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J R Lemos

Publications and source records attributed to J R Lemos.

At least 19 recordsLinked to original sources
PubMed2005

Micro-opioid receptor preferentially inhibits oxytocin release from neurohypophysial terminals by blocking R-type Ca2+ channels.

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Analgesics, Opioid↗
PubMed2003

mu-Opioid receptor modulates peptide release from rat neurohypophysial terminals by inhibiting Ca(2+) influx.

The activation of opioid receptors in neurones of the central nervous system leads to a variety of effects including the modulation of diuresis and parturition, processes that are directly controlled by the hypothalamic-neurohypophysial system (HNS). The effects of mu-opioid receptor activation on peptide release, voltage-gated Ca2+ currents and intracellular calcium levels ([Ca2+]i) were studied in isolated nerve terminals of the HNS. The mu-receptor agonist, DAMGO ([d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin) inhibited high K+-induced peptide release in a dose-dependent manner, with oxytocin release being more sensitive to block than vasopressin release at all concentrations tested. The addition of the mu-receptor antagonist CTOP (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide) was able to overcome the inhibitory effects of DAMGO. By contrast to previous results, voltage-gated Ca2+ currents were sensitive to blockage by DAMGO and this inhibition was also prevented by CTOP. Furthermore, [Ca2+]i measurements with Fura-2 corroborated the inhibition by DAMGO of calcium entry and its reversal by the micro -receptor antagonist in these nerve terminals. Thus, the decrease in neuropeptide release, particularly for oxytocin, induced by the activation of mu-opioid receptors in neurohypophysial terminals is mediated, at least in part, by a corresponding decrease in Ca2+ entry due to the inhibition of voltage-gated Ca2+ channels.

Animals↗
PubMed2000

Excitatory versus inhibitory modulation by ATP of neurohypophysial terminal activity in the rat.

Much is now known about the electrophysiological properties of the magnocellular neurones of the hypothalamus. Oxytocin neurones are characterized by an intermittent high frequency discharge during suckling that leads to the pulsatile release of oxytocin into the blood and to subsequent milk ejection. Vasopressin neurones are characterized by their asynchronous phasic activity (bursting) during maintained vasopressin release and the subsequent regulation of water balance. In both cases, it is the clustering of spikes, albeit with different time courses for each peptide, that facilitates hormone release. The mechanism underlying this differential facilitation is one of the major unanswered questions in neuroendocrinology. This paper considers recent evidence that indicates that ATP, co-secreted with vasopressin and oxytocin, may play a key role in the regulation of stimulus-secretion coupling in the neurohypophysis. The activity of the type (II) Ca2+-activated K+ (K(Ca)) channel found in the nerve terminals was significantly increased in the presence of ATP on the cytoplasmic side of the channel. Extracellular ATP, in contrast, inhibited the type II K(Ca) current in a dose-dependent manner. Thus, intracellular and extracellular ATP exert opposite effects on the type II K(Ca) channel of neurohypophysial terminals. Furthermore, ATP opens P2X2 channels to increase intracellular [Ca2+] in the nerve terminals and subsequent arginine vasopressin (AVP) release. In contrast, adenosine, acting via A1 receptors, specifically inhibits only the N-type Ca2+ channel, thus decreasing neuropeptide release. These multiple, conflicting effects of ATP and its metabolite adenosine could explain the patterns of AVP release observed during physiological stimulation in vivo.

Adenosine Triphosphate↗
PubMed2000

Tolerance to acute ethanol inhibition of peptide hormone release in the isolated neurohypophysis.

BACKGROUND: Acute ethanol (EtOH) exposure reduces the evoked release of vasopressin (AVP) and oxytocin (OT) from excised neurohypophyses and from dissociated neurohypophysial terminals of the rat. METHODS AND RESULTS: Rats placed on a diet that maintained blood levels of 30 mM EtOH for 20 to 40 days developed tolerance to acute EtOH inhibition of release. In the presence of 10 mM EtOH, high (50 mM) K+-induced release of AVP from isolated neurohypophysial terminals of EtOH-naive rats was reduced by 77.7+/-1.4%, whereas in the chronic EtOH group, release was reduced by only 9.4+/-8.7%. Similar tolerance was evident during acute challenge with 75 mM EtOH, as well as for release of OT from isolated terminals. Animals treated with an intraperitoneal injection of EtOH and sacrificed 90 min postinjection did not exhibit the reduced EtOH inhibition of release from dissociated terminals during a 75 mM EtOH acute challenge. CONCLUSIONS: The altered component responsible for the tolerance to inhibition of release resides in the isolated terminal, because tolerance measured in vitro from intact neurohypophyses was similar to that seen in isolated terminals. The failure of EtOH-injected animals to exhibit reduced inhibition of release in response to an acute EtOH challenge indicates that short-term elevated blood alcohol level does not induce this tolerance. The finding of tolerance to EtOH-induced inhibition of release from the intact neurohypophysis and isolated terminals provides a physiological preparation in which to examine the molecular targets of acute drug action modified after chronic exposure to the drug.

Animals↗
PubMed1999

An R-type Ca(2+) current in neurohypophysial terminals preferentially regulates oxytocin secretion.

Multiple types of voltage-dependent Ca(2+) channels are involved in the regulation of neurotransmitter release (Tsien et al., 1991; Dunlap et al., 1995). In the nerve terminals of the neurohypophysis, the roles of L-, N-, and P/Q-type Ca(2+) channels in neuropeptide release have been identified previously (Wang et al., 1997a). Although the L- and N-type Ca(2+) currents play equivalent roles in both vasopressin and oxytocin release, the P/Q-type Ca(2+) current only regulates vasopressin release. An oxytocin-release and Ca(2+) current component is resistant to the L-, N-, and P/Q-type Ca(2+) channel blockers but is inhibited by Ni(2+). A new polypeptide toxin, SNX-482, which is a specific alpha(1E)-type Ca(2+) channel blocker (Newcomb et al., 1998), was used to characterize the biophysical properties of this resistant Ca(2+) current component and its role in neuropeptide release. This resistant component was dose dependently inhibited by SNX-482, with an IC(50) of 4.1 nM. Furthermore, SNX-482 did not affect the other Ca(2+) current types in these CNS terminals. Like the N- and P/Q-type Ca(2+) currents, this SNX-482-sensitive transient Ca(2+) current is high-threshold activated and shows moderate steady-state inactivation. At the same concentrations, SNX-482 blocked the component of oxytocin, but not of vasopressin, release that was resistant to the other channel blockers, indicating a preferential role for this type of Ca(2+) current in oxytocin release from neurohypophysial terminals. Our results suggest that an alpha(1E) or "R"-type Ca(2+) channel exists in oxytocinergic nerve terminals and, thus, functions in controlling only oxytocin release from the rat neurohypophysis.

Animals↗
PubMed1999

Rat supraoptic magnocellular neurones show distinct large conductance, Ca2+-activated K+ channel subtypes in cell bodies versus nerve endings.

1. Large conductance, Ca2+-activated K+ (BK) channels were identified in freshly dissociated rat supraoptic neurones using patch clamp techniques. 2. The single channel conductance of cell body BK channels, recorded from inside-out patches in symmetric 145 mM K+, was 246.1 pS, compared with 213 pS in nerve ending BK channels (P<0.01). 3. At low open probability (Po), the reciprocal of the slope in the ln(NPo)-voltage relationship (N, number of available channels in the patch) for cell body and nerve ending channels were similar: 11 vs. 14 mV per e-fold change in NPo, respectively. 4. At 40 mV, the [Ca2+]i producing half-maximal activation was 273 nM, as opposed to >1.53 microM for the neurohypophysial channel, indicating the higher Ca2+ sensitivity of the cell body isochannel. 5. Cell body BK channels showed fast kinetics (open time constant, 8.5 ms; fast closed time constant, 1.6 and slow closed time constant, 12.7 ms), identifying them as 'type I' isochannels, as opposed to the slow gating (type II) of neurohypophysial BK channels. 6. Cell body BK activity was reduced by 10 nM charybdotoxin (NPo, 37% of control), or 10 nM iberiotoxin (NPo, 5% of control), whereas neurohypophysial BK channels are insensitive to charybdotoxin at concentrations as high as 360 nM. 7. Whilst blockade of nerve ending BK channels markedly slowed the repolarization of evoked single spikes, blockade of cell body channels was without effect on repolarization of evoked single spikes. 8. Ethanol reversibly increased neurohypophysial BK channel activity (EC50, 22 mM; maximal effect, 100 mM). In contrast, ethanol (up to 100 mM) failed to increase cell body BK channel activity. 9. In conclusion, we have characterized BK channels in supraoptic neuronal cell bodies, and demonstrated that they display different electrophysiological and pharmacological properties from their counterparts in the nerve endings.

Alkaloids↗
PubMed1998

ATP-evoked increases in [Ca2+]i and peptide release from rat isolated neurohypophysial terminals via a P2X2 purinoceptor.

1. The effect of externally applied ATP on cytosolic free Ca2+ concentration ([Ca2+]i) was tested in single isolated rat neurohypophysial nerve terminals by fura-2 imaging. The release of vasopressin (AVP) and oxytocin (OT) upon ATP stimulation was also studied from a population of terminals using specific radioimmunoassays. 2. ATP evoked a sustained [Ca2+]i increase, which was dose dependent in the 1-100 microM range (EC50 = 4.8 microM). This effect was observed in only approximately 40 % of the terminals. 3. Interestingly, ATP, in the same range (EC50 = 8.6 microM), evoked AVP, but no significant OT, release from these terminals. 4. Both the [Ca2+]i increase and AVP release induced by ATP were highly and reversibly inhibited by suramin, suggesting the involvement of a P2 purinergic receptor in the ATP-induced responses. Pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), another P2 purinergic receptor antagonist, strongly reduced the ATP-induced [Ca2+]i response. 5. To further characterize the receptor, different agonists were tested, with the following efficacy: ATP = 2-methylthio-ATP > ATP-gamma-S > alpha, beta-methylene-ATP > ADP. The compounds adenosine, AMP, beta, gamma-methylene-ATP and UTP were ineffective. 6. The ATP-dependent [Ca2+]i increase was dependent on extracellular Ca2+ concentration ([Ca2+]o). Fluorescence-quenching experiments with Mn2+ showed that externally applied ATP triggered a Mn2+ influx. The ATP-induced [Ca2+]i increase and AVP release were independent of and additive to a K+-induced response, in addition to being insensitive to Cd2+. The ATP-induced [Ca2+]i increase was strongly reduced in the presence of Gd3+. These results suggest that the observed [Ca2+]i increases were elicited by Ca2+ entry through a P2X channel receptor rather than via a voltage-dependent Ca2+ channel. 7. We propose that ATP, co-released with neuropeptides, could act as a paracrine-autocrine messenger, stimulating, via Ca2+ entry through a P2X2 receptor, the secretion of AVP, in particular, from neurohypophysial nerve terminals.

Adenosine Triphosphate↗
PubMed1998

Ethanol reduces the duration of single evoked spikes by a selective inhibition of voltage-gated calcium currents in acutely dissociated supraoptic neurons of the rat.

The effects of ethanol were studied on single evoked spikes recorded at 20 degrees C with the perforated-patch method in acutely dissociated rat supraoptic neurons. In seven out of eight neurons, ethanol (50 mM) significantly reduced the spike duration by selectively decreasing the decay time (82+/-2% of the control), leaving the amplitude and rise time unaffected. Resting potential and threshold did not change. Similarly, CdCl2 at a concentration of 100 microM, which blocks all voltage-activated calcium current in the supraoptic neurons, reduced the decay time of single evoked spikes (76+/-3% of the control, n=10) without modifying the other above-mentioned parameters. In addition, exposure to 100 microM CdCl2 prevented any subsequent effect of 50 mM ethanol (n = 5). Exposure to apamin (10 nM) and iberiotoxin (10 nM) did not have any effect on single evoked spikes. Because these concentrations are effective in blocking, respectively, small (SK) and large (BK) conductance calcium-dependent potassium channels in these neurons, this result shows that these currents are not involved in either the shaping of single evoked spikes or the actions of ethanol on spike shape. The sustained component of whole-cell recorded calcium current measured at -10 mV (hp -60 mV) was inhibited by ethanol in a dose-dependent manner, with a significant effect detectable at 25 mM. Exposure to 50 mM ethanol significantly reduced the sustained current to 70+/-5% of the control (n=12), without any apparent shift of the current-voltage relationship. Control exposure of the neurons to either 50 mM urea or 50 mM sucrose did not affect the voltage-gated calcium currents. We conclude that ethanol reduces the duration of single evoked spikes by a specific inhibition of voltage-activated calcium currents. The results suggest that, in addition to its direct effects on release of vasopressin and oxytocin from neurohypophysial terminals, ethanol could also affect hormonal release via changes in firing patterns arising in the cell bodies.

Animals↗
PubMed1998

Ethanol potentiation of calcium-activated potassium channels reconstituted into planar lipid bilayers.

We examined the actions of ethanol on the single channel properties of large conductance Ca2+-activated K+ (BK) channels isolated from skeletal muscle T-tubule membranes and incorporated into planar lipid bilayer membranes. We have taken advantage of this preparation, because it lacks most elements of cellular complexity, including cytoplasmic constituents and complex membrane lipid composition and architecture, to examine the minimum requirements for the effects of alcohol. Clinically relevant concentrations (25-200 mM) of ethanol increased the activity of BK channels incorporated into bilayers composed of phosphatidylethanolamine (PE) alone or PE and phosphatidylserine. The potentiation of channel activity by ethanol was attributable predominantly to a decrease in the average amount of time spent in closed states. Ethanol did not significantly affect the current amplitude-voltage relationship for BK channels, indicating that channel conductance for K+ was unaffected by the drug. Although base-line characteristics of BK channels incorporated into bilayers composed only of PE differed from those of channels in PE/ phosphatidylserine in a manner expected from the change in bilayer charges, the actions of ethanol on channel activity were qualitatively similar in the different lipid environments. The effects of ethanol on single channel properties of BK channels in the planar bilayer are very similar to those reported for the action of ethanol on neurohypophysial BK channels studied in native membrane, and for cloned BK channels expressed in Xenopus laevis oocytes, which suggests that ethanol's site and mechanism of action are preserved in this greatly simplified preparation.

Animals↗
PubMed1997

Role of Q-type Ca2+ channels in vasopressin secretion from neurohypophysial terminals of the rat.

1. The nerve endings of rat neurohypophyses were acutely dissociated and a combination of pharmacological, biophysical and biochemical techniques was used to determine which classes of Ca2+ channels on these central nervous system (CNS) terminals contribute functionally to arginine vasopressin (AVP) and oxytocin (OT) secretion. 2. Purified neurohypophysial plasma membranes not only had a single high-affinity binding site for the N-channel-specific omega-conopeptide MVIIA, but also a distinct high-affinity site for another omega-conopeptide (MVIIC), which affects both N- and P/Q-channels. 3. Neurohypophysial terminals exhibited, besides L- and N-type currents, another component of the Ca2+ current that was only blocked by low concentrations of MVIIC or by high concentrations of omega-AgaIVA, a P/Q-channel-selective spider toxin. 4. This Ca2+ current component had pharmacological and biophysical properties similar to those described for the fast-inactivating form of the P/Q-channel class, suggesting that in the neurohypophysial terminals this current is mediated by a 'Q'-type channel. 5. Pharmacological additivity studies showed that this Q-component contributed to rises in intraterminal Ca2+ concentration ([Ca2+]i) in only half of the terminals tested. 6. Furthermore, the non-L- and non-N-component of Ca(2+)-dependent AVP release, but not OT release, was effectively abolished by the same blockers of Q-type current. 7. Thus Q-channels are present on a subset of the neurohypophysial terminals where, in combination with N- and L-channels, they control AVP but not OT peptide neurosecretion.

Animals↗
PubMed1997

Voltage-dependent modulation of T-type calcium channels by protein tyrosine phosphorylation.

A T-type Ca2+ channel is expressed during differentiation of the male germ lineage in the mouse and is retained in sperm, where is it activated by contact with the the egg's extracellular matrix and controls sperm acrosomal exocytosis. Here, we examine the regulation of this Ca2+ channel in dissociated spermatogenic cells from the mouse using the whole-cell patch-clamp technique. T currents were enhanced, or facilitated, after strong depolarizations or high frequency stimulation. Voltage-dependent facilitation increased the Ca2+ current by an average of 50%. The same facilitation is produced by antagonists of protein tyrosine kinase activity. Conversely, antagonists of tyrosine phosphatase activity block voltage-dependent facilitation of the current. These data are consistent with the presence of a two-state model, in which T channels are maintained in a low (or zero) conductance state by tonic tyrosine phosphorylation and can be activated to a high conductance state by a tyrosine phosphatase activity. The positive and negative modulation of this channel by the tyrosine phosphorylation state provides a plausible mechanism for the control of sperm activity during the early stages of mammalian fertilization.

Animals↗
PubMed1997

Voltage dependent calcium and potassium currents in Y-1 adrenocortical cells are unresponsive to ACTH.

In this report we use both whole cell and perforated patch clamp recording techniques to characterize calcium and potassium channels in Y-1 adrenocortical cells in order to assess their responsiveness to ACTH. Both transient and long-lasting components of an inward calcium current were identified which were similar to T and L-type Ca2+ currents. With Ba2+ as the charge carrier, the transient current activated at voltages more hyperpolarized than -50 mV with V1/2 for activation at -78.1 mV, and for steady state inactivation at -52.3 mV. The L-type current activated at -20 mV, with a V1/2 for activation at -29.9 mV and steady state inactivation at -44.2 mV. Under perforated patch conditions the response was shifted to more depolarized voltages. Both currents were responsive to agents which usually affect T- or L-type Ca2+ currents. The transient current was completely blocked by 50 microM lanthanum or 200 microM nickel and partially blocked by 300 mM amiloride. Cadmium (100 microM) and nifedipine (300 nM) completely blocked the long-lasting current while omega-conotoxin GVIA (1992 nM) inhibited the current by only 20-25%. The agonist, Bay K 8644 was stimulatory at 50 nM. Both transient and sustained outward potassium currents similar to A-type and delayed rectifier currents, respectively, were present. The transient current demonstrated fast activation at voltages more positive than -10 mV, inactivation with continued depolarization and steady state inactivation at V1/2 = -50 mV. The sustained current activated rapidly and had minimal inactivation with continued depolarization. The transient current was blocked by 5 mM 4AP and the sustained by 25 mM TEA. While Y-1 cells contain both calcium and potassium currents similar to those found in other adrenocortical cells, none of the currents were affected by ACTH or AII, secretagogues which stimulate steroidogenesis. These data, combined with the inability of both Ca2+ and K+ channel blockers to alter ACTH-induced steroidogenesis as reported earlier, suggests that neither calcium nor potassium currents are responsive to ACTH in Y-1 cells.

Adrenal Cortex↗
PubMed1996

Activation of mouse sperm T-type Ca2+ channels by adhesion to the egg zona pellucida.

The sperm acrosome reaction is a Ca(2+)-dependent exocytotic event that is triggered by adhesion to the mammalian egg's zona pellucida. Previous studies using ion-selective fluorescent probes suggested a role of voltage-sensitive Ca2+ channels in acrosome reactions. Here, wholecell patch clamp techniques are used to demonstrate the expression of functional T-type Ca2+ channels during mouse spermatogenesis. The germ cell T current is inhibited by antagonists of T-type channels (pimozide and amiloride) as well as by antagonists whose major site of action is the somatic cell L-type Ca2+ channel (1,4-dihydropyridines, arylalkylamines, benzothiazapines), as has also been reported for certain somatic cell T currents. In sperm, inhibition of T channels during gamete interaction inhibits zona pellucida-dependent Ca2+ elevations, as demonstrated by ion-selective fluorescent probes, and also inhibits acrosome reactions. These studies directly link sperm T-type Ca2+ channels to fertilization. In addition, the kinetics of channel inhibition by 1,4-dihydropyridines suggests a mechanism for the reported contraceptive effects of those compounds in human males.

Acrosome↗
PubMed1996

Calcium channels do not play a role in the steroid response to ACTH IN Y1 adrenocortical cells.

Y1 cells derived from mouse tumor zona fasciculata cells (ZF) were used to assess the importance of extracellular (EC) calcium availability via voltage-dependent calcium channels (VDCC) on steroidogenesis. The steroidogenic response to ACTH was investigated in the presence of blockers known to affect both calcium and potassium channels in Y1 cells. Y1 cells respond to either ACTH (100 pM) or cAMP (300 microM) at low EC Ca2+ (1 microM) suggesting that EC Ca2+ is not absolutely necessary for a steroidogenic response. However, increases in Ca2+ from 0.05-2.2 mM induced a small but significant biphasic response, first stimulating then inhibiting steroidogenesis. Nickel and amiloride, blockers of T-type Ca2+ channels in Y1 cells, did not depress ACTH-induced steroidogenesis. The dihydropyridine, nifedipine, which is an L-type channel antagonist did not affect ACTH-induced steroidogenesis while the agonist, Bay K 8644 was consistently inhibitory. Neither pimozide nor omega-conotoxin which suppressed Ca2+ currents in Y1 cells inhibited ACTH-induced steroidogenesis. Depolarization of the membrane which would activate VDCC's was inhibitory rather than stimulatory. The present studies using blockers of both voltage-dependent Ca2+ and K+ channels suggest that EC Ca2+ plays a modulatory role in ACTH-induced steroidogenesis in Y1 cells but the data do not support the concept that activation of voltage dependent calcium channels are an important mechanism for steroidogenesis.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1996

Ethanol increases the activity of large conductance, Ca(2+)-activated K+ channels in isolated neurohypophysial terminals.

Large conductance, Ca(2+)-activated K+ channels are believed to underlie interburst intervals and, thus, contribute to the control of hormone release from neurohypophysial terminals. Because ethanol inhibits the release of vasopressin and oxytocin, we studied its effects on large conductance, Ca(2+)-activated K+ channels from these terminals using patch-clamp techniques. Ethanol (10-100 mM) applied to the cytosolic surface of excised, inside-out patches reversibly increases channel activity in a concentration-dependent manner, reaching a plateau at 50-100 mM. This activation is not mediated by freely diffusible cytosolic second messengers or the release of Ca2+ from intracellular stores. Rather, it likely reflects a direct interaction of ethanol with the channel protein or a closely associated component. Neither the unitary conductance nor the characteristics of the voltage-current relationship are modified by the drug. The increase of channel activity by ethanol results from a modification of channel gating properties: the contribution of long openings to the total time spent in the open state is increased, the average duration of the fast openings is slightly increased, and long closures disappear in the presence of the drug. The activation of large conductance, Ca(2+)-activated K+ channels by ethanol, in conjunction with the previously reported inhibition of voltage-dependent Ca2+ channels, can explain the reduced release of vasopressin and oxytocin after ethanol ingestion.

Animals↗
PubMed1995

Tetrandrine: a new ligand to block voltage-dependent Ca2+ and Ca(+)-activated K+ channels.

Extensive pharmacological investigations on tetrandrine, one of the traditional medicinal alkaloids, are reviewed. Tetrandrine has been used clinically in China for centuries in the treatment of many diseases. A recent series of studies has revealed major mechanisms underlying its multiple pharmacological and therapeutic actions. One of the most interesting discoveries is that tetrandrine is a new kind blocker of the voltage-activated, L-type Ca2+ channel in a variety of excitable cells, such as cardiac, GH3 anterior pituitary and neuroblastoma cells, as well as in rat neurohypophysial nerve terminals. Although tetrandrine does not belong to any of the three classical Ca2+ channel blocker groups, electrophysiological and radioligand binding studies show that tetrandrine is an L-type Ca2+ channel blocker with its binding site located at the benzothiazepine receptor on the alpha 1-subunit of the channel. In addition, tetrandrine is a blocker of the voltage-dependent T-type Ca2+ channel. It is clear that tetrandrine's actions in the treatment of cardiovascular diseases, including hypertension and supraventricular arrhythmia, are due primarily to its blocking of voltage-activated L-type and T-type Ca2+ channels. Furthermore, this alkaloid is a potent blocker of the Ca(2+)-activated K+ (K(Ca)) channels of neurohypophysial nerve terminals. The blocking kinetics of tetrandrine on the K(Ca) channel is quite different from that of typical K(Ca) channel blockers such as tetraethylammonium and Ba2+. Although the clinical role of tetrandrine as a blocker of the K(Ca) channels is unclear, it is a promising ligand for the study of K(Ca) channel function.

Alkaloids↗
PubMed1994

Effects of funnel web spider toxin on Ca2+ currents in neurohypophysial terminals.

Funnel web spider toxin (FTX) is reportedly a specific blocker of P-type Ca2+ channels. The effects of FTX on the Ca2+ currents of isolated neurohypophysial nerve terminals of the rat were investigated using the 'whole-cell' patch-clamp technique. Both the transient and long-lasting Ca2+ current components were maximally elicited by depolarization from a holding potential equal to the normal terminal resting potential (-90 mV). Externally applied FTX inhibited the high-voltage-threshold, transient component of the Ca2+ current in a concentration-dependent manner, with a half-maximal inhibition at a dilution of approximately 1:10000. FTX also shifted the peak current of the I-V relationship by +10 mV. The long-lasting Ca2+ current component, which is sensitive to L-type Ca2+ channel blockers, was insensitive to FTX. The transient current, which is sensitive to omega-conotoxin GVIA, was completely blocked by FTX. These results suggest that there could be a novel, inactivating Ca2+ channel in the rat neurohypophysial terminals which is affected by both N-type and P-type Ca2+ channel blockers.

Alkaloids↗