A novel secretagogue increases cardiac contractility by enhancement of L-type Ca2+ current.
ABSTRACT N'1-(3,3,6,8-tetramethyl-1-oxo-1,2,3,4-tetrahydronaphthalen-2-yliden)-2-cyanoethanohydrazide (TTYC) increases secretion of glucagon-like peptide-1 and intracellular Ca(2+) concentration in GLUTag cells. The purpose of the present study was to examine if TTYC exerts positive inotropic effects on isolated rabbit ventricular myocytes and in vivo heart in anesthetized rats, and if so to further define the potential mechanism of action. Contractility was assessed in vitro using changes in fractional shortening (FS) of myocyte sarcomere length and in vivo using changes in the velocity of left ventricular pressure. Changes in L-type Ca(2+) current of ventricular myocytes were evaluated using whole-cell voltage-clamp techniques. TTYC increased FS of myocyte sarcomere length in a concentration-dependent manner. The positive inotropic effect was not abrogated by beta-adrenergic blockade (propranolol) or protein kinase A inhibition. TTYC enhanced peak L-type Ca(2+) current in a voltage-dependent manner (current amplitudes increased by 4.0-fold at -10 mV and 1.5-fold at +10 mV). Voltage-dependence of steady-state activation of L-type Ca(2+) current was shifted by 15 mV in the negative direction. Inactivation time course of the L-type Ca(2+) currents at voltages of -10 to 20 mV was significantly slowed by 0.3 microM TTYC. In vivo studies demonstrated that TTYC increased cardiac contractility in a dose-dependent manner. In conclusion, TTYC is a novel L-type Ca(2+) current activator with positive cardiac inotropic effects. Negative shifting of the voltage-dependence of L-type Ca(2+) current activation and reduced inactivation are two mechanisms responsible for the enhanced L-type Ca(2+) current that contribute to the positive inotropic effects.
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ABSTRACT: Delayed cardiac repolarization is an established risk factor for proarrhythmia and Torsades-de-Pointes (TdeP) that is typically measured in vitro during slow, regular stimulation. We have developed an alternative, novel, and rapid cellular-based approach for predicting drug-induced proarrhythmia that detects changes in electrical refractoriness based on mechanical responses (measured optically) during increasingly rapid trains of stimulation interspersed with pauses (mimicking the clinically observed short-long-short (SLS) stimulation sequence associated with the TdeP initiation). Acutely isolated rabbit ventricular myocytes were superfused and electrically stimulated using an accelerating pacing protocol (APP) consisting of 12 consecutive pacing segments (10 beats per segment) with incrementally faster cycle lengths; trains were separated by pauses to identify loss of stimulus capture as well as to mimic clinically observed SLS sequences. Drug effects were evaluated based on a myocyte's ability to contract during progressively faster pacing segments (rate-adaptation); the earliest rate during which the myocyte fails to respond (longest cycle length with incomplete capture (CLIC)) was used to quantify electrophysiologic effects. Torsadogenic drugs known to delay repolarization during slow stimulation prolonged CLIC and dramatically limited the ability to respond to progressively rapid stimulation. The recognized proarrhythmic compounds E-4031, cisapride, grepafloxacin, and haloperidol rapidly prolonged CLIC at and above therapeutic concentrations in a concentration-dependent manner, while negative controls (captopril, indomethacin, and loratidine) do not affect rate-adaptation. Ventricular rate adaptation represents a novel approach for rapidly detecting drugs with torsadogenic risk using rapid rhythms that are typically not employed when evaluating proarrhythmic risk. This method is well suited for detecting and avoiding potential cardiac liabilities early in drug discovery ("frontloading") prior to final selection of candidate drugs.Journal of pharmacological and toxicological methods 03/2011; 64(1):68-73. · 2.32 Impact Factor
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ABSTRACT: Chronic psychosocial stress triggers cardiovascular diseases although underlying mechanisms are still elusive. This study examined the effect of social stress on cardiomyocyte contractile function and pathological changes in myocardium using the visible burrow system (VBS) model. Chronic social stress was induced using a mixed-sex VBS housing in adult Sprague-Dawley (SD) rats. Contractile and intracellular Ca(2+) properties were evaluated in isolated cardiomyocytes including peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90)), maximal velocity of shortening/relengthening (± dL/dt), Fura-2 fluorescence intensity, and intracellular Ca(2+) decay. Myocardial histology was evaluated using Masson trichrome staining. Social stress led to depressed PS, ± dL/dt, shortened TPS and prolonged TR(90) compared with the unstressed controls. Baseline and electrically-stimulated rise in Ca(2+) were reduced whereas intracellular Ca(2+) decay was delayed in stressed rats. Histological analyses exhibited overt interstitial fibrosis and cardiomyocyte hypertrophy in stressed rats. The GSH/GSSG ratio (indicative of oxidative stress status) was reduced whereas oxidative protein carbonyl formation was elevated in stressed rats. Western blot analysis showed unchanged expression of superoxide dismutase 1 (SOD1), β(1)-adrenoceptor (β(1)-AR) levels, reduced sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) levels, and elevated phosphorylation of the stress signaling protein kinase JNK but not ERK in myocardium from stressed rats. Short-term in vitro treatment of cardiomyocytes with the stress inducer phenylephrine mimicked cell damage and intracellular Ca(2+) mishandling, the effects of which were mitigated by antioxidant, JNK inhibition, carvedilol and SERCA2a adenovirus. These findings indicate that chronic social stress is detrimental to cardiac structure and function possibly via mechanisms associated with oxidative injury and intracellular Ca(2+) mishandling.Physiology & Behavior 09/2011; 105(2):498-509. · 3.03 Impact Factor
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ABSTRACT: Cardiac toxicity is a major concern in drug development and it is imperative that clinical candidates are thoroughly tested for adverse effects earlier in the drug discovery process. In this report, we investigate the utility of an impedance-based microelectronic detection system in conjunction with mouse embryonic stem cell-derived cardiomyocytes for assessment of compound risk in the drug discovery process. Beating of cardiomyocytes was measured by a recently developed microelectronic-based system using impedance readouts. We used mouse stem cell-derived cardiomyocytes to obtain dose-response profiles for over 60 compounds, including ion channel modulators, chronotropic/ionotropic agents, hERG trafficking inhibitors and drugs known to induce Torsades de Pointes arrhythmias. This system sensitively and quantitatively detected effects of modulators of cardiac function, including some compounds missed by electrophysiology. Pro-arrhythmic compounds produced characteristic profiles reflecting arrhythmia, which can be used for identification of other pro-arrhythmic compounds. The time series data can be used to identify compounds that induce arrhythmia by complex mechanisms such as inhibition of hERG channels trafficking. Furthermore, the time resolution allows for assessment of compounds that simultaneously affect both beating and viability of cardiomyocytes. Microelectronic monitoring of stem cell-derived cardiomyocyte beating provides a high throughput, quantitative and predictive assay system that can be used for assessment of cardiac liability earlier in the drug discovery process. The convergence of stem cell technology with microelectronic monitoring should facilitate cardiac safety assessment.British Journal of Pharmacology 08/2011; 165(5):1424-41. · 5.07 Impact Factor