Single-channel recording by the patch-clamp technique has now characterized three kinds of membrane potassium channels activated by intracellular calcium ions in animal cells. These play a crucial part in the regulation of membrane potential and of secretion.

Ca2+ release from the envelope of isolated pancreatic acinar nuclei could be activated by nicotinic acid adenine dinucleotide phosphate (NAADP) as well as by inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose (cADPR). Each of these agents reduced the Ca2+ concentration inside the nuclear envelope, and this was associated with a transient rise in the nucleoplasmic Ca2+ concentration. NAADP released Ca2+ from the same thapsigargin-sensitive pool as IP3. The NAADP action was specific because, for example, nicotineamide adenine dinucleotide phosphate was ineffective. The Ca2+ release was unaffected by procedures interfering with acidic organelles (bafilomycin, brefeldin, and nigericin). Ryanodine blocked the Ca2+-releasing effects of NAADP, cADPR, and caffeine, but not IP3. Ruthenium red also blocked the NAADP-elicited Ca2+ release. IP3 receptor blockade did not inhibit the Ca2+ release elicited by NAADP or cADPR. The nuclear envelope contains ryanodine and IP3 receptors that can be activated separately and independently; the ryanodine receptors by either NAADP or cADPR, and the IP3 receptors by IP3.

No abstract

Nervous or hormonal stimulation of many exocrine glands evokes release of cellular K+ (ref. 1), as originally demonstrated in mammalian salivary glands2,3, and is associated with a marked increase in membrane conductance1,4,5. We now demonstrate directly, by using the patch-clamp technique6, the existence of a K+ channel with a large conductance localized in the baso-lateral plasma membranes of mouse and rat salivary gland acinar cells. The K+ channel has a conductance of approximately 250 pS in the presence of high K+ solutions on both sides of the membrane. Although mammalian exocrine glands are believed not to possess voltage-activated channels1,7, the probability of opening the salivary gland K+ channel was increased by membrane depolarization. The frequency of channel opening, particularly at higher membrane potentials, was increased markedly by elevating the internal ionized Ca2+ concentration, as previously shown for high-conductance K+ channels from cells of neural origin8-10. The Ca2+ and voltage-activated K+ channel explains the marked cellular K+ release that is characteristically observed when salivary glands are stimulated to secrete.

Ca2+- and voltage-activated K+ channels are found in many electrically excitable cells and have an important role in regulating electrical activity. Recently, the large K+ channel has been found in the baso-lateral plasma membranes of salivary gland acinar cells, where it may be important in the regulation of salt transport. Using patch-clamp methods to record single-channel currents from excised fragments of baso-lateral acinar cell membranes in combination with current recordings from isolated single acinar cells and two- and three-cell clusters, we have now for the first time characterized the K+ channels quantitatively. In pig pancreatic acini there are 25-60 K+ channels per cell with a maximal single channel conductance of about 200 pS. We have quantified the relationship between internal ionized Ca2+ concentration [( Ca2+]i) membrane potential and open-state probability (p) of the K+ channel. By comparing curves obtained from excised patches relating membrane potential to p, at different levels of [Ca2+]i, with similar curves obtained from intact cells, [Ca2+]i in resting acinar cells was found to be between 10(-8) and 10(-7) M. In microelectrode experiments acetylcholine (ACh), gastrin-cholecystokinin (CCK) as well as bombesin peptides evoked Ca2+-dependent opening of the K+ conductance pathway, resulting in membrane hyperpolarization. The large K+ channel, which is under strict dual control by internal Ca2+ and voltage, may provide a crucial link between hormone-evoked increase in internal Ca2+ concentration and the resulting NaCl-rich fluid secretion.