Optimization of hERG and Pharmacokinetic Properties for Basic Dihydro-8H-purin-8-one Inhibitors of DNA-PK

The DNA-PK complex is activated by double-strand DNA breaks and regulates the non-homologous end-joining repair pathway; thus, targeting DNA-PK by inhibiting the DNA-PK catalytic subunit (DNA-PKcs) is potentially a useful therapeutic approach for oncology. A previously reported series of neutral DNA-PKcs inhibitors were modified to incorporate a basic group, with the rationale that increasing the volume of distribution while maintaining good metabolic stability should increase the half-life. However, adding a basic group introduced hERG activity, and basic compounds with modest hERG activity (IC50 = 10-15 μM) prolonged QTc (time from the start of the Q wave to the end of the T wave, corrected by heart rate) in an anaesthetized guinea pig cardiovascular model. Further optimization was necessary, including modulation of pK a, to identify compound 18, which combines low hERG activity (IC50 = 75 μM) with excellent kinome selectivity and favorable pharmacokinetic properties.

Pharmacological inhibition of MERTK induces in vivo retinal degeneration: a multimodal imaging ocular safety assessment

The receptor tyrosine kinase, MERTK, plays an essential role in homeostasis of the retina via efferocytosis of shed outer nuclear segments of photoreceptors. The Royal College of Surgeons rat model of retinal degeneration has been linked to loss-of-function of MERTK, and together with the MERTK knock-out mouse, phenocopy retinitis pigmentosa in humans with MERTK mutations. Given recent efforts and interest in MERTK as a potential immuno-oncology target, development of a strategy to assess ocular safety at an early pre-clinical stage is critical. We have applied a state-of-the-art, multi-modal imaging platform to assess the in vivo effects of pharmacological inhibition of MERTK in mice. This involved the application of mass spectrometry imaging (MSI) to characterize the ocular spatial distribution of our highly selective MERTK inhibitor; AZ14145845, together with histopathology and transmission electron microscopy to characterize pathological and ultra-structural change in response to MERTK inhibition. In addition, we assessed the utility of a human retinal in vitro cell model to identify perturbation of phagocytosis post MERTK inhibition. We identified high localized total compound concentrations in the retinal pigment epithelium (RPE) and retinal lesions following 28 days of treatment with AZ14145845. These lesions were present in 4 of 8 treated animals, and were characterized by a thinning of the outer nuclear layer, loss of photoreceptors (PR) and accumulation of photoreceptor outer segments at the interface of the RPE and PRs. Furthermore, the lesions were very similar to that shown in the RCS rat and MERTK knock-out mouse, suggesting a MERTK-induced mechanism of PR cell death. This was further supported by the observation of reduced phagocytosis in the human retinal cell model following treatment with AZ14145845. Our study provides a viable, translational strategy to investigate the pre-clinical toxicity of MERTK inhibitors but is equally transferrable to novel chemotypes.

Discovery and pharmacological characterization of AZD3229, a potent KIT/PDGFRα inhibitor for treatment of gastrointestinal stromal tumors

Gastrointestinal stromal tumor (GIST) is the most common human sarcoma driven by mutations in KIT or platelet-derived growth factor α (PDGFRα). Although first-line treatment, imatinib, has revolutionized GIST treatment, drug resistance due to acquisition of secondary KIT/PDGFRα mutations develops in a majority of patients. Second- and third-line treatments, sunitinib and regorafenib, lack activity against a plethora of mutations in KIT/PDGFRα in GIST, with median time to disease progression of 4 to 6 months and inhibition of vascular endothelial growth factor receptor 2 (VEGFR2) causing high-grade hypertension. Patients with GIST have an unmet need for a well-tolerated drug that robustly inhibits a range of KIT/PDGFRα mutations. Here, we report the discovery and pharmacological characterization of AZD3229, a potent and selective small-molecule inhibitor of KIT and PDGFRα designed to inhibit a broad range of primary and imatinib-resistant secondary mutations seen in GIST. In engineered and GIST-derived cell lines, AZD3229 is 15 to 60 times more potent than imatinib in inhibiting KIT primary mutations and has low nanomolar activity against a wide spectrum of secondary mutations. AZD3229 causes durable inhibition of KIT signaling in patient-derived xenograft (PDX) models of GIST, leading to tumor regressions at doses that showed no changes in arterial blood pressure (BP) in rat telemetry studies. AZD3229 has a superior potency and selectivity profile to standard of care (SoC) agents-imatinib, sunitinib, and regorafenib, as well as investigational agents, avapritinib (BLU-285) and ripretinib (DCC-2618). AZD3229 has the potential to be a best-in-class inhibitor for clinically relevant KIT/PDGFRα mutations in GIST.

Parent and Metabolite Concentration-QT Modeling to Evaluate QT-Interval Prolongation at Savolitinib Therapeutic Doses

Savolitinib is an oral, potent, and highly selective MET-tyrosine kinase inhibitor under investigation in various tumor types. A thorough QT study evaluated effects on QT interval after a 600-mg single savolitinib dose in healthy subjects. We report exposure-response (E-R) modeling from this study to characterize the effects of savolitinib and its metabolites, M2 and M3, on QTc changes. In a novel application, in vitro potencies against hERG current provided mechanistic support to model the metabolites' effects. The hERG IC50 estimates (95% CI) were 25.8 (22.2-29.9) and 22.6 (14.7-34.6) μM for parent and M2, respectively. The E-R was described by both linear and E_max models, with exposure captured by an active moiety that consisted of savolitinib and M2 concentrations, weighted by the hERG IC50 ratio (1.14). The maximal increase in ΔΔQTcF and EC50 estimates (95% CI) was 18.5 (9.2-27.7) ms and 5709 (2889-8529) nM, respectively. Ignoring M2 contribution resulted in under prediction of QTcF prolongation in the hypothetical case of inhibited M2 clearance; at 300 mg C_max, the mean (90% CI) of ∆∆QTcF was 9.0 (5.7-12.6) and 5.9 (2.9-8.9) ms using the hERG-informed and parent-only linear models, respectively. Simulations in normal setting confirmed modest QTcF prolongation with 600 mg, but not 300 mg. Using the linear model, the mean (90% CI) maximum ΔΔQTcF were 12.3 (8.6-16.2) and 5.5 (2.6-8.5) ms for 600 and 300 mg, respectively. Further clinical studies will monitor cardiac safety to assess the clinical significance of QT-interval prolongation with savolitinib.

Abstract 1718: Design and optimization of a dendrimer-conjugated dual Bcl-2/Bcl-xL inhibitor, AZD0466, with improved therapeutic index

Dual Bcl-2/Bcl-xL inhibitors are expected to deliver therapeutic benefit in many hematological and solid tumors, but their clinical application has been limited by tolerability issues, including thrombocytopenia. AZD4320, a potent dual Bcl-2/Bcl-xL inhibitor, showed good efficacy but encountered dose limiting cardiovascular toxicity in preclinical species, and had challenging physicochemical properties which prevented its clinical development. Nanocarriers can provide prolonged circulation time, controlled release, tumor accumulation and retention. Consequently, they have been explored to improve the therapeutic index of small molecules in oncology. This work describes the design and development of AZD0466, a novel drug-dendrimer conjugate, where AZD4320 is chemically conjugated to Starpharma's DEP® dendrimer platform, a 5-generation PEGylated poly-lysine dendrimer via a hydrolytically labile linker. Release of AZD4320 is through hydrolytic cleavage of the linker, which is characterized by a “release half-life”, defined as the time to release 50% of the active moiety. This release half-life can be modified through linker design.

Initially, three drug-dendrimer conjugates with a range of AZD4320 release half-lives (from 1.7 h to 217 h) were synthesized and efficacy was investigated in C.B-17 SCID mice bearing RS4;11 tumors while cardiovascular parameters and tolerance were assessed in a telemetered rat model. A mathematical model was developed and used to optimize the desired release rate of the active moiety, AZD4320, from the dendrimer conjugate for maximal therapeutic index in terms of preclinical anti-tumor efficacy and cardiovascular profile. Based on this modeling, AZD0466, with a release half-life of 25.5 h, was synthesized and selected for further in vivo efficacy and tolerability assessment.

Efficacy studies in the RS4;11 xenograft model showed similar efficacy to AZD4320, while cardiovascular studies in rat and dog demonstrated that AZD0466 was tolerated at doses of active-moiety that are more than ten-fold higher. In addition, it can be easily formulated for intravenous administration due to significantly enhanced solubility.

The AZD4320-dendrimer conjugate, AZD0466, identified in this study has resulted in an improved therapeutic index and enabled progression of this promising Bcl-2/Bcl-xL inhibitor into clinical development.

Citation Format: Marianne B. Ashford, Srividya B. Balachander, Lorraine Graham, Iain Grant, Francis D. Gibbons, Kathryn J. Hill, Alexander R. Harmer, Sonya Gales, Sean Redmond, Brian Kelly, William McCoull, Shenghua Wen, Martin Wild, Eric Gangl, David J. Owen, Barry R. Davies. Design and optimization of a dendrimer-conjugated dual Bcl-2/Bcl-xL inhibitor, AZD0466, with improved therapeutic index [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1718.

Quantifying the relationship between inhibition of VEGF receptor 2, drug-induced blood pressure elevation and hypertension

BACKGROUND AND PURPOSE

Several anti-angiogenic cancer drugs that inhibit VEGF receptor (VEGFR) signalling for efficacy are associated with a 15-60% incidence of hypertension. Tyrosine kinase inhibitors (TKIs) that have off-target activity at VEGFR-2 may also cause blood pressure elevation as an undesirable side effect. Therefore, the ability to translate VEGFR-2 off-target potency into blood pressure elevation would be useful in development of novel TKIs. Here, we have sought to quantify the relationship between VEGFR-2 inhibition and blood pressure elevation for a range of kinase inhibitors.

EXPERIMENTAL APPROACH

Porcine aortic endothelial cells overexpressing VEGFR-2 (PAE) were used to determine IC₅₀ for VEGFR-2 phosphorylation. These IC₅₀ values were compared with published reports of exposure attained during clinical use and the corresponding incidence of all-grade hypertension. Unbound average plasma concentration (C_av,u ) was selected to be the most appropriate pharmacokinetic parameter. The pharmacokinetic-pharmacodynamic (PKPD) relationship for blood pressure elevation was investigated for selected kinase inhibitors, using data derived either from clinical papers or from rat telemetry experiments.

KEY RESULTS

All-grade hypertension was predominantly observed when the C_av,u was >0.1-fold of the VEGFR-2 (PAE) IC₅₀ . Furthermore, based on the PKPD analysis, an exposure-dependent blood pressure elevation >1 mmHg was observed only when the C_av,u was >0.1-fold of the VEGFR-2 (PAE) IC₅₀ .

CONCLUSIONS AND IMPLICATIONS

Taken together, these data show that the risk of blood pressure elevation is proportional to the amount of VEGFR-2 inhibition, and a margin of >10-fold between VEGFR-2 IC₅₀ and C_av,u appears to confer a minimal risk of hypertension.

Assessment of cardiomyocyte contraction in human-induced pluripotent stem cell-derived cardiomyocytes

Functional changes to cardiomyocytes are a common cause of attrition in preclinical and clinical drug development. Current approaches to assess cardiomyocyte contractility in vitro are limited to low-throughput methods not amenable to early drug discovery. Human-induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) were used to assess their suitability to detect drug-induced changes in cardiomyocyte contraction. Application of field stimulation and measurement of cardiac contraction (IonOptix edge detection) and Ca(2+) transients confirmed hiPS-CMs to be a suitable model to investigate drug-induced changes in cardiomyocyte contractility. Using a live cell, fast kinetic fluorescent assay with a Ca(2+) sensitive dye to test 31 inotropic and 20 non-inotropic compounds in vivo, we report that hiPS-CMs provide a high-throughput experimental model to detect changes in cardiomyocyte contraction that is applicable to early drug discovery with a sensitivity and specificity of 87% and 70%, respectively. Moreover, our data provide evidence of the detection of this liability at therapeutically relevant concentrations with throughput amenable to influencing chemical design in drug discovery. Measurement of multiple parameters of the Ca(2+) transient in addition to the number of Ca(2+) transients offered no insight into the mechanism of cardiomyocyte contraction.

The contribution of VEGF signalling to fostamatinib-induced blood pressure elevation

BACKGROUND AND PURPOSE

Fostamatinib is an inhibitor of spleen tyrosine kinase (TK). In patients, fostamatinib treatment was associated with increased BP. Some TK inhibitors cause BP elevation, by inhibiting the VEGF receptor 2 (VEGFR2). Here, we have assessed the mechanistic link between fostamatinib-induced BP elevation and inhibition of VEGF signalling.

EXPERIMENTAL APPROACH

We used conscious rats with automated blood sampling and radio telemetry and anaesthetized rats to measure cardiovascular changes. Rat isolated aorta and isolated hearts, and human resistance vessels in vitro were also used. NO production by human microvascular endothelial cells was measured with the NO-dependent probe, DAF-FM and VEGFR2 phosphorylation was determined in mouse lung, ex vivo.

KEY RESULTS

In conscious rats, fostamatinib dose-dependently increased BP. The time course of the BP effect correlated closely with the plasma concentrations of R406 (the active metabolite of fostamatinib). In anaesthetized rats, infusion of R406 increased BP and decreased femoral arterial conductance. Endothelial function was unaffected, as infusion of R406 did not inhibit hyperaemia- or ACh-induced vasodilatation in rats. R406 did not affect contraction of isolated blood vessels. R406 inhibited VEGF-stimulated NO production from human endothelial cells in vitro, and treatment with R406 inhibited VEGFR2 phosphorylation in vivo. R406 inhibited VEGF-induced hypotension in anaesthetized rats.

CONCLUSIONS AND IMPLICATIONS

Increased vascular resistance, secondary to reduced VEGF-induced NO release from endothelium, may contribute to BP increases observed with fostamatanib. This is consistent with the elevated BP induced by other drugs inhibiting VEGF signalling, although the contribution of other mechanisms cannot be excluded.

Effects of type 1 diabetes, sprint training and sex on skeletal muscle sarcoplasmic reticulum Ca2+ uptake and Ca2+-ATPase activity

Calcium cycling is integral to muscle performance during the rapid muscle contraction and relaxation of high-intensity exercise. Ca(2+) handling is altered by diabetes mellitus, but has not previously been investigated in human skeletal muscle. We investigated effects of high-intensity exercise and sprint training on skeletal muscle Ca(2+) regulation among men and women with type 1 diabetes (T1D, n = 8, 3F, 5M) and matched non-diabetic controls (CON, n = 8, 3F, 5M). Secondarily, we examined sex differences in Ca(2+) regulation. Subjects undertook 7 weeks of three times-weekly cycle sprint training. Before and after training, performance was measured, and blood and muscle were sampled at rest and after high-intensity exercise. In T1D, higher Ca(2+)-ATPase activity (+28%) and Ca(2+) uptake (+21%) than in CON were evident across both times and days (P < 0.05), but performance was similar. In T1D, resting Ca(2+)-ATPase activity correlated with work performed until exhaustion (r = 0.7, P < 0.01). Ca(2+)-ATPase activity, but not Ca(2+) uptake, was lower (-24%, P < 0.05) among the women across both times and days. Intense exercise did not alter Ca(2+)-ATPase activity in T1D or CON. However, sex differences were evident: Ca(2+)-ATPase was reduced with exercise among men but increased among women across both days (time × sex interaction, P < 0.05). Sprint training reduced Ca(2+)-ATPase (-8%, P < 0.05), but not Ca(2+) uptake, in T1D and CON. In summary, skeletal muscle Ca(2+) resequestration capacity was increased in T1D, but performance was not greater than CON. Sprint training reduced Ca(2+)-ATPase in T1D and CON. Sex differences in Ca(2+)-ATPase activity were evident and may be linked with fibre type proportion differences.

On the relationship between block of the cardiac Na⁺ channel and drug-induced prolongation of the QRS complex

BACKGROUND AND PURPOSE

Inhibition of the human cardiac Na(+) channel (hNa(v) 1.5) can prolong the QRS complex and has been associated with increased mortality in patients with underlying cardiovascular disease. The safety implications of blocking hNa(v) 1.5 channels suggest the need to test for this activity early in drug discovery in order to design out any potential liability. However, interpretation of hNa(v) 1.5 blocking potency requires knowledge of how hNa(v) 1.5 block translates into prolongation of the QRS complex.

EXPERIMENTAL APPROACH

We tested Class I anti-arrhythmics, other known QRS prolonging drugs and drugs not reported to prolong the QRS complex. Their block of hNa(v) 1.5 channels (as IC(50) values) was measured in an automated electrophysiology-based assay. These IC(50) values were compared with published reports of the corresponding unbound (free) plasma concentrations attained during clinical use (fC(max)) to provide an IC(50) : fC(max) ratio. KEY RESULTS For 42 Class I anti-arrhythmics and other QRS prolonging drugs, 67% had IC(50) : fC(max) ratios <30. For 55 non-QRS prolonging drugs tested, 72% had ratios >100. Finally, we determined the relationship between the IC(50) value and the free drug concentration associated with prolongation of the QRS complex in humans. For 37 drugs, QRS complex prolongation was observed at free plasma concentrations that were about 15-fold lower than the corresponding IC(50) at hNa(v) 1.5 channels.

CONCLUSIONS AND IMPLICATIONS

A margin of 30- to 100-fold between hNa(v) 1.5 IC(50) and fC(max) appears to confer an acceptable degree of safety from QRS prolongation. QRS prolongation occurs on average at free plasma levels 15-fold below the IC(50) at hNa(v) 1.5 channels.

LINKED ARTICLE

This article is commented on by Gintant et al., pp. 254-259 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2011.01433.x.

Validity of the modified 20-metre shuttle test: Assessment of cardiorespiratory fitness in people who have sustained a traumatic brain injury

PRIMARY OBJECTIVE

To validate the modified 20-metre shuttle test in adults who have sustained a traumatic brain injury (TBI).

DESIGN

Single-sample validity study.

SETTING

Brain injury rehabilitation unit.

PARTICIPANTS

Twenty-four adults with severe TBI, discharged from hospital for at least 6-months.

PROTOCOL

Participants attended the facility for a familiarization session, followed by a symptom-limited treadmill test and a modified shuttle test on two separate days. The treadmill test was based on an individualised protocol which used a physiotherapist-selected speed and increments in gradient every minute until volitional fatigue. The modified shuttle test was externally-paced and commenced with a speed of 2.4 km h(-1) which increased every minute until volitional fatigue.

MAIN MEASURES

Four primary measures were taken from both tests: peak oxygen uptake, peak heart rate, maximal velocity and rating of perceived exertion.

RESULTS

All participants completed the study. There were no adverse events. A high correlation was observed between the modified shuttle test and the treadmill test for peak oxygen uptake, peak heart rate and maximal velocity (r = 0.96, r = 0.80, r = 0.82, respectively; p < 0.001), but not for rating of perceived exertion (r = 0.013, p = 0.952).

CONCLUSION

The modified shuttle test is a valid measure of cardiorespiratory fitness in people who have sustained a TBI.

Fitness training for cardiorespiratory conditioning after traumatic brain injury

BACKGROUND

Cardiorespiratory deconditioning is a common sequelae after traumatic brain injury (TBI). Clinically, fitness training is implemented to address this impairment, however this intervention has not been subject to rigorous review.

OBJECTIVES

The primary objective was to evaluate whether fitness training improves cardiorespiratory fitness in people who have sustained a TBI.

SEARCH STRATEGY

We searched ten electronic databases (Cochrane Injuries Group Trials Register; Cochrane Central Register of Controlled Trials (CENTRAL); EMBASE; PubMed (MEDLINE); CINAHL; AMED; SPORTDiscus; PsycINFO; PEDro and PsycBITE) and two clinical trials registers (TrialsCentral and Current Controlled Trials). The last search was August 2007. In addition we screened reference lists from included studies and contacted trialists to identify further studies.

SELECTION CRITERIA

Randomised controlled studies with TBI participants were eligible if they compared an exercise programme incorporating cardiorespiratory fitness training to usual care, a non-exercise intervention or no intervention.

DATA COLLECTION AND ANALYSIS

Two authors independently screened the search output, extracted data and assessed quality. All trialists were contacted for additional information. Mean difference and 95% confidence intervals (CI) were calculated for continuous data and risk difference or odds ratio and 95% CI were calculated for dichotomous data. Data were pooled when there were sufficient studies with clinical and statistical homogeneity.

MAIN RESULTS

Six studies, incorporating 303 participants, were included. The participants were primarily males, in their mid thirties who had sustained a severe TBI. The studies were clinically diverse with regard to the interventions, time post-injury and the outcome measures used; therefore, the primary outcome could not be pooled. Three of the six studies indirectly assessed change in cardiorespiratory fitness after fitness training using the peak power output obtained during cycle ergometry (either at volitional fatigue or at a predetermined endpoint, that is, a percentage of predicted heart rate maximum). Cardiorespiratory fitness was improved after fitness training in one study (mean difference 59 watts, 95% CI 24 to 94), whilst there was no significant improvement in the other two studies. Four of the six studies had no drop-outs from their intervention group and no adverse events were reported in any study.

AUTHORS' CONCLUSIONS

There is insufficient evidence to draw any definitive conclusions about the effects of fitness training on cardiorespiratory fitness. Whilst it appears to be a safe and accepted intervention for people with TBI, more adequately powered and well-designed studies are required to determine the effects across a range of outcome measures.

Local and global calcium signals and fluid and electrolyte secretion in mouse submandibular acinar cells

Polarized Ca(2+) signals that originate at and spread from the apical pole have been shown to occur in acinar cells from lacrimal, parotid, and pancreatic glands. However, "local" Ca(2+) signals, that are restricted to the apical pole of the cell, have been previously demonstrated only in pancreatic acinar cells in which the primary function of the Ca(2+) signal is to regulate exocytosis. We show that submandibular acinar cells, in which the primary function of the Ca(2+) signal is to drive fluid and electrolyte secretion, are capable of both Ca(2+) waves and local Ca(2+) signals. The generally accepted model for fluid and electrolyte secretion requires simultaneous Ca(2+)-activation of basally located K(+) channels and apically located Cl(-) channels. Whereas a propagated cell-wide Ca(2+) signal is clearly consistent with this model, a local Ca(2+) signal is not, because there is no increase in intracellular Ca(2+) concentration at the basal pole of the cell. Our data provide the first direct demonstration, in submandibular acinar cells, of the apical and basal location of the Cl(-) and K(+) channels, respectively, and confirm that local Ca(2+) signals do not Ca(2+)-activate K(+) channels. We reevaluate the model for fluid and electrolyte secretion and demonstrate that Ca(2+)-activation of the Cl(-) channels is sufficient to voltage-activate the K(+) channels and thus demonstrate that local Ca(2+) signals are sufficient to support fluid secretion.

Correlations between the functional integrity of the endoplasmic reticulum and polarized Ca2+ signalling in mouse lacrimal acinar cells: a role for inositol 1,3,4,5-tetrakisphosphate

Ca(2+) signalling in exocrine acinar cells has been shown to be both polarized and pulsatile in all cell types tested, except acutely isolated mouse lacrimal acinar cells. Lacrimal cells are also unusual in that they display a very low sensitivity to Ins(1,4,5) P (3) (Ins P (3)) that may be enhanced by placing the cells in primary culture for 12-72 h or by intracellular infusion of a low concentration of Ins(1,3,4,5) P (4) (Ins P (4)). We have proposed previously that this atypical behaviour stemmed from vesiculation of the endoplasmic reticulum (ER) incurred during isolation of the cells and, furthermore, that time in culture or Ins P (4) increased sensitivity to Ins P (3) by increasing ER integrity [Smith, Harmer, Letcher and Irvine (2000) Biochem. J. 347, 77-82]. We have measured the half time for fluorescence recovery after photobleaching (FRAP) of a fluorescent marker (Mag-fluo 4) loaded into the ER lumen in order to determine directly the functional integrity of the ER in lacrimal cells. The half-time for FRAP was increased (indicating a reduction in the functional integrity of the ER) following exposure to anti-microtubule agents (taxol and nocodazole) known to perturb ER structure and decreased (indicating an increase in the functional integrity of the ER) by time in culture and exposure to Ins P (4). The action of Ins P (4) was particularly pronounced because it occurred under patch-clamp whole-cell conditions that were themselves found to reduce ER functional integrity. These data show that ER remodelling could be a physiological regulator of Ca(2+) signalling and indicate a role for Ins P (4) in control of this process.

Acetylcholine-evoked calcium mobilization and ion channel activation in human labial gland acinar cells from patients with primary Sjögren's syndrome

Recent evidence has indicated that the salivary gland dysfunction associated with Sjögren's syndrome (SjS) is not necessarily due to immune-mediated destruction of acinar tissue. SjS sufferers may possess substantial reserves of acinar tissue but nevertheless be incapable of maintaining salivary flow rates in the normal range. We have investigated the ability of isolated labial gland acinar cells from SjS patients to fluid secrete by measuring agonist-evoked changes in intracellular Ca(2+) ([Ca(2+)](i)) using fura-2 microfluorimetry and activation of K(+) and Cl(-) channels using the patch-clamp whole cell technique. We can confirm that stimulation with a super-maximal dose of acetylcholine (ACh) increased [Ca(2+)]i equally in both control acinar cells and those derived from SjS patients. However, at submaximal concentrations, the dose-response curve for ACh was shifted to the right by approximately one order of magnitude in acinar cells from SjS patients compared to control acinar cells. Patch-clamp measurements consistent with the presence of Ca(2+)-activated K(+) and Cl(-) conductances were obtained from both control acinar cells and those obtained from SjS patients. Dose-dependent activation of the ion channels by acetylcholine was also right-shifted in acinar cells from SjS patients compared to control cells. Our data show that labial gland acinar cells from SjS patients were capable of responding to agonist stimulation by mobilizing [Ca(2+)](i) and activating K(+) and Cl(-) channels consistent with the requirements of fluid secretion. However, the persistent loss of sensitivity to ACh observed in from SjS patients may account for the lack of saliva production observed in these patients in vivo.

Effect of 10-day cast immobilization on sarcoplasmic reticulum calcium regulation in humans

This study investigated the effects of 10-day lower limb cast immobilization on sarcoplasmic reticulum (SR) Ca2+ regulation. Muscle biopsies were analysed in eight healthy females for maximal rates of SR Ca2+ release, Ca2+ uptake and Ca2+ ATPase activity at control, during immobilization at day 3 (IM 3), day 6 (IM 6) and day 10 (IM 10). Quadriceps muscle cross-sectional area (CSA) and 1-repetition maximum (1RM) leg extension strength were measured to determine the extent of muscle size and strength adaptations. Muscle CSA and strength decreased following 10 days of immobilization (11.8 and 41.6%, respectively, P < 0.01). A decrease in SR Ca2+ uptake rate (analysed per g wet wt) was found at IM 3 (13.2%, P=0.05), with a further decrease at IM 10 (19.8% from control, P < 0.01). At IM 10, a decrease in SR Ca2+ uptake rate (per mg protein) also occurred (19.9%, P < 0.01). Sarcoplasmic reticulum Ca2+ ATPase activity and rate of Ca2+ release were not altered with 10 days of immobilization. This study observed a decrease in SR Ca2+ uptake rate, muscular atrophy and strength loss over 10 days of immobilization in humans.

Role of Ins(1,4,5)P3, cADP-ribose and nicotinic acid-adenine dinucleotide phosphate in Ca2+ signalling in mouse submandibular acinar cells

cADP-ribose (cADPr) and nicotinic acid-adenine dinucleotide phosphate (NAADP) are two putative second messengers; they were first shown to stimulate Ca(2+) mobilization in sea urchin eggs. We have used the patch-clamp whole-cell technique to determine the role of cADPr and NAADP in relation to that of Ins(1,4,5)P(3) in mouse submandibular acinar cells by measuring agonist-evoked and second-messenger-evoked changes in Ca(2+)-dependent K(+) and Cl(-) currents. Both Ins(1,4,5)P(3) and cADPr were capable of reproducing the full range of responses normally seen in response to stimulation with acetylcholine (ACh). Low concentrations of agonist (10-20 nM ACh) or second messenger [1-10 microM Ins(1,4,5)P(3) or cADPr] elicited a sporadic transient activation of the Ca(2+)-dependent currents; mid-range concentrations [50-500 nM ACh, 50 microM Ins(1,4,5)P(3) or 50-100 microM cADPr] elicited high-frequency (approx. 2 Hz) trains of current spikes; and high concentrations [more than 500 nM ACh, more than 50 microM Ins(1,4,5)P(3) or more than 100 microM cADPr] gave rise to a sustained current response. The response to ACh was inhibited by antagonists of both the Ins(1,4,5)P(3) receptor [Ins(1,4,5)P(3)R] and the ryanodine receptor (RyR) but could be completely blocked only by an Ins(1,4,5)P(3)R antagonist (heparin). NAADP (50 nM to 100 microM) did not itself activate the Ca(2+)-dependent ion currents, nor did it inhibit the activation of these currents by ACh. These results show that, in these cells, both Ins(1,4,5)P(3)R and RyR are involved in the propagation of the Ca(2+) signal stimulated by ACh and that cADPr can function as an endogenous regulator of RyR. Furthermore, although NAADP might have a role in hormone-stimulated secretion in pancreatic acinar cells, it does not contribute to ACh-evoked secretion in submandibular acinar cells.

Skeletal muscle metabolic and ionic adaptations during intense exercise following sprint training in humans

The effects of sprint training on muscle metabolism and ion regulation during intense exercise remain controversial. We employed a rigorous methodological approach, contrasting these responses during exercise to exhaustion and during identical work before and after training. Seven untrained men undertook 7 wk of sprint training. Subjects cycled to exhaustion at 130% pretraining peak oxygen uptake before (PreExh) and after training (PostExh), as well as performing another posttraining test identical to PreExh (PostMatch). Biopsies were taken at rest and immediately postexercise. After training in PostMatch, muscle and plasma lactate (Lac(-)) and H(+) concentrations, anaerobic ATP production rate, glycogen and ATP degradation, IMP accumulation, and peak plasma K(+) and norepinephrine concentrations were reduced (P<0.05). In PostExh, time to exhaustion was 21% greater than PreExh (P<0.001); however, muscle Lac(-) accumulation was unchanged; muscle H(+) concentration, ATP degradation, IMP accumulation, and anaerobic ATP production rate were reduced; and plasma Lac(-), norepinephrine, and H(+) concentrations were higher (P<0.05). Sprint training resulted in reduced anaerobic ATP generation during intense exercise, suggesting that aerobic metabolism was enhanced, which may allow increased time to fatigue.

The effect of inositol 1,3,4,5-tetrakisphosphate on inositol trisphosphate-induced Ca2+ mobilization in freshly isolated and cultured mouse lacrimal acinar cells

Earlier reports have shown a remarkable synergism between InsP(4) and InsP(3) [either Ins(1,4,5)P(3) or Ins(2,4,5)P(3)] in activating Ca(2+)-dependent K(+) and Cl(-) currents in mouse lacrimal cells [Changya, Gallacher, Irvine, Potter and Petersen (1989) J. Membr. Biol. 109, 85-93; Smith (1992) Biochem. J. 283, 27-30]. However, Bird, Rossier, Hughes, Shears, Armstrong and Putney [(1991) Nature (London) 352, 162-165] reported that they could see no such synergism in the same cell type. A major experimental difference between the two laboratories lies in whether or not the cells were maintained in primary culture before use. Here we have compared directly the responses to inositol polyphosphates in freshly isolated cells versus cells cultured for 6-72 h. In the cultured cells, Ins(2,4,5)P(3) at 100 microM produced a robust stimulation of K(+) and Cl(-) currents, as much as an order of magnitude greater than that observed in the freshly isolated cells. However, the freshly isolated cells could be restored to a sensitivity similar to cultured cells by the addition of InsP(4) at a concentration two orders of magnitude lower than that of Ins(2,4,5)P(3). We discuss the implications of this with respect to the actions of InsP(4), including the possibility that disruption of the cellular structure during the isolation of the cells exposes an extreme manifestation of a possible physiological role for InsP(4) in controlling calcium-store integrity.

Human skeletal sarcoplasmic reticulum Ca2+ uptake and muscle function with aging and strength training

This study investigated the adaptations of skeletal muscle sarcoplasmic reticulum (SR) Ca2+ uptake, relaxation, and fiber types in young (YW) and elderly women (EW) to high-resistance training. Seventeen YW (18-32 yr) and 11 EW (64-79 yr) were assessed for 1) electrically evoked relaxation time and rate of the quadriceps femoris; and 2) maximal rates of SR Ca2+ uptake and Ca2+-ATPase activity and relative fiber-type areas, analyzed from muscle biopsies of the vastus lateralis. EW had significantly slower relaxation rates and times, decreased SR Ca2+ uptake and Ca2+-ATPase activity, and a larger relative type I fiber area than did YW. A subgroup of 9 young (YWT) and 10 elderly women (EWT) performed 12 wk of high-resistance training (8 repetition maximum) of the quadriceps and underwent identical testing procedures pre- and posttraining. EWT significantly increased their SR Ca2+ uptake and Ca2+-ATPase activity in response to training but showed no alterations in speed of relaxation or relative fiber-type areas. In YWT none of the variables was altered after resistance training. These findings suggest that 1) a reduced SR Ca2+ uptake in skeletal muscle of elderly women was partially reversed with resistance training and 2) SR Ca2+ uptake in the vastus lateralis was not the rate-limiting mechanism for the slowing of relaxation measured from electrically evoked quadriceps muscle of elderly women.

Impaired calcium pump function does not slow relaxation in human skeletal muscle after prolonged exercise

This study examined the effects of prolonged exercise on human quadriceps muscle contractile function and homogenate sarcoplasmic reticulum Ca2+ uptake and Ca2+-adenosinetriphosphatase activity. Ten untrained men cycled at 75 +/- 2% (SE) peak oxygen consumption until exhaustion. Biopsies were taken from the right vastus lateralis muscle at rest, exhaustion, and 20 and 60 min postexercise. Peak tension and half relaxation time of the left quadriceps muscle were measured during electrically evoked twitch and tetanic contractions and a maximal voluntary isometric contraction at rest, exhaustion, and 10, 20, and 60 min postexercise. At exhaustion, homogenate Ca2+ uptake and Ca2+ adenosinetriphosphatase activity were reduced by 17 +/- 4 and 21 +/- 5%, respectively, and remained depressed after 60 min recovery (P </= 0.01). Muscle ATP, creatine phosphate, and glycogen were all depressed at exhaustion (P </= 0.01). Peak tension during a maximal voluntary contraction, a twitch, and a 10-Hz stimulation were reduced after exercise by 28 +/- 3, 45 +/- 6, 65 +/- 5%, respectively (P </= 0.01), but no slowing of half relaxation times were found. Thus fatigue induced by prolonged exercise reduced muscle Ca2+ uptake, but this did not cause a slower relaxation of evoked contractions.

Effects of training on potassium, calcium and hydrogen ion regulation in skeletal muscle and blood during exercise

Ionic regulation is critical to muscle excitation, contraction and metabolism, and thus for muscle function during exercise. This review focuses on the effects of training upon K+, Ca2+ and H+ ion regulation in muscle and K+ regulation in blood during exercise. Training enhances K+ regulation in muscle and blood and reduces muscular fatiguability. Endurance, sprint and strength training in humans induce an increased muscle Na+, K+ pump concentration, usually associated with a reduced rise in plasma [K+] during exercise. Although impaired muscle Ca2+ regulation plays a vital role in fatigue, little is known about possible training effects. In rat fast-twitch muscle, overload-induced hypertrophy and endurance training were associated with reduced sarcoplasmic reticulum Ca2+ uptake, consistent with fast-to-slow fibre transition. In human muscle, endurance and strength training had no effect on muscle Ca2+ ATPase concentration. Whilst muscle Ca2+ uptake, release and Ca2+ ATPase activity were depressed by fatigue, no differences were found between strength athletes and untrained individuals. Muscle H+ accumulation may contribute to fatigue during intense exercise and is also modified by sprint training. Sprint training may increase muscle Lac- and work output with exhaustive exercise, but the rise in muscle [H+] is unchanged or attenuated, indicating a reduced rise in muscle [H+] relative to work performed. Muscle buffering capacity can be dissociated from this improved H+ regulatory capacity after training. Thus, training enhances muscle and blood K+ and muscle H+ regulation during exercise, consistent with improved muscular performance and reduced fatiguability; however, little is known about training effects on muscle Ca2+ regulation during contraction.