Blockade by funnel web toxin of a calcium current in the intermediate pituitary of the rat

Critical role of L-type voltage-dependent Ca2+ channels in neural progenitor cell proliferation induced by hypoxia

Hypoxia can promote proliferation of neural progenitor cells in vitro and in vivo, however, the mechanisms underlying this phenomenon remain largely unknown. Calcium ions are important for the proliferation of progenitor cells. In this study, we reported that Ca(2+) influx through L-type voltage-dependent Ca(2+) channels mediated hypoxia-promoted proliferation of neural progenitor cells isolated from embryonic day 14.5 rat mesencephalon. Cell number was greatly increased in the cultured neural progenitor cells exposed to physiological hypoxia (3% O(2), 72 h) compared with normoxia exposure (20% O(2), 72 h). Increased intracellular Ca(2+) concentration was also observed when the cells were exposed to hypoxia. Moreover, removal of extracellular Ca(2+) or administration of nicardipine, an agent known to block L-type Ca(2+) channels, resulted in suppression of the hypoxia-induced increase in intracellular Ca(2+) and cell numbers. These results suggest that hypoxia promoted the proliferation of neural progenitor cells by increasing Ca(2+) influx, which was likely a result of upregulation of L-type voltage-dependent Ca(2+) channel function.

Voltage-activated Ca(2+) channels and their role in the endocrine function of the pituitary gland in newborn and adult mice

We have prepared fresh pituitary gland slices from adult and, for the first time, from newborn mice to assess modulation of secretory activity via voltage-activated Ca(2+) channels (VACCs). Currents through VACCs and membrane capacitance have been measured with the whole-cell patch-clamp technique. Melanotrophs in newborns were significantly larger than in adults. In both newborn and adult melanotrophs activation of VACCs triggered exocytosis. All pharmacologically isolated VACC types contributed equally to the secretory activity. However, the relative proportion of VACCs differed between newborns and adults. In newborn cells L-type channels dominated and, in addition, an exclusive expression of a toxin-resistant R-type-like current was found. The expression of L-type VACCs was up-regulated by the increased oestrogen levels observed in females, and was even more emphasized in the cells of pregnant females and oestrogen-treated adult male mice. We suggest a general mechanism modulating endocrine secretion in the presence of oestrogen and particularly higher sensitivity to treatments with L-type channel blockers during high oestrogen physiological states.

The role of voltage-gated Ca2+ channels in anoxic injury of spinal cord white matter

Dorsal column axons of the rat spinal cord are partially protected from anoxic injury following blockade of voltage-sensitive Na+ channels and the Na+/--Ca2+ exchanger. To examine the potential contribution of voltage-gated Ca2+ channels to anoxic injury of spinal cord axons, we studied axonal conduction in rat dorsal columns in vitro following a 60-min period of anoxia. Glass microelectrodes were used to record field potentials from the dorsal columns following distal local surface stimulation. Perfusion solutions containing blockers of voltage-gated Ca2+ channels were introduced 60 min prior to onset of anoxia and continued until 10 min after reoxygenation. Pharmacological blocking agents which are relatively selective for L- (verapamil, diltiazem, nifedipine) and N- (omega-conotoxin GVIA) type calcium channels were significantly protective against anoxia-induced loss of conduction, as was non-specific block using divalent cations. Other Ca2+ channel blockers (neomycin and omega-conotoxin MVIIC) that affect multiple Ca2+ channel types were also neuroprotective. Ni2+, which preferentially blocks R-type Ca2+ channels more than T-type channels, was also protective in a dose-dependent manner. These data suggest that the influx of Ca2+, through L-, N- and possibly R-type voltage-gated Ca2+ channels, participates in the pathophysiology of the Ca2+-mediated injury of spinal cord axons that is triggered by anoxia.

Two distinct Na+ currents control cytosolic Ca2+ pulsing in Xenopus laevis pituitary melanotrophs

Studies with the Na+ channel blocker tetrodotoxin (TTx) on Ca(2+)-dependent hormone release by mammalian and amphibian pituitary melanotrophs have suggested that the Na+ spikes these cells generate are not responsible for triggering Ca2+ influx and consequently secretion. In contrast, we found in Xenopus laevis melanotrophs that the spontaneously occurring elevations in cytosolic free Ca2+ concentration ("Ca2+ pulses') were dependent on the presence of extracellular Na+ and sensitive to TTx and the Na+ channel activator, veratridine. However, an inhibitory effect of TTx could only be demonstrated when the extracellular Na+ concentration was lowered to near-threshold levels. In voltage-clamp experiments, two distinct Na+ currents were recorded, one sensitive to TTx and the other insensitive to TTx but blocked by micromolar concentrations of Cd2+. Together they appeared to control action potential activity and spontaneous Ca2+ pulsing. These data strongly suggest that Na+ action potentials do regulate cytosolic free Ca2+ concentration in melanotrophs.

Involvement of N- and P/Q- but not L- or T-type voltage-gated calcium channels in ischaemia-induced striatal dopamine release in vitro

Calcium influx and transmitter efflux are central events in the neuropathological cascade that occurs during and following cerebral ischaemia. This study explored the role of voltage-gated calcium channels (VGCCs) in ischaemia-induced striatal dopamine (DA) release in vitro. Slices (350 microm thickness) of rat neostriatum were superfused (400 ml/h) with an artificial cerebrospinal fluid (aCSF) at 34 degrees C and subjected to episodes of 'ischaemia' by reduction of the glucose concentration from 4 to 2 mM and gassing with 95% N2/5% CO2. DA release was monitored with fast cyclic voltammetry at implanted carbon fibre microelectrodes. The time to onset, time to peak, rate and magnitude of DA release were measured. Non-selective blockade of VGCCs with a high concentration of Ni2+ (2.5 mM), markedly delayed (P < 0.01) and slowed (P < 0.05) DA release but preferential blockade of T-type VGCCs with a lower concentration (200 microM) had no effect. DA release was also unaffected by selective antagonism of L-type VGCCs with nimodipine and nicardipine (10 microM each). Selective blockade of N-type VGCCs with omega-conotoxin GVIA (100 nM) delayed DA release (P < 0.05) but did not affect its rate or magnitude. Blockade of P- and possibly Q-type VGCCs with omega-agatoxin IVA (up to 200 nM) both delayed (P < 0.05) and slowed (P < 0.05) DA release. Preferential blockade of P- type VGCCs with neomycin (500 microM) also delayed (P < 0.05) and slowed (P < 0.05) DA release. These findings suggest that N-, P- and possibly Q- but not L- or T-type VGCCs mediate ischaemia-induced DA release. Although it is not possible to say, on the basis of these results, that the effects are directly upon the dopamine terminals, these calcium channels nevertheless constitute promising targets for therapeutic intervention.

Polyamine FTX-3.3 and polyamine amide sFTX-3.3 inhibit presynaptic calcium currents and acetylcholine release at mouse motor nerve terminals

FTX-3.3 is the proposed structure of a calcium-channel blocking toxin that has been isolated from the funnel web spider (Agelenopsis aperta). The effects of FTX-3.3 and one of its analogues, sFTX-3.3, on acetylcholine release, on presynaptic currents at mouse motor nerve terminals and on whole-cell sodium currents in SK.N.SH cells (a human neuroblastoma cell line) have been studied. FTX-3.3 (10-30 microM) and sFTX-3.3 (100-300 microM) reversibly reduced release of acetylcholine by approximately 70-90% and 40-60%, respectively. FTX-3.3 (10 microM) blocked the fast component of presynaptic calcium currents by approximately 60%. sFTX-3.3 (100 microM) reduced the duration of the slow component of presynaptic calcium currents by about 50% of the control and also reduced presynaptic sodium current by approximately 20% of the control. sFTX-3.3 (100 microM) reduced whole-cell sodium current recorded from SK.N.SH cells by approximately 15%, whereas FTX-3.3, even at 200 microM, did not affect this current. Since the only difference in chemical structures of these toxins is that sFTX-3.3 has an amide function which is absent in FTX-3.3, the amide function may be responsible for the reduced potency and selectivity of sFTX-3.3. This study also provides further support for the existence of P-type calcium channels at mouse motor nerve terminals.

Dihydropyridine block of omega-agatoxin IVA- and omega-conotoxin GVIA-sensitive Ca2+ channels in rat pituitary melanotropic cells

High voltage-activated Ca2+ currents in rat melanotropic cells consist of a sustained and an inactivating component. In this study the pharmacological properties of the high voltage-activated Ca2+ channels underlying these components are investigated with whole-cell recordings. We report that melanotropes express four pharmacologically distinct high voltage-activated Ca2+ channels. Non-inactivating L-type channels account for 35% of the total high voltage-activated channel population. These channels have a very high affinity for the dihydropyridine nimodipine (EC50 approximately 3 pM). The cone snail toxin omega-conotoxin GVIA irreversibly blocked an inactivating high voltage-activated component which accounted for 26% of the total whole-cell high voltage-activated Ca2+ current. The spider toxin omega-agatoxin IVA reversibly blocked an additional 31% of the total high voltage-activated current. The current blocked by omega-agatoxin IVA was not homogenous and consisted of a sustained component with a high affinity for omega-agatoxin IVA (< 10 nM) and an inactivating current with a low affinity for omega-agatoxin IVA (> 100 nM). Both the omega-agatoxin IVA and omega-conotoxin GVIA-blocked currents were very sensitive to nimodipine and nitrendipine with a half maximal block at 200-500 nM. 10 microM nimodipine blocked 70% of the omega-conotoxin GVIA-sensitive current and 90% of the omega-agatoxin IVA-sensitive current. Thus, omega-conotoxin GVIA- and omega-agatoxin IVA-sensitive high voltage-activated Ca2+ channels in melanotropes have an unusual high affinity for dihydropyridines compared to N-, P-, and Q-type channels in other preparations.

Selective inhibition of high voltage-activated L-type and Q-type Ca2+ currents by serotonin in rat melanotrophs

1. Whole-cell Ca2+ currents (ICa) from cultured rat melanotrophs were identified by their sensitivity to Ca2+ channel blockers, and their modulation by serotonin (5-HT) was studied. All cells displayed high voltage-activated (HVA; > -30 mV) Ca2+ currents. A low voltage-activated (LVA; > -60 mV) Ca2+ current was detected in 92% of the cells. 2. The whole-cell ICa was insensitive to omega-conotoxin GVIA (0.5-1 microM) indicating the absence of N-type Ca2+ channels. 3. At a holding potential (Vh) of -70 mV, the L-type channel blocker nifedipine reduced ICa in a dose-dependent manner with a half-maximal effective concentration (IC50) of 28 nM. The L-type current represented 39% of the total ICa. 4. omega-Agatoxin IVA (omega-Aga IVA) produced a biphasic dose-dependent inhibition of ICa, with IC50 values of 0.4 and 91 nM, indicating the presence of P-type and Q-type Ca2+ channels, which accounted respectively for 16 and 45% of the total ICa. The P-type current was also blocked by synthetic funnel-web spider toxin (sFTX 3.3; 1-10 microM) and was present only in a subpopulation (60-70%) of cells. 5. All cells possessed a Ca2+ current which was resistant to nifedipine (10 microM) and omega-Aga IVA (50 nM). This current was not affected by Ni2+ (40 microM) but was abolished by a low concentration of Cd2+ (10 microM) and by omega-conotoxin MVIIC (1 microM) indicating that it was a Q-type Ca2+ current. 6. 5-HT (10 microM) inhibited the whole-cell ICa in 70% of the cells tested (n = 120) by activating 5-HT1A and 5-HT2C receptors. 5-HT produced either a kinetic slowing of the activation phase (37% of the cells) or a scaling down (14% of the cells) of ICa. In the majority of cells (49%) both types of inhibition were found to coexist. 7. The effects of 5-HT were voltage dependent, rendered irreversible when GTP-gamma-S (30 microM) was present in the pipette solution and abolished by pretreatment of the cells with pertussis toxin (PTX; 150 ng ml-1, 18 h). 8. Low concentrations of omega-Aga IVA (20 nM), which blocked mainly P-type channels, did not reduce the effect of 5-HT on ICa. The scaling down effect of 5-HT on ICa was eliminated in the presence of nifedipine (10 microM) and the kinetic slowing effect of 5-HT persisted after blockade of L- and P-type channels but was abolished by omega-conotoxin MVIIC (1 microM). 9. We conclude that rat melanotrophs possess functional L-, P- and Q-type Ca2+ channels and that 5-HT inhibits selectively L-type and Q-type Ca2+ currents with different modalities. These effects are voltage dependent and mediated by a PTX-sensitive G-protein.

Voltage dependence of G-protein-mediated inhibition of high-voltage-activated calcium channels in rat pituitary melanotropes

Dopamine D2 receptor stimulation inhibited high-threshold, slowly inactivating (L-type) barium currents of isolated, rat pituitary melanotropes in primary culture. The extent of inhibition depended on the concentration of LY 171555 applied. Current activation in the presence of LY 171555 was described by two time constants, a fast one, also observed under control conditions, and a slow one, induced by LY 171555. The slow time constant did not depend on the concentration of LY 171555. Guanosine-5'-O-(3-thiotriphosphate) (100 microM) induced a similar inhibition of the barium currents. Depolarizing prepulses more positive than -20 mV counteracted the inhibition induced by LY 171555 as well as guanosine-5'-O-(3-thiotriphosphate). The voltage dependence and time course of this disinhibition were obtained. The results suggest that the slow time course of activation during current inhibition reflects a voltage-dependent conversion of the channel from the inhibited state to an available state. This voltage dependence is probably an intrinsic property of the calcium channel. The voltage-dependent rate constants of a first-order kinetic model which describes the voltage-dependent inhibition and disinhibition were estimated.

The secretion of alpha-MSH from xenopus melanotropes involves calcium influx through omega-conotoxin-sensitive voltage-operated calcium channels

The secretory activity of endocrine cells largely depends on the concentration of free cytosolic calcium. We have studied the mechanisms that are involved in supplying the calcium necessary for the secretion of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells in the pituitary intermediate lobe of the amphibian Xenopus laevis. Using whole-cell voltage clamp, high-voltage activated calcium currents were observed, with a peak current between 0 and +20 mV. Two types of Ca(2+)-currents appeared, depending on the experimental setup. An inactivating current, which was observed after a 10 msec depolarizing prepulse, resembled currents through N-type channels as it was clearly inhibited by 1 microM omega-conotoxin. The second type was a non-inactivating current, which was blocked up to 50% by 1 microM nifedipine, indicating its L-type nature. Only a small component of this inactivating current could be blocked by omega-conotoxin. No evidence was found for the presence of transient, low-voltage activated currents. The spontaneous secretion of alpha-MSH from superfused neurointermediate lobes was dependent on extracellular calcium, as low calcium conditions (10(-4)-10(-8) M) rapidly inhibited this process. Under these conditions, secretion was not affected by depolarizing concentrations of potassium chloride. The calcium ionophore A23187 increased secretion under low calcium conditions, but had no effect on spontaneous alpha-MSH release. These results suggest that spontaneous alpha-MSH release depends on influx of calcium through voltage-operated calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)