Interaction of dihydropyridine Ca2+ agonist Bay K 8644 with normal and transformed pituitary cells

Ca²⁺-dependent regulation of Ca²⁺ currents in rat primary afferent neurons: role of CaMKII and the effect of injury

Currents through voltage-gated Ca²⁺ channels (I(Ca)) may be regulated by cytoplasmic Ca²⁺ levels ([Ca²⁺](c)), producing Ca²⁺-dependent inactivation (CDI) or facilitation (CDF). Since I(Ca) regulates sensory neuron excitability, altered CDI or CDF could contribute to pain generation after peripheral nerve injury. We explored this by manipulating [Ca²⁺](c) while recording I(Ca) in rat sensory neurons. In uninjured neurons, elevating [Ca²⁺](c) with a conditioning prepulse (-15 mV, 2 s) inactivated I(Ca) measured during subsequent test pulses (-15 mV, 5 ms). This inactivation was Ca²⁺-dependent (CDI), since it was decreased with elimination of Ca²⁺ influx by depolarization to above the I(Ca) reversal potential, with high intracellular Ca²⁺ buffering (EGTA 10 mm or BAPTA 20 mm), and with substitution of Ba²⁺ for extracellular Ca²⁺, revealing a residual voltage-dependent inactivation. At longer latencies after conditioning (>6 s), I(Ca) recovered beyond baseline. This facilitation also proved to be Ca²⁺-dependent (CDF) using the protocols limiting cytoplasmic Ca²⁺ elevation. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) blockers applied by bath (KN-93, myristoyl-AIP) or expressed selectively in the sensory neurons (AIP) reduced CDF, unlike their inactive analogues. Protein kinase C inhibition (chelerythrine) had no effect. Selective blockade of N-type Ca²⁺ channels eliminated CDF, whereas L-type channel blockade had no effect. Following nerve injury, CDI was unaffected, but CDF was eliminated in axotomized neurons. Excitability of sensory neurons in intact ganglia from control animals was diminished after a similar conditioning pulse, but this regulation was eliminated by injury. These findings indicate that I(Ca) in sensory neurons is subject to both CDI and CDF, and that hyperexcitability following injury-induced loss of CDF may result from diminished CaMKII activity.

Pharmacological modulation of nitric oxide synthesis by mechanism-based inactivators and related inhibitors

Nitric oxide synthase (NOS) (EC 1.14.13.39) is a homodimeric cytochrome P450 monooxygenase analog that generates nitric oxide (NO) from the amino acid L-arginine. Enzymatically produced NO acts as an intracellular messenger in neuronal networks, blood pressure regulatory mechanisms, and immune responses. Isoform-selective pharmacological modulation of NO synthesis has emerged as a new therapeutic strategy for the treatment of diverse clinical conditions associated with NO overproduction. Mechanism-based inactivators (MBIs) represent a class of NOS mechanistic inhibitors that require catalytic turnover to produce irreversible inactivation of the ability of NOS to generate NO. Diverse isoform-selective NOS MBIs have been characterized with respect to their kinetic parameters and chemical mechanisms of inactivation. In studies with isolated and purified NOS isoforms, MBIs produce irreversible inactivation of NOS enzymatic activities. The inactivation process is associated with covalent modification of the NOS active site and proceeds either through heme destruction, its structural alteration, or covalent modification of the NOS protein chain. The behavior of NOS MBIs in intact cells is different from their behavior observed with the isolated NOS isoforms. In cytokine-induced RAW 264.7 macrophages, treatment with MBIs produces a complete loss of cellular NOS synthetic competence and inducible NOS activity. However, following drug removal, cells can recover at least partially in the absence of protein synthesis. In GH3 cells containing the neuronal NOS isoform, calcium transients are too low and abbreviated to allow significant NOS inactivation; hence, the cellular effects of MBIs on the neuronal isoform are almost completely and immediately reversible.

Neuronal nitric oxide synthase is refractory to mechanism-based inactivation in GH3 pituitary cells

Nitric oxide formation by GH3 pituitary cells is stimulated by depolarizing concentrations of K+ and by the L-channel Ca2+ agonist Bay kappa 8644 in an additive manner that depends on extracellular Ca2+. Ca(2+)-dependent NO formation at 100 microM arginine was inhibited 50% over a 30-min period by 5 microM NG-amino-L-arginine, 30 microM N6-iminoethyl-L-ornithine (NIO) and 520 microM N5-iminoethyl-L-lysine (NIL) but required concentrations of aminoguanidine (AG) greater than 3 mM. As measured at 100 microM extracellular arginine, intracellular neuronal nitric oxide synthase (nNOS) was inactivated 50% over a 30-min period by 150 microM NG-amino-L-arginine and 1500 microM NIO, but required concentrations of NIL or AG greater than 5 mM. The inactivation of nNOS by these agents occurred only under conditions that mobilized extracellular Ca2+ and was inhibited by increased extracellular arginine. Thus these agents inhibit cellular Ca(2+)-dependent NO formation at concentrations far lower than those required to inactivate the cellular nNOS. Inhibition of NO formation by these agents was not attributable to effects on cellular arginine uptake. In contrast diphenyliodonium chloride produced a rapid and complete inactivation of cellular NO formation and nNOS activity. These inactivations produced by diphenyliodonium chloride occurred with identical kinetics but displayed no requirement for Ca2+. These data support the assertion that neuronal NO synthase is refractory to mechanism-based inactivation in GH3 pituitary cells.

Maitotoxin, a calcium channel activator, inhibits cell cycle progression through the G1/S and G2/M transitions and prevents CDC2 kinase activation in GH4C1 cells

Calcium regulates progression through several checkpoints in the cell cycle, including the G1/S-phase transition, G2/M-phase transition, and exit from mitosis. In the GH4C1 rat pituitary cell line, calcium mobilizing polypeptides and calcium channel activation inhibit cell proliferation. This report examines the effects of maitotoxin (MTX), an activator of type L voltage-dependent calcium channels (L-VDCC), on calcium influx and cell cycle progression in GH4C1 cells. MTX causes both a block from G1 to S-phase and a concentration-dependent accumulation of cells in G2+M. MTX does not increase the mitotic index; thus, sustained calcium channel activation by MTX results in an accumulation of cells in G2. In order to temporally localize the MTX-induced G2 block relative to cell cycle regulatory events at the G2/M transition, we assessed the relative activity of two cell cycle regulatory protein kinases, CDC2 and CDK2, in MTX-treated cells. CDC2-specific histone kinase activity in MTX-treated cells is lower than either in cells blocked in mitosis with the microtubule destabilizing agent demecolcine or in randomly cycling cells. In contrast, the activity of CDK2 is highest in MTX-treated cells, consistent with a G2 block prior to CDC2 activation. Together, these results implicate with a G2 block prior to CDC2 activation. Together, these results implicate calcium as an intracellular signal required for progression through G2 phase of the cell cycle prior to CDC2 kinase activation.

Evidence that TRH controls prolactin release from rat lactotrophs by stimulating a calcium influx

Prolactin (PRL) release and intracellular free calcium concentration [Ca2+]i were measured in two populations of normal rat lactotrophs (light and heavy fractions) in culture. Spontaneous PRL release of heavy fraction cells was more sensitive to dihydropyridines (DHPs; Bay K 8644 and nifedipine) when compared to the light fraction lactotrophs. The stimulatory effect of thyrotropin-releasing hormone (TRH) on PRL release from heavy fraction cells was inhibited by Cd2+ and mimicked by Bay K 8644. Indo-1 experiments revealed that TRH-increased [Ca2+]i was reversibly inhibited by Cd2+. In a Ca(2+)-free EGTA-containing medium, TRH did not modify [Ca2+]i.

Development of rapid and sensitive high throughput pharmacologic assays for marine phycotoxins

The lack of rapid, high throughput assays is a major obstacle to many aspects of research on marine phycotoxins. Here we describe the application of microplate scintillation technology to develop high throughput assays for several classes of marine phycotoxin based on their differential pharmacologic actions. High throughput "drug discovery" format microplate receptor binding assays developed for brevetoxins/ciguatoxins and for domoic acid are described. Analysis for brevetoxins/ciguatoxins is carried out by binding competition with [3H] PbTx-3 for site 5 on the voltage dependent sodium channel in rat brain synaptosomes. Analysis of domoic acid is based on binding competition with [3H] kainic acid for the kainate/quisqualate glutamate receptor using frog brain synaptosomes. In addition, a high throughput microplate 45Ca flux assay for determination of maitotoxins is described. These microplate assays can be completed within 3 hours, have sensitivities of less than 1 ng, and can analyze dozens of samples simultaneously. The assays have been demonstrated to be useful for assessing algal toxicity and for assay-guided purification of toxins, and are applicable to the detection of biotoxins in seafood.

Ca2+ channel agonists enhance thyrotropin-releasing hormone-induced inositol phosphates and prolactin secretion

The dihydropyridine Ca2+ channel activator BAY K 8644 (1 microM) stimulated basal prolactin secretion from perifused primary cultures of anterior pituitary cells and potentiated the stimulation of prolactin secretion by 1 microM thyrotropin-releasing hormone (TRH) 5-fold over 30 min. This potentiation was mimicked by other dihydropyridine agonists CGP 28392 and (+)-SDZ 202-791 and by (-)-BAY K 8644 (1 microM), but not by (+)-BAY K 8644. The Ca2+ channel antagonist nimodipine, at a concentration sufficient to block BAY K 8644-stimulated 45Ca2+ uptake in GH4C1 anterior pituitary tumor cells, decreased basal prolactin secretion and blocked the enhancement of basal and TRH-stimulated secretion by BAY K 8644. These results suggest that dihydropyridine agonists potentiate TRH-induced secretion through interaction with known stereospecific sites on Ca2+ channels. In GH4C1 cells, BAY K 8644 alone did not affect inositol polyphosphate accumulation, but potentiated TRH-stimulated accumulation of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Accumulation of the Ca(2+)-mobilizing isomer inositol 1,4,5-trisphosphate was not potentiated, suggesting that potentiation of TRH-stimulated hormone secretion by BAY K 8644 does not result from synergistic stimulation of phospholipase C, but may correlate with enhanced inositol trisphosphate-3-kinase activity.

Voltage-dependent calcium channels regulate GH4 pituitary cell proliferation at two stages of the cell cycle

Calcium is an intracellular signal implicated in the regulation of cell proliferation. We have examined the growth regulatory role of voltage-dependent calcium channels (VDCC) in a rat pituitary cell line (GH4C1) that expresses two well-characterized VDCC subtypes (L and T) and is growth-inhibited by several agents known to enhance calcium entry. Thyrotropin-releasing hormone (TRH), tetradecanoylphorbol acetate (TPA), and epidermal growth factor (EGF), each known to enhance calcium entry in GH4 cells, decrease GH4 cell number and incorporation of [3H]-thymidine. The growth inhibitory action of these agents is cytostatic with a predominant effect to block G1 cells from entering S-phase. We next examined the growth regulatory action of pharmacologic agents that interact directly and specifically with type L VDCC. Activation of type L VDCC with the dihydropyridine BAY K8644 inhibits GH4 proliferation as measured by cell number and [3H]-thymidine incorporation. This action of BAY K8644 is enhanced by a submaximal K(+)-maintained depolarization, and the growth inhibitory action of these agents is also cytostatic as evident by the block of G1 cells from entering S-phase. Nimodipine, an antagonist specific for type L VDCC blocks (IC50 = 30 nM) BAY K8644-inhibited cell proliferation by substantially reducing the S-phase block. Taken together these findings indicate that calcium entry through type L VDCC inhibits GH4 cell proliferation by blocking entry into S-phase. By contrast, nimodipine caused only a small reversal of the TRH-induced S-phase block, suggesting that TRH inhibits proliferation by a mechanism that differs at least in part from L-channel activation. Unexpectedly, nimodipine, given alone, caused a substantial inhibition of GH4 cell proliferation. This action of nimodipine was cytostatic, yet differed from calcium channel activators in that the percentage of S-phase cells was unchanged whereas G2-M-phase cells increased with a parallel decrease in G1-phase cells. Similar effects were also observed with other classes of calcium channel blockers. Taken together these results indicate that calcium entry through VDCC regulates GH4 cell proliferation differently depending on the stage of the cell cycle. In G1-phase cells, sustained entry of calcium through type L VDCC blocks entry into S-phase. In G2-M-phase cells entry of calcium promotes progression through mitosis.

L-type calcium channels may regulate neurite initiation in cultured chick embryo brain neurons and N1E-115 neuroblastoma cells

The intracellular free Ca2+ concentration, [Ca2+]i, plays an important role in regulating neurite growth in cultured neurons. Insofar as [Ca2+]i is partly a function of Ca2+ influx through voltage-sensitive calcium channels (VSCC), Ca2+ entry through VSCC should influence neurite growth. Vertebrate neurons may possess several types of VSCC. The most frequently described VSCC types are usually designated L, T and N. In most preparations, these VSCC types respond differently to certain pharmacological agents, including Cd2+, Ni2+, the dihydropyridines nifedipine and BAY K8644, and the aminoglycoside antibiotics. We used these agents to study the role of Ca2+ influx in regulating neurite initiation and length in cultures of chick embryo brain neurons and N1E-115 mouse neuroblastoma cells. In chick neurons, nifedipine and Cd2+ (less than 50 microM), which have been reported to inhibit L-type channels, reduced neurite initiation, but not mean neurite length. Ni2+ (less than 100 microM), reported to inhibit T-type channels, had no effect on either initiation or length. Low concentrations of most aminoglycosides (less than 300 microM), reported to inhibit N-type channels, had no effect on neurite initiation, but high concentrations of streptomycin (great than 300 microM), reported to inhibit both L- and N-type channels, reduced neurite initiation. BAY K8644, which enhances current flow through L-type channels, had no effect except at high concentration (50 microM), which inhibited initiation. N1E-115 neuroblastoma cells have been reported to contain L-type and T-type channels, but thus far no channel similar to the N-type has been described. In cultured N1E-115 cells, nifedipine (5 microM), Cd2+ (5 microM), and streptomycin (200 microM) reduced neurite initiation, while nickel (50 microM) and neomycin (100 microM) did not affect initiation. None of these agents altered neurite length. In N1E-115 cells, whole-cell voltage clamp recordings showed that nifedipine and Cd2+ inhibited L-type channels but not T-type channels, while Ni2+ inhibited T-type channels but not L-type channels. Streptomycin slightly inhibited L-type channels but enhanced current flow through T-type channels. Neomycin slightly inhibited both channel types. These data indicated that neurite initiation in these two cell types may be modulated by Ca2+ influx through L-type channels, but not T- or N-type channels. Neurite length was not significantly influenced by any of the agents tested, suggesting that Ca2+ influx through VSCC may not affect neurite elongation.

Three components of the calcium current in cultured glomerulosa cells from rat adrenal gland

1. Ca2+ channels were studied in cultured glomerulosa cells from the rat adrenal gland. The whole-cell configuration of the patch-clamp technique was used. Cs+-filled pipettes were used in order to block K+ channels. 2. Three Ca2+ components were found, namely, T, L and N, according to the nomenclature proposed by Nowycky, Fox & Tsien (1985). The T-component was a fast transient component activated in the range -60 to -40 mV; the L-component did not inactivate for a sustained depolarization and activated at voltages around -30 mV; the third component, the N-component, was transient and was activated at voltages close to -20 mV. 3. A statistical analysis made on seventy-one experiments showed that the L-component was the most frequent (65% of the experiments), followed by the T- and finally the N- components (59 and 29% of the experiments, respectively). 4. The substitution of Ba2+ ions for Ca2+ ions greatly enhanced the L-component's amplitude (iBa/iCa = 4) while the N-component was unaffected and the T-component was reduced (iBa/iCa = 0.4). 5. A comparison of the voltage-dependent steady-state inactivation of the three components showed that the T-component was inactivated at -60 mV while the inactivation of the L- and N-components was complete at -25 and 0 mV, respectively. 6. A run-down effect was detected in some cells. The time stability of the L-component was lower than that of the T-component. The N-component seemed to be insensitive for at least 1 h. The results for the L- and T-components were obtained in cells which presented no run-down of the current or only a weak one. 7. Cd2+ ions (5 x 10(-5)M) completely blocked the long-lasting component (L-component) and slightly decreased the T-component. 8. Bay K 8644, a dihydropyridine agonist, enhanced the L-component at a concentration of 2.5 microM but decreased it for a higher concentration (5 microM). The T-component was decreased in a reversible way by 1 microM-Bay K 8644. Nifedipine, a well-known antagonist, blocked completely the L-component. This effect was reversed by the addition of Bay K 8644 to the perfusion medium. The T-component was also blocked by nifedipine, a result which is in keeping with the fact that Bay K 8644 has a weak effect on this current.

Single nisoldipine-sensitive calcium channels in smooth muscle cells isolated from rabbit mesenteric artery

Single smooth muscle cells were enzymatically isolated from the rabbit mesenteric artery. At physiological levels of external Ca, these cells were relaxed and contracted on exposure to norepinephrine, caffeine, or high levels of potassium. The patch-clamp technique was used to measure unitary currents through single channels in the isolated cells. Single channels were selective for divalent cations and exhibited two conductance levels, 8 pS and 15 pS. Both types of channels were voltage-dependent, and channel activity occurred at potentials positive to -40 mV. The activity of both channel types was almost completely inhibited by 50 nM nisoldipine. These channels appear to be the pathways for voltage-dependent Ca influx in vascular smooth muscle and may be the targets of the clinically used dihydropyridines.