Depletion of intracellular polyamines relieves inward rectification of potassium channels.

Two different approaches were used to examine the in vivo role of polyamines in causing inward rectification of potassium channels. In two-microelectrode voltage-clamp experiments, 24-hr incubation of Xenopus oocytes injected with 50 nl of difluoromethylornithine (5 mM) and methylglyoxal bis(guanylhydrazone) (1 mM) caused an approximate doubling of expressed Kir2.1 currents and relieved rectification by causing an approximately +10-mV shift of the voltage at which currents are half-maximally inhibited. Second, a putrescine auxotrophic, ornithine decarboxylase-deficient Chinese hamster ovary (O-CHO) cell line was stably transfected with the cDNA encoding Kir2.3. Withdrawal of putrescine from the medium led to rapid (1-day) loss of the instantaneous phase of Kir2.3 channel activation, consistent with a decline of intracellular putrescine levels. Four days after putrescine withdrawal, macroscopic conductance, assessed using an 86Rb+ flux assay, was approximately doubled, and this corresponded to a +30-mV shift of V1/2 of rectification. With increasing time after putrescine withdrawal, there was an increase in the slowest phase of current activation, corresponding to an increase in the spermine-to-spermidine ratio over time. These results provide direct evidence for a role of each polyamine in induction of rectification, and they further demonstrate that in vivo modulation of rectification is possible by manipulation of polyamine levels using genetic and pharmacological approaches.

Intracellular polyamines mediate inward rectification of Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that lack the glutamate receptor GluR2 subunit are Ca(2+)-permeable and exhibit inwardly rectifying current responses to kainate and AMPA. A proportion of cultured rat hippocampal neurons show similar Ca(2+)-permeable inwardly rectifying AMPA receptor currents. Inward rectification in these neurons was lost with intracellular dialysis and was not present in excised outside-out patches but was maintained in perforated-patch whole-cell recordings, suggesting that a diffusible cytoplasmic factor may be responsible for rectification. Inclusion of the naturally occurring polyamines spermine and spermidine in the recording pipette prevented loss of rectification in both whole-cell and excised-patch recordings; Mg2+ and putrescine were without effect. Inward rectification of Ca(2+)-permeable AMPA receptors may reflect voltage-dependent channel block by intracellular polyamines.

Rectification and signal averaging of weak electric fields by biological cells.

Oscillating electric fields can be rectified by proteins in cell membranes to give rise to a dc transport of a substance across the membrane or a net conversion of a substrate to a product. This provides a basis for signal averaging and may be important for understanding the effects of weak extremely low frequency (ELF) electric fields on cellular systems. We consider the limits imposed by thermal and "excess" biological noise on the magnitude and exposure duration of such electric field-induced membrane activity. Under certain circumstances, the excess noise leads to an increase in the signal-to-noise ratio in a manner similar to processes labeled "stochastic resonance." Numerical results indicate that it is difficult to reconcile biological effects with low field strengths.