Preston R R, Saimi Y, Kung C
Laboratory of Molecular Biology, University of Wisconsin-Madison 53706.
J Gen Physiol. 1992 Aug;100(2):233-51. doi: 10.1085/jgp.100.2.233.
Hyperpolarization of Paramecium tetraurelia under conditions where K+ currents are suppressed elicits an inward current that activates rapidly toward a peak at 25-80 ms and decays thereafter. This peak current (Ihyp) is not affected by removing Cl ions from the microelectrodes used to clamp membrane potential, or by changing extracellular Cl- concentration, but is lost upon removing extracellular Ca2+. Ihyp is also lost upon replacing extracellular Ca2+ with equimolar concentrations of Ba2+, Co2+, Mg2+, Mn2+, or Sr2+, suggesting that the permeability mechanism that mediates Ihyp is highly selective for Ca2+. Divalent cations also inhibit Ihyp when introduced extracellularly, in a concentration- and voltage-dependent manner. Ba2+ inhibits Ihyp with an apparent dissociation constant of 81 microM at -110 mV, and with an effective valence of 0.42. Ihyp is also inhibited reversibly by amiloride, with a dissociation constant of 0.4 mM. Ihyp is not affected significantly by changes in extracellular Na+, K+, or H+ concentration, or by EGTA injection. Also, it is unaffected by manipulations or mutations that suppress the depolarization-activated Ca2+ current or the various Ca(2+)-dependent currents of Paramecium. We suggest that Ihyp is mediated by a novel, hyperpolarization-activated calcium conductance that is distinct from the one activated by depolarization.
在抑制钾离子电流的条件下，四膜虫的超极化会引发一种内向电流，该电流在25 - 80毫秒内迅速激活至峰值，随后衰减。这个峰值电流（Ihyp）不受用于钳制膜电位的微电极去除氯离子或改变细胞外氯离子浓度的影响，但在去除细胞外钙离子后会消失。当用等摩尔浓度的钡离子、钴离子、镁离子、锰离子或锶离子取代细胞外钙离子时，Ihyp也会消失，这表明介导Ihyp的通透机制对钙离子具有高度选择性。二价阳离子在细胞外引入时也会以浓度和电压依赖的方式抑制Ihyp。在-110 mV时，钡离子抑制Ihyp的表观解离常数为81 microM，有效价为0.42。Ihyp也会被氨氯吡咪可逆性抑制，解离常数为0.4 mM。细胞外钠离子、钾离子或氢离子浓度的变化，或EGTA注射对Ihyp没有显著影响。此外，抑制四膜虫去极化激活的钙离子电流或各种钙依赖性电流的操作或突变对其也没有影响。我们认为Ihyp是由一种新型的超极化激活钙电导介导的，它与去极化激活的钙电导不同。
