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 Naf-free bathing solution but enhanced when the extracellular K+ concentration was increased to 60 mM. The current was significantly reduced by I mill Csf and 1 mu M ZD7288 but not by 1.8 mM Ba2+. 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 mu M 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.