Using FM1-43 fluorescence, we have optically detected single exocytic and endocytic events in rat pituitary lactotrophs. About fifty discrete fluorescent spots abruptly appear around the entire surface of a cell bathed in FM1-43 and high-potassium saline. The spots, which also immunostain for prolactin, reflect the labeling of dense cores as well as membranes of exocytosed secretory granules. Stained cores are not released, but remain attached to the cell and are eventually endocytosed. However, in cells exposed to dopamine (or an analog, bromocriptine), the cores dissolve and are secreted after several seconds. Solubilization of dense cores is mediated through a reduction in cytoplasmic cyclic AMP. Thus, the composition of secretions from individual secretory granules is regulated.

In human liver, Ca(2+)-dependent changes in membrane K(+) permeability play a central role in coordinating functional interactions between membrane transport, metabolism, and cell volume. On the basis of the observation that K(+) conductance is partially sensitive to the bee venom toxin apamin, we aimed to assess whether small-conductance Ca(2+)-sensitive K(+) (SK(Ca)) channels are expressed endogenously and contribute to volume-sensitive K(+) efflux and cell volume regulation. We isolated a full-length 2,140-bp cDNA (hSK2) highly homologous to rat brain rSK2 cDNA, including the putative apamin-sensitive pore domain, from a human liver cDNA library. Identical cDNAs were isolated from primary human hepatocytes, human HuH-7 hepatoma cells, and human Mz-ChA-1 cholangiocarcinoma cells. Transduction of Chinese hamster ovary cells with a recombinant adenovirus encoding the hSK2-green fluorescent protein fusion construct resulted in expression of functional apamin-sensitive K(+) channels. In Mz-ChA-1 cells, hypotonic (15% less sodium glutamate) exposure increased K(+) current density (1.9 +/- 0.2 to 37.5 +/- 7.1 pA/pF; P < 0.001). Apamin (10-100 nM) inhibited K(+) current activation and cell volume recovery from swelling. Apamin-sensitive SK(Ca) channels are functionally expressed in liver and biliary epithelia and likely contribute to volume-sensitive changes in membrane K(+) permeability. Accordingly, the hSK2 protein is a potential target for pharmacological modulation of liver transport and metabolism through effects on membrane K(+) permeability.