Search PubMed⌕ Search

Biomedical subjects

Steven R Houser

Publications and source records attributed to Steven R Houser.

36 records · Page 2Linked to original sources
PubMed2004

Myocyte nitric oxide synthase 2 contributes to blunted beta-adrenergic response in failing human hearts by decreasing Ca2+ transients.

BACKGROUND: Human heart failure (HF) usually exhibits blunted response to beta-adrenergic receptor (AR) stimulation. Here, we examined whether expression of nitric oxide synthase-2 (NOS2, or inducible NOS) contributes to this loss of inotropic reserve in human HF. METHODS AND RESULTS: Failing human hearts were obtained at transplantation. Contraction and [Ca2+]i measurements were performed in isolated cardiac myocytes and trabeculae. In HF myocytes and muscle, isoproterenol (ISO), a beta-AR agonist, led to small inotropic and lusitropic responses. Specific inhibition of NOS2 by aminoguanidine (AG) or L-NIL dramatically increased the ISO-induced inotropy and lusitropy, such that the ISO+AG response in HF approached that seen with ISO alone in nonfailing human myocytes or muscles. Ca2+ transient data directly paralleled these results, indicating that altered cellular Ca2+ handling is responsible. In nonfailing human hearts, NOS2 inhibition had no effects. In addition, NOS2 inhibition also had no effect in 30% of failing hearts, but in these myocytes and muscles, the ISO response alone was similar to that of nonfailing hearts. In line with these functional findings, NOS2 protein expression measured by Western blotting was induced in HF when AG/L-NIL had a functional effect but not when AG/L-NIL had no effect on contractility and Ca2+ transients. CONCLUSIONS: NOS2 expression strongly limited ISO-induced increases in contraction, twitch Delta[Ca2+]i, and lusitropy in trabeculae and isolated myocytes from failing human hearts. Thus, the beta-AR hyporesponsiveness in human HF is mediated in large part by NO (or related congeners) produced within cardiac myocytes via NOS2.

Adrenergic beta-Agonists↗
PubMed2004

Abnormal frequency-dependent responses represent the pathophysiologic signature of contractile failure in human myocardium.

BACKGROUND: - The normal increase in isometric developed force (DF) with faster pacing rates, known as the positive force-frequency response/relationship (FFR), is altered in failing myocardium, as shown by its negative response to increased pacing. The objective of this study was to determine if increasing Ca2+ influx with L-type Ca2+ channel (L-CaCh) agonists: BayK 8644 (BayK) and FPL 64176 (FPL) or increased extracellular Ca2+ could increase contractility and normalize the FFR in failing myocardium. METHODS: - Isometric DF was measured in right ventricular trabeculae from failing (n = 28) and non-failing (n = 12) human hearts at various stimulation frequencies (0.5-2.5 Hz) before and after bath application of BayK (250 nM), FPL (100 nM), or high Ca2+ (7.0 mM). Post-rest (PR) experiments were also conducted on several trabeculae. RESULTS: - In trabeculae from failing hearts, the DF decreased with an increase in pacing. Addition of L-CaCh agonists increased DF to similar levels in trabeculae from both failing and non-failing hearts at slow pacing rates, but did not alter the negative FFR in the failing group. During increased rest intervals, the amount of PR potentiation was diminished in trabeculae from failing hearts as compared to the non-failing preparations. CONCLUSION: - This study demonstrates that the abnormal FFR observed in trabeculae from failing hearts is a reliable physiologic signature of the cardiomyopathic state even when DF, at slow stimulation frequencies, is relatively high. These studies further demonstrate that the impaired FFR is not due to an inability to further increase contractility. Rather, our findings suggest that the abnormal FFR and blunted PR potentiation alike are a reflection of an altered functional balance between Ca2+ re-uptake and Ca2+ extrusion.

Calcium↗
PubMed2003

Dynamic regulation of sodium/calcium exchange function in human heart failure.

BACKGROUND: Sarcolemmal Na/Ca exchange (NCX) regulates cardiac Ca and contractility. NCX function during the cardiac cycle is determined by intracellular [Ca] and [Na] ([Ca]i, and [Na]i) and membrane potential (Em), which all change in human heart failure (HF). Therefore, changes in NCX function may contribute to abnormal Ca regulation in human HF. METHODS AND RESULTS: We assessed the cellular bases of differences in NCX function in ventricular myocytes from failing (F) and nonfailing (NF) human hearts. Allosteric activation of NCX by [Ca]i was comparable in F and NF myocytes (K1/2=150+/-31 nmol/L, n=7). The steady-state relation between [Ca]i and NCX current (INCX) was used to infer the local submembrane [Ca]i ([Ca]sm) that is sensed by NCX dynamically during the action potential (AP) and Ca transient (37 degrees C). This involved "tail" INCX measurement during abrupt repolarization of APs and Ca transients, where peak inward INCX indicates [Ca]sm. This allows inference of the direction of Ca transport by the NCX during the AP. In NF myocytes, NCX extrudes Ca for most of the AP. Three factors shift the direction of NCX-mediated Ca transport (to favor more Ca influx) in F versus NF myocytes, as follows: (1) reduced [Ca]sm, (2) prolonged AP duration, and (3) elevated [Na]i. CONCLUSIONS: These results show that Ca entry through NCX may limit systolic dysfunction due to reduced sarcoplasmic reticulum Ca stores in HF but could contribute to slow decay of the [Ca]i transient and to diastolic dysfunction.

Action Potentials↗
PubMed2003

Phenotypic differences in transient outward K+ current of human and canine ventricular myocytes: insights into molecular composition of ventricular Ito.

The Ca(2+)-independent transient outward K(+) current (I(to)) plays an important electrophysiological role in normal and diseased hearts. However, its contribution to ventricular repolarization remains controversial because of differences in its phenotypic expression and function across species. The dog, a frequently used model of human cardiac disease, exhibits altered functional expression of I(to). To better understand the relevance of electrical remodeling in dogs to humans, we studied the phenotypic differences in ventricular I(to) of both species with electrophysiological, pharmacological, and protein-chemical techniques. Several notable distinctions were elucidated, including slower current decay, more rapid recovery from inactivation, and a depolarizing shift of steady-state inactivation in human vs. canine I(to). Whereas recovery from inactivation of human I(to) followed a monoexponential time course, canine I(to) recovered with biexponential kinetics. Pharmacological sensitivity to flecainide was markedly greater in human than canine I(to), and exposure to oxidative stress did not alter the inactivation kinetics of I(to) in either species. Western blot analysis revealed immunoreactive bands specific for Kv4.3, Kv1.4, and Kv channel-interacting protein (KChIP)2 in dog and human, but with notable differences in band sizes across species. We report for the first time major variations in phenotypic properties of human and canine ventricular I(to) despite the presence of the same subunit proteins in both species. These data suggest that differences in electrophysiological and pharmacological properties of I(to) between humans and dogs are not caused by differential expression of the K channel subunit genes thought to encode I(to), but rather may arise from differences in molecular structure and/or posttranslational modification of these subunits.

Animals↗
PubMed2003

Calcium entry via Na/Ca exchange during the action potential directly contributes to contraction of failing human ventricular myocytes.

UNLABELLED: Prolongation of the Ca2+ transient and action potential (AP) durations are two characteristic changes in myocyte physiology in the failing human heart. The hypothesis of this study is that Ca2+ influx via reverse mode Na+/Ca2+ exchanger (NCX) or via L-type Ca2+ channels directly activates contraction in failing human myocytes while in normal myocytes this Ca2+ is transported into the sarcoplasmic reticulum (SR) to regulate SR Ca2+ stores. METHODS: Myocytes were isolated from failing human (n=6), nonfailing human (n=3) and normal feline hearts (n=9) and whole cell current and voltage clamp techniques were used to evoke and increase the duration of APs (0.5 Hz, 37 degrees C). Cyclopiazonic acid (CPA 10(-6) M), nifedipine (NIF;10(-6) M) and KB-R 7943 (KB-R; 3x10(-6) M) were used to reduce SR Ca2+ uptake, Ca2+ influx via the L-type Ca2+ current and reverse mode NCX, respectively. [Na+)i was changed by dialyzing myocytes with 0, 10 and 20 mM Na(+) pipette solutions. RESULTS: Prolongation of the AP duration caused an immediate prolongation of contraction and Ca2+ transient durations in failing myocytes. The first beat after the prolonged AP was potentiated by 21+/-5 and 27+/-5% in nonfailing human and normal feline myocytes, respectively (P<0.05), but there was no significant effect in failing human myocytes (+5+/-4% vs. steady state). CPA blunted the potentiation of the first beat after AP prolongation in normal feline and nonfailing human myocytes, mimicking the failing phenotype. NIF reduced steady state contraction in feline myocytes but the potentiation of the first beat after AP prolongation was unaltered (21+/-3% vs. base, P<0.05). KB-R reduced basal contractility and abolished the potentiation of the first beat after AP prolongation (2+/-1% vs. steady state). Increasing [Na+]i shortened AP, Ca2+ transient and contraction durations and increased steady state and post AP prolongation contractions. Dialysis with 0 Na+ eliminated these effects. CONCLUSIONS: Ca2+ enters both normal and failing cardiac myocytes during the late portion of the AP plateau via reverse mode NCX. In (normal) myocytes with good SR function, this Ca(2+) influx helps maintain and regulate SR Ca2+ load. In (failing) human myocytes with poor SR function this Ca2+ influx directly contributes to contraction. These studies suggest that the Ca2+ transient of the failing human ventricular myocytes has a higher than normal reliance on Ca2+ influx via the reverse mode of the NCX during the terminal phases of the AP.

Action Potentials↗
PubMed2003

[Na+]i handling in the failing human heart.

Proper contractile function of the heart depends on intact excitation-contraction processes and ion homeostasis of the myocytes. The Ca2+ ion activates contraction through its binding to troponin C. However, Ca2+ homeostasis is tightly linked to Na+ regulation because the primary mechanism for Ca2+ efflux in cardiac myocytes is via electrogenic Na+/Ca2+-exchange. While altered Ca2+-homeostasis has been demonstrated in animal models of heart failure and failing human cardiac tissue, the role of dysfunctional Na+ handling processes in altered excitation-contraction coupling remains obscure. Furthermore, altered Na+ handling has been implicated in a wide range of cellular processes, such as regulation of membrane potential, pH, and growth. This review will discuss (1) the evidence for altered [Na+]i homeostasis in the failing human heart, (2) how alterations in the Na+ electrochemical gradient can influence Ca2+ handling, contractile function, and a number of other cellular processes, and (3) the potential defects in Na+ channels and transporters that may underlie altered [Na+]i in the failing human heart.

Animals↗
PubMed2003

Is depressed myocyte contractility centrally involved in heart failure?

This review examines the evidence for and against the hypothesis that abnormalities in cardiac contractility initiate the heart failure syndrome and drive its progression. There is substantial evidence that the contractility of failing human hearts is depressed and that abnormalities of basal Ca2+ regulation and adrenergic regulation of Ca2+ signaling are responsible. The cellular and molecular defects that cause depressed myocyte contractility are not well established but seem to culminate in abnormal sarcoplasmic reticulum uptake, storage, and release. There are also strong links between Ca2+ regulation, Ca2+ signaling pathways, hypertrophy, and heart failure that need to be more clearly delineated. There is not substantial direct evidence for a causative role for depressed contractility in the initiation and progression of human heart failure, and some studies show that heart failure can occur without depressed myocyte contractility. Stronger support for a causal role for depressed contractility in the initiation of heart failure comes from animal studies where maintaining or improving contractility can prevent heart failure. Recent clinical studies in humans also support the idea that beneficial heart failure treatments, such as beta-adrenergic antagonists, involve improved contractility. Current or previously used heart failure treatments that increase contractility, primarily by increasing cAMP, have generally increased mortality. Novel heart failure therapies that increase or maintain contractility or adrenergic signaling by selectively modulating specific molecules have produced promising results in animal experiments. How to reliably implement these potentially beneficial inotropic therapies in humans without introducing negative side effects is the major unanswered question in this field.

Animals↗
PubMed2003

Cellular basis of abnormal calcium transients of failing human ventricular myocytes.

Depressed contractility is a central feature of the failing human heart and has been attributed to altered [Ca2+]i. This study examined the respective roles of the L-type Ca2+ current (ICa), SR Ca2+ uptake, storage and release, Ca2+ transport via the Na+-Ca2+ exchanger (NCX), and Ca2+ buffering in the altered Ca2+ transients of failing human ventricular myocytes. Electrophysiological techniques were used to measure and control V(m) and measure I(m), respectively, and Fluo-3 was used to measure [Ca2+]i in myocytes from nonfailing (NF) and failing (F) human hearts. Ca2+ transients from F myocytes were significantly smaller and decayed more slowly than those from NF hearts. Ca2+ uptake rates by the SR and the amount of Ca2+ stored in the SR were significantly reduced in F myocytes. There were no significant changes in the rate of Ca2+ removal from F myocytes by the NCX, in the density of NCX current as a function of [Ca2+]i, ICa density, or cellular Ca2+ buffering. However, Ca2+ influx during the late portions of the action potential seems able to elevate [Ca2+]i in F but not in NF myocytes. A reduction in the rate of net Ca2+ uptake by the SR slows the decay of the Ca2+ transient and reduces SR Ca2+ stores. This leads to reduced SR Ca2+ release, which induces additional Ca2+ influx during the plateau phase of the action potential, further slowing the decay of the Ca2+ transient. These changes can explain the defective Ca2+ transients of the failing human ventricular myocyte.

Action Potentials↗
PubMed2003

Gender differences in post-infarction hypertrophy in end-stage failing hearts.

OBJECTIVES: We explored whether there are gender differences in cardiac remodeling and whether etiology influences organ and cellular remodeling in advanced heart failure (HF). BACKGROUND: Several studies have shown a survival benefit for women compared to men with symptomatic HF. This observation may be related to gender differences in cardiac remodeling. METHODS: We studied hearts from 100 patients (72 men and 28 women) receiving cardiac transplantation at our institution. Cardiac morphology was assessed with echocardiography and direct measurement of cardiac mass. Cardiac myocyte volume, length, width, cross-sectional area, and contraction were measured using previously validated techniques. RESULTS: Among 50 patients with idiopathic cardiomyopathy (CM), we observed no gender-based differences in cardiac or cellular remodeling. In contrast, among 50 patients with ischemic cardiomyopathy (ICM), the heart weight index was significantly greater in men, and there was a strong trend toward an increased left ventricular (LV) mass index as well. These gender differences in cardiac and LV mass were paralleled by marked gender differences in myocyte volume, such that average myocyte volume was 36% greater in men than in women, in association with a 14% increase in resting cell length. CONCLUSIONS: Our studies demonstrate a multilevel gender difference in post-infarction remodeling, with women exhibiting reduced hypertrophy. Our studies further demonstrate that gender differences in cardiac remodeling in ICM are largely related to fundamental differences in cellular remodeling rather than simply differences in infarct size or expansion. Distinctions observed between ischemic and idiopathic CM suggest that gender may influence local myocardial responses to injury.

Cardiac Catheterization↗
PubMed2003

Differential effects of exercise training on skeletal muscle SERCA gene expression.

INTRODUCTION: Exercise training induces significant changes in the performance of skeletal muscle. PURPOSE: To determine whether changes in the gene expression of rat hind-limb muscle sarcoplasmic reticulum Ca2+ -ATPase isoforms (SERCA1a and SERCA2a) in response to either moderate- or high-intensity exercise training underlie the functional remodeling. METHODS: SERCA1a and SERCA2a isoform mRNA expression was determined in predominantly fast-twitch, gastrocnemius muscle and predominantly slow-twitch, soleus muscle with northern analysis. SERCA mRNA was normalized by the 18S rRNA measured in the same sample. RESULTS: Significant increases in the gastrocnemius SERCA2a mRNA expression were observed after both moderate- and high-intensity training. No significant change in SERCA1a expression was found under any conditions. CONCLUSION: These results indicate that both moderate and high intensity exercise increase the relative SERCA2a expression in the gastrocnemius.

Animals↗
PubMed2002

L-type Ca2+ channel density and regulation are altered in failing human ventricular myocytes and recover after support with mechanical assist devices.

Ca2+ influx through the L-type calcium channel (LTCC) induces Ca2+ release from the sarcoplasmic reticulum (SR) and maintains SR Ca2+ loading. Alterations in LTCC properties, their contribution to the blunted adrenergic responsiveness in failing hearts and their recovery after support with LV assist devices (LVAD) were studied. L-type Ca2+ current (I(Ca,L)) was measured under basal conditions and in the presence of isoproterenol (ISO), dibutyryl-cAMP (db-cAMP), Bay K 8644 (BayK), Okadaic acid (OA, a phosphatase inhibitor), and phosphatase 2A (PP2A) in nonfailing (NF), failing (F), and LVAD-supported human left ventricular myocytes (HVMs). Basal I(Ca,L) density was not different in the 3 groups but I(Ca,L) was activated at more negative voltages in F- and LVAD- versus NF-HVMs (V(0.5): -7.18+/-1.4 and -7.0+/-0.9 versus 0.46+/-1.1 mV). Both ISO and db-cAMP increased I(Ca,L) in NF- and LVAD- significantly more than in F-HVMs (NF >LVAD> F: ISO: 90+/-15% versus 77+/-19% versus 24+/-12%; db-cAMP: 235%>172%>90%). ISO caused a significant leftward shift of the I(Ca,L) activation curve in NF- and LVAD- but not in F-HVMs. After ISO and db-cAMP, the I(Ca,L) activation was not significantly different between groups. BayK also increased I(Ca,L) more in NF- (81+/-30%) and LVAD- (70+/-15%) than in F- (51+/-8%) HVMs. OA increased I(Ca, L) by 85.6% in NF-HVMs but had no effect in F-HVMs, while PP2A decreased I(Ca, L) in F-HVMs by 35% but had no effect in NF-HVMs. These results suggest that the density of LTCC is reduced in F-HVMs but basal I(Ca,L) density is maintained by increasing in LTCC phosphorylation.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed2002

Overexpression of sorcin, a calcium-binding protein, induces a low level of paclitaxel resistance in human ovarian and breast cancer cells.

Paclitaxel, an antimitotic, anticancer agent, induces cell cycle arrest in the mitotic phase by binding to the beta-tubulin subunit and forming highly stable microtubule polymers that resist depolymerization. The overexpression of the P-glycoprotein (P-gp) and/or alteration in the cellular microtubules is associated with the development of paclitaxel resistance. However, we have established a paclitaxel-resistant human ovarian carcinoma subline (2008/13/4) wherein the degree of resistance could not be correlated with overexpression of P-gp, alterations in the alpha- and beta-tubulin isotypes, or changes in the drug-binding affinity of the microtubules. mRNA differential display analysis revealed the overexpression of sorcin, a calcium-binding protein in the 2008/13/4 cells. However, no detectable changes in the intracellular calcium levels were detected in the parental and the paclitaxel-resistant variant. Furthermore, co-treatment with A23187, a calcium ionophore, did not alter the cytotoxicity of paclitaxel against the parental and the paclitaxel-resistant cells. Transfection of the parental 2008 cells with full-length sorcin cDNA induced a low level (3-5-fold) of paclitaxel resistance. In addition, transfection of human breast cancer cells with the full-length sorcin cDNA also led to the induction of a low level of paclitaxel resistance in the transfectants. Although the overexpression of sorcin did not produce high levels of paclitaxel resistance, the results obtained present compelling evidence of the involvement of sorcin in developing low-level paclitaxel resistance in a variety of tumor cells. The precise biochemical mechanism(s) by which sorcin overexpression induces low-level paclitaxel resistance is currently under investigation.

Antineoplastic Agents, Phytogenic↗
PubMed2002

L-type Ca(2+) currents overlapping threshold Na(+) currents: could they be responsible for the "slip-mode" phenomenon in cardiac myocytes?

Phosphorylation of Na channels has been suggested to increase their Ca permeability. Termed "slip-mode conductance" (SMC), this hypothesis predicts that Ca influx via protein kinase A (PKA)-modified Na channels can induce sarcoplasmic reticulum (SR) Ca release. We tested this hypothesis by determining if SR Ca release is graded with I(Na) in the presence of activated PKA (with Isoproterenol, ISO). V(m), I(m), and [Ca](i) were measured in feline (n=26) and failing human (n=19) ventricular myocytes. Voltage steps from -70 through -40 mV were used to grade I(Na). Na channel antagonists (tetrodotoxin), L-type Ca channel (I(Ca,L)) antagonists (nifedipine, cadmium, verapamil), and agonists (Bay K 8644, FPL 64176) were used to separate SMC from I(Ca,L). In the absence of ISO, I(Na) was associated with SR Ca release in human but not feline myocytes. After ISO, graded I(Na) was associated with small amounts of SR Ca release in feline myocytes and the magnitude of release increased in human myocytes. I(Na)-related SR Ca release was insensitive to tetrodotoxin (n=10) but was blocked by nifedipine (n=10) and cadmium (n=3). SR Ca release was induced over the same voltage range in the absence of ISO with Bay K 8644 and FPL 64176 (n=9). Positive voltage steps (to 0 mV) to fully activate Na channels (SMC) in the presence of ISO and Verapamil only caused SR Ca release when block of I(Ca,L) was incomplete. We conclude that PKA-mediated increases in I(Ca,L) and SR Ca loading can reproduce many of the experimental features of SMC.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed2002

Na(+)-Ca(2+) exchange current and submembrane [Ca(2+)] during the cardiac action potential.

Na(+)-Ca(2+) exchange (NCX) is crucial in the regulation of [Ca(2+)](i) and cardiac contractility, but key details of its dynamic function during the heartbeat are not known. In the present study, we assess how NCX current (I(NCX)) varies during a rabbit ventricular action potential (AP). First, we measured the steady-state voltage and [Ca(2+)](i) dependence of I(NCX) under conditions when [Ca(2+)](i) was heavily buffered. We then used this relationship to infer the submembrane [Ca(2+)](i) ([Ca(2+)](sm)) sensed by NCX during a normal AP and [Ca(2+)](i) transient (when the AP was interrupted to produce an I(NCX) tail current). The [Ca(2+)](i) dependence of I(NCX) at -90 mV allowed us to convert the peak inward I(NCX) tail currents to [Ca(2+)](sm). Peak [Ca(2+)](sm) measured via this technique was >3.2 micromol/L within < 32 ms of the AP upstroke (versus peak [Ca(2+)](i) of 1.1 micromol/L at 81 ms measured with the global Ca(2+) indicator indo-1). The voltage and [Ca(2+)](sm) dependence of I(NCX) allowed us to infer I(NCX) during the normal AP and Ca(2+) transient. The early rise in [Ca(2+)](sm) causes I(NCX) to be inward for the majority of the AP. Thus, little Ca(2+) influx via NCX is expected under physiological conditions, but this can differ among species and in pathophysiological conditions.

Action Potentials↗
PubMed2002

Modulation of contractility in failing human myocytes by reverse-mode Na/Ca exchange.

A decrease in the peak systolic [Ca](i) and slow decay of the Ca(i) transient are common features of the end-stage failing human ventricular myocyte and may underlie the contractile abnormalities observed in congestive heart failure. The role of the Na/Ca exchanger has been a great area of interest given the changes observed at the molecular level. Results from these experiments have been inconsistent, however, and therefore cellular-based experiments may be required to characterize the role of the Na/Ca exchanger in failing human myocardium. We review recent data that suggest an increased ability of the Na/Ca exchanger to transport Ca into the cytoplasm in failing human myocytes. We hypothesize that this increased Ca influx can explain the slowed decay and impaired relaxation of failing human ventricular myocytes.

Animals↗