Hyperpolarisation‐activated inward current in isolated sheep mesenteric lymphatic smooth muscle

1 Freshly isolated sheep lymphatic smooth muscle cells were studied using the perforated patch‐clamp technique. Hyperpolarisation with constant‐current pulses caused a time‐dependent rectification evident as a depolarising ‘sag’ followed by an anode‐break overshoot at the end of the pulse. Both sag and overshoot were blocked with 1 mM Cs + . 2 Cells were voltage clamped at −30 mV and stepped to −120 mV in 10 mV steps of 2 s duration. Steps negative to −60 mV evoked a slowly activating, non‐inactivating inward current which increased in size and rate of activation with increasing hyperpolarisation. 3 The slowly activating current was reduced in Na + ‐free bathing solution but enhanced when the extracellular K + concentration was increased to 60 mM. The current was significantly reduced by 1 mM Cs + and 1 μM ZD7288 but not by 1·8 mM Ba 2+ . 4 The steady‐state activation curve of the underlying conductance showed a threshold at −50 mV and half‐maximal activation at −81 mV. Neither threshold nor half‐maximal activation was significantly affected by increasing the external K + concentration to 60 mM. 5 The frequency of spontaneous contractions and fluid propulsion in isolated cannulated segments of sheep mesenteric lymphatics were decreased by 1 mM Cs + and by 1 μM ZD7288. 6 We conclude that sheep lymphatics have a hyperpolarisation‐activated inward current similar to the I f seen in sinoatrial node cells of the heart. Blockade of this current slows spontaneous pumping in intact lymphatic vessels suggesting that it is important in normal pacemaking.