We have learned a lot in recent years, using the patch-clamp technique, about the conductances underlying excitation in isolated smooth muscle cells dispersed from sheep mesenteric lymphatics (Hollywood et al. 1997; McCloskey et al. 1999; Toland et al. 2000). This technique, however, does not allow us to assess the relevance of any given conductance to the function of the syncitium. In the present study we have used intracellular microelectrodes to record the effects of some ion channel blockers on spontaneous and evoked action potentials in segments of sheep mesenteric lymphatic vessels. Sheep mesenteries were obtained from the local abattoir and 1-2 cm lengths of lymphatic were mounted in a modified Abe & Tomita bath (1968) and superfused with Krebs solution at 37 °C. Smooth muscle cells were impaled with glass microelectrodes filled with 3 M KCl having resistances of 80-120 M¢. Resting membrane potential varied from -49 to -65 mV (mean ± S.D. -57.5 ± 4.1, n = 57),

Approximately 50 % of the vessels showed spontaneous electrical and mechanical activity at frequencies of 2.5-8 min^-1 (4.8 ± 1.4, n = 32). The basic pattern of electrical activity consisted of slow depolarisations (dV/dt 0.33 ± 0.06 V s^-1) of relatively low amplitude (16.3 ± 2.0 mV, n = 12) and with duration at half-amplitude of 0.79 ± 0.44 s (n = 9). In most preparations between 1 and 3 spikes were superimposed on the slow oscillations. These were much faster and of shorter duration than the slow oscillations (maximum rate of depolarisation varied from 9.8 to 20 V s^-1, duration at half-amplitude 6.6 ± 0.1 ms). Responses evoked by electrical stimulation (single 20 ms depolarising pulse) consisted of a single spike followed by a declining plateau. Rate of rise was less than spontaneous spikes (5 V s^-1) and duration at half-amplitude was longer (42 ms). TTX (1 µM) initially decreased the amplitude of and then abolished spontaneous spikes while it did not affect the evoked spike. Nifedipine (1 µM) abolished both spontaneous and evoked spikes. The calcium-activated chloride channel blockers niflumic acid and A-9-C abolished spontaneous action potential firing (presumably by depressing the pacemaker mechanism) but it was still possible to evoke an action potential by electrical stimulation although the spike duration was decreased by both drugs.

Abe, Y. & Tomita, T. (1968). J. Physiol. 196, 87-100. abstract

Hollywood, M.A., Cotton, K.D., Thornbury, K.D. & McHale, N.G. (1997). J. Physiol. 503, 13-20. abstract

McCloskey, K.D., Toland, H.M., Hollywood, M.A., Thornbury, K.D. & McHale, N.G. (1999). J. Physiol. 521, 201-211. abstract

Toland, H.M., McCloskey, K.D., Thornbury, K.D., McHale, N.G. & Hollywood, M.A. (2000). Am. J. Physiol. 279, C1327-1335.