Inward rectifier K+ channels mediate the K+ conductance at resting potential in many types of cell. Since these K+ channels do not pass outward currents (inward rectification) when the cell membrane is depolarized beyond a trigger threshold, they play an important role in controlling excitability. Both a highly voltage-dependent block by intracellular Mg2+ and an endogenous gating process are presently assumed to underly inward rectification. It is shown that strong voltage dependence of rectification found under physiological conditions is predominantly due to the effect of intracellular spermine. Physiological concentrations of free spermine mediate strong rectification of IRK1 inward rectifier K+ channels even in the absence of free Mg2+ and in IRK1 mutant channels that have no endogenous rectification.

Potassium (K+) homeostasis is controlled by the secretion of K+ ions across the apical membrane of renal collecting duct cells through a low‐conductance inwardly rectifying K+ channel. The sensitivity of this channel to intracellular pH is particularly high and assumed to play a key role in K+ homeostasis. Recently, the apical K+ channel has been cloned (ROMK1,2,3 = Kir1.1a, Kir1.1b and Kir1.1c) and the pH dependence of ROMK1 was shown to resemble closely that of the native apical K+ channel. It is reported here that the steep pH dependence of ROMK channels is determined by a single amino acid residue located in the N‐terminus close to the first hydrophobic segment M1. Changing lysine (K) at position 80 to methionine (M) removed the sensitivity of ROMK1 channels to intracellular pH. In pH‐insensitive IRK1 channels, the reverse mutation (M84K) introduced dependence on intracellular pH similar to that of ROMK1 wild‐type. A detailed mutation analysis suggests that a shift in the apparent pKalpha of K80 underlies the pH regulation of ROMK1 channels in the physiological pH range.

P2X receptors are ion channels gated by extracellular ATP. We report here cloning of a P2X 2 receptor splice variant (P2X2_2) carrying a 207 bp deletion in the intracellular Cterminus and the analysis of the corresponding genomic structure of the P2X 2 gene. P2X 2 _2 is as highly expressed as the original P2X 2 sequence in various tissues. ATP-activated currents mediated by heterologous expressed P2X 2 or P2X2_ 2 receptors showed significant differences in desensitization time constants and steady-state currents in the continuous presence of ATP. These results imply functional differences between cells differentially expressing these P2X 2 isoforms.

Large subtype-specific differences in the sensitivity of cloned inward-rectifier K' channels of the IRKl, BIRlO and ROMKl subtype to being blocked by intracellular spermine (SPM) are described. It is shown, by site-directed mutagenesis, that the four orders of magnitude larger SPM sensitivity of BIRlO channels compared to ROMKl channels may be explained by a difference in a single amino acid in the putative transmembrane segment TMII. This residue, a negatively charged glutamate in BIRlO, is homologous to the residue in IRK1 and ROMKl which has previously heen shown to change gating properties and Me sensitivity. Differential block by physiological SPM concentrations is suggested as a major functional difference between subtypes of inward-rectifier K' channels.

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The P2X 3 receptor is an ATP-gated ion channel predominantly expressed in nociceptive neurons from the dorsal root ganglion. P2X 3 receptor channels are highly expressed in sensory neurons and probably contribute to the sensation of pain. Kinetics of P2X 3 currents are characterized by rapid desensitization (<100 ms) and slow recovery (>20 s). Thus, any mechanism modulating rate of desensitization and/or recovery may have profound effect on susceptibility of nociceptive neurons expressing P2X 3 to ATP. Here we show that currents mediated by P2X 3 receptor channels and the heteromeric channel P2X 2/3 composed of P2X 2 and P2X 3 subunits are potentiated by the neuropeptides substance P and bradykinin, which are known to modulate pain perception. The effect is mediated by the respective neuropeptide receptors, can be mimicked by phorbol ester and blocked by inhibitors of protein kinases. Together with data from site-directed mutagenesis our results suggest that inflammatory mediators sensitize nociceptors through phosphorylation of P2X 3 and P2X 2/3 ion channels or associated proteins.P2X receptors mediate fast responses of excitable cells to application of ATP. So far, seven different P2X subunits have been cloned (P2X 1-7 ), which form a gene family (1). They have two transmembrane domains with intracellular termini and a rather large extracellular loop (2, 3) and form non-selective cation channels with a high permeability to Ca 2ϩ . Various P2X receptors show differences both in the affinity to ATP and in the kinetics of activation and inactivation. P2X 1 and P2X 3 show high agonist affinity (EC 50 Ϸ 1 M) and rapid activation and desensitization with full activation in less than 10 ms and almost complete desensitization in less than 1 s (4, 5). Moreover, repeated application of the agonist leads to the disappearance of the ATP-gated currents (4, 5). In contrast, P2X 2 and P2X 4 through P2X 7 have a low agonist affinity (EC 50 Ϸ 10 M), are slowly activating with time constants in the order of 100 ms, and show only partial desensitization (1, 6). Heterooligomeric P2X 2/3 channels show a mixed phenotype with high affinity to the agonist ␣,␤-methylene ATP (␣,␤-meATP) 1 like P2X 3 receptors and incomplete desensitization like P2X 2 receptors (5).The desensitization properties of the rapidly activating and inactivating P2X 1 and P2X 3 are likely to be of physiological importance as synaptic transmission takes place in a few milliseconds. Modulation of the rate of desensitization and/or recovery of these receptors may contribute to synaptic efficacy. P2X 3 is almost exclusively expressed in sensory neurons (7, 8), mostly in capsaicin-sensitive nociceptors (9 -11). P2X 3 -mediated currents in sensory neurons are large (12), and P2X 3 shows the strongest expression level in dorsal root ganglion (DRG) neurons compared with other P2X receptors (10). The P2X 3 protein is found on the sensory endings as well as on the presynaptic membrane in inner lamina II of the spinal horn (9), and its activation by ATP might contribute t...

The channels that control K ؉ homeostasis by mediating K ؉ secretion across the apical membrane of renal tubular cells have recently been cloned and designated ROMK1, -2, and -3. Native apical K ؉ channels are indirectly regulated by the K ؉ concentration at the basolateral membrane through a cascade of intracellular second messengers. It is shown here that ROMK1 (K ir 1.1) channels are also directly regulated by the extracellular (apical) K ؉ concentration, and that this K ؉ regulation is coupled to intracellular pH. The K ؉ regulation and its coupling to pH were assigned to different structural parts of the channel protein. K ؉ regulation is determined by the core region, which comprises the two hydrophobic segments M1 and M2 and the P region. Decoupling from pH was achieved by exchanging the N terminus of ROMK1 by that of the pH-insensitive channel IRK1 (K ir 2.1). These results suggest an allosteric regulation of ROMK1 channels by extracellular K ؉ and intracellular pH, which may represent a novel link between K ؉ homeostasis and pH control.

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