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current in
sheep lymphatic smooth muscle
Smooth Muscle Group, Department of Physiology, Queen's University, Belfast BT9 7BL, United Kingdom
Freshly dispersed sheep mesenteric lymphatic
smooth muscle cells were studied at 37°C using the perforated
patch-clamp technique with Cs+- and K+-filled
pipettes. Depolarizing steps evoked currents that consisted of
L-type Ca2+ [ICa(L)]
current and a slowly developing current. The slow current reversed at
1 ± 1.5 mV with symmetrical Cl
concentrations
compared with 23.2 ± 1.2 mV (n = 5) and
34.3 ± 3.5 mV (n = 4) when external
Cl was substituted with either glutamate (86 mM) or
I (125 mM). Nifedipine (1 µM) blocked and BAY K 8644 enhanced ICa(L), the slow-developing sustained
current, and the tail current. The Cl channel blocker
anthracene-9-carboxylic acid (9-AC) reduced only the slowly developing
inward and tail currents. Application of caffeine (10 mM) to
voltage-clamped cells evoked currents that reversed close to the
Cl equilibrium potential and were sensitive to 9-AC.
Small spontaneous transient depolarizations and larger action
potentials were observed in current clamp, and these were blocked by
9-AC. Evoked action potentials were triphasic and had a prominent
plateau phase that was selectively blocked by 9-AC. Similarly, fluid
output was reduced by 9-AC in doubly cannulated segments of
spontaneously pumping sheep lymphatics, suggesting that the
Ca2+-activated Cl current plays an important
role in the electrical activity underlying spontaneous activity in this tissue.
lymphatics; pacemaking; action potentials; spontaneous activity
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