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1. Human and murine neuroblastoma cell lines were used to investigate, by the whole‐cell patch‐clamp technique, the properties of a novel inward‐rectifying K+ current (IIR) in the adjustment of cell resting potential (Vrest), which was in the range ‐40 to ‐20 mV. 2. When elicited from a holding potential of 0 mV, IIR was completely inactivated with time constants ranging from 13 ms at ‐140 mV to 4.5 s at ‐50 mV. The steady‐state inactivation curve (h(V)) was found to be independent of [Na+]o and [K+]o (2‐80 mM) and could be fitted to a Boltzmann curve with a steep slope factor of 5‐6, and a V1/2 around Vrest. Divalent ion‐free extracellular solutions shifted h(V) to the left by about 28 mV. 3. Peak chord conductance, whose maximal value was approximately proportional to the square root of [K+]o, could be fitted to a Boltzmann curve independently of [K+]o, with a V1/2 value around ‐48 mV and a slope factor of 18. Extracellular Cs+ and Ba2+ blocked the IIR in a concentration‐ and voltage‐dependent manner, but Ba2+ was less effective than it is on classical inward‐rectifier channels. 4. Under control culture conditions the values of Vrest and V1/2 of h(V) varied widely among cells. The knowledge of V1/2 proved crucial or the theoretical prediction of Vrest. After cell synchronization in the G0‐G1 phase of the cell cycle, or at the G1‐S boundaries, the cells reduced their variability of h(V). The same occurred after cell synchronization in G1 by treatment with retinoic acid. 5. The experimental data could be fitted to a classical model of an inward rectifier, after removing the dependence of conductance activation on (V‐EK), and incorporating an inactivation with an intrinsic voltage dependence. Moreover, the model predicts, for this novel inward rectifier and in contrast with the classical inward rectifier, the incapacity of maintaining, in physiological media, a Vrest more negative than ‐35 to ‐40 mV, which is an important feature of cancer cells.

1. The relationships between the K+ inward rectifier current present in neuroblastoma cells (IIR) and the current encoded by the human ether‐á‐go‐go‐related gene (HERG), IHERG, and the rapidly activating repolarizing cardiac current IK(r), were investigated in a rat dorsal root ganglion (DRG) x mouse neuroblastoma hybrid cell line (F‐11) using pharmacological and biophysical treatments. 2. IIR shared the pharmacological features described for IK(r), including the sensitivity to the antiarrhythmic drugs E4301 and WAY‐123,398, whilst responding to Cs+, Ba2+ and La3+ in a similar way to IHERG. 3. The voltage‐dependent gating properties of IIR were similar to those of IK(r) and IHERG, although IIR outward currents were negligible in comparison. 4. In high K+ extracellular solutions devoid of divalent cations, IIR deactivation kinetics were removed resulting in long‐lasting currents apparently activated in hyperpolarization, with a marked (2.7‐fold) increase in conductance, as recorded from the instantaneous linear current‐voltage relationship at ‐120 mV. Re‐addition of Ca2+ restored the original closure of the channel whereas re‐addition of Mg2+ reduced the peak current. 5. The IIR described here, the heart IK(r) and the IHERG could be successfully predicted by a unique kinetic model where the voltage dependencies of the activation/inactivation gates were properly voltage shifted. On the whole, IIR seems to be the first example of a HERG‐type current constitutively expressed and operating in mammalian cells of the neuronal lineage.

Post‐ganglionic neurones of the isolated rat superior cervical ganglion were voltage clamped at 37 degrees C using separate intracellular voltage and current micro‐electrodes. Control experiments in current clamp suggested that the neurone is electrotonically compact, the soma and the proximal dendritic membranes being under good spatial voltage uniformity. Depolarizing voltage steps from membrane potentials near ‐50 mV evoked: (i) a voltage‐dependent inward Na+ current, (ii) an inward Ca2+ current, (iii) a voltage‐dependent outward K+ current, (iv) a Ca2+‐activated K+ outward current. Depolarizations from holding potentials more negative than ‐60 mV elicited, besides the currents mentioned above, a fast transient outward current IA which peaked in 1‐2.5 ms and then decayed to zero following an exponential time course. The IA current was shown to be primarily, if not exclusively, carried by K+. It was unaffected by removal of external Ca2+ or addition of Cd2+ and was weakly blocked by tetraethylammonium ions and partially by 4‐aminopyridine. The IA current showed a linear instantaneous current‐voltage relationship. Its activation ranged from ‐60 to 0 mV with a mid‐point at ‐30 mV. The A conductance could be described in terms of a simple Boltzmann distribution for a single gating particle with a valency of +3. Both the development and removal of inactivation followed a single exponential time course with a voltage‐dependent time constant which was large near the resting potential (42 ms at ‐70 mV) and small (11 ms) near ‐100 and ‐40 mV. Steady‐state inactivation h infinity ranged from ‐100 to ‐50 mV, with a mid‐point at ‐78 mV, suggesting that approximately 50% of the IA channels are available at the physiological resting potential. Action potentials elicited from various holding potentials showed maximal repolarization rates dependent on the holding potential itself. This voltage dependence was found to be in reasonably good agreement with that of h infinity curve. These data are consistent with the view that in the rat sympathetic neurone, under physiological conditions, it is the IA current rather than the delayed outward current that is responsible for the fast action potential repolarization.

Post‐ganglionic neurones of the isolated rat superior cervical ganglion were studied at 37 degrees C under two‐electrode voltage‐clamp conditions. Membrane depolarization beyond ‐40 mV from holding levels between ‐50 and ‐100 mV produced a delayed outward current which exhibited no inactivation within this voltage range. The current is carried primarily by K+ ions and its instantaneous I‐V relation is linear. The total outward current could be separated into two distinct components on the basis of ion‐substitution experiments. A voltage‐dependent component of the delayed current, termed IK(V), is activated by membrane depolarization beyond ‐40 mV when Ca2+ fluxes are selectively blocked by Cd2+ or in Ca2+‐free solution. IK(V) develops following first‐order kinetics and rises to a peak with a voltage‐dependent delay (239 ms at ‐30 mV and 23 ms at +10 mV). GK(V) attains a saturating value of the order of 17 mS/cm2 at about +20 mV and can be described in terms of a simple Boltzmann distribution for a single gating particle with a valency equal to +2.5. A second component of the delayed outward current, termed IK(Ca), depends on Ca2+ entry for its activation and was isolated as difference current before and after block of Ca2+ movements across the membrane. IK(Ca) is larger and faster than IK(V): it is strictly related to Ca2+ influx and also depends on membrane potential depolarization. A distinct Ca2+ current, ICa, was recorded from the neurone exposed to Na+‐free or tetrodotoxin solution. ICa was activated by membrane depolarization beyond ‐30 mV and reached a maximum value near 0 mV. Its activation agrees with fourth‐order kinetics and becomes faster with increasing depolarization. The Ca2+ current developed with a voltage‐dependent time to peak of 2.9‐1.8 ms and thereafter completely inactivated. The relationship between ICa and IK(Ca) is discussed. The Ca2+‐k+ repolarizing system is expected to be mainly associated with action potentials arising from a depolarized neurone, whereas the IA current (Belluzzi, Sacchi & Wanke, 1985) dominates the repolarization mechanism at the normal membrane potential. The effect of muscarine was examined. Muscarine (10‐50 microM) produced a fall in conductance with a voltage dependence similar to that exhibited by GK(Ca) and was ineffective when removing extracellular Ca2+ or adding Cd2+. A partial suppression of ICa by muscarine is demonstrated. It is suggested that the decrease of the outward current magnitude in the presence of muscarine may be accounted for qualitatively by the reduction in ICa.

1. The spectral density of current noise power from 20 mm segments of giant axons of the squid Loligo vulgaris has been measured under space‐clamp and voltage‐clamp conditions. From 4 to 1000 Hz the measured noise is larger by several orders of magnitude than the theoretical thermal noise. The amplifier's noise, which may yield appreciable contributions above 200 Hz, could be evaluated and subtracted from the total noise using direct measurements of the membrane impedance...