POS0436 PATIENT CHARACTERISTICS AND OUTCOMES IN PATIENTS WITH RHEUMATOID ARTHRITIS TREATED WITH UPADACITINIB: THE OM1 RA REGISTRY

Background:

Upadacitinib (UPA) has demonstrated efficacy in the treatment of rheumatoid arthritis (RA) in randomized controlled trials,1-6 but there are limited data available on its real-world use and effectiveness in patients with RA.

Objectives:

To describe the characteristics and clinical outcomes at 3 months among real-world patients with RA initiating UPA.

Methods:

The data source for this study was the OM1 RA Registry, a subset of the OM1 Real-World Data Cloud (OM1, Inc, Boston, MA, US), a large, linked clinical and administrative dataset derived from medical and pharmacy claims, electronic medical record data, and death data. This analysis includes data collected in patients who initiated UPA during or after August 2019. Patients had ≥1 prescription for UPA (index date was first UPA prescription), were ≥18 years of age at index date, had ≥6 months of available data in the OM1 RA Registry prior to index date (ie, baseline period), ≥1 baseline disease activity measure, and ≥1 follow-up disease activity measure (3 or 6 months post-index). Disease activity was based on RAPID3 or CDAI. Multivariate analyses were conducted using a mixed-effects linear model adjusting for age, sex, and baseline scores. Outcomes were also assessed by therapy status (monotherapy or combination therapy) and targeted immunomodulator (TIM) use (naïve vs experienced).

Results:

Inclusion criteria were met by 1,102 patients, of whom 620 were on monotherapy and 482 were on combination therapy at index. Mean age was 57.7 years, 83% were female, 75% had prior treatment with a biologic, and 47% had prior treatment with a Janus kinase inhibitor. Of 651 patients with known disease activity category, 113 (17%) were in low disease activity (LDA)/remission. At baseline, overall mean±SD scores were 19.9±12.3 for CDAI, 4.5±2.4 for RAPID3, 5.7±2.8 for pain, 5.2±3.0 for fatigue, 3.1±2.7 for MDHAQ Physician Global Assessment (PGA), 5.2±2.8 for MDHAQ Patient Global Assessment (PtGA), and 3.1±2.3 for MDHAQ Functional Index. At 3 months post-UPA initiation, mean (95% CI) change in CDAI was –5.1 (–7.5 to –2.7) in the monotherapy group and –5.9 (–8.7 to –3.0) in the combination group. At 3 months, 29% (109/374) of patients were in LDA/remission and 32% (120/374) of patients showed improvement in disease activity. Of 94 patients with moderate disease at baseline, 34 (36%) were in LDA/remission at 3 months. Of 215 patients with high disease at baseline, 30 (14%) were in LDA/remission and 49 (23%) had moderate disease at 3 months. RAPID3 and other outcomes also improved at 3 months in the monotherapy and combination therapy groups (Figure 1). Improvements in disease activity were observed at 3 months and maintained at 6 months post-UPA initiation. Of 1,102 patients, 16% were TIM naïve and 84% TIM experienced. Both TIM-naïve and TIM-experienced patients achieved significant mean changes in CDAI (–5.7 [–10.8 to –0.6] and–5.0 [–7.0 to –3.0], respectively) and RAPID3 (–1.0 [–1.6 to –0.4] and –0.5 [–0.8 to –0.1]) at 3 months (Table 1). Improvements in clinical outcomes were maintained at 6 months in both TIM-naïve and TIM-experienced patients.

Conclusion:

Significant improvements in disease activity were consistently observed at 3 months and maintained at 6 months post-UPA initiation regardless of monotherapy, combination therapy, or prior TIM use.

References:

[1]Fleischmann R. Arthritis Rheumatol. 2019;71:1788–800.

[2]Smolen JS. Lancet. 2019;393:2303–11.

[3]Burmester GR. Lancet. 2018;382:2505–12.

[4]Genovese MC. Lancet. 2018;391:2513–24.

[5]van Vollenhoven R. Arthritis Rheumatol. 2020;72:1607–20.

[6]Rubbert-Roth A. N Engl J Med. 2020;383:1511–21.

Table 1.

Change in clinical outcomes from baseline at 3 months: TIM-naïve and TIM-experienced groups

TIM naïve(N=179)TIM experienced(N=923)nMean changenMean changeCDAI36–5.7*160–5.0*RAPID367–1.0*189–0.5*Pain (VAS)76–1.5*237–0.9*Fatigue46–0.7149–0.5MDHAQ PGA65–0.7*251–0.7*MDHAQ PtGA97–0.6*383–0.3MDHAQ Functional Index72–0.7*215–0.2

*Statistically significant change from baseline (P<0.05).

Acknowledgements:

Funding statement: Financial support for the study was provided by AbbVie. AbbVie participated in the interpretation of data, review, and approval of the abstract. All authors contributed to the development of the publication and maintained control over the final content.

Acknowledgment:

Medical writing services were provided by Joann Hettasch of Fishawack Facilitate Ltd, part of Fishawack Health, and funded by AbbVie.

Disclosure of Interests:

Martin Bergman Shareholder of: JNJ (parent of Janssen), Speakers bureau: AbbVie, Amgen, BMS, Genentech, Gilead, Janssen, Merck, Novartis, Pfizer, Regeneron, Sanofi, Sandoz, Consultant of: AbbVie, Amgen, BMS, Genentech, Gilead, Janssen, Merck, Novartis, Pfizer, Regeneron, Sanofi, Sandoz, Namita Tundia Shareholder of: AbbVie, Employee of: AbbVie, Allison Bryant: None declared, Ia Topuria: None declared, Tom Brecht: None declared, Kendall Dunlap Shareholder of: AbbVie, Employee of: AbbVie, Allan Gibofsky Shareholder of: AbbVie, Amgen, Horizon, J&J, Pfizer, Regeneron, Speakers bureau: AbbVie, Acquist, Amgen, Lilly, Merck, Pfizer, Sandoz, Samumed, Consultant of: AbbVie, Acquist, Amgen, Lilly, Merck, Pfizer, Sandoz, Samumed

POS0666 TREATMENT EFFECTIVENESS OF UPADACITINIB AT 3 MONTHS IN US PATIENTS WITH RHEUMATOID ARTHRITIS FROM THE UNITED RHEUMATOLOGY NORMALIZED INTEGRATED COMMUNITY EVIDENCE (NICE[TM]) REAL-WORLD DATA

Background:

Upadacitinib (UPA), an oral Janus kinase inhibitor (JAKi), has demonstrated efficacy in the phase 3 SELECT clinical program, conducted across a range of patients (pts) with rheumatoid arthritis (RA).1–6 Real-world data for UPA, including in pts previously treated with a JAKi, have not yet been reported since global approvals beginning in 2019.

Objectives:

To assess the characteristics of US-based pts receiving UPA and its effectiveness in clinical practice at 3 months.

Methods:

This observational study included US-based pts from the United Rheumatology Normalized Integrated Community Evidence (UR-NICE) database who initiated UPA 15 mg once daily from FDA approval (August 2019) to July 31, 2020 and had ≥6-month pre-baseline data available. Effectiveness was assessed in pts with a reported Clinical Disease Activity Index (CDAI) score at 3 months after UPA initiation and included proportions of pts achieving CDAI remission (≤2.8), CDAI low disease activity (≤10), other disease activity measures, and pt-reported outcomes. A subgroup analysis assessed UPA effectiveness in pts with or without prior tofacitinib (TOFA) treatment.

Results:

This analysis included 252 pts treated with UPA 15 mg, of whom 98 (38.9%) received UPA monotherapy and 154 (61.1%) received UPA combined with conventional synthetic (cs) disease-modifying antirheumatic drugs (DMARDs). 64.3% of pts were from the Southern region of the USA. 86.1%, 72.2%, and 47.6% of pts had been previously treated with csDMARDs, biologic DMARDs, and JAKis, respectively. Baseline characteristics were largely similar between UPA monotherapy and combination therapy groups and those with or without prior TOFA treatment (Table 1). Pts with prior TOFA treatment had a longer duration of RA since diagnosis and higher steroid use versus those without. UPA 15 mg improved disease activity scores (including CDAI) and pt-reported outcomes (including physical function and pain) after 3 months of treatment (Figure 1). Similar effectiveness was observed with UPA 15 mg in pts with or without prior TOFA treatment.

Conclusion:

In the UR-NICE real-world database of US-based pts, improvements in clinical and pt-reported outcomes were observed at 3 months in UPA-treated pts with RA, including those with or without prior TOFA treatment, despite the treatment-refractory population included in this dataset.

References:

[1]Burmester GR, et al. Lancet 2018;391:2503–12.

[2]Smolen JS, et al. Lancet 2019;393:2303–11.

[3]Fleischmann R, et al. Arthritis Rheumatol 2019;71:1788–800.

[4]Genovese MC, et al. Lancet 2018;391:2513–24.

[5]van Vollenhoven R, et al. Arthritis Rheumatol 2020;72:1607–20.

[6]Rubbert-Roth A, et al. N Engl J Med 2020;383:1511–21.

Table 1.

Baseline characteristics

n (%), unless otherwise statedFull analysis set(n=252)Pts with prior TOFA treatment(n=113)Pts without prior TOFA treatment (n=139)Mean (SD) exposure, days219.7 (112.1)215.7 (116.7)222.9 (108.5)Female199 (79.0)85 (75.2)114 (82.0)Age ≥65 years75 (29.8)34 (30.1)41 (29.5)Oral steroid use140 (55.6)70 (61.9)70 (50.4)Prior csDMARDs217 (86.1)102 (90.3)115 (82.7)Prior TOFA113 (44.8)113 (100.0)0Prior biologic DMARDs182 (72.2)86 (76.1)96 (69.1)Tumor necrosis factor inhibitor147 (58.3)66 (58.4)81 (58.3)Interleukin-6 receptor inhibitor87 (34.5)47 (41.6)40 (28.8)nMean (SD)nMean (SD)nMean (SD)Duration of RA diagnosis, years1884.0 (3.0)895.1 (2.9)993.1 (2.8)Methotrexate dose, mg/week8817.0 (5.1)2817.8 (5.0)6016.6 (5.2)SJC282394.8 (5.7)1084.5 (5.0)1315.0 (6.2)TJC282376.5 (6.7)1076.5 (6.8)1306.5 (6.6)CDAI22520.4 (13.4)10520.2 (13.5)12020.6 (13.3)Routine assessment of patient index data 31654.2 (2.3)724.2 (2.4)934.3 (2.2)Disease Activity Score in 28 joints based on C-reactive protein1673.9 (1.5)833.9 (1.5)843.9 (1.5)Health Assessment Questionnaire-Disability Index1702.5 (2.1)742.4 (2.2)962.5 (2.1)Pain(0–10)22956.5 (28.5)10456.9 (29.3)12556.1 (28.0)SD, standard deviation; S/TJC, swollen/tender joint count

Acknowledgements:

AbbVie funded this study; contributed to its design; participated in data collection, analysis, and interpretation of the data; and participated in the writing, review, and approval of the abstract. No honoraria or payments were made for authorship. Medical writing support was provided by Hilary Wong, PhD, of 2 the Nth (Cheshire, UK), and was funded by AbbVie.

Disclosure of Interests:

Allan Gibofsky Shareholder of: AbbVie, Amgen, Johnson & Johnson, and Pfizer, Consultant of: AbbVie, Celgene, Eli Lilly, Flexion, Pfizer, Relburn Pharma, and Samumed. Paid consultant with investment analysts on behalf of the Gerson Lehrman Group, Bhavna Dhillon Shareholder of: May own stock or options in United Rheumatology, Employee of: United Rheumatology, Mark E. Pearson Shareholder of: May own AbbVie stock or options, Namita Tundia Shareholder of: May own stock or options in AbbVie, Employee of: AbbVie, Yanna Song Shareholder of: May own stock or options in AbbVie, Employee of: AbbVie, Kendall Dunlap Shareholder of: May own stocks or shares in AbbVie, Employee of: AbbVie, Grace Wright Speakers bureau: AbbVie, Amgen, Bristol-Myers Squibb, Eli Lilly, Exagen, Myriad Autoimmune, Novartis, Sanofi/Regeneron, UCB, and Vindico, Consultant of: AbbVie, Amgen, Bristol-Myers Squibb, Eli Lilly, Exagen, Gilead, Janssen, Myriad Autoimmune, Novartis, Pfizer, Sanofi/Regeneron, and UCB, Employee of: President and Founder of the Association of Women in Rheumatology

FRI0140 IMPACT OF BASELINE DEMOGRAPHICS AND DISEASE ACTIVITY ON OUTCOMES IN PATIENTS WITH RHEUMATOID ARTHRITIS RECEIVING UPADACITINIB

Background:

Upadacitinib (UPA), an oral selective JAK1 inhibitor, has demonstrated favorable efficacy and acceptable safety in five Phase 3 global studies in patients with moderately to severely active rheumatoid arthritis (RA).1–5

Objectives:

This analysis reports the efficacy and safety of UPA in predefined RA patient subgroups based on differences in baseline demographics and disease activity.

Methods:

Data were pooled from three pivotal, double-blind, PBO-controlled, multicenter, Phase 3 studies in patients with RA who had an inadequate response(IR) to conventional synthetic DMARDs (csDMARD-IR: SELECT-NEXT [N=661]), MTX(MTX-IR; SELECT-COMPARE[N=1629]), or biologic DMARDs(bDMARD-IR: SELECT-BEYOND[N=498]). Two integrated analysis sets were evaluated: one comparing UPA 15 mg QD vs PBO(SELECT-NEXT, SELECT-COMPARE, SELECT-BEYOND) and the other comparing UPA 15 mg QD and UPA 30 mg QD vs PBO(SELECT-NEXT, SELECT-BEYOND). All patients received background treatment with csDMARDs. The proportion of patients achieving ACR20 and DAS28(CRP) ≤3.2 at Week 12 was evaluated by predefined baseline demographics and disease activity measure groups, including age, sex, weight, BMI, race, geographic region, duration of RA, RF, and ACPA status, and level of high sensitivity CRP. Non-responder imputation was used for missing data. Subgroup analyses for safety were performed for age, race, sex, weight, BMI, and Asian region.

Results:

Across the three Phase 3 studies, 1036, 384, and 1041 patients received UPA 15 mg QD, UPA 30 mg QD or PBO, respectively. The demographic and baseline disease characteristics in the two integrated analysis sets were balanced across treatment groups. ACR20 and DAS28 ≤3.2 response rates at Week 12 were consistently higher with UPA 15 mg and UPA 30 mg vs PBO across the evaluated demographic and baseline disease characteristics(Figure 1a,Figure 1b). The efficacy of UPA 15 mg QD was generally similar to that observed with UPA 30 mg QD. At 12 weeks, the proportion of patients with treatment-emergent AEs, serious AEs, severe AEs, and AEs leading to discontinuation were generally comparable across different age, sex, race, weight, and BMI groups. Compared with the global population, patients receiving UPA in the Asian region had a higher rate of CPK elevations(UPA 30 mg only) and herpes zoster; herpes zoster also has been observed to be higher in the Asian region with other JAK inhibitors.6,7

Conclusion:

In this analysis of pooled integrated efficacy data in csDMARD-IR or bDMARD-IR patients with RA, UPA 15 mg or 30 mg QD in combination with csDMARDs improved efficacy outcomes at Week 12 when compared with PBO across all predefined subgroups evaluated.

References:

[1]Burmester GR, et al. Lancet 2018 23;391:2503–2512;

[2]Genovese MC, et al. Lancet 2018; 391:2513–24;

[3]Smolen JS, et al. Lancet 2019 May 23[Epub ahead of print];

[4]van Vollenhoven R, et al. Arthritis Rheumatol 2018;70(Suppl. 10): Abstract 891;

[5]Fleischmann R, et al. Arthritis Rheumatol 2018;70(Suppl. 10): Abstract 890;

[6]Winthrop KL, et al. Arthritis Rheum 2014;66:2675-84;

[7] Winthrop KL, et al. ACR 2016 [Abstract 3027]

Disclosure of Interests:

Michael Weinblatt Grant/research support from: Amgen, Bristol-Myers Squibb, Crescendo, Lily, Sanofi/Regeneron, Consultant of: AbbVie, Amgen, Bristol-Myers Squibb, Crescendo, Gilead, Horizon, Lily, Pfizer, Roche, Eduardo Mysler Grant/research support from: AbbVie, Amgen, Bristol Myers Squibb, Roche, Eli Lilly, Novartis, Janssen, Sanofi, and Pfizer., Speakers bureau: AbbVie, Amgen, Bristol Myers Squibb, Roche, Eli Lilly, Novartis, Janssen, Sanofi, and Pfizer, Andrew Ostor Consultant of: MSD, Pfizer, Lilly, Abbvie, Novartis, Roche, Gilead and BMS, Speakers bureau: MSD, Pfizer, Lilly, Abbvie, Novartis, Roche, Gilead and BMS, Aaron Broadwell Grant/research support from: Janssen Research & Development, LLC, Janssen, Eli Lilly, Consultant of: AbbVie, Amgen, AstraZeneca, Celgene, Eli Lilly, Janssen, Novartis, Pfizer, Sandoz, Speakers bureau: AbbVie, Amgen, Celgene, GSK, Horizon, Janssen, Mallinckrodt, Novartis, Pfizer, Radius, Sanofi-Regeneron, UCB, Sławomir Jeka Grant/research support from: AbbVie, Pfizer, Roche, Novartis, MSD, Sandoz, Eli Lilly, Egis, UCB, Celgene, Speakers bureau: AbbVie, Pfizer, Roche, Novartis, MSD, Sandoz, Eli Lilly, Egis, UCB, Celgene, Kendall Dunlap Shareholder of: AbbVie Inc., Employee of: AbbVie Inc., Jessica Suboticki Shareholder of: AbbVie Inc., Employee of: AbbVie Inc., Jeffrey Enejosa Shareholder of: AbbVie Inc., Employee of: AbbVie Inc., Barbara Hendrickson Shareholder of: AbbVie Inc., Employee of: AbbVie Inc., Sheng Zhong Shareholder of: AbbVie Inc., Employee of: AbbVie Inc., Katya Cherny Shareholder of: AbbVie Inc., Employee of: AbbVie Inc., Grace Wright Consultant of: AbbVie, Amgen, BMS, Exagen, Janssen, Lilly, Medac, Myriad Autoimmune, Novartis, Pfizer, Sanofi Genzyme Regeneron, and UCB, Speakers bureau: AbbVie, Amgen, BMS, Exagen, Lilly, Medical Education Resource, Myriad Autoimmune, Novartis, Sanofi Genzyme Regeneron, UCB, and Vindico

Altered functional expression of Purkinje cell calcium channels precedes motor dysfunction in tottering mice

In tottering mice, a point mutation in the gene encoding P-type (Ca(v)2.1) voltage-gated calcium channels results in ataxia, absence epilepsy, and motor dystonia that appear 3-4 weeks postnatally. The aberrant motor behaviors have been linked to cerebellar dysfunction, and adult Purkinje cells (PCs) of tottering mice exhibit calcium-dependent changes in gene transcription suggestive of altered calcium homeostasis. In an attempt to identify early postnatal events important for the development of the behavioral phenotype, we examined calcium channel expression in cerebellar PCs from postnatal days 6-15 (P6-15). Whole cell recording was combined with selective calcium channel antagonists to allow discrimination of the various voltage-activated calcium channels types; early age-dependent differences between tottering and wild-type PCs were found. Wild-type PCs experienced a steady increase in P current density over this period, resulting in a twofold change by P15. In tottering, by contrast, P current density remained unchanged from P6-8 and was only 25% of the wild-type level by P8. A developmental delay in functional expression was implicated in this early deficit, since ensuing gains over the subsequent week brought tottering P current density close to the wild-type level by P15. At this age, tottering PCs also exhibited a 2.2-fold higher L-type calcium current density than that expressed by wild-type PCs. Increases in N current were apparent at some ages, most strikingly within a subset of tottering PCs at P15. Functional R- and T-type calcium current densities were equivalent to wild-type levels at all ages. We conclude that the tottering mutation brings about selective changes in functional calcium channel expression 1 to 2 weeks prior to the appearance of the behavioral deficits, raising the possibility that they represent an early, primary event along the path to motor dysfunction in tottering.

Local immune responsiveness following intravaginal challenge with Candida antigen in adult women at different stages of the menstrual cycle

Recurrent vulvovaginal candidiasis (RVVC) is a significant problem in women of childbearing age and is most often caused by Candida albicans that asymptomatically colonizes mucosal tissues. Although some form of local immune dysfunction is postulated to precipitate bouts of RVVC, the normal protective vaginal host response to C. albicans is poorly understood. In an effort to stimulate the natural adaptive response to yeast in healthy women without a history of VVC, commercial Candida skin test antigen was introduced intravaginally and changes in cytokines/immunomodulators were monitored in vaginal lavage fluid pre- and post-antigen challenge. In an earlier pilot study using small numbers of women without controlling for stages of the menstrual cycle, we reported elevated cytokines in vaginal secretions of antigen challenged women. The present study, however, that employed a similar design in a large number of women during each stage of the menstrual cycle showed no evidence of local immune stimulation, including changes in Th and proinflammatory cytokines, IgE, histamine, and prostaglandin, despite a natural modulation of vaginal cytokines over the course of the menstrual cycle. Taken together, these results suggest that either some form of vaginal immunoregulation/tolerance is evident in response to yeast or that more advanced clinical designs are required to detect the normal protective vaginal response to C. albicans.

Comparative analysis of methods for collection and measurement of cytokines and immunoglobulins in cervical and vaginal secretions of HIV and HPV infected women

The goal of these studies was to distinguish which of two techniques [cervicovaginal lavage (CVL) and cervical wick (SS)] is the optimal collection method for the measurement of the local immunological response in human papillomavirus (HPV) and HIV infected women. The following parameters were measured in 24 paired samples from 15 women (9 HIV+, 6 HIV-): total protein, immunoglobulin levels, HPV-specific antibodies, and Th1-Th2 cytokines. In addition, relative mRNA levels from CVL cell pellets were compared to protein levels from CVL supernatants. The total protein (2-fold) and IgG concentration (10-fold) are higher in the SS samples, were reproducible (%CV<3) and these levels correlated (P<0.0001) with their paired CVL sample. Type-specific HPV-L1 IgG and IgA antibodies were detected in CVL and SS (r>0.28, P<0.008) with excellent reproducibility (CV<3.0%). However, SS (%CV>18) failed to yield reproducible results for the cytokine assays as compared to the CVL (%CV<5.0). Furthermore, no correlations were found between relative mRNA levels from CVL cell pellet and cytokine protein levels in CVL supernatants. The CVL sample's superior reproducibility in the cytokine assays makes this the better collection method. In addition, cytokine protein level's failure to correlate with mRNA suggests tight regulation of cytokine genes or production from a different cell population.

Growth inhibition of Candida albicans by human vaginal epithelial cells

Vulvovaginal candidiasis (VVC) is a common mucosal infection caused by Candida species in women of childbearing age. Although acute VVC affects a large number of women and is often precipitated by hormonal fluctuations involving high estrogen levels, recurrent VVC (RVVC) affects another 5%-10% of women without any known predisposing factors. We have recently reported that vaginal epithelial cells from nonhuman primates and mice inhibit the growth of Candida albicans in vitro, which may represent an innate host defense mechanism against C. albicans at the vaginal mucosa. In the present study, we show that vaginal epithelial cells collected from healthy women with no history of VVC also exhibit anti-Candida activity, with no differences in activity at various stages of the menstrual cycle. Women diagnosed with RVVC, on the other hand, have reduced epithelial cell anti-Candida activity. These results are further evidence that vaginal epithelial cells provide an innate host resistance mechanism against Candida and that reduced activity may contribute to RVVC.

Syntaxin 1A supports voltage-dependent inhibition of alpha1B Ca2+ channels by Gbetagamma in chick sensory neurons

N-type Ca(2+) channels are modulated by a variety of G-protein-coupled pathways. Some pathways produce a transient, voltage-dependent (VD) inhibition of N channel function and involve direct binding of G-protein subunits; others require the activation of intermediate enzymes and produce a longer-lasting, voltage-independent (VI) form of inhibition. The ratio of VD:VI inhibition differs significantly among cell types, suggesting that the two forms of inhibition play unique physiological roles in the nervous system. In this study, we explored mechanisms capable of altering the balance of VD and VI inhibition in chick dorsal root ganglion neurons. We report that (1) VD:VI inhibition is critically dependent on the Gbetagamma concentration, with VI inhibition dominant at low Gbetagamma concentrations, and (2) syntaxin-1A (but not syntaxin-1B) shifts the ratio in favor of VD inhibition by potentiating the VD effects of Gbetagamma. Variations in expression levels of G-proteins and/or syntaxin provide the means to alter over a wide range both the extent and the rate of Ca(2+) influx through N channels.

Selective coupling of G protein beta gamma complexes to inhibition of Ca2+ channels

Several mechanisms couple heterotrimeric guanine nucleotide-binding proteins (G proteins) to cellular effectors. Although alpha subunits of G proteins (Galpha) were the first recognized mediators of receptor-effector coupling, Gbetagamma regulation of effectors is now well known. Five Gbeta and 12 Ggamma subunit genes have been identified, suggesting through their diversity that specific subunits couple selectively to effectors. The molecular determinants of Gbetagamma-effector coupling, however, are not well understood, and most studies of G protein-effector coupling do not support selectivity of Gbetagamma action. To explore this issue further, we have introduced recombinant Gbetagamma complexes into avian sensory neurons and measured the inhibition of Ca(2+) currents mediated by an endogenous phospholipase Cbeta- (PLCbeta) and protein kinase C-dependent pathway. Activities of Gbetagamma in the native cells were compared with enzyme assays performed in vitro. We report a surprising selective activation of the PLCbeta pathway by Gbetagamma complexes containing beta(1) subunits, whereas beta(2)-containing complexes produced no activation. In contrast, when assayed in vitro, PLCbeta and type II adenylyl cyclase did not discriminate among these same Gbetagamma complexes, suggesting the possibility that additional cellular determinants confer specificity in vivo.

Cloning and functional expression of novel N-type Ca(2+) channel variants

Voltage-dependent Ca(2+) channels are structurally and functionally diverse. As Ca(2+) currents recorded from embryonic chick dorsal root ganglion (DRG) neurons differ significantly from their mammalian counterparts, information on the primary sequence of the chick channels will help define the structural underpinnings of Ca(2+) channel function. Here, we report the cloning and functional expression of full-length Ca(2+) channel alpha(1B) subunit cDNAs derived from chick DRGs. Two variable regions (A and B) have been identified in the cytoplasmic linker between repeats I and II; a third (C) in the carboxyl terminus extends the open reading frame by 525 nucleotides. The A and C inserts are absent, and the B insert is present in all other class B clones reported to date. The unique shorter channels appear to predominate in DRG neurons. Results represent a requisite first step in defining the structural elements that underlie variations in function and modulation of Ca(2+) channels.

Dexamethasone-mediated inhibition of calcium transients and ACTH release in a pituitary cell line (AtT-20)

In the corticotroph-like murine pituitary tumor cell line, AtT-20, adrenocorticotropic hormone release is triggered by corticotropin-releasing hormone and is attenuated by the synthetic adrenal steroid dexamethasone. The precise mechanisms by which dexamethasone inhibits secretion are under investigation. We examined whether dexamethasone can modulate release via regulation of calcium homeostasis. More specifically, we have evaluated the effects of dexamethasone on calcium current, intracellular calcium concentration, and adrenocorticotropic hormone release. Using perforated patch-clamp and calcium imaging with fura PE3/AM, we found that dexamethasone decreases calcium current and intracellular calcium levels. The inhibition of current by dexamethasone is not, however, altered by the calcium channel antagonists nifedipine (L-type) or omega-agatoxin IVA (P/Q-type), despite the presence of these calcium channel subtypes in AtT-20 cells and the exclusive coupling of adrenocorticotropic hormone release to the L-type channel in these cells. We also evaluated the temporal relationship between dexamethasone-mediated inhibition of secretion and calcium influx. Whereas a prolonged (2 h) incubation with dexamethasone inhibits corticotropin-induced release by approximately 40%, a rapid (10 min) incubation (a time interval sufficient for dexamethasone-mediated inhibition of calcium transients) does not inhibit release. These data suggest, therefore, that dexamethasone does, indeed, modulate calcium homeostasis in AtT-20 cells, but that this effect is not responsible for its inhibition of secretion.

Plasticity of the electric organ discharge: implications for the regulation of ionic currents

Weakly electric fish emit electric organ discharges (EODs) to locate objects around themselves and for communication. The EOD is generated by a simple hierarchically organized, neurophysiologically accessible circuit, the electromotor system. A number of forms of plasticity of the EOD waveform are initiated by social or environmental factors and mediated by hormones or neurotransmitters. Because the behavior itself is in the form of electric discharges, behavioral observations easily lead to testable hypotheses about the biophysical bases of these plasticities. This allows us to study ionic channels in their native cellular environments, where the regulation of various parameters of these currents have obvious functional consequences. In this review, we discuss three types of plasticity: a rapidly occurring, long-lasting, N-methyl-d-aspartate (NMDA)-receptor-dependent increase in baseline firing frequency of neurons in the pacemaker nucleus that underlies a readjustment of the baseline EOD frequency after long bouts of the jamming avoidance response; a rapidly occurring diurnal change in amplitude and duration of the EOD pulse that depends in part on modulation of the magnitude of the electrocyte Na+ current by a protein kinase; and a slowly occurring, hormonally modulated tandem change in pacemaker firing frequency and in the duration of the EOD pulse in which changes in EOD pulse duration are mediated by coordinated shifts in the activation and inactivation kinetics of the electrocyte Na+ and K+ currents.

Dynamic regulation of calcium influx by G-proteins, action potential waveform, and neuronal firing frequency

The time course of Ca2+ channel activation and the amplitude and rate of change of Ca2+ influx are primarily controlled by membrane voltage. G-protein-coupled signaling pathways, however, modulate the efficacy of membrane voltage on channel gating. To study the interactions of membrane potential and G-proteins on Ca2+ influx in a physiological context, we have measured N-type Ca2+ currents evoked by action potential waveforms in voltage-clamped chick dorsal root ganglion neurons. We have quantified the effect of varying action potential waveforms and frequency on the shape of Ca2+ current in the presence and absence of transmitters (GABA or norepinephrine) that inhibit N current. Our results demonstrate that both the profile of Ca2+ entry and the time course and magnitude of its transmitter-induced inhibition are sensitive functions of action potential waveform and frequency. Increases in action potential duration enhance total Ca2+ entry, but they also prolong and blunt Ca2+ signals by slowing influx rate and reducing peak amplitude. Transmitter-mediated inhibition of Ca2+ entry is most robust with short-duration action potentials and decreases exponentially with increasing duration. Increases in action potential frequency promote a voltage-dependent inactivation of Ca2+ influx. In channels exposed to GABA or norepinephrine, however, this inactivation is counteracted by a time- and frequency-dependent relief of modulation. Thus, multiple stimuli are integrated by Ca2+ channels, tuning the profile of influx in a changing physiological environment. Such variations are likely to be significant for the control of Ca2+-dependent cellular responses in all tissues.

The practice of nursing theory in the operating room

Nursing theory is a set of ideas, experiences, or observations regarding how and why nurses engage in certain activities and exhibit various behavioral attributes in providing patient care. The individualized caregiving activities perioperative nurses express during all phases of the surgical experience are supported by nursing theory.

Class A calcium channel variants in pancreatic islets and their role in insulin secretion

The initiation of insulin release from rat islet beta cells relies, in large part, on calcium influx through dihydropyridine-sensitive (alpha1D) voltage-gated calcium channels. Components of calcium-dependent insulin secretion and whole cell calcium current, however, are resistant to L-type channel blockade, as well as to omega-conotoxin GVIA, a potent inhibitor of alpha1B channels, suggesting the expression of additional exocytotic calcium channels in the islet. We used a reverse transcription-polymerase chain reaction-based strategy to ascertain at the molecular level whether the alpha1A calcium channel isoform was also present. Results revealed two new variants of the rat brain alpha1A channel in the islet with divergence in a putative extracellular domain and in the carboxyl terminus. Using antibodies and cRNA probes specific for alpha1A channels, we found that the majority of cells in rat pancreatic islets were labeled, indicating expression of the alpha1A channels in beta cells, the predominant islet cell type. Electrophysiologic recording from isolated islet cells demonstrated that the dihydropyridine-resistant current was sensitive to the alpha1A channel blocker, omega-agatoxin IVA. This toxin also inhibited the dihydropyridine-resistant component of glucose-stimulated insulin secretion, suggesting functional overlap among calcium channel classes. These findings confirm the presence of multiple high voltage-activated calcium channels in the rat islet and implicate a physiologic role for alpha1A channels in excitation-secretion coupling in beta cells.

Vaginal-associated immunity in women with recurrent vulvovaginal candidiasis: evidence for vaginal Th1-type responses following intravaginal challenge with Candida antigen

Studies from women with recurrent vulvovaginal candidiasis (RVVC) and from an animal model of experimental vaginitis suggest that deficiencies in immune function should be examined at the local rather than systemic level. Evidence of vaginal cell-mediated immunity (CMI) was evaluated for the first time in cervicovaginal lavage (CVL) fluid from RVVC patients. Results showed that although constitutive Th1- and Th2-type cytokine expression was detectable in CVL fluid from normal women, and differences in cytokines were observed in RVVC patients, limitations in experimental design of such de novo analyses urged caution in interpretation. Alternatively, attempts were made to establish conditions in control subjects whereby vaginal immunity could be detected after intravaginal challenge with Candida antigen. Preliminary results showed that Th1-type cytokines (interleukin-2 and -12, interferon-gamma) and histamine were increased 16-18 h after intravaginal introduction of Candida skin test antigen. Intravaginal antigenic challenge represents a novel approach for studying Candida-specific vaginal CMI.

Novel form of crosstalk between G protein and tyrosine kinase pathways

Neuronal Ca2+ channels are inhibited by a variety of transmitter receptors coupled to Go-type GTP-binding proteins. Go has been postulated to work via a direct interaction between an activated G protein subunit and the Ca2+ channel complex. Here we show that the inhibition of sensory neuron N-type Ca2+ channels produced by gamma-aminobutyric acid involves a novel, rapidly activating tyrosine kinase signaling pathway that is mediated by Galphao and a src-like kinase. In contrast to other recently described G protein-coupled tyrosine kinase pathways, the Galphao-mediated modulation requires neither protein kinase C nor intracellular Ca2+. The results suggest that this pathway mediates rapid receptor-G protein signaling in the nervous system and support the existence of a previously unrecognized form of crosstalk between G protein and tyrosine kinase pathways.

A CNS catecholaminergic cell line expresses voltage-gated currents

CATH.a is a central nervous system (CNS) catecholaminergic cell line derived from a transgenic mouse carrying the SV40 T antigen oncogene under the transcriptional control of regulatory elements from the rat tyrosine hydroxylase gene (Suri et al., 1993). CATH.a cells express several differentiated neuronal characteristics including medium and light chain neurofilament proteins, synaptophysin, tyrosine hydroxylase, and dopamine beta-hydroxylase; they synthesize dopamine and norepinephrine. Conversely, they do not express glial-specific fibrillary acidic protein. To establish definitively that CATH.a cells are of neuronal origin, we characterized the repertoire of voltage-gated inward currents expressed by CATH.a cells. Such inward currents are necessary for neuronal excitability. We report that all CATH.a cells possess a tetrodotoxin-sensitive sodium current (peak amplitude = 590 +/- 319 pA) and 68% possess a high voltage-activated calcium current (peak amplitude = 175 +/- 67 pA). Pharmacological analyses suggest that individual cells express varying levels of L- and N-type calcium current, but no P-type current. In addition, in 55% of the cells with a calcium current, about a half of this current is resistant to selective antagonists for L- and N-type currents, suggesting that another calcium current exists in these CATH.a cells which is not L-, N-, or P-type. The heterogeneous pattern of current detected persisted in several CATH. a subclones, suggesting that factors other than genetic variability influence current expression. The demonstration that CATH.a cells express these currents indicates that they have excitable membrane properties characteristic of neurons. Although many peripheral nervous system (PNS) cell lines exist, very few CNS cell lines with differentiated neuronal properties exist. Since the CATH.a cells can be grown continuously in large amounts, they may be useful for purifying, characterizing, and/or cloning various neuronal-specific molecules and thereby may add to our understanding of CNS catecholaminergic neurons.

Calcium currents in a pituitary cell line (AtT-20): differential roles in stimulus-secretion coupling

The purpose of the present investigation was to identify voltage-dependent calcium channel subtypes that control the release of ACTH in AtT-20 cells, a clonal mouse pituitary cell line. Using the perforated patch-clamp technique, we identified dihydropyridine (nimodipine)-, omega-Agatoxin IVA-, and omega-Conotoxin MVIIC-sensitive calcium currents. No omega-Conotoxin GVIA-sensitive currents are present in these cells. There also existed a considerable resistant component to the recorded inward current that was inhibited by cadmium, a nonselective calcium channel antagonist. Using RIA, we examined the contributions of each of the pharmacologically distinct calcium channel populations to CRH- or potassium chloride (KCI)-stimulated release of ACTH at various time intervals (10 sec to 60 min). We found that nimodipine markedly inhibited ACTH release at all intervals tested, whereas omega-Agatoxin IVA, omega-Conotoxin MVIIC, and omega-Conotoxin GVIA had no significant effect. Moreover, the inhibition by nimodipine was comparable to that seen after cadmium application, and the effects of these two antagonists were not additive. These data suggest that although AtT-20 cells possess dihydropyridine-, omega-Agatoxin IVA-, and omega-Conotoxin MVIIC-sensitive calcium channels as well as a considerable toxin-resistant current, only the dihydropyridine-sensitive calcium channels appear to be coupled to CRH- or KCI-induced ACTH release.

G protein-coupled receptor kinase mediates desensitization of norepinephrine-induced Ca2+ channel inhibition

G protein-coupled receptors are essential signaling molecules at sites of synaptic transmission. Here, we explore the mechanisms responsible for the use-dependent termination of metabotropic receptor signaling in embryonic sensory neurons. We report that the inhibition of voltage-dependent Ca2+ channels mediated by alpha2-adrenergic receptors desensitizes slowly with prolonged exposure to the transmitter and that the desensitization is mediated by a G protein-coupled receptor kinase (GRK). Intracellular introduction of recombinant, purified kinases or synthetic blocking peptides into individual neurons demonstrates the specific involvement of a GRK3-like protein. These results suggest that GRK-mediated termination of receptor-G protein coupling is likely to regulate synaptic strength and, as such, may provide one effective mechanism for depression of synaptic transmission.

Pharmacological characterization of presynaptic calcium channels using subsecond biochemical measurements of synaptosomal neurosecretion

The recent development of peptide antagonists that selectively block subtypes of neuronal calcium channel has provided tools to study the role of presynaptic calcium channels in triggering exocytosis. A variety of methods have consistently demonstrated that multiple channel types participate in exocytosis. We have studied the subsecond kinetics of [3H]glutamate release from rat cortical synaptosomes as an assay for presynaptic calcium channel activity. The system has been characterized over a broad range of conditions in an effort to compare biochemical measurements of transmitter release with electrophysiological measurements of synaptic currents. The efficacies of omega-agatoxin IVA and omega-conotoxins GVIA and MVIIC were increased when Ca2+ influx was decreased by: (1) decreasing the KCl concentration to diminish the extent of depolarization, (2) decreasing the Ca2+ concentration, or (3) partially blocking Ca2+ influx with one of the other antagonists. By using these toxins in combination, we found that at least three types of pharmacologically distinct channel participate in exocytosis. The largest fraction of glutamate release is blocked by omega-agatoxin IVA (IC50 = 12.2 nM) and by omega-conotoxin MVIIC (IC50 = 35 nM), consistent with the pharmacology of a P type channel. The effects of saturating concentrations (1 microM) of omega-agatoxin IVA or omega-conotoxin MVIIC occlude each other, suggesting that these peptides overlap completely. The specific N type antagonist omega-conotoxin GVIA inhibits a significant portion of release (IC50 less than 1 nM) but only under conditions of reduced Ca2+ concentration. These results suggest that the N type channel in nerve terminals is distinct from that found in hippocampal somata, since it appears to be resistant to by omega-conotoxin MVIIC. The combination of omega-conotoxin GVIA (100 nM) and either omega-agatoxin IVA or omega-conotoxin MVIIC (1 microM each) blocked approx 90% of release when the Ca2+ concentration was reduced (0.46 mM or less), but 30-40% of release remained when the concentration of Ca2+ in the stimulus buffer was 1 mM or greater, indicating that a resistant channel type(s) also participates in exocytosis. Specific inhibitors of this resistant phenotype will be useful for further refinement of our understanding of the role of presynaptic calcium channels in mediating neurosecretion.

Shielding and radiation safety around a superconducting cyclotron neutron therapy facility

Prior to routine operation of the neutron therapy unit a radiation survey was performed in order to confirm the shielding design and to assure the safety of the personnel involved in the operation of the unit. The shielding requirements were calculated in accordance with NCRP Report No. 51. The contributions of the neutron and gamma dose equivalents have been measured separately outside the treatment room. The exposure outside the shield is negligible. In general, the measured values were lower than those derived from the shielding calculations. The highest total dose equivalents were registered at locations corresponding to the highest calculated values.

Prolonged time course of glutamate release from nerve terminals: relationship between stimulus duration and the secretory event

The kinetics of synaptosomal [3H]glutamate release were measured on a subsecond time scale to study the relationship between the length of depolarization and the duration of the secretory event. The time course of release evoked by elevated K+ was complex, proceeding for several seconds after a 200-ms depolarization. We developed a protocol for depolarizing excitable membranes on a millisecond time scale to deliver brief depolarizations, termed the synthetic action potential, by using batrachotoxin to activate Na+ channels. Depolarization is achieved by superfusing with solutions containing elevated concentrations of Na+, and the duration of the depolarization is limited by including tetrodotoxin (TTX) in the superfusion solution to block Na+ current and membrane depolarizations were made in batrachotoxin-treated sensory neurons using patch clamp recording methods. Rapid increases in Na+ and TTX concentrations produced transient increases in inward Na+ current that decayed with a time course proportional to TTX concentration. Current clamp measurements indicated that, with 10 microM TTX, depolarizations last approximately 30 ms. Nonetheless, synaptosomal release of [3H]glutamate triggered by the synthetic action potential remained prolonged. Brief neuronal action potentials at some synapses may trigger transmitter release that persists for several seconds.

Interaction of convergent pathways that inhibit N-type calcium currents in sensory neurons

Norepinephrine and GABA inhibit omega-conotoxin GVIA-sensitive (N-type) calcium current in embryonic sensory neurons by separate pathways. We have investigated the mechanisms that limit the modulation of N current by varying the level of activation for a single pathway or simultaneously activating multiple pathways. Calcium currents were measured with tight-seal, whole-cell recording methods. Simultaneous application of the two transmitters at saturating concentrations produced a larger inhibition of the current than either transmitter by itself, but the maximal inhibition was not linearly additive. Maximal, direct activation of GTP-binding proteins by intracellular application of guanosine 5'-(3-O-thio)-triphosphate (GTP gamma S) resulted in a similar limit to the inhibition; furthermore, GTP gamma S did not enhance the maximal inhibition produced by co-application of transmitters. Interventions downstream in the modulatory pathway (e.g. direct activation of protein kinase C or inhibition of protein phosphatases) were also unable to alter the maximal limit for inhibition. These results suggest that transmitter-mediated inhibition is not limited by receptor number, levels of G-protein or protein kinase C activation, or degree of phosphorylation; rather, the extent of inhibition may be limited by the structural properties of the N channels themselves.

Characterization of presynaptic calcium channels with omega-conotoxin MVIIC and omega-grammotoxin SIA: role for a resistant calcium channel type in neurosecretion

The peptide Ca2+ channel antagonists omega-conotoxin (omega-CTX) MVIIC and omega-grammotoxin (omega-GTX) SIA were studied by measuring their effects on the release of [3H]glutamate from rat brain synaptosomes. The pseudo-first-order association constant for omega-CTX MVIIC (1.1 x 10(4) M-1 sec-1) was small, relative to that for omega-GTX SIA (3.6 x 10(5) M-1 sec-1). Equilibrium experiments showed that omega-CTX MVIIC blocked approximately 70% of Ca(2+)-dependent glutamate release evoked by 30 mM KCl (IC50 approximately 200 nM), whereas omega-GTX SIA virtually eliminated release, with lower potency (IC50 approximately 700 nM). At stronger depolarizations (60 mM KCl), neither toxin (at 1 microM) showed significant block of release, but when these or other Ca2+ channel antagonists (omega-CTX GVIA or omega-agatoxin IVA) were used in combination a substantial fraction of release was blocked. [3H]Glutamate release that was resistant to omega-CTX MVIIC was characterized with respect to its sensitivity to block by omega-GTX SIA and the inorganic blocker Ni2+. Both omega-GTX SIA and Ni2+ were relatively weak blockers of the resistant release. These results suggest that a previously uncharacterized Ca2+ channel exists in nerve terminals and can be distinguished on the basis of its resistance to omega-CTX MVIIC and its weak sensitivity to omega-GTX SIA and Ni2+. Thus, at least three channel types (P, N, and a "resistant" type) contribute to excitation-secretion coupling in nerve terminals.

Exocytotic Ca2+ channels in mammalian central neurons

Intracellular Ca2+ initiates physiological events as diverse as gene transcription, muscle contraction, cell division and exocytosis. Predictably, the metabolic machinery that elicits and responds to changes in intracellular Ca2+ is correspondingly heterogeneous. This review focuses on one element of this complex web that is of particular importance to neurobiologists: identifying which members of the voltage-dependent Ca(2+)-channel superfamily are responsible for the Ca2+ that enters nerve terminals and elicits vesicular release of chemical transmitters.

Transmitter-mediated inhibition of N-type calcium channels in sensory neurons involves multiple GTP-binding proteins and subunits

The modulation of voltage-activated Ca2+ channels by neurotransmitters and peptides is very likely a primary means of regulating Ca(2+)-dependent physiological functions such as neurosecretion, muscle contraction, and membrane excitability. In neurons, N-type Ca2+ channels (defined as omega-conotoxin GVIA-sensitive) are one prominent target for transmitter-mediated inhibition. This inhibition is widely thought to result from a shift in the voltage independence of channel gating. Recently, however, voltage-independent inhibition has also been described for N channels. As embryonic chick dorsal root ganglion neurons express both of these biophysically distinct modulatory pathways, we have utilized these cells to test the hypothesis that the voltage-dependent and -independent actions of transmitters are mediated by separate biochemical pathways. We have confirmed this hypothesis by demonstrating that the two modulatory mechanisms activated by a single transmitter involve not only different classes of G protein but also different G protein subunits.

Two subcellular mechanisms underlie calcium-dependent facilitation of bioluminescence

Epithelial calcium action potentials in Obelia geniculata trigger brief light flashes from specialized cells by direct activation of cytoplasmic calcium-activated photoprotein obelin. During a series of action potentials, sequential flashes undergo characteristic facilitation and decrement with no change in associated spike waveform. Analysis of the subcellular light distribution shows that facilitation results from two processes: recruitment of calcium entry sites and increased light from previously responding localized sites. We propose a model that accounts for the localized flash facilitation and decrement observed in vivo and is based upon the kinetics of calcium binding and emission of obelin. In this model, obelin emits light only when three calcium ions are bound. Changes in flash intensity during successive action potentials result from calcium bound persistently to unexpended obelin, effectively lowering the number of calcium ions required for subsequent activation. Accordingly, facilitation or decrement results from the time-dependent availability of singly and doubly bound obelin.

Sensory neuron N-type calcium currents are inhibited by both voltage-dependent and -independent mechanisms

The voltage dependence of gamma-aminobutyric-acid- and norepinephrine-induced inhibition of N-type calcium current in cultured embryonic chick dorsal-root ganglion neurons was studied with whole-cell voltage-clamp recording. The inhibitory action of the neurotransmitters was comprised of at least two distinct modulatory components, which were separable on the basis of their differential voltage dependence. The first component, which we term "kinetic slowing", is associated with a slowing of the activation kinetics--an effect that subsides during a test pulse. The kinetic-slowing component is largely reversed at depolarized voltages (i.e., it is voltage-dependent). The second component, which we term "steady-state inhibition", is by definition not associated with a change in activation kinetics and is present throughout the duration of a test pulse. The steady-state inhibition is not reversed at depolarized voltages (i.e., it is voltage-independent). Although the two components can be separated on the basis of their voltage dependence, they appear to be indistinguishable in their time courses for onset and recovery as well as their rates of desensitization following multiple applications of transmitter. Furthermore, neither component requires cell dialysis, as both are observed using perforated-patch as well as whole-cell recording configurations. The co-existence in nerve terminals of both voltage-dependent and -independent mechanisms to modulate calcium channel function could offer a means of differentially controlling synaptic transmission under conditions of low- and high-frequency presynaptic discharge.

Inactivation of N-type calcium current in chick sensory neurons: calcium and voltage dependence

We have studied the inactivation of high-voltage-activated (HVA), omega-conotoxin-sensitive, N-type Ca2+ current in embryonic chick dorsal root ganglion (DRG) neurons. Voltage steps from -80 to 0 mV produced inward Ca2+ currents that inactivated in a biphasic manner and were fit well with the sum of two exponentials (with time constants of approximately 100 ms and > 1 s). As reported previously, upon depolarization of the holding potential to -40 mV, N current amplitude was significantly reduced and the rapid phase of inactivation all but eliminated (Nowycky, M. C., A. P. Fox, and R. W. Tsien. 1985. Nature. 316:440-443; Fox, A. P., M. C. Nowycky, and R. W. Tsien. 1987a. Journal of Physiology. 394:149-172; Swandulla, D., and C. M. Armstrong. 1988. Journal of General Physiology. 92:197-218; Plummer, M. R., D. E. Logothetis, and P. Hess. 1989. Neuron. 2:1453-1463; Regan, L. J., D. W. Sah, and B. P. Bean. 1991. Neuron. 6:269-280; Cox, D. H., and K. Dunlap. 1992. Journal of Neuroscience. 12:906-914). Such kinetic properties might be explained by a model in which N channels inactivate by both fast and slow voltage-dependent processes. Alternatively, kinetic models of Ca-dependent inactivation suggest that the biphasic kinetics and holding-potential-dependence of N current inactivation could be due to a combination of Ca-dependent and slow voltage-dependent inactivation mechanisms. To distinguish between these possibilities we have performed several experiments to test for the presence of Ca-dependent inactivation. Three lines of evidence suggest that N channels inactivate in a Ca-dependent manner. (a) The total extent of inactivation increased 50%, and the ratio of rapid to slow inactivation increased approximately twofold when the concentration of the Ca2+ buffer, EGTA, in the patch pipette was reduced from 10 to 0.1 mM. (b) With low intracellular EGTA concentrations (0.1 mM), the ratio of rapid to slow inactivation was additionally increased when the extracellular Ca2+ concentration was raised from 0.5 to 5 mM. (c) Substituting Na+ for Ca2+ as the permeant ion eliminated the rapid phase of inactivation. Other results do not support the notion of current-dependent inactivation, however. Although high intracellular EGTA (10 mM) or BAPTA (5 mM) concentrations suppressed the rapid phase inactivation, they did not eliminate it. Increasing the extracellular Ca2+ from 0.5 to 5 mM had little effect on this residual fast inactivation, indicating that it is not appreciably sensitive to Ca2+ influx under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple calcium channel types control glutamatergic synaptic transmission in the hippocampus

N-type calcium channels play a dominant role in controlling synaptic transmission in many peripheral neurons. Transmitter release from mammalian central nerve terminals, however, is relatively resistant to the N channel antagonist omega-conotoxin GVIA. We studied the sensitivity of glutamatergic synaptic transmission in rat hippocampal slices to omega-conotoxin and to omega-Aga-IVA, a P channel antagonist. Both toxins reduced the amplitude of excitatory postsynaptic potentials in CA1 pyramidal neurons, but omega-Aga-IVA was the more rapid and efficacious. These results were corroborated by biochemical studies measuring subsecond, calcium-dependent [3H]glutamate release from hippocampal synaptosomes. Thus, at least two calcium channel types trigger glutamate release from hippocampal neurons, but P-type plays a more prominent role. Eliminating synaptic transmission in the CNS, therefore, may require inhibiting more than a single calcium channel type.

Multiple Ca2+ channel types coexist to regulate synaptosomal neurotransmitter release

The regulation of excitation-secretion coupling by Ca2+ channels is a fundamental property of the nerve terminal. Peptide toxins that block specific Ca2+ channel types have been used to identify which channels participate in neurotransmitter release. Subsecond measurements of [3H]-glutamate and [3H]dopamine release from rat striatal synaptosomes showed that P-type channels, which are sensitive to the Agelenopsis aperta venom peptide omega-Aga-IVA, trigger the release of both transmitters. Dopamine (but not glutamate) release was also controlled by N-type, omega-conotoxin-sensitive channels. With strong depolarizations, where neither toxin was very effective alone, a combination of omega-Aga-IVA and omega-conotoxin produced a synergistic inhibition of 60-80% of Ca(2+)-dependent dopamine release. The results suggest that multiple Ca2+ channel types coexist to regulate neurosecretion under normal physiological conditions in the majority of nerve terminals. P- and N-type channels coexist in dopaminergic terminals, while P-type and a omega-conotoxin- and omega-Aga-IVA-resistant channel coexist in glutamatergic terminals. Such an arrangement could lend a high degree of flexibility in the regulation of transmitter release under diverse conditions of stimulation and modulation.

Distinct, convergent second messenger pathways modulate neuronal calcium currents

Norepinephrine (NE) and gamma-aminobutyric acid (GABA) inhibit N-type calcium channels in embryonic chick sensory neurons. We demonstrate here that the modulatory actions of the two transmitters are mediated through distinct biochemical pathways. Intracellular application of the pseudosubstrate inhibitor for protein kinase C blocks the inhibition produced by NE (and the protein kinase C activator oleoylacetylglycerol), but not that produced by GABA. Calcium current inhibition produced by oleoylacetylglycerol occludes inhibition by subsequent application of NE; GABA-mediated inhibition, however, is not eliminated by prior activation of protein kinase C. These results demonstrate that multiple biochemical pathways converge to control N-type calcium channel function.

Calcium channels coupled to glutamate release identified by omega-Aga-IVA

Presynaptic calcium channels are crucial elements of neuronal excitation-secretion coupling. In mammalian brain, they have been difficult to characterize because most presynaptic terminals are too small to probe with electrodes, and available pharmacological tools such as dihydropyridines and omega-conotoxin are largely ineffective. Subsecond measurements of synaptosomal glutamate release have now been used to assess presynaptic calcium channel activity in order to study the action of peptide toxins from the venom of the funnel web spider Agelenopsis aperta, which is known to inhibit dihydropyridine and omega-conotoxin-resistant neuronal calcium currents. A presynaptic calcium channel important in glutamate release is shown to be omega-Aga-IVA sensitive and omega-conotoxin resistant.

Pharmacological discrimination of N-type from L-type calcium current and its selective modulation by transmitters

GABA and norepinephrine inhibit high voltage-activated calcium current in chick sensory neurons. Using specific pharmacological tools, we have dissected this current into two components: the major one is omega-conotoxin sensitive and dihydropyridine resistant (N-type) while the minor one is dihydropyridine sensitively and omega-conotoxin resistant (L-type). The ability to selectively eliminate these two components has allowed us to determine whether the transmitters target the same or different channel types. Both GABA and norepinephrine modulate the N-type component as evidenced by their lack of effect on (1) omega-conotoxin-resistant current and (2) pure L-type tail current, prolonged by a dihydropyridine calcium channel agonist. This simple pharmacological profile will allow future tests of the significance of the two channel types in regulating sensory neuron functions.

Multiple actions of extracellular ATP on calcium currents in cultured bovine chromaffin cells

Hormone secretion from chromaffin cells is evoked by calcium influx through voltage-dependent channels in the plasma membrane. Previous studies have shown that ATP, cosecreted with catecholamines from chromaffin granules, can modulate the secretion resulting from depolarization by nicotinic agonists. The immediate effect of ATP is to enhance secretion; more prolonged exposure to the nucleotide results in inhibition. These receptor-mediated actions of ATP involve the activation of at least two separate classes of GTP-binding protein. Results from electrophysiological experiments reported here demonstrate that the modulatory actions of ATP can, in large part, be explained by the effects of the nucleotide on inward calcium current. ATP shows a rapid enhancement and a slower, persistent inhibition of the depolarization-induced inward current.

Interleukin-1 augments gamma-aminobutyric acidA receptor function in brain

Interleukin-1 (IL-1), a cytokine involved in the acute phase reaction to injury and infection, has multiple effects in the central nervous system, including induction of fever and sleep and the release of several neuropeptides. We evaluated effects of IL-1 beta on inhibitory postsynaptic function at the gamma-aminobutyric acidA (GABAA) receptor. IL-1 (100 pg/ml to 10 ng/ml) augmented GABAA receptor function in cortical synaptic preparations. This effect of IL-1 was largely prevented by incubation with a specific IL-1 receptor antagonist. The related cytokines interleukin-6 and tumor necrosis factor did not augment GABA-dependent chloride transport. Similar enhancement of GABAA receptor function was observed in tissue prepared from mice previously injected intraperitoneally with IL-1 (1 microgram). Electrophysiological studies in cultured primary cortical neurons demonstrated that IL-1 enhanced the GABA-mediated increase in chloride permeability, whereas IL-1 alone produced no alterations in resting conductance. Behavioral studies indicated that IL-1 is similarly active in vivo; mice treated with IL-1 showed a decrease in open-field activity and an increase in the threshold for pentylenetetrazol-induced seizures. The interaction of IL-1 with GABAA receptors might account for the somnogenic and motor-depressant effects of this cytokine.

Intercellular signaling as visualized by endogenous calcium-dependent bioluminescence

Bioluminescence in the hydrozoan coelenterate Obelia results from calcium activation of a photoprotein contained in light-emitting cells (photocytes) scattered in the animal's endoderm. The influx of calcium into nonluminescent endodermal cells through conventional voltage-dependent calcium channels is required for the excitation-luminescence coupling. Our results suggest that the subsequent diffusion of this calcium, via gap junctions, into the neighboring photocytes triggers a localized luminescence response. Following intense stimulation, the local rise in calcium elicits a secondary wave of luminescence that is supported by a voltage-independent calcium permeability mechanism in the photocyte plasma membrane. These two mechanisms for elevating internal calcium in light-emitting cells can account for the spatial and temporal features of intracellular luminescence in Obelia.