The inhibitor of connexin Cx36 channels, mefloquine, inhibits voltage-dependent Ca2+ channels and insulin secretion. (5th September 2018) - Seemann, Nele
Abstract: The antimalarial agent, mefloquine, inhibits the function of connexin Cx36 gap junctions and hemichannels and has thus become a tool to investigate their physiological relevance in pancreatic islets. In view of earlier reports on a KATP channel-block by mefloquine, the specificity of mefloquine as a pharmacological tool was investigated. Mouse pancreatic islets and single beta cells were used to measure membrane potential, whole cell currents, Ca 2+ channel activity, cytosolic Ca 2+ concentration ([Ca 2+ ]i ) and insulin secretion. Mefloquine was tested in the concentration range of 5–50 μM 25 μM mefloquine was as effective as 500 μM tolbutamide to depolarize the plasma membrane of beta cells, but did not induce action potentials. Rather, it abolished tolbutamide-induced action potentials and the associated increase of [Ca 2+ ]i . In the range of 5–50 μM mefloquine inhibited voltage-dependent Ca 2+ currents in primary beta cells as effectively as 1 μM nisoldipine, a specific blocker of L-type Ca 2+ channels. The Ca 2+ channel opening effect of Bay K8644 was completely antagonized by mefloquine. Likewise, the increase of [Ca 2+ ]i and of insulin secretion stimulated by 40 mM KCl, but not that by 30 mM glucose was antagonized by 50 μM mefloquine. Neither at 5 μM nor at 50 μM did mefloquin stimulate insulin secretion at basal glucose. In conclusion, mefloquine blocks KATP channels and L-type Ca 2+ channels in pancreatic beta cells in the range from 5 to 50 μM. Thus it inhibits depolarization-induced insulin secretion, but in the presence of a stimulatory glucose concentration additional effects of mefloquine, possibly on intracellular Ca 2+ mobilization, and the metabolic amplification by glucose permit a sustained rate of secretion. Graphical abstract: Multiple effects of mefloquine on the pancreatic beta cell. The closure of ATP-sensitive potassium channels (KATP channels) depolarizes the plasma membrane and opens voltage-dependent Ca 2+ channels leading to the increase of the cytosolic Ca 2+ concentration ([Ca 2+ ]i ). This is called "triggering" of insulin secretion, additional signals from the glucose metabolism have "amplifying" function. Similar to the sulfonylureas (e.g. tolbutamide) mefloquine closes KATP channels and generates the triggering signal but at the same time inhibits Ca 2+ influx via voltage-dependent Ca 2+ channels, thus inhibiting depolarization-induced insulin secretion. At high glucose mefloquine does not diminish [Ca 2+ ]i and secretion, apparently by additional mechanisms enabled by the metabolism of glucose. In the range of 5–50 μM mefloquine is no selective blocker of Cx36 hemichannels. Highlights: Mefloquine is used as a tool to block Cx36 gap junctions and hemichannels. These may have important physiological roles in pancreatic islets and beta cells. However mefloquine inhibits also KATP channels and voltage-dependent Ca2+ channels. Mefloquine inhibits depolarization-induced, but not glucose-induced insulin secretion. These complex effects may lead to erroneous conclusions concerning the roles of Cx36.