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Characterization of Hyperpolarization-Activated Cation Currents in Mouse Anterior Pituitary, AtT20 D16:16 Corticotropes¹

Membrane Biology Group, Department of Biomedical Sciences, University of Edinburgh Medical School, Edinburgh, Scotland EH8 9AG, United Kingdom

Address all correspondence and requests for reprints to: Michael J. Shipston, Ph.D., Membrane Biology Group, Department of Biomedical Sciences, University of Edinburgh Medical School, Teviot Place, Edinburgh, Scotland EH8 9AG, United Kingdom. E-mail: mike.shipston{at}ed.ac.uk

The properties of the hyperpolarization-activated inward cation current (I_h) in mouse anterior pituitary, AtT20 D16:16 corticotropes was characterized by whole cell patch clamp recording. In response to hyperpolarizing steps a large, slowly activating, voltage-dependent inward current was activated with a half maximal activation voltage (V_0.5) of -96.2 ± 3.1 mV with a time constant of 168 ± 13 msec determined at -140 mV at room temperature. I_h had a reversal potential of -35.5 ± 1.0 mV and -23.3 ± 1.4 mV using 5 mM and 25 mM extracellular K⁺, respectively, with a relative permeability ratio for Na⁺ and K⁺ of 0.24. The current was completely blocked by 2 mM extracellular CsCl and partially blocked by ZD7288 (100 µM) but was unaffected by TEA (10 mM) or Ba²⁺ (1 mM). RT-PCR analysis revealed robust expression of HCN1, but not HCN2 or HCN3, subunits of hyperpolarization-activated cation channels. The endogenous I_h current was weakly activated by cAMP but robustly inhibited by the cAMP antagonist, Rp-8-CPT-cAMPS. Activation or suppression of protein kinase C activity had no significant effect on the I_h current. The data suggest that in AtT20 D16:16 corticotropes I_h is tonically regulated by the cAMP-signaling cascade and may serve to limit excessive hyperpolarization.

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