The inwardly rectifying K channel provides the resting K conductance in a variety of cells. This channel acts as a valve or diode, permitting entry of K+ under hyperpolarization, but not its exit under depolarization. This behaviour, termed inward rectification, permits long depolarizing responses which are of physiological significance for the pumping function of the heart and for fertilization of egg cells. Little is known about the outward currents through the inwardly rectifying K channel, despite their great physiological importance, and the mechanism of inward rectification itself is unknown. We have used improved patch clamp techniques to control the intracellular media, and have recorded the outward whole-cell and single-channel currents. We report here that the channel conductance is ohmic and that the well-known inward rectification of the resting K conductance is caused by rapid closure of the channel accompanied by a voltage-dependent block by intracellular Mg2+ ions at physiological concentrations.

In a wide variety of cells, various intracellular agents, such as Ca2+, ATP and cyclic nucleotides, regulate ionic conductances of the membrane. In cardiac cells, the intracellular Na+ concentration [( Na+]i) frequently increases when a disturbance occurs in the electrogenic Na-K pump activity or the Na-Ca exchange mechanism. We have investigated a possible role of [Na+]i in controlling ion channels by using a patch-clamp method, and have found a K+ channel that is gated by [Na+]i greater than 20 mM, but not by the intracellular Ca2+ concentration (approximately 10(-4) M). We report here that the channel has a unitary conductance of 207 +/- 19 pS (n = 16) with K+ concentrations of 150 mM outside and 49 mM inside, and shows no detectable voltage-dependent kinetics. The Na+-activated K+ channel represents a novel class of ionic channel.

SUMMARY1. Outward single-channel currents through inwardly rectifying K+ channels of cardiac myocytes were studied in the open cell-attached configuration to clarify the mechanism of the rectification. The outward currents, which were not recorded in the cell-attached configuration, appeared after the inner surface of the patch was exposed to low-Mg2+ solution by rupturing a part of the cell membrane.2. The single-channel current-voltage (I-V) relation was linear in the absence of Mg2+ and crossed the voltage axis near the equilibrium potential for K+ (EK). The channel conductance was 22 and 16 pS (15-16°C) at external K+ concentrations of 150 and 40 mm, respectively.3. The channel rapidly closed on stepping the membrane potential of the patch to values more positive than EK. Decay of the average current during depolarization was fitted with a single-exponential function. The time constant appeared voltage dependent, but also tended to increase slowly with time after opening the cell to the bath solution.4. Mg2+ on the cytoplasmic side blocked the outward currents without affecting the inward currents. The half-saturation concentration of the Mg2+ block was 1-7 /4M as examined by measuring the mean patch current at +70 mV.5. In the presence of internal Mg2+ at a micromolar level (2-10 ftM), the outward single-channel current fluctuated between four levels including two intermediate levels (sublevels) in addition to the fully open channel current and the zero-current levels. The I-V relations of each sublevel were equally spaced with an interval of about 7 pS. Corresponding sublevels were found spontaneously in the inward direction. 6. Occupancy at each level was estimated from reconstructed traces at various Mg2+ concentrations and voltages, and compared with the value predicted from the binomial theorem. At different probabilities for the blocked state, the distribution of the current levels showed reasonable agreement with the binomial theorem. These findings suggest that the inwardly rectifying K+ channel of cardiac cells is composed of three identical conducting subunits and each subunit is blocked by Mg2+ independently.

Background:The physiological role of trehalose as a hemolymph sugar during insect development remains unclear. Results: Mutants of the trehalose-synthesizing enzyme Tps1 failed to produce trehalose. Conclusion: Drosophila larvae lacking the hemolymph sugar trehalose exhibit diet-dependent phenotypes of growth and survival. Significance: Tps1 mutant flies are particularly useful in unraveling a wide range of physiological processes, such as homeostasis, aging, and stress resistance.