Prolongation of the QT interval corrected for heart rate (QTc) can lead to the development of torsades de pointes, a life-threatening form of polymorphic ventricular tachycardia. However, the QTc interval duration exhibits a high degree of spontaneous variability and is not necessarily a direct predictor of the risk of torsades. This observation holds implications for the assessment of the potential proarrhythmic effects of noncardiac pharmacologic agents. To date, the antihistamine terfenadine is the only noncardiac drug that has undergone a comprehensive and systematic evaluation related to the consequences of its causing QTc prolongation. The results suggest that QTc prolongation resulting solely from terfenadine at clinical doses does not have an important impact on clinically relevant endpoints. The risk of serious ventricular arrhythmias with terfenadine using epidemiologic data is the same or less than that associated with traditional first-generation antihistamines. The risk of a clinical cardiac event (QTc prolongation, ventricular arrhythmias, syncope, or sudden death) with terfenadine is similar to that of other antihistamines. Factors associated with increased risk in patients taking terfenadine include significant liver disease, hypokalemia, overdose, and concomitant administration of ketoconazole-like agents or erythromycin; use of terfenadine is relatively contraindicated in these settings. No increased risk of serious arrhythmias has been confirmed in conjunction with the use of terfenadine in patients with cardiac disease.

Voltage-dependent potassium currents of neurons enzymatically isolated from the medial and dorsal subnuclei of the solitary tract (mNTS) of adult guinea pig have been characterized with respect to their voltage dependence, time dependence, and sensitivity to specific blocking agents. This region of the medulla receives baroreceptor afferent input and is involved in cardiovascular regulation. Our results showed the presence of three types of potassium currents. First, in all neurons studied (n = 58) a slowly developing outward current was present at potentials more positive than -30 mV. The time to half-peak current decreased with depolarization [24.8 ms at 0 mV; 19.2 ms at +10 mV; 12.5 ms at +20 mV; 9.9 ms at +30 mV (n = 4)]. This current required 20 mM tetraethylammonium (TEA) for full block and failed to show significant inactivation for voltage commands up to 300 ms. Second, a rapidly activating, 4-aminopyridine (4-AP)-sensitive transient outward potassium current was present in 83% of the cells examined (n = 39/47). Threshold for activation was -30 mV. The current relaxation consisted of three components: tau 1 = 14-49 ms; tau 2 = 174-362; tau 3 = 1.1-2.4 s. Finally, in all cells tested calcium activated a large nontransient outward potassium current that was inhibited by charybdotoxin. The studies reported here will be used in conjunction with studies describing sodium and calcium currents to understand the basis for generation of activity in the mNTS in response to baroreceptor input.

We examined the effects of the benzolpyrrole-type Ca2+ channel activator FPL 64176 on voltage-dependent L-type Ca2+ channels in rat anterior pituitary (GH3) cells. FPL 64176 increased K(+)-dependent Ca2+ influx into GH3 cells with an EC50 value of 1.2 x 10(-7) M but had no effect on the binding of [3H]PN200-110 to GH3 cell membranes at concentrations up to 10(-6) M. Whole-cell patch-clamp electrophysiology revealed that FPL 64176 (1 microM) increased L-type Ca2+ channel current amplitude and shifted the current-voltage relationship in the hyperpolarizing direction. Furthermore, Ca2+ channel current activation and deactivation were prolonged. Single-channel analysis showed that FPL 64176 increased both the probability of channel opening and the mean channel open time. Interestingly, the effect of FPL 64176 on channel open time was highly voltage dependent, with much longer openings being observed at more hyperpolarized potentials. We conclude that FPL 64176 represents a new class of L-type Ca2+ channel activator with a novel site and mechanism of action.

Voltage-dependent Ca 2+ channels (VDCCs) are modulators of synaptic plasticity, oscillatory behavior, and rhythmic firing in brain regions such as the hippocampus. The distribution and lateral mobility of VDCCs on CA1 hippocampal neurons have been determined with biologically active fluorescent and biotinylated derivatives of the selective probe ω-conotoxin in conjunction with circular dityndallism, digital fluorescence imaging, and photobleach recovery microscopy. On noninnervated cell bodies, VDCCs were found to be organized in multiple clusters, whereas after innervation the VDCCs were concentrated and immobilized at synaptic contact sites. On dendrites, VDCC distribution was punctate and was interrupted by extensive bare regions or abruptly terminated. More than 85% of the dendritic VDCCs were found to be immobile by fluorescence photobleach recovery. Thus, before synaptic contact, specific mechanisms target, segregate, and immobilize VDCCs to neuronal cell bodies and to specialized dendritic sites. Regulation of this distribution may be critical in determining the firing activity and integrative properties of hippocampal CA1 neurons.

Although there is compelling pharmacological evidence based on Ca2+-channel antagonist studies suggesting that the voltage-dependent Ca2+ channels regulate insulin release, no direct comparison with Ca2+ currents exists. This is particularly important because of the recent demonstration in other cell types of one and possibly two Ca2+ channels that are insensitive to Ca2+-channel antagonists, the dihydropyridines and the phenylalkylamines. Using an SV40-transformed pancreatic β-cell line (HIT cells), we determined how voltage-dependent Ca2+ channels are involved in stimulus-secretion coupling. Ca2+ currents were measured with the tight-seal technique for wholecell recording. The cytosolic free-Ca2+ concentration ([Ca2+]1) was followed with the fluorescent probe Fura 2, and the measurements were compared with insulin secretion stimulated by depolarizing the cells with K+. The Ca2+ current contained two components: arapidly decaying current activated at –50 to –40 mV that decayed with a time constant of 25 ms anda very slowly decaying component activated at –40 mV. Both components were sensitive to the Ca2+-channel antagonist nimodipine. There is excellent agreement in the concentration of nimodipine that inhibited Ca2+ current and the increase in [Ca2+1], in response to K+ depolarization (IC50 of 15 and 6 nM, respectively). Nimodipine inhibited insulin release over a similar dose-response range with an IC50of 1.5 × 10 9 M. These studies indicate that the increase in [Ca2+1], in response to β-cell depolarization can be accounted for by the influx of this ion through a single class of dihydropyridine-sensitive Ca2+ channels in the cell membrane.

In most studies of synaptic currents in mammalian central neurons, preparations have been used in which synaptic currents are recorded at some distance from the synapse itself. This procedure introduces problems in interpretation of the kinetics and voltage-dependent properties of the synaptic current. These problems have now been overcome by the development of a preparation in which presynaptic vesicle-containing boutons have been coisolated with the soma of individual neurons, thus providing the opportunity to study synaptic currents under conditions of both adequate voltage control and internal ionic perfusion. Spontaneous synaptic currents mediated by γ-aminobutyric acid and excitatory amino acids were recorded from neurons isolated from a mammalian medial solitary tract nucleus. Calcium- and depolarization-dependent spontaneous currents of several to hundreds of picoamperes occurred with rapid rise times of 0.8 to 3 milliseconds and decays at least ten times as long.

To investigate whether cardiac sodium channels have dihydropyridine (DHP) receptors we studied the effects of the optically pure (greater than 95%) enantiomers of the DHPs PN200-110 and BAY-K 8644 and the racemic DHP nitrendipine (NTD). Whole cell and single-channel sodium currents were recorded from cultured ventricular cells of neonatal rats using the patch-clamp method. NTD reduced cardiac sodium currents in a voltage-dependent manner. Inhibitory effects were due to an increase in traces without activity. The unit conductance remained unchanged. At negative holding potentials, NTD transiently increased the probability of channel opening. Both (+) and (-) PN 200-110 blocked sodium channels, although the (-) isomer was about one order of magnitude less effective. The blocking effects were voltage dependent. (+) BAY-K 8644 had similar blocking effects. (-) BAY-K 8644 produced an increase in sodium currents due to an increased frequency of channel openings and a marked prolongation of open time without any significant change in unit conductance. The DHPs have effects on cardiac sodium whole cell and single-channel currents that appear identical to and are as stereospecific as their effects on cardiac calcium currents, although the concentrations required are larger. In contrast the inwardly rectifying potassium channel (IK1) is unaffected by these DHPs. We conclude that functionally equivalent DHP receptors are present in cardiac sodium and calcium channels but not potassium channels and take this as evidence of the homology between sodium and calcium channels.

We studied the effects of dihydropyridine Ca channel ligands (DHPs), mainly nitrendipine and Bay K8644, on whole cell and single channel Ca currents on single myocytes isolated from the adult guinea‐pig ventricle. Nitrendipine had dual effects, stimulatory or inhibitory, depending upon the membrane potential. At low frequencies (less than 0.03 Hz) and negative holding potentials (‐90 mV or more), nitrendipine increased the Ca currents in a dose‐dependent manner. The dose‐response curve was best fitted by a Langmuir adsorption isotherm model which was the sum of two independent one‐to‐one drug‐receptor sites with median effective doses (ED50S) of 1.0 X 10(‐9) M and 1.4 X 10(‐6) M respectively. When the membrane potential was held at ‐30 mV or less, nitrendipine inhibited the Ca currents, also in a dose‐dependent manner. The dose‐response curve was fitted by a single binding site model having a median inhibitor concentration (IC50) of 1.5 X 10(‐9) M. At holding potentials between ‐70 and ‐40 mV, nitrendipine produced mixed effects on Ca currents; an increase occurred initially and this was followed by a decrease. When rundown was excluded, Bay K8644 showed only stimulatory effects on the Ca currents between holding potentials of ‐120 and ‐30 mV. When the test potential was zero or +10 mV the Ca currents reached peak values and the dose‐response curve was best fitted by a single binding site model having an ED50 of 3 X 10(‐8) M. When the effects were measured at negative test potentials of ‐30 to ‐10 mV, the curve was best fitted by a two‐site model with ED50S of 3 X 10(‐9) and 9 X 10(‐7) M. At the single Ca channel level the stimulatory effect of nitrendipine was due to an increased probability that a Ca channel which had opened once would reopen, a reduction in records without activity and an increase in the mean open time. There were no changes in unit conductance. Inhibitory effects were due to a large increase in nulls. At lower concentrations the main effect of Bay K8644 was an increase in the probability of opening. At doses above 10(‐6) M, a pronounced increase in the open time was observed. The effects we observed are attributed to at least two sites for DHP related to Ca channels; one with high affinity and one with a lower affinity. The low affinity site mediates a stimulatory effect due to greatly prolonged openings.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium tall currents have a large, fast component which Is not detectable when the currents are turned on or activated from the completely rested state CBrown et al, 1983). This is probably because a rate limiting process In the earlier stages of activation dominates the turn-on. There is also evidence that Ca channels are regulated by transmitters (Reuter, 1983) and by Ca ions themselves (Brehm and Eckert, 1978; Tillotson, 1979; Brown et al, 1981), although Lux and Brown (1984) have recently speculated that the latter effect may not be due to a direct action of entering Ca ions. These findings suggest a chemical reaction may be involved in Ca current activation and expectation is that the kinetics of Ca currents might be strongly affected by changes in temperature. Moreover the temperature effects might have specificity for certain kinetic components. Therefore, in these experiments we have examined the effects of varying temperature on single Ca channels and whole cell Ca currents.

Inactivation of single Ca channels in snail neurons was examined to test the idea that entering Ca ions react directly with the channel to produce this effect. Simulations of specific models were used for comparison with the experimental data. The Ca-dependent model predicts time-dependent changes in the single-channel events which were not found experimentally. It is possible that Ca-dependent inactivation is mediated by a Ca-binding protein that is associated with but not part of the channel itself.

Tetrodotoxin (TTX)-sensitive Na currents were examined in single dissociated ventricular myocytes from neonatal rats . Single channel and whole cell currents were measured using the patch-clamp method . The channel density was calculated as 2/jrn1, which agreed with our usual finding of four channels per membrane patch . At 20°C, the single channel conductance was 20 pS . The open time distributions were fit by a single-exponential function with a mean open time of -., 1 .0 ms at membrane potentials from -60 to -40 mV . Averaged single channel and whole cell currents were similar when scaled and showed both fast and slow rates of inactivation . The inactivation and activation gating shifted quickly to hyperpolarized potentials for channels in cell-attached as well as excised patches, whereas a much slower shift occurred in whole cells . Slowly inactivating currents were present in both whole cell and single channel current measurements at potentials as positive as -40 mV . In whole cell measurements, the potential range could be extended, and slow inactivation waspresent at potentials as positive as -10mV . The curves relating steady state activation and inactivation to membrane potential had very little overlap, and slow inactivation occurred at potentials that were positive to the overlap. Slow inactivation is in this way distinguishable from the overlap or window current, and the slowly inactivating current may contribute to the plateau of the rat cardiac action potential . On rare occasions, a second set of Na channels having a smaller unit conductance and briefer duration was observed . However, a separate set of threshold channels, as described by Gilly and Armstrong (1984 . Nature (Loud.) . 309:448), was not found . For the commonly observed Na channels, the number of openings in some samples far exceeded the number of channels per patch and the latencies to first opening or waiting times were not sufficiently dispersed to account for the slowly inactivating currents ; the slow inactivation wasproduced by channel reopening . A general model was developed to predict the number of openings in each sample . Models in which the number of openings per sample was due to a dispersion of waiting times combined with a rapid transition from an open to

Tetrodotoxin (TTX)-sensitive Na currents were examined in single dissociated ventricular myocytes from neonatal rats. Single channel and whole cell currents were measured using the patch-clamp method. The channel density was calculated as 2/micron 2, which agreed with our usual finding of four channels per membrane patch. At 20 degrees C, the single channel conductance was 20 pS. The open time distributions were fit by a single-exponential function with a mean open time of approximately 1.0 ms at membrane potentials from -60 to -40 mV. Averaged single channel and whole cell currents were similar when scaled and showed both fast and slow rates of inactivation. The inactivation and activation gating shifted quickly to hyperpolarized potentials for channels in cell-attached as well as excised patches, whereas a much slower shift occurred in whole cells. Slowly inactivating currents were present in both whole cell and single channel current measurements at potentials as positive as -40 mV. In whole cell measurements, the potential range could be extended, and slow inactivation was present at potentials as positive as -10 mV. The curves relating steady state activation and inactivation to membrane potential had very little overlap, and slow inactivation occurred at potentials that were positive to the overlap. Slow inactivation is in this way distinguishable from the overlap or window current, and the slowly inactivating current may contribute to the plateau of the rat cardiac action potential. On rare occasions, a second set of Na channels having a smaller unit conductance and briefer duration was observed. However, a separate set of threshold channels, as described by Gilly and Armstrong (1984. Nature [Lond.]. 309:448), was not found. For the commonly observed Na channels, the number of openings in some samples far exceeded the number of channels per patch and the latencies to first opening or waiting times were not sufficiently dispersed to account for the slowly inactivating currents: the slow inactivation was produced by channel reopening. A general model was developed to predict the number of openings in each sample. Models in which the number of openings per sample was due to a dispersion of waiting times combined with a rapid transition from an open to an absorbing inactivated state were unsatisfactory and a model that was more consistent with the results was identified.

Recent studies show that the arterial baroreceptor reflex cannot be defined by a single buffer curve. The reflex blood pressure and heart rate curves depend on the pressure to which the baroreceptors are exposed. If arterial pressure is elevated for longer than 3-5 min the threshold and the entire buffer curve are shifted to higher pressures. On the other hand, a reduced arterial pressure shifts the buffer curve to lower pressures. Part of this phenomenon, which has been called rapid or acute resetting, may be explained by changes in the baroreceptor discharge in response to exposure to sustained alterations in pressure. The reflex response, however, resets more than can be explained by changes in the baroreceptor discharge. A central component to the resetting process is suggested. Resetting allows the baroreceptor reflex to operate over a wide range of arterial pressures rather than being confined to a single range defined by one buffer curve. Resetting is not complete. That is, if the receptors are exposed to a change in pressure of 30 mm Hg the buffer curves shift by less than 30 mm Hg. Thus a signal concerning mean pressure is not eliminated by the resetting process.

We studied TTX-sensitive Na channels in dissociated single ventricular cells from neonatal rats using the patch clamp method for single channel and whole cell recording. In both cases, slowly inactivating or window currents were observed that decayed in a biexponential fashion. Customary models of Na current kinetics such as the Hodgkin-Huxley model attribute activation to a process that is much faster than inactivation. The model of Aldrich, Corey and Stevens(1), says that inactivation is fast and final and activation is dispersed. We found activation too associated, deactivation too quick, and reopenings too frequent to accept this model for cardiac Na channels. We also found that the predominant set of single Na channels had one open state and two inactivated states. Rarely, a second set of Na channels having about 2/3 the conductance and mean open time of the predominant set was found.