Activation of Ca-dependent K channels by carbamoylcholine in rat lacrimal glands

Electrical properties of the membranes of lacrimal gland cells were investigated using patch-clamp techniques [Hamill, O.P., Marty A., Neher, E., Sakmann, B. & Sigworth, F.J. (1981) Pflügers Arch. 391, 85-100]. The membranes were found to contain a specific kind of voltage- and Ca2+ -activated K+ channel ("BK channels"). These channels account for the strong rectification of the cell current-voltage curve as obtained in tight-seal whole-cell recordings. Application of low concentrations of carbamoylcholine (CbmCho, 0.5 microM) activated the BK channels. No effect was obtained in the presence of atropine (2 microM) or when dialyzing the cell with a strong CaEGTA buffer. The latter result, together with other findings, suggests that CbmCho exerts its action on BK channels by increasing the intracellular Ca2+ concentration. This Ca2+ concentration increase presumably occurred via liberation from a cytoplasmic Ca2+ store, because the response remained unaffected in the absence of extracellular Ca2+. At higher CbmCho concentration (2 microM), an inward current was observed, which was assumed to result from activation of another type of Ca2+ -regulated channel.