-adrenergic inhibitory stimulation. The possibility of nonadrenergic inhibitory innervation in bovine mesenteric lymphatics is also discussed.