Electrophysiological properties of neuroendocrine cells of the intact rat pars intermedia: multiple calcium currents

Intracellular recordings for current and voltage clamping were obtained from 130 neuroendocrine cells of the pars intermedia (PI) in intact pituitaries maintained in vitro. Spontaneous and evoked action potentials were blocked by TTX or by intracellular injection of a local anesthetic, QX-222. After potassium (K+) currents were blocked by tetraethylammonium (TEA), 4-aminopyridine, and intracellular cesium (Cs+), 2 distinct calcium (Ca2+) spikes were observed which were differentiated by characteristic thresholds, durations, and amplitudes. Both Ca2+ spikes were blocked by cobalt (Co2+) but were unaffected by TTX or QX-222. The low-threshold spike (LTS) had a smaller amplitude and inactivated when membrane potential was depolarized past -40 mV or when evoked at a fast rate (greater than 0.5 Hz). The high-threshold spike (HTS) typically had a larger amplitude and longer duration, was not inactivated at potentials which inactivated the LTS, and could be evoked at rates of up to 10 Hz. Single-electrode voltage-clamp analysis revealed that 3 distinct components of the Ca2+ current were present in most cells. From a negative holding potential (-90 mV), 2 separate peak inward currents were observed; a low-threshold transient current, similar to a T-type Ca2+ current, activated at -40 mV, whereas a large-amplitude inactivating current activated above -20 mV. This large inactivating Ca2+ current was significantly inactivated at a holding potential of -40 mV or by brief prepulses to positive potentials, and was similar to an N-type Ca2+ current. A sustained Ca2+ current (L-type) was observed which was not altered by different holding potentials.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium currents and graded synaptic transmission between heart interneurons of the leech.

Synaptic transmission between reciprocally inhibitory heart interneurons (HN cells) of the medicinal leech was examined in the absence of Na-mediated action potentials. Under voltage clamp, depolarizing steps from a holding potential of -60 mV elicited 2 kinetically distinct components of inward current in the presynaptic HN cell: an early transient current that inactivates within 200 msec and a persistent current that only partially decays over several seconds. Both currents begin to activate near -60 mV. Steady-state inactivation occurs over the voltage range between -70 and -45 mV and is completely removed by 1-2-sec hyperpolarizing voltage steps to -80 mV. The inward currents are carried by Ca2+, Ba2+, or Sr2+ ions, but not by Co2+, Mn2+, or Ni2+. These same inward currents underlie the burst-generating plateau potentials previously described in HN cells (Arbas and Calabrese, 1987a,b). With a presynaptic holding potential of -60 mV, the threshold for transmitter release is near -45 mV. Postsynaptic currents in the contralateral HN cell have a reversal potential near -60 mV. The largest postsynaptic currents (300-400 pA) exhibit an initial peak response that is followed by a more slowly decaying component. The persistent component of Ca2+ current in the presynaptic neuron is strongly correlated with the prolonged component of the postsynaptic current, while the transient presynaptic Ca2+ current appears to correspond to the early peak of postsynaptic current. These data are consistent with the hypothesis that voltage-dependent calcium currents contribute to the oscillatory capability of reciprocally inhibitory HN cells by (1) generating the plateau potential that drives the burst of action potentials and (2) underlying the release of inhibitory transmitter onto the contralateral cell.     

Intracellular Ca2+ suppressed a transient potassium current in hippocampal neurons

The effects of intracellular Ca2+ (Ca2+i) on K+ currents in hippocampal cells were examined using acutely isolated cells obtained from adult guinea pigs. Whole-cell voltage-clamp recordings were carried out in a configuration that allowed a continuous perfusion of the intracellular medium. Recording media were made to block inward currents and allowed selective activation of K(+)-dependent outward currents. Voltage-dependent outward currents consisted of an initial rapidly decaying component followed by a sustained component. The time constant of decay of the transient current was about 25 msec, and previous studies (Numann et al., 1987) showed that the kinetic and pharmacological properties of this current closely resembled the A current recorded in invertebrate neurons (Connor and Stevens, 1971; Thompson, 1982). Intracellular perfusion of hippocampal cells with a solution containing elevated Ca2+ (about 4.5 x 10(-4) M) elicited outward currents at the holding potential (-45 to -55 mV) and produced changes in voltage-dependent K+ currents. The transient outward current (IA) activated by depolarization was suppressed with increases in Ca2+i. Delayed, sustained K+ currents were greatly potentiated. Data also showed that, among the 3 effects elicited by Ca2+i, suppression of IA was most sensitive to Ca2+i elevation. Previous results (Numann et al., 1987) showed that IA had a lower threshold (about -45 mV) than sustained currents (about -40 mV). By using low levels of depolarization (-40 mV), IA can be selectively activated, and the suppressive effect of Ca2+i on IA was confirmed on the kinetically isolated IA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of maintained voltage-dependent K(+)-channels induced in Xenopus oocytes by rat brain mRNA.

The voltage-dependent K+ currents encoded by rat brain mRNA were studied in Xenopus oocytes after the voltage-dependent Na+ currents and the Ca(2+)-activated Cl- currents were eliminated pharmacologically. This paper describes the maintained K+ currents (IK), defined primarily by resistance to inactivation for 1 s at a holding potential of -40 mV. IK activates at potentials more positive than -60 to -70 mV and consists of both low-threshold and high-threshold components. IK is partially blocked by both tetraethyl ammonium (TEA) and 4-aminopyridine (4-AP), which appear to be blocking the same component. Long depolarizing pulses result in incomplete inactivation of IK; the inactivating component is inhibited by TEA. Sucrose density gradient fractionation partially resolves the RNA encoding the several components of IK; most IK arises from size classes between 3.8 and 9.5 kb. The study gives further evidence for the existence of numerous distinct RNA populations that encode brain K+ channels different from previously reported cloned K+ channels that have been expressed in Xenopus oocytes.     

Outward currents in isolated ventral cochlear nucleus neurons

Neurons of the ventral cochlear nucleus (VCN) perform diverse information processing tasks on incoming activity from the auditory nerve. We have investigated the cellular basis for functional diversity in VCN cells by characterizing the outward membrane conductances of acutely isolated cells using whole-cell, tight-seal, current- and voltage-clamp techniques. The electrical responses of isolated cells fall into two broad categories. Type 1 cells respond to small depolarizations with a regular train of action potentials. Under voltage clamp, these cells exhibit a noninactivating outward current for voltage steps positive to -35 mV. Analysis of tail currents reveals two exponentially decaying components with slightly different voltage dependence. These currents reverse at -73 mV, near the potassium equilibrium potential of -84 mV, and are blocked by tetraethylammonium (TEA). The major outward current in Type I cells thus appears to be mediated by potassium channels. In contrast to Type I cells, Type II cells respond to small depolarizations with only one to three short-latency action potentials and exhibit strong rectification around -70 mV. Under voltage clamp, these cells exhibit a noninactivating outward current with a threshold near -70 mV. Analysis of tail currents reveals two components with different voltage sensitivity and kinetics. A low-threshold current with slow kinetics is partly activated at rest. This current reverses at -77 mV and is blocked by 4-aminopyridine (4-AP) but is only partly affected by TEA. The other component is a high-threshold current activated by steps positive to -35 mV. This current is blocked by TEA, but not by 4-AP. A simple model based on the voltage dependence and kinetics of the slow low-threshold outward current in Type II cells was developed. The model produces current- and voltage-clamp responses that resemble those recorded experimentally. Our results indicate that the two major classes of acoustic response properties of VCN neurons are in part attributable to the types of outward (potassium) conductances present in these cells. The low-threshold conductance in the Type II (bushy) cells probably plays a role in the preservation of information about the acoustic stimulus phase from the auditory nerve to central auditory nuclei involved in low-frequency sound localization.     

Calcium currents in cultured rat cortical neurons

Rat neocortical neurons grown in dissociated cell culture for 4-12 weeks were studied with whole-cell patch-clamp techniques in order to characterize the calcium currents present in these cells. When voltage-dependent Na and K currents were inhibited, depolarizations from negative holding potentials induced inward currents which had 3 components: a low threshold activated, small, relatively persistent component, which was completely inactivated at holding potentials more positive then -60 mV; a higher threshold, relatively persistent component (which was not inactivated at VH = -50 mV); and a higher threshold, larger, transient component. All 3 components were reduced by removal of Ca, and blocked by Cd and Ni at appropriate concentrations. The components were differentially affected by low concentrations of Ni (500 microM), nifedipine (500 microM) and Ba (1.8 mM). Only the first two components were present in very young neurons.     

The low-threshold Ca current in isolated amygdaloid neurons in the rat

Two types of voltage-dependent Ca currents were recorded from isolated rat amygdaloid neurons under single-electrode voltage-clamp. A low-threshold Ca current was elicited at -60 mV or more positive potentials and inactivated rapidly. At -30 mV or more positive potentials, a high-threshold Ca current was also activated. In steady-state inactivation curve of the low-threshold Ca current, the half-inhibition value (h0.5) was -71 mV. The low-threshold Ca current was inhibited by organic and inorganic Ca blockers in a dose-dependent manner. The inhibitory effect of these Ca blockers was completely reversible while that of flunarizine was partly so. It is concluded that the membrane and pharmacological properties of the low-threshold Ca channel in the rat amygdaloid neurons are quite similar to those in the hypothalamic neurons.     

Low- and high-threshold calcium current in the membrane of cultured neurons of Helix pomatia

Two components of the inward calcium current were observed in the membrane of cultured Helix pomatia neurons by the patch clamp method in its whole cell configuration. The first one was activated at high-negative membrane potentials (-70- -60 mV), had low amplitude (200 pA at 15 mmol/l of Ca2+ ions as a charge carrier) and displayed time-dependent inactivation. A second, larger current component (2 nA, the same conditions) appeared at more positive potentials (-20 mV). Its time-dependent inactivation was much less expressed. The tail currents of the low amplitude component were slower than those associated with the high-amplitude component.     

Dihydropyridine actions on calcium currents of frog sympathetic neurons.

Dihydropyridines (DHPs) generally have little effect on whole-cell calcium currents of neurons, even at concentrations far higher than those effective on muscle. Either neuronal calcium currents are much less sensitive to DHPs, or only a small proportion of the current is DHP-sensitive. We find that DHP agonists and antagonists act at low concentration on calcium currents in frog sympathetic neurons but that the effects are small even at optimal concentrations. The half-maximal dose (EC50) of the agonist Bay K 8644 is approximately 50 nM, and the effect of Bay K 8644 is blocked by 50% at approximately 300 nM nifedipine, from a holding potential of -80 mV. Nifedipine is more effective from a holding potential of -50 mV. These results suggest the presence of an L-type calcium current, with DHP sensitivity similar to L-currents in cardiac muscle. The predominant (greater than 90%) calcium current in frog sympathetic neurons is a DHP-resistant N-type current. However, high concentrations of DHPs (10 microM) partially block N-type calcium current, as well as voltage-dependent sodium and potassium currents.     

Membrane currents in identified lactotrophs of rat anterior pituitary

Qualitative features of the primary inward and outward current components of identified lactotrophs of the rat anterior pituitary were examined. Identification of lactotrophs in heterogeneous dissociated anterior pituitary cultures was accomplished by application of the reverse hemolytic plaque assay. Currents in lactotrophs were subsequently examined using whole-cell or patch recording techniques. Two components of inward calcium current were observed: a transient component and a sustained component. The transient component activated at voltages as negative as -50 mV and was the major contributor to total lactotroph calcium current. The sustained component activated at voltages above about -10 mV. The 2 currents could be qualitatively separated by differences in inactivation properties and in sensitivity to cadmium. At least 3 components of outward current were distinguished. Either 30 mM TEA or 0 calcium eliminated a major portion of sustained outward current. This is likely to represent primarily calcium- and voltage-activated potassium current. The remaining current could be further differentiated into a transient current component that could be inactivated with conditioning potentials above -60 mV. A slowly activating and deactivating potassium current remained following inactivation of the transient current. Although the time course of the transient current is reminiscent of "A" current, activation of this current required potentials above -30 mV. Candidates for the single-channel currents that underlie the whole-cell outward currents were observed in cell-attached recordings. When combined with patch-clamp electrophysiological methods, the reverse hemolytic plaque assay promises to be a powerful technique for the electrophysiological characterization of specific cell subtypes in heterogeneous dissociated cell populations.     

Membrane currents induced by pentylenetetrazol in identified neurons of Helix pomatia

After systemic application of pentylenetetrazol (PTZ), mammalian as well as molluscan neurons generate epileptic paroxysmal depolarization shifts. For a further analysis of these potential oscillations the membrane currents induced by local application of PTZ onto identified neurons of Helix pomatia were investigated. Different types of responses were obtained at membrane potentials negative and positive to ca. -30 mV. At holding potentials more negative than -30 mV, PTZ as a rule evoked an inward current, sometimes preceded by a brief outward current. In a few experiments only a solitary outward current was found. The amplitudes of the inward and outward currents increased towards more negative potentials. The inward current was associated with a decrease and the outward current with an increase in membrane resistance. Besides these findings pharmacological and ion substitution experiments indicate that the inward current represents an unspecific current. At holding potentials more positive than -30 mV, PTZ evoked a sequence of currents which was the same in all neurons. This stereotyped current sequence consisted of (i) an early inward current, (ii) an intermediate outward current, and (iii) a late long-lasting inward current. The amplitudes of all these components increased towards more positive potentials with the outward current being particularly enhanced. The early inward current and the following outward current were associated with a decrease and the late inward current with an increase of the membrane resistance. Besides these pharmacological and ion substitution experiments suggest that the early inward current represents a mixed sodium and calcium current, the intermediate outward current a calcium activated potassium current. The late inward current is assumed to be due to a decreased potassium conductance. On the basis of the present results, it may be concluded that the unspecific inward current in the negative potential range is involved in the initiation and the calcium dependent potassium current in the termination of spontaneously occurring paroxysmal depolarization shifts.     

Deactivation of calcium currents in the soma of spinal ganglion neurons after elimination of the depolarizing shift in the membrane potential

Kinetic and voltage-dependent characteristics of deactivation of calcium inward currents with the removal of membrane depolarization were studied in the somatic membrane of rat dorsal root ganglion neurons by intracellular dialysis technique. The "tail" of low-threshold calcium current could be described reliably by one exponent with time constant tau 1-1.2-1.8 ms at repolarization to --90 mV. The "tail" of the high-threshold calcium current represented a sum of several exponents; the time constant of the main component tau h was in the range of 250-380 microseconds. tau 1 and tau h remained practically unchanged for repolarization potentials in the subthreshold region; however, they increased if it was in the range of potentials at which the corresponding component of the calcium current started to activate. A dependence of tau 1 and tau h on the duration of depolarizing shift was observed. The results obtained are discussed in the framework of a three-level kinetic model of calcium channels.     

Voltage-dependent calcium channels regulate melatonin output from cultured chick pineal cells

Chick pineal cells maintained in primary culture display a circadian rhythm of melatonin production and release, and the nocturnal increase in melatonin output is enhanced by elevating extracellular K+. The divalent cations, Co2+, Cd2+, and Mn2+, each reduce nocturnal melatonin output. Nitrendipine and nifedipine also prevent the nocturnal rise in melatonin output, while Bay K 8644 increases it, suggesting a role for voltage-dependent Ca2+ channels in regulating melatonin output. The whole-cell patch-clamp technique was used to record from individual chick pineal cells. Under conditions designed to isolate currents through voltage-dependent Ca2+ channels, biphasic inward currents are elicited by large depolarizing commands (e.g., to 0 mV) from a holding potential of -90 mV; from a holding potential of -40 mV, only a sustained inward current is elicited by steps to 0 mV. Both components of the inward current are blocked by Co2+ or Cd2+. The sustained current is increased in amplitude by Bay K 8644 and blocked by nifedipine, while the transient current is unaffected. Since there is no evidence for vesicular release of melatonin, the "L-type" calcium channels mediating the sustained calcium current appear to be involved in the pathways regulating melatonin synthesis in chick pineal cells.     

Differences in a K current in schwann cells from normal and neurofibromatosis-infected damselfish

Patch clamp techniques were used to study whole cell ionic currents in Schwann cells (SC) from a tropical marine fish, the bicolor damselfish, Pomacentrus partitus. The bicolor damselfish is affected by a disease termed damselfish neurofibromatosis (DNF), being developed as an animal model of neurofibromatosis-type 1 (NF1) in humans. NF1 affects SC, fibroblasts, and perineurial cells. The sole depolarization-activated ionic current present in cultured SC from normal fish peripheral nerve and from neurofibromas of fish with induced or spontaneously occurring DNF was an inactivating K⁺ current (K current), with a strong dependence on the Nernst potential for K⁺. This K current activated at depolarizations to -40 mV and above and inactivated during a maintained test pulse (0.2-1 s), but inactivation was significantly greater in tumored SC. Both currents were inhibited by 4-aminopyridine (K_d ? 1 mM) and by dendrotoxin (15 μM) but were insensitive to extracellular tetraethyammonium (≤ 150 mM), indicating that the whole cell currents were similar pharmacologically. The currents could be distinguished on the basis of their sensitivity to depolarized holding potential, with normal cells less sensitive. Half-inactivation of the current was -32 mV in normal cells and -38 mV in tumored cells. Inactivation curves constructed from the average normalized current for many SC were significantly different in normal and tumored cells. When the depolarized holding potential was maintained between test depolarizations, greater voltage-dependent inactivation in tumored cells was apparent. Normal cells maintained an average of 36% of peak current at a holding voltage of ?40 mV, while in tumored cells this average was 12%, a significant difference. © 1994 Wiley-Liss, Inc.     

Two components of calcium channel current in embryonic chick skeletal muscle cells developing in culture

The properties of the Ca channel currents in chick skeletal muscle cells (myoballs) in culture were studied using a suction pipette technique which allows internal perfusion and voltage clamp. The Ca channel currents as carried by Ba ions were recorded, after suppression of currents through ordinary Na, K and Cl channels by absence of Na, K and Cl ions, by external TEA, by internal EGTA and by observing the Ba currents instead of the Ca currents. Two components of Ba current could be distinguished. One was present only if the myoballs were held at relatively negative holding potentials below -50 mV. This component first became detectable at clamp potentials of about -50 mV and reached a maximum between -10 and -20 mV. During long clamp steps, it became inactivated completely. The inactivation process of this component at a clamp potential of -30 mV was well fitted to a single exponential with a time constant of about -20 ms. Half-maximal steady-state inactivation was observed at -63 mV. The other component persisted even at relatively positive holding potentials above -40 mV, was observed during clamp pulses to -20 mV and above, and reached a maximum between +10 and +20 mV. This component inactivated very little; a substantial fraction of this component remained at the end of clamp pulses lasting 1 s. The inactivation process of this component at a clamp potential of -10 mV apparently followed a single exponential with a time constant of about 1 s. Half-maximal steady-state inactivation was attained at -33 mV. Both components of Ba current were blocked by Co ions, but organic Ca channel blocker D600 preferentially blocked the high-threshold, slowly inactivating component. The relationship between the current amplitude and the concentration of the external Ba ions was different between the two components. Furthermore, the two components of Ba current also differed in their developmental profile. These findings demonstrate the existence of two distinct types of Ca channels in the early stages of chick muscle cell development.     

Swelling-induced changes in electrophysiological properties of cultured astrocytes and oligodendrocytes. II. Whole-cell currents

Using whole cell patch-clamp recordings we have found that swelling cultured cerebrocortical astrocytes or mouse spinal cord oligodendrocytes by perfusing them with hypotonic medium induced inward currents at the normal resting potential of -60 mV. The currents in the oligodendrocytes were always less than for astrocytes. We examined the reversal potentials of these responses by rapidly jumping the holding potential to different values and measuring the currents. We found that the hypotonic medium-induced conductance increase was always preceded by a conductance decrease in the case of oligodendrocytes, but only sometimes preceded by a conductance decrease in cultured astrocytes. The reversal potential of the conductance increase for astrocytes was around -40 mV, while the conductance decrease had a more negative reversal potential of -60 mV or less. For oligodendrocytes the reversal potential for the conductance increase was around -50 mV while the conductance decrease had a reversal potential of -90 mV or less. This suggests that K+ conductance decreased in the initial phase, while the conductance increase was due to additional channel openings. Ion substitution experiments in the case of the astrocytes showed that the reversal potential was shifted to a more positive value when medium K+ was increased, but was unaffected when Na+ was substituted by N-methyl-D-glucamine or Cl- by D-glucuronate, when corrected for liquid junction potential changes. Thus, the channels opened in these cells are likely to include non-specific cation channels. It is of interest that the two cells show a difference in their responses, and in the case of astrocytes these are likely to be involved in the regulatory volume decrease processes documented in these cells.     

Three types of voltage-dependent calcium current in cultured rat hippocampal neurons

Voltage-dependent calcium (Ca2+) currents in cultured rat hippocampal neurons were studied with the whole-cell recording mode of the patch-clamp technique. On the basis of the voltage-dependence of activation, kinetics of inactivation and pharmacology, 3 types of Ca2+ currents were distinguished. The low-threshold Ca2+ current (Il) was activated at -60 mV, and completely inactivated during a 100-ms depolarization to -40 mV (time constant: tau = 16 +/- 1 ms). The high-threshold currents (Ih), which were activated at -20 mV, could be separated into two types. The high-threshold, fast inactivating current (Ih,f) decayed quickly during a maintained depolarization (tau = 33 +/- 3 ms at 0 mV), whereas the high-threshold, slowly inactivating current (Ih,s) decayed with a much slower time constant (tau = 505 +/- 42 ms at 0 mV). The inactivations of Ih,f and Ih,s exhibited different time- and voltage-dependencies. Nickel ions (Ni2+, 25 microM) markedly suppressed Il, but little affected Ih. Cadmium ions (Cd2+, 10 microM) almost completely suppressed Ih, but left a small amount of Il. Lanthanum ions (La3+, 10 microM) almost completely suppressed both Il and Ih. Ih,s was sensitive to block by the dihydropyridine antagonist nicardipine (10 microM).     

Calcium current in type I hair cells isolated from the semicircular canal crista ampullaris of the rat

The low voltage gain in type I hair cells implies that neurotransmitter release at their afferent synapse should be mediated by low voltage activated calcium channels, or that some peculiar mechanism should be operating in this synapse. With the patch clamp technique, we studied the characteristics of the Ca(2+) current in type I hair cells enzymatically dissociated from rat semicircular canal crista ampullaris. Calcium current in type I hair cells exhibited a slow inactivation (during 2-s depolarizing steps), was sensitive to nimodipine and was blocked by Cd(2+) and Ni(2+). This current was activated at potentials above -60 mV, had a mean half maximal activation of -36 mV, and exhibited no steady-state inactivation at holding potentials between -100 and -60 mV. This data led us to conclude that hair cell Ca(2+) current is most likely of the L type. Thus, other mechanisms participating in neurotransmitter release such as K(+) accumulation in the synaptic cleft, modulation of K(+) currents by nitric oxide, participation of a Na(+) current and possible metabotropic cascades activated by depolarization should be considered.     

Inactivation of calcium channels in rat pituitary intermediate lobe cells

The voltage-dependent inactivation of Ca currents was explored in dissociated intermediate lobe (IL) cells from the rat pituitary. On the basis of current-voltage relations two main inward currents could be identified in this cell, a transient current, (I-t), and a sustained current, (I-s). Inactivation was explored either by changing the holding potential and testing the change in the inward currents during a brief test pulse, or, by depolarizing the membrane and following the decay of the evoked inward current. Three current decay rates were identified, each with a characteristic dependence on membrane potential. The fastest decay rate (tau 1), was attributed to the inactivation of the I-t current and had a value of 57 ms at -40 mV, decreasing to 10 ms at -10 mV (extrapolated value of 6 ms at 0 mV). The other two decay rates, tau 2 and tau 3, decreased monotonically with depolarization of the membrane potential and reflected the inactivation of the I-s current with values of 1.8 and 20 s at 0 mV. I-s inactivation and reactivation was found to occur even in the normal resting potential range of this cell. These properties of the calcium channels can explain the voltage-dependent inactivation of secretion that has been observed previously in this and other secretory cells. In addition, they suggest that calcium currents, and hence secretion, may be modulated by external factors that cause small, but sustained, changes in the resting potential of the IL cell.     

Halothane inhibits two components of calcium current in clonal (GH3) pituitary cells.

The effect of halothane on isolated calcium (Ca2+) current of clonal (GH3) pituitary cells was investigated using standard whole-cell clamp techniques at room temperature. Halothane (0.1-5.0 mM) reversibly reduced both the low-threshold, transient [low-voltage-activated (LVA)] component and the high-threshold [high-voltage-activated (HVA)] component of Ca2+ current. Halothane had little effect on the voltage dependence of activation or inactivation of either component of Ca2+ current. Inhibition of the peak high-threshold Ca2+ current was half-maximal at about 0.8 mM halothane, with maximal inhibition (100%) occurring with 5 mM halothane. When measured at the end of a 190-msec command step, half-maximal reduction of high-threshold current occurred at less than 0.5 mM halothane. The low-threshold transient current was less sensitive to halothane, with half-maximal inhibition of peak transient current activated at -30 mV occurring at approximately 1.3 mM. The effect of halothane on the HVA current was apparently not mediated by changes in intracellular Ca2+ concentration. The ability of halothane to inhibit Ca2+ current was unaffected by either the inclusion of the rapid Ca2+ buffer 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA) in the recording pipette or exposure of the cell to 10 mM caffeine. To assess the selectivity of the effect of halothane, the actions of halothane on two components of voltage-activated potassium (K+) current observed in the absence of extracellular Ca2+ and on voltage-dependent sodium (Na+) current were also examined. Halothane had no effect on the voltage-dependent, inactivating K+ current of GH3 cells at concentrations up to 1.2 mM. In contrast, the non-inactivating K+ current, though less sensitive to halothane than either Ca2+ current, was reduced by about 40% by 1.2 mM halothane at +20 mV. Peak Na+ current was also blocked by halothane, but 50% block required around 2.6 mM halothane with little effect at 1.6 mM. Reduction of Na+ current was associated with a substantial negative shift in the steady-state inactivation curve. Although the results indicate that a number of voltage-dependent ionic currents are sensitive to halothane, both components of Ca2+ current exhibit a greater sensitivity to halothane than any of three other voltage-dependent currents in GH3 cells. These results show that GH3 cell Ca2+ currents are selectively inhibited by clinically appropriate concentrations of halothane and that the reduction of Ca2+ current can account for the inhibition by halothane of TRH- or KCl-induced prolactin secretion in GH3 cells.