A NOVEL SLOW HYPERPOLARIZATION-ACTIVATED POTASSIUM CURRENT (I-K(SHA))FROM A MOUSE HIPPOCAMPAL CELL-LINE

1. A slow hyperpolarization-activated inwardly rectifying K+ current ( I-K(SHA)) with novel characteristics was identified from the mouse emb ryonic hippocampus x neuroblastoma cell line HN9.10e. 2. The non-inact ivating current activated negative to a membrane potential of -80 mV w ith slow and complex activation kinetics (tau(act) approximate to 1-7 s) and a characteristic delay of 1-10 s (-80 to -140 mV) that was line arly dependent on the membrane potential. 3. Tail currents and instant aneous open channel currents determined through fast voltage ramps rev ersed at the K+ equilibrium potential (E-K) indicating that primarily K+, but not Na+, permeated the channels. 4. I-K(SHA) was unaffected by altering the intracellular Ca2+ concentration between similar to 0 an d 10 mu M, but was susceptible to block by 5 mM extracellular Ca2+, Ba 2+ (K-i = 0.42 mM), and Cs+ (K-i = 2.77 mM) 5. In cells stably transfo rmed with M2 muscarinic receptors, I-K(SHA) was rapidly, but reversibl y, suppressed by application of micromolar concentrations of muscarine . 6. At the single channel level II(SHA) channel openings were observe d with the characteristic delay upon membrane hyperpolarization. Analy sis of unitary currents revealed an inwardly rectifying I-V profile an d a channel slope conductance of 7 pS. Channel activity persisted in t he inside-out configuration for many minutes. 7. It is concluded that I-K(SHA) in HN9.10e cells represents a novel K+ current, which is acti vated upon membrane hyperpolarization. It is functionally different fr om both classic inwardly rectifying I-Kir currents and other cationic hyperpolarization-activated I-H currents that have been previously des cribed in neuronal or glial cells.