Neuromuscular transmission in bovine mesenteric lymphatics

Neuromuscular transmission in bovine mesenteric lymphatics was investigated using the double sucrose-gap technique. Single pulses of 0.3 msec duration (35----90 V) elicited excitatory junction potentials (EJP's) with a time to peak of about 1 sec. The EJPs showed facilitation and at stimulus frequencies greater than about 0.25 Hz could summate to reach threshold for action potential firing. The action potential so produced was followed by a phasic contraction of an "all or none" type. Increasing the frequency of stimulation did not increase the force of contraction but the resulting sustained depolarization increased the probability of a second or third action potential (and thus contraction) being elicited. EJPs and their electrical and mechanical consequences could be blocked by tetrodotoxin (10(-6) M) and by phentolamine (5 X 10(-7) M) confirming their neural mediation and dependence on postjunctional alpha-receptors.