High-Conductance K+ Channels Show a Graded Sensitivity to Cell Volume Changes

Abstract

Slick (Slo2.1) and Slack (Slo2.2) Na+/Cl−-activated high-conductance K+ channels have been co-localized in CNS and are also probably present in the heart and epithelia. Furthermore, it has been found that these two channels can associate and form functional heteromers. A number of physiological processes, such as salt and water transport, central for neuronal activity, migration and apoptosis, involve changes in cell volume. We have previously shown that homomeric Slick channels are strongly regulated by changes in cell volume while the highly homologous channel Slack is totally insensitive. It is the aim of this work to evaluate if volume sensitive Slick subunits confer volume sensitivity to the otherwise insensitive Slack channels in tetrameric channels. For this purpose different configurations of Slick/Slack heteromeric channels were co-expressed with aquaporin1 in Xenopus laevis oocytes and cell volume changes of aprox. 5% were induced by exposure to hypotonic or hypertonic buffers. Whole-cell currents were measured by two electrode voltage clamp. Co-injections of Slick and Slack mRNA in different ratios (1:1-1:0.5-0.5:1) resulted in heteromeric channels sensitive to cell volume changes but to a smaller degree compared to homomeric Slick channels. Concatemeric Slick/Slack channels were also constructed and successfully expressed in oocytes. These chimeric channels showed, as co-injection experiments, higher whole cell currents than homomeric Slick or Slack channels and slower activation kinetics. Heteromeric channels resulting from the association Slick/Slack concatemers showed intermediate volume sensitivity between Slick and Slack channels, which seems to reflect the number of Slick subunits in the quatrameric channels. In conclusion, we have identified a heteromeric K+ channel with graded sensitivity to small and fast changes in cell volume, a mechanism related with the number of volume sensitive subunits in the quatrameric channels.