Three types of calcium‐dependent channel in rat lacrimal glands.

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Abstract
Isolated cells from rat lacrimal glands were studied with patch-clamp techniques. Whole-cell and cell-attached recordings were obtained while the cells were stimulated by application of carbamylcholine or of the Ca ionophore, A23187 [calcimycin]. The results were compared with recordings of Ca-dependent channels obtained in isolated patches. Whole-cell recordings revealed 2 types of carbamylcholine-induced current. At low levels of stimulation, a specific class of Ca-dependent K channels was selectively activated (BK channels). With more intense stimulation an inward current, Ii, was obtained at the cell resting potential. Ii rose rather abruptly after a long delay. In several cells, Ii currents presented spontaneous oscillations. Both K and Ii current responses to carbamylcholine were due to activation of muscarinic receptors. Both responses were elicited by a rise of the intracellular Ca concentration. The immediate source of Ca was intracellular. Replacement of intracellular K with either Na or Cs blocked BK channels entirely, thus allowing the study of Ii currents free from K currents. Ii responses to carbamylcholine were less frequently obtained in Na- or Cs-dialyzed cells than in K-dialyzed cells. In symmetrical NaCl solutions, Ii inverted at 0 mV. When replacing part of the intracellular or extracellular Cl with Glu the reversal potential, Ei, was found to vary in the same direction as the equilibrium potential for Cl ions, ECl. In some experiments, Ei was close to ECl but in others Ei deviated strongly from ECl. This suggested that Ii was mainly due to a Cl-selective conductance and that another conductance type was contributing to Ii in variable proportions. It was found that, in K-free solutions, Ii had a reversal potential very close to ECl. Noise analysis showed that the Cl channels involved in Ii current had a unit conductance of .apprx. 1-2 pS [siemens] in symmetrical NaCl solutions. At -60 mV, the mean channel open time derived from noise power spectra was .apprx. 200 ms. The activation of the Ca-dependent Cl channels was increased by depolarization. Voltage jumps elicited slow exponential relaxations. At -60 mV, the time constants of the relaxations were in the range 100-250 ms. Cell-attached recordings suggested that internal Ca activated 3 types of channels depending on the Ca concentration: BK channels, 2-4 pS channels and 25 pS channels. Inside-out and outside-out patch conditions allowed a rough estimate to be made of the Ca concentration needed to activate each class of channel. The 25 pS channels appeared to be cation selective and not to discriminate between Na and K. Taken together, the results allow the characterization of 3 types of Ca-dependent channel, 1 selective for K (BK channels), 1 selective for Cl and 1 selective for cations. A 4th type of Ca-dependent conductance, which is presumably cation selective and sensitive to external K ions was also indicated. Possible roles of the different classes of Ca-dependent channels in fluid secretion were discussed.