Expression of cardiac Na channels with appropriate physiological and pharmacological properties in Xenopus oocytes

Developmental changes in two voltage-dependent sodium currents in utricular hair cells

Two kinds of sodium current (I(Na)) have been separately reported in hair cells of the immature rodent utricle, a vestibular organ. We show that rat utricular hair cells express one or the other current depending on age (between postnatal days 0 and 22, P0-P22), hair cell type (I, II, or immature), and epithelial zone (striola vs. extrastriola). The properties of these two currents, or a mix, can account for descriptions of I(Na) in hair cells from other reports. The patterns of Na channel expression during development suggest a role in establishing the distinct synapses of vestibular hair cells of different type and epithelial zone. All type I hair cells expressed I(Na,1), a TTX-insensitive current with a very negative voltage range of inactivation (midpoint: -94 mV). I(Na,2) was TTX sensitive and had less negative voltage ranges of activation and inactivation (inactivation midpoint: -72 mV). I(Na,1) dominated in the striola at all ages, but current density fell by two-thirds after the first postnatal week. I(Na,2) was expressed by 60% of hair cells in the extrastriola in the first week, then disappeared. In the third week, all type I cells and about half of type II cells had I(Na,1); the remaining cells lacked sodium current. I(Na,1) is probably carried by Na(V)1.5 subunits based on biophysical and pharmacological properties, mRNA expression, and immunoreactivity. Na(V)1.5 was also localized to calyx endings on type I hair cells. Several TTX-sensitive subunits are candidates for I(Na,2).

Effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes

BDF 9198 (a congener of DPI 201 - 106 and BDF 9148) was found to be a positive inotrope on guinea-pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes were studied using the whole cell patch clamp method. In normal external solution, at 37 degrees C, action potential duration at 50% repolarization (APD(50)) was 167.4+/-8.36 ms (n=37). BDF 9198 produced a concentration-dependent increase in APD(50) (no significant increase at 1x10(-10) M; and APD(50) values of 273.03+/-35.8 ms at 1x10(-9) M; n=6, P<0.01 and 694.7+/-86.3 ms at 1x10(-7) M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after-depolarizations. Qualitative measurements of I(Na) with physiological [Na](o) showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations. Quantitative recording conditions for I(Na) were established using low external [Na] and by making measurements at room temperature. The current - voltage relation, activation parameters and time-course of I(Na) were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 microM), despite a 2 fold difference in current amplitude. This suggests that voltage-clamp during flow of I(Na) was adequately maintained under our conditions. Selective measurements of I(Na) at room temperature showed that BDF 9198 induced a concentration-dependent, sustained component of I(Na) (I(Late)) and caused a slight left-ward shift in the current - voltage relation for peak current. The drug-induced I(Late) showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and I(Late) were abolished by 30 microM TTX and were sensitive to external [Na]. Inactivation of control I(Na) during a 200 ms test pulse to -30 mV followed a bi-exponential time-course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time-constant unchanged, but increased the magnitude of the slow inactivation time-constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, I(Na) inactivation may be comprised of more than two phases. No significant effects of 1x10(-6) M BDF 9198 were observed on the L-type calcium current, or delayed and inward rectifying potassium currents measured at 37 degrees C. It is concluded that the prolongation of APD(50) by BDF 9198 resulted from selective modulation of I(Na). Reduced current inactivation induced a persistent I(Na), increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for 'QT prolongation' of the ECG. The observation of after-depolarizations suggests a potential for proarrhythmia at some drug concentrations.

TTX resistivity of Na+ channel in newt retinal neuron

We examined voltage-dependent, TTX-resistant Na+ channels of newt retina (nRNaCh) electrophysiologically. IC50-TTX value of nRNaCh is more than 20 microM. We determined partial cDNA sequences of nRNaCh restricted to TTX binding sites (SS2 regions of all four repeats). While the amino acid sequences of SS2 regions of repeats II, III and IV of nRNaCh are identical to those of TTX-sensitive Na+ channels, only one amino acid in SS2 of repeat I of nRNaCh is different. nRNaCh have nonaromatic amino acid (Ala) in this site instead of the aromatic amino acid in a case of TTX-sensitive Na+ channels. Many studies suggested that the differences of TTX sensitivity of Na+ channels are decided by whether amino acid in this site is aromatic or not. Therefore nRNaCh acquire their TTX resistivity with the same mechanism as TTX-resistant cardiac Na+ channels do.

Developmental temperature selectively regulates a voltage-activated potassium current in Drosophila

Ionic currents are regulated by many conditions including disease states, aging, learning and memory, and chronic drug treatment. Here we describe a novel phenomenon of regulation of ionic currents by developmental temperature. Raising Drosophila larvae at 28 degrees C instead of 18 degrees C increased one of the two voltage-activated K(+)-currents, the delayed sustained IK, in their muscles by up to 3.5-fold, with little effect on the early transient current, IA. Consistent with this increase in IK, the amplitude and the duration of the action potentials were reduced. The major increase in IK occurred between a rather abrupt interval from 25 degrees to 28 degrees C. The activation curve of the increased current was shifted towards hyperpolarizing potentials. There was no change in activation kinetics. This phenomenon has mechanistic implications for activity-dependent neuronal plasticity, expression of ion channels in cultured cells and heterologous systems, phototransduction, and behavior. The specificity of the regulation suggests a discrete mechanism geared to affect excitability such that it can respond to altered external stimuli such as temperature.

Structure, function and expression of voltage-dependent sodium channels

Voltage-dependent sodium channels control the transient inward current responsible for the action potential in most excitable cells. Members of this multigene family have been cloned, sequenced, and functionally expressed from various tissues and species, and common features of their structure have clearly emerged. Site-directed mutagenesis coupled with in vitro expression has provided additional insight into the relationship between structure and function. Subtle differences between sodium channel isoforms are also important, and aspects of the regulation of sodium channel gene expression and the modulation of channel function are becoming topics of increasing importance. Finally, sodium channel mutations have been directly linked to human disease, yielding insight into both disease pathophysiology and normal channel function. After a brief discussion of previous work, this review will focus on recent advances in each of these areas.

State-dependent block underlies the tissue specificity of lidocaine action on batrachotoxin-activated cardiac sodium channels

We have identified two kinetically distinct modes of block, by lidocaine, of cardiac sodium channels, activated by batrachotoxin and incorporated into planar lipid bilayers. Here, we analyze the slow blocking mode which appears as a series of nonconducting events that increase in frequency and duration with increasing lidocaine concentrations. This type of block occurred rarely, if at all, for the skeletal muscle sodium channel subtype. Kinetic analysis showed that a linear open-closed-blocked model is sufficient to account for the major features of our data. Slow block occurs from a long closed state that is a distinguishing characteristic of cardiac channels under these conditions. Slow block showed no significant voltage dependence in the range of -60 to -20 mV for which the detailed kinetic analysis was performed, and was not elicited by application of the permanently charged lidocaine derivative QX-314. By contrast, the fast block, described in the companion paper, results from drug binding to the open state, and is similar for cardiac and skeletal muscle sodium channels. Application of trypsin to the cytoplasmic end of the channel eliminates both the spontaneous, long, gating closures and slow block. Thus, the lidocaine-sensitive closed state of batrachotoxin-activated cardiac sodium channels exhibits a protease susceptibility resembling that of the inactivated state of unmodified sodium channels. It is the slow block caused by lidocaine binding to this closed state that underlies the channel-subtype specificity of lidocaine action in our experiments.

Neuroanatomical distribution and binding properties of saxitoxin sites in the rat and turtle CNS

Since saxitoxin (STX) binds to voltage-sensitive sodium channels and blocks their function, it has been widely used in the study of these channels. There is, however, limited information on STX binding properties and the neuroanatomical distribution of the Na+ channel as a function of brain region in the rat and in lower vertebrates such as the turtle. In the present study, we used a broad range of 3H-STX concentration (up to 64 nM) to examine saturation profiles and density distribution in both adult rat and turtle brains. We found that (1) STX sites do not vary greatly in affinity (most Kds = 2 to 5 nM) in various regions of the adult rat brain; (2) STX binding distribution was very heterogeneous in the rat with much higher density in the cortex, hippocampus, amygdala, and cerebellum than in the brainstem and spinal cord; (3) STX sites are mostly localized in layers made mostly of neurons with low density in white matter; and (4) turtle brain STX sites had similar binding properties, but its brain had much fewer STX sites than the rat, especially in the cerebellum and rostral areas such as the cortex. We conclude that (a) adult brain sodium channels have similar STX binding affinity in spite of the existence of multiple sodium channel subtypes; (b) the brainstem is very different from rostral brain areas in channel density; and (c) the turtle brain has a much lower sodium channel density than the rat brain.

Characterization of endogenous sodium channel gene expressed in Chinese hamster ovary cells

Chinese hamster ovary (CHO-K1) cells were observed to display transient inward Na+ currents of average amplitude (-92 +/- 20 pA), which activated at voltages more than -40 mV, and peak inward currents were observed at potentials equal to or more than +10 mV. Inward Na+ currents in these cells were eliminated after treatment with 500 or 50 nM tetrodotoxin (TTX), whereas 5 nM TTX resulted in 64 +/- 10% inhibition of Na+ current. Using DNA primers designed to bind to the rat brain IIA Na+ channel subtype, we amplified specific polymerase chain reaction (PCR) fragments from CHO-K1 poly-(A)+RNA. The cloning and sequencing of two of these fragments confirmed the presence of an endogenously expressed Na+ channel gene in these cells, which we have termed cho 1. Comparison of the DNA sequence of cho 1 PCR fragments with other known Na+ channel genes indicated a high degree of homology with rat brain Na+ channel subtypes. Northern blots using riboprobes generated from the cho 1 PCR fragments revealed the presence of a specific 9-kb mRNA in these cells. The molecular and electrophysiological data suggest that the cho 1 Na+ channel gene from CHO-K1 cells is closely related to brain-type Na+ channels.

The cloned cardiac Na channel alpha-subunit expressed in Xenopus oocytes show gating and blocking properties of native channels

The neonatal rat cardiac Na channel alpha-subunit directed currents in oocytes show characteristic cardiac relative resistance to tetrodotoxin (TTX) block. TTX-sensitive currents obtained by expression in Xenopus oocytes of the alpha-subunits of the rat brain (BrnIIa) and adult skeletal muscle (microI) Na channels show abnormally slow decay kinetics. In order to determine if currents directed by the cardiac alpha-subunit (RHI) exhibit kinetics in oocytes like native currents, we compared RHI-directed currents in oocytes to Na currents in freshly isolated neonatal rat myocytes. The decay rate of RHI currents approached that of neonatal myocytes and was faster than BrnIIa and microI currents in oocytes. The voltage dependence of availability and activation was the same as that in the rat myocytes except for a 12-19 mV shift in the depolarizing direction. The RHI Na currents were sensitive to Cd2+ block, and they showed use dependence of TTX and lidocaine block similar to native currents. The current expressed in oocytes following injection of the cRNA encoding for the alpha-subunit of the cardiac Na channel possesses most of the characteristic kinetic and pharmacological properties of the native cardiac Na current.

Primary structure and functional expression of the human cardiac tetrodotoxin-insensitive voltage-dependent sodium channel

The principal voltage-sensitive sodium channel from human heart has been cloned, sequenced, and functionally expressed. The cDNA, designated hH1, encodes a 2016-amino acid protein that is homologous to other members of the sodium channel multigene family and bears greater than 90% identity to the tetrodotoxin-insensitive sodium channel characteristic of rat heart and of immature and denervated rat skeletal muscle. Northern blot analysis demonstrates an approximately 9.0-kilobase transcript expressed in human atrial and ventricular cardiac muscle but not in adult skeletal muscle, brain, myometrium, liver, or spleen. When expressed in Xenopus oocytes, hH1 exhibits rapid activation and inactivation kinetics similar to native cardiac sodium channels. The single channel conductance of hH1 to sodium ions is about twice that of the homologous rat channel and hH1 is more resistant to block by tetrodotoxin (IC50 = 5.7 microM). hH1 is also resistant to mu-conotoxin but sensitive to block by therapeutic concentrations of lidocaine in a use-dependent manner.