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 r abbit superior lacrimal gland. METHODS. New Zealand White rabbits of b oth sexes were killed by injecting 45 mg/kg pentobarbital sodium, and the glands were excised. Single acinar cells were isolated enzymatical ly from these glands. Standard patch-clamp techniques were used to rec ord ion currents. RESULTS. Hyperpolarizing voltages evoked K-IR curren ts that had a conductance of 2.7 +/- 0.16 nS (n = 6) in the range -50 mV to -160 mV. The K-IR current was activated with steep voltage depen dence on hyperpolarization, and the conductance was approximately depe ndent on the square root of the external K+ concentration. Increasing the pipette Ca2+ concentration from 10(-9) M to 10(-6) M increased the conductance to 5.3 +/- 0.45 nS (n = 7). Internal substitution of K+ w ith various cations gave the following permeability sequence: K+ (1.0) > Rb+ (0.83) > Li+ (0.15). The K-IR current was inhibited by Ba2+ (10 0 mu m), tetraethylammonium (10 mM), and Cs+ (5 mM) but was insensitiv e to 4-aminopyridine (5 mM). The single-channel conductance was 43 +/- 2.7 pS (n = 11), and the relationship between between single-channel conductance (gamma) and external K+ concentration ([K](0)) is given by : gamma = 7.04[K](0)(0.37) (pS, r(2) = 0.99, P < 0.05). The relationsh ip between [K](0) and zero current potential (E-rev) is given by: E-re v = 35.5 log[K](0) - 77.8 (mV; r(2) = 0.99, P < 0.05). CONCLUSIONS. Th e K-IR current identified in these lacrimal acini has a similar depend ence on [K](0) as other inward rectifiers of excitable tissues and exo crine glands. However, this study highlights that there are interspeci es variations and similarities between K-IR channels that could be rel ated to their individual physiological roles. The authors investigatio ns suggest that one role of the K-IR channel in the rabbit superior la crimal gland acinar cells is to set and stabilize the resting membrane potential. However, this K-IR channel may also be involved in secreti on, as has been shown to occur in the sheep parotid gland.