Polyamine flux in Xenopus oocytes through hemi-gap junctional channels

Diverse polyamine transport systems have been described in different cells, but the molecular entities that mediate polyamine influx and efflux remain incompletely defined. We have previously demonstrated that spermidine efflux from oocytes is a simple electrodiffusive process, inhibitable by external Ca2+, consistent with permeation through a membrane cation channel. Hemi-gap junctional channels in Xenopus oocytes are formed from connexin 38 (Cx38), and produce a calcium-sensitive (Ic) current that is inhibited by external Ca2+. Spermidine efflux is also calcium sensitive, and removal of external calcium increases both Ic currents and spermidine efflux in Xenopus oocytes. Injection of Cx38 cRNA or Cx38 antisense oligonucleotides (to increase or decrease, respectively, Cx38 expression) also increases or decreases spermidine efflux in parallel. Spermidine efflux has a large voltage-dependent component, which is abolished with injection of Cx38 antisense oligonucleotides. In addition, spermidine uptake is significantly increased in Cx38 cRNA-injected oocytes in the absence of external calcium. The data indicate that hemi-gap junctional channels provide the Ca2+-inhibited pathway for electrodiffusive efflux of polyamines from oocytes, and it is likely that hemi-gap junctional channels provide Ca2+ and metabolism-sensitive polyamine permeation pathways in other cells.

Tolerance for ATP-insensitive K(ATP) channels in transgenic mice

To examine the role of sarcolemmal K(ATP) channels in cardiac function, we generated transgenic mice expressing GFP-tagged Kir6.2 subunits with reduced ATP sensitivity under control of the cardiac alpha-myosin heavy chain promoter. Four founder mice were isolated, and both founders and progeny were all apparently normal and fertile. Electrocardiograms from conscious animals also appeared normal, although mean 24-hour heart rate was approximately 10% lower in transgenic animals compared with littermate controls. In excised membrane patches, K(ATP) channels were very insensitive to inhibitory ATP: mean K(1/2) ([ATP] causing half-maximal inhibition) was 2.7 mmol/L in high-expressing line 4 myocytes, compared with 51 micromol/L in littermate control myocytes. Counterintuitively, K(ATP) channel density was approximately 4-fold lower in transgenic membrane patches than in control. This reduction of total K(ATP) conductance was confirmed in whole-cell voltage-clamp conditions, in which K(ATP) was activated by metabolic inhibition. K(ATP) conductance was not obvious after break-in of either control or transgenic myocytes, and there was no action potential shortening in transgenic myocytes. In marked contrast to the effects of expression of similar transgenes in pancreatic beta-cells, these experiments demonstrate a profound tolerance for reduced ATP sensitivity of cardiac K(ATP) channels and highlight differential effects of channel activity in the electrical activity of the 2 tissues.

ATP interaction with the open state of the K(ATP) channel

The mechanism of ATP-sensitive potassium (K(ATP)) channel closure by ATP is unclear, and various kinetic models in which ATP binds to open or to closed states have previously been presented. Effects of phosphatidylinositol bisphosphate (PIP2) and multiple Kir6.2 mutations on ATP inhibition and open probability in the absence of ATP are explainable in kinetic models where ATP stabilizes a closed state and interaction with an open state is not required. Evidence that ATP can in fact interact with the open state of the channel is presented here. The mutant Kir6.2[L164C] is very sensitive to Cd2+ block, but very insensitive to ATP, with no significant inhibition in 1 mM ATP. However, 1 mM ATP fully protects the channel from Cd2+ block. Allosteric kinetic models in which the channel can be in either open or closed states with or without ATP bound are considered. Such models predict a pedestal in the ATP inhibition, i.e., a maximal amount of inhibition at saturating ATP concentrations. This pedestal is predicted to occur at >50 mM ATP in the L164C mutant, but at >1 mM in the double mutant L164C/R176A. As predicted, ATP inhibits Kir6.2[L164C/R176A] to a maximum of approximately 40%, with a clear plateau beyond 2 mM. These results indicate that ATP acts as an allosteric ligand, interacting with both open and closed states of the channel.

The kinetic and physical basis of K(ATP) channel gating: toward a unified molecular understanding

K(ATP) channels can be formed from Kir6.2 subunits with or without SUR1. The open-state stability of K(ATP) channels can be increased or reduced by mutations throughout the Kir6.2 subunit, and is increased by application of PIP(2) to the cytoplasmic membrane. Increase of open-state stability is manifested as an increase in the channel open probability in the absence of ATP (Po(zero)) and a correlated decrease in sensitivity to inhibition by ATP. Single channel lifetime analyses were performed on wild-type and I154C mutant channels expressed with, and without, SUR1. Channel kinetics include a single, invariant, open duration; an invariant, brief, closed duration; and longer closed events consisting of a "mixture of exponentials," which are prolonged in ATP and shortened after PIP(2) treatment. The steady-state and kinetic data cannot be accounted for by assuming that ATP binds to the channel and causes a gate to close. Rather, we show that they can be explained by models that assume the following regarding the gating behavior: 1) the channel undergoes ATP-insensitive transitions from the open state to a short closed state (C(f)) and to a longer-lived closed state (C(0)); 2) the C(0) state is destabilized in the presence of SUR1; and 3) ATP can access this C(0) state, stabilizing it and thereby inhibiting macroscopic currents. The effect of PIP(2) and mutations that stabilize the open state is then to shift the equilibrium of the "critical transition" from the open state to the ATP-accessible C(0) state toward the O state, reducing accessibility of the C(0) state, and hence reducing ATP sensitivity.