Characterization of an inwardly rectifying potassium channel in the rabbit superior lacrimal gland

PURPOSE. To characterize the properties of an inwardly rectifying K + (K IR ) current in fresh, enzymatically isolated acinar cells from the rabbit superior lacrimal gland. METHODS. New Zealand White rabbits of both sexes were killed by injecting 45 mg/kg pentobarbital sodium, and the glands were excised. Single acinar cells were isolated enzymatically from these glands. Standard patch-clamp techniques were used to record ion currents. Results. Hyperpolarizing voltages evoked K IR currents that had a conductance of 2.7 ± 0.16 nS (n = 6) in the range -50 mV to - 160 mV mV. The K IR current was activated with steep voltage dependence on hyperpolarization, and the conductance was approximately dependent on the square root of the external K + concentration. Increasing the pipette Ca 2+ concentration from 10 9 M to 10 6 M increased the conductance to 5.3 ± 0.45 nS (n = 7). Internal substitution of K' with various cations gave the following permeability sequence: K' (1.0) > Rb' (0.83) Li + (0.15). The K IR current was inhibited by Ba 2+ (100 μm), tetraethylammonium (10 mM), and Cs + (5 mM) but was insensitive to 4-aminopyridine (5 mM). The single-channel conductance was 43 ± 2.7 pS (n = 11), and the relationship between between single-channel conductance (γ) and external K + concentration ([K] o ) is given by: γ = 7.04[K] 0 0.37 (pS, r 2 = 0.99, P < 0.05), The relationship between [K] o and Zero current potential (E rev ) is given by: E rev = 35.5 log[K] o - 77.8 (mV; r 2 = 0.99, P < 0.05). CONCLUSIONS. The K IR current identified in these lacrimal acini has a similar dependence on [K] o as other inward rectifiers of excitable tissues and exocrine glands. However, this study highlights that there are interspecies variations and similarities between K IR channels that could be related to their individual physiological roles. The authors' investigations suggest that one role of the K IR channel in the rabbit superior lacrimal gland acinar cells is to set and stabilize the resting membrane potential. However, this K IR channel may also be involved in secretion, as has been shown to occur in the sheep parotid gland.