Biomedical subjects

S E Howlett

Publications and source records attributed to S E Howlett.

At least 37 records · Page 2Linked to original sources

PubMed1995

Calcium currents in ventricular myocytes of prehypertrophic cardiomyopathic hamsters.

Possible changes in Ca2+ currents (ICa), which might contribute to Ca2+ overload in young (70- to 100-day-old) cardiomyopathic (CM) hamster hearts, were determined in isolated ventricular myocytes with conventional recording and discontinuous single-electrode voltage-clamp techniques. Action potential duration at 90% repolarization (APD90) was significantly longer in CM myocytes compared with normal cells (APD90 = 120.3 +/- 4.5 vs. 98.2 +/- 5.9 ms, P < 0.01). Input resistance, membrane time constant, and membrane capacitance were similar in normal and CM myocytes. Current-voltage (I-V) relations for peak ICa were depressed in CM cells compared with normal cells; this difference was statistically significant at the peak of the I-V curve. Activation and inactivation relations for ICa and recovery from inactivation were similar in myocytes from normal and CM hearts. These changes occurred in myocytes from young CM animals before development of heart failure. Results indicate that increased Ca2+ influx through L-type Ca2+ channels does not account for the longer APD in cardiomyopathy and is not involved in the development of Ca2+ overload in cardiomyopathy.

Action Potentials↗

PubMed1995

Effects of adenosine in simulated ischemia and reperfusion in guinea pig ventricular myocytes.

Effects of adenosine (ADN) on cardiac cellular electrical and contractile activity were determined during ischemia and reperfusion. Electrical activity was recorded with conventional and voltage-clamp techniques. Contractions were monitored with a video edge detector. Myocytes were exposed to simulated ischemia (20 min), in the presence or absence of ADN (1-50 microM), and reperfused with Tyrode solution. ADN had no effects under control conditions. However, action potential abbreviation during ischemia was greater in the presence of ADN than for control, and recovery was delayed. In ischemia, Ca2+ current declined equally, and contractions were abolished in control and ADN-treated myocytes. In early reperfusion, oscillatory afterpotentials (OAP), transient inward current (ITI) and aftercontractions appeared, and contractions increased above preischemic levels. ADN abolished contractile overshoot and reduced incidence of OAP, ITI, and aftercontractions from 78 to 37.5%. The effects of exogenous ADN were inhibited by ADN A1-receptor blockade. Inhibition of endogenous ADN by 8-phenyltheophylline only increased incidence of ITI. Thus exogenous ADN in ischemia may protect the myocardium in reperfusion via A1 receptors.

Adenosine↗

PubMed1995

Saralasin suppresses arrhythmias in an isolated guinea pig ventricular free wall model of simulated ischemia and reperfusion.

The effects of saralasin on electrophysiological changes and arrhythmias induced by simulated ischemia and reperfusion were examined in an isolated tissue model. Segments of guinea pig right ventricles, stimulated regularly, were exposed to simulated ischemia for 15 min and then were reperfused with normal Tyrode's solution for 30 min. Transmembrane electrical activity and a high-gain electrogram were recorded. Arrhythmias and electrophysiological changes accompanying simulated ischemia and reperfusion in control preparations were compared to those in preparations treated with 0.1 or 1 microM saralasin. Simulated ischemia caused abbreviation of action potential duration measured at 90% repolarization, abbreviation of endocardial effective refractory period (ERP) and prolongation of transmural conduction time. Premature ventricular beats, ventricular tachycardia and conduction block were observed in approximately 35% of control preparations during simulated ischemia. Rapid sustained or nonsustained ventricular tachycardia occurred in approximately 60% of control preparations in early reperfusion. The overall incidence of arrhythmias and the incidence of ventricular tachycardia in early reperfusion were significantly decreased by 1 microM but not 0.1 microM saralasin. Saralasin (1 microM) prolonged the ERP in normoxic tissues, but it did not alter changes induced by ischemia or reperfusion in ERP or the action potential duration at 90% repolarization. Prolongation of transmural conduction time during ischemia and early reperfusion was significantly inhibited by both concentrations of saralasin. However, only 1 microM saralasin reduced the ratio of transmural conduction time to ERP enough to prevent arrhythmias. Our observations demonstrate that saralasin exerts antiarrhythmic effects in myocardial reperfusion by a mechanism independent of circulatory and central actions.

Action Potentials↗

PubMed1995

Molecular geometries of dibenzothiazepinone and dibenzoxazepinone calcium antagonists.

A number of dibenzothiazepinones and dibenzoxazepinones have been designed, synthesized and evaluated as calcium antagonists. Molecular geometries of these dibenzotricyclic calcium antagonists have been studied using X-ray crystallography, molecular modeling and two-dimensional NMR spectroscopy. X-Ray diffraction reveals dibenzothiazepinone 1 and dibenzoxazepinone 2 to have, respectively, flexure angles of 108 degrees and 116.9 degrees between the two benzene rings. The molecular mechanics-optimized geometry of dibenzothiazepinone 1 shows a 7 degrees smaller flexure angle than the X-ray crystallographic result, while that of dibenzoxazepinone 2 has an angle only 2 degrees smaller than the X-ray result. AM1 and ab initio calculations show that the side chains can affect the geometry of the tricyclic nucleus and both 1 and 2 have negative electrostatic potentials around the bridged portion of the tricyclics. Two-dimensional NOESY NMR spectroscopy supports the extended geometry of the 6 carbon spacer as obtained from X-ray crystallography and molecular mechanics calculations. Vasorelaxation properties among these compounds appear to be relatively insensitive to the flexure angle and to chain length. Vasorelaxation is profoundly influenced by the nature of the basic terminal moiety.

Animals↗

PubMed1994

Density of ryanodine receptors is increased in sarcoplasmic reticulum from prehypertrophic cardiomyopathic hamster heart.

Calcium overload has been linked to the development of cardiomyopathy in the cardiomyopathic (CM) hamster, but the site or sites of the lesion remain obscure. To determine whether the number of sarcoplasmic reticulum (SR) calcium release channels (ryanodine receptors) changes in the CM heart, we compared the density (Bmax) and affinity (Kd) of [3H]-ryanodine binding sites in heavy SR fractions from 40-65 day-old normal and CM hamster hearts. Results showed that the Bmax was significantly increased in CM heart when compared to normal (Bmax = 2489 +/- 159 fmol/mg protein in normal heart and 3360 +/- 223 fmol/mg protein in CM heart, mean +/- S.E., P = 0.01). [3H]-Ryanodine bound to a single, high affinity site in SR from both normal and CM hearts; values for Kd were similar in both groups. Sensitivity of [3H]-ryanodine binding to Ca2+ was unchanged, but the density of binding was increased at all Ca2+ concentrations which potentiated binding in CM heart. Similarly, potentiation of [3H]ryanodine binding by ATP and inhibition of binding by Mg2+ were intact in membranes from CM heart. Results demonstrate that the density [3H]-ryanodine receptors is increased in SR from CM hearts early in the development of cardiomyopathy, although the properties of these receptors are unchanged. This suggests an increase in the amount or velocity of Ca2+ release from SR may contribute to the development of Ca2+ overload in this model of cardiomyopathy.

Age Factors↗

PubMed1994

Simulated ischaemia and reperfusion in isolated guinea pig ventricular myocytes.

OBJECTIVE: The objectives were (1) to develop a cellular model of simulated ischaemia and reperfusion in isolated ventricular myocytes; (2) to determine effects of simulated ischaemia and reperfusion on calcium current (ICa), transient inward current (ITI) and contraction; and (3) to determine whether pharmacological agents which alter intracellular sodium and calcium loading affect signs of calcium overload in reperfusion in this model. METHODS: Electrical activity was recorded with conventional and voltage clamp techniques. Cell shortening was measured with a video edge detector. Myocytes were equilibrated in Tyrode solution, exposed to simulated ischaemia (hypoxia, acidosis, lactate, hyperkalaemia, glucose-free) for 20 min, and reperfused with Tyrode solution. RESULTS: Ischaemia depolarised myocytes [-89(SEM 1) to -67(4) mV, p < 0.05], abbreviated action potential duration [APD90, 257(14) to 188(12) ms, p < 0.05], and abolished contractions. Contractions elicited by voltage clamp steps also were abolished in ischaemia; however, ICa decreased by only 51% [-0.98(0.08) to -0.50(0.06) nA, p < 0.05]. Signs of calcium overload, including aftercontractions, oscillatory afterpotentials, and ITI, occurred in 69% of myocytes in reperfusion. Upon reperfusion, both APD90 and ICa recovered slowly; however, contractions returned quickly and temporarily exceeded control. Amiloride during ischaemia and reperfusion lowered incidence of ITI in reperfusion, whereas nifedipine and lignocaine had no effect on ITI. CONCLUSIONS: This model of ischaemia and reperfusion in ventricular myocytes shows many features of multicellular preparations, such as membrane depolarisation and action potential duration shortening during ischaemia, and appearance of oscillatory afterpotentials upon reperfusion. Inhibition of contraction during ischaemia and recovery of contraction in reperfusion are independent of changes in APD90 or ICa. Induction of aftercontractions, oscillatory afterpotentials, and ITI in reperfusion is associated with reduced peak ICa. Amiloride most probably decreased signs of calcium overload in early reperfusion by inhibiting sodium loading via Na+/H+ exchange. Additionally, amiloride may inhibit ITI directly by blocking Na+/Ca2+ exchange.

Action Potentials↗

PubMed1993

Dibenzothiazepinones as potential calcium channel antagonists. I.

Certain dibenzo[a,d]cycloheptenes and dibenzo[b,e]thiepins (e.g., 1 and 2) substituted in the 5 and 11-positions, respectively, are potent calcium channel antagonists. It has previously been shown that potency increases with a decrease in the angle between the planes of the flanking aromatic rings, as computed by semi-empirical AM1 calculations with geometry minimization. In the present study, we evaluated a series of dibenzo[b,f]-1,4-thiazepinones (10-14) as potential bioisosteres of these compounds having a slightly more acute angle of flexure. X-Ray diffraction reveals the crystal structure of the parent dibenzo[b,f]-1,4-thiazepinone 4 to have a flexure angle of 108.4 degrees. Due to electron delocalization, the amide moiety in the seven-membered ring adopts a planar conformation. The synthesis of a series of 10-[omega-[4-(4-fluorophenyl)piperazin-1-yl]alkyl]dibenzo[b,f]-1,4 - thiazepin-11(10H)-ones (10-14), and results of their evaluation for calcium channel antagonistic activity on hamster aorta, are presented.

Animals↗

PubMed1992

Synthesis, characterization, and Ca2+ antagonistic activity of diltiazem metabolites.

Diltiazem is a calcium antagonist widely used in the treatment of angina and hypertension. The contributions of metabolites of diltiazem to the vasorelaxant effects of diltiazem were investigated. The synthesis and spectroscopic characterization of eight major cis-diltiazem metabolites are described. Three of the compounds--N, O-didemethylated metabolite (21), O-demethylated metabolite (22), and diltiazem N-oxide (27)--have been recently reported and have not previously been synthesized. The identities of all eight synthetic metabolites have been verified with samples obtained from human urine using combined LC-MS/MS. The Ca2+ antagonistic activities of diltiazem and its metabolites (except 27) were studied on hamster aorta preparations depolarized with KCl. The order of potencies (IC50 +/- SE, microM) is as follows: diltiazem (0.98 +/- 0.47) greater than 17 (2.46 +/- 0.38) greater than or equal to 23 (3.27 +/- 1.02) greater than 26 (20.2 +/- 10.5) greater than 22 (40.4 +/- 15.4) greater than or equal to 25 (45.5 +/- 18.1) greater than 21 (112.2 +/- 33.2) greater than or equal to 24 (126.7 +/- 24.2). Structure-activity relationships are also discussed.

Animals↗

PubMed1992

Density of 1,4-dihydropyridine receptors decreases in the hearts of aging hamsters.

To determine whether density and properties of myocardial voltage-sensitive calcium channels may change in the hearts of aged animals, we have compared specific binding of [3H]-nitrendipine ([3H]-NTP) to 1,4-dihydropyridine (DHP) receptors in ventricular homogenate and membrane fractions prepared from the hearts of adult hamsters of different ages. In crude homogenate preparations from young adult animals (70 days) the density (Bmax) of [3H]-NTP binding was 82 +/- 6 fmol/mg protein (n = 5 hearts). By contrast, Bmax values for 400-, 500- and 600-day-old animals were 54 +/- 5, 59 +/- 11 and 50 +/- 2 fmol/mg protein, respectively (significantly lower than control, P = 0.004, n = 4-5 hearts/group). The affinity (Kd) of [3H]-NTP binding was not affected by increasing age. Similar results were obtained in a partially purified membrane fraction. Allosteric modulation of [3H]-NTP binding by the phenylalkylamine, verapamil and the benzothiazepine, diltiazem were preserved in aging heart. These studies demonstrate that the density of myocardial DHP receptors declines in the aged hamster, although the properties of these receptors are unchanged. This suggests that a decrease in voltage-sensitive calcium channels may be associated with normal myocardial aging.

Aging↗

PubMed1992

Unblock of the slow inward current induces the arrhythmogenic transient inward current in isolated guinea-pig myocytes.

This study investigated whether abrupt changes in extracellular Ca2+ concentration or washout of the Ca2+ antagonists Mn2+ or verapamil, could induce transient inward current (ITI) in enzymatically disaggregated guinea-pig myocytes. Single electrode voltage-clamp techniques were used. ITI was elicited upon repolarization to various voltage steps from an activating step to +20 mV. The holding potential was -80 mV. Slow inward current (ICa) was induced by steps to -10 mV. Continuous exposure to either 2.5 or 6.0 mM Ca2+ did not induce ITI; however, following exposure of cells to 0.5 mM Ca2+ for 20 min which decreased ICa, return to 2.5 or 6.0 mM Ca2+ induced ITI. ITI could be observed for 10 to 20 min following sudden elevations of Ca2+. Similar effects also were seen when Ca2+ was increased from 2.5 to 6.0 mM. Exposure to 2.0 mM Mn2+ or 2.0 microM verapamil blocked ICa. Washout of either blocker induced ITI, particularly in 6.0 mM Ca2+. Peak ITI occurred upon repolarization at c. -70 mV; a reversal potential could not be demonstrated. Thus, abrupt changes in Ca2+ influx, produced either by sudden changes in external Ca2+ or by washout of Ca2+ antagonists, induced ITI with characteristics similar to those described for ITI induced by toxic concentrations of cardiac glycosides.

Animals↗

PubMed1992

The impact of pacemaker implantation on cognitive functioning in elderly patients.

OBJECTIVE: To describe and quantify the impact of pacemaker implantation on cognitive functioning in the elderly. DESIGN: Prospective case-control, non-randomized trial. Data were collected from clinical and family interviews and from a psychological test battery. SETTING: Pacemaker clinic in a tertiary care hospital. PARTICIPANTS: Nineteen elderly (65+ years) patients undergoing new or replacement pacemaker implantation for dysrhythmias and volunteer controls matched for age, sex, and short Mental Status Questionnaire test results, without dysrhythmia or intervention. MAIN OUTCOME MEASURES: Subjective and clinical impressions based on family interviews; results of psychological test battery before and 6-12 months after pacemaker implantation. RESULTS: Prior to pacemaker implantation, three patients met DSM-III criteria for dementia and two for delirium. Paced patients demonstrated deficiency in immediate memory, language, memory for less structured information, and learning of abstract materials. These deficits were due primarily to the poor performance of patients with complete heart block. Despite clinical and subjective impressions of improvement, there was no change in psychologic test performance subsequent to pacemaker implantation. CONCLUSIONS: Impaired cognitive functioning is not always clinically apparent but appears common in patients with cardiac dysrhythmias; it is not altered 6-12 months after pacemaker implantation.

Aged↗

PubMed1991

Force-interval relation in normal and cardiomyopathic hamster atria.

The purpose of this study was to determine how cardiomyopathy affects the beat-to-beat regulation of contractile force in cardiac muscle. Isometric force produced by left atria from 80- to 85-day-old normal and cardiomyopathic (CM) hamsters was measured in vitro at 29 degrees C in 2.5 and 6.0 mM Ca2+. During steady-state stimulation at 1 Hz, single test stimuli were interpolated at varying test intervals (0.3-600 s). The force-interval curves were fitted with an equation using five parameters to define the curve and were compared under different conditions; the recovery of force after long rest intervals was fitted with a single exponential curve. Results showed that the force-interval curves were similar in normal and CM atria except that force was depressed at all intervals in 2.5 mM external Ca2+ concentration ([Ca2+]e) and that the parameter U(0), reflecting force produced at short test intervals, tended to be lower in CM muscles. At high [Ca2+]e (6.0 mM) the force-interval curves were similar, but recovery of steady-state force after long test intervals was much slower in CM atria (tau = 77.3 +/- 8.5 s, n = 11) than in normal atria (tau = 30.5 +/- 3.9 s, n = 11). Recovery was also slower at 2.5 mM [Ca2+]e. These findings suggest that, on a beat-to-beat basis, there is less Ca2+ available in intracellular compartments in the CM heart.

Animals↗

PubMed1991

Possible mechanisms underlying differences in force production between normal and cardiomyopathic hamster atria.

The rate of recovery of force after a long rest interval is lower than normal in left atria from 80- to 85-day-old cardiomyopathic (CM) golden Syrian hamsters. To determine whether this difference was due to a reduced amount of Ca2+ available for release with each beat, we manipulated the amount of Ca2+ entering excised atria using the Ca2+ agonist BAY K 8644 and the antagonist nifedipine. We also simulated altered Ca2+ influx in a recent model of cardiac excitation-contraction coupling (V. J. A. Schouten, J. K. Van Deen, P. de Tombe, and A. A. Verveen Biophys. J. 51: 13-26, 1987) by varying the parameter representing Ca2+ influx and observing the effect on the force it predicted. Steady-state force of normal and CM atria was recorded in response to 1-Hz stimulation and recovery of steady-state force was monitored after a 600-s rest interval. Ca2+ fluxes were manipulated by raising external Ca2+ or by the presence and absence of drug. The recovery of force after a 600-s rest interval was digitized and fitted with an exponential function, and the time constant and steady-state force to which the muscle recovered after the pause were compared. Inclusion of the Ca2+ agonist BAY K 8644 (0.25 or 2.0 microM) made the response of CM atria similar to that of normal, while inclusion of the Ca2+ antagonist nifedipine (0.8 microM) made the response of normal atria similar to that of the CM. Similarly, decreasing the simulated Ca2+ influx in the model produced all of the differences observed between normal and CM muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

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

PubMed1990

[3H]-nitrendipine binding in normal and cardiomyopathic hamster hearts: modulation by temperature, verapamil and diltiazem.

The characteristics of high affinity dihydropyridine binding sites were compared in normal and cardiomyopathic hamster hearts to probe for possible defects in the calcium channel which could lead to calcium overload and, in turn, to the muscle necrosis characteristic of cardiomyopathy. Kinetic studies of the temperature dependence of [3H]-nitrendipine binding to ventricular homogenates from 60-day-old normal and cardiomyopathic hamsters showed that, in normal hamsters, the rate of dissociation (0.049 +/- 0.006/min at 25 degrees C) was highly temperature-dependent (Q10 = 4.40 +/- 0.69) and that neither the rate nor the temperature dependence was influenced by disease. The rate of association (1.12 +/- 0.11/min/nM at 25 degrees C) was weakly temperature-dependent (Q10 = 1.25 +/- 0.04) and similarly unaffected by disease. The rate of dissociation of [3H]-nitrendipine was increased by verapamil and decreased by diltiazem with little effect on the association rate. Allosteric interactions of diltiazem and verapamil with the dihydropyridine receptor were identical in normal and cardiomyopathic hearts and, together with the normal temperature sensitivity, show that there is no abnormality at the related binding sites for nitrendipine, verapamil and diltiazem in the calcium channel of the cardiomyopathic heart.

Allosteric Regulation↗

PubMed1988

Radioligand binding to muscle homogenates to quantify receptor and ion channel numbers.

Quantitation of the number of receptor or ion channel proteins in muscle that may be changed as a result of disease, development, or experimental manipulation can be achieved by radioligand binding assays. The problem of apparent specific binding to filters, which severely distorts these assays, is described. Results show that when techniques are applied to minimize both high nonspecific binding and spurious specific binding to filters, equilibrium and nonequilibrium binding assays can be effectively used to measure binding site densities in muscle homogenates. As no sites are lost during homogenate preparation, changes in binding site density that are not apparent when normalized per mg protein are revealed by normalizing the number of sites either per muscle or by muscle fiber diameter. Thus, the resolution of problems inherent in homogenate binding allows the use of these preparations to compare the plasticity and control of ion channels and receptors under a wide variety of experimental conditions.

Animals↗

PubMed1988

Responsiveness of normal and dystrophic avian muscle to acetylcholine, carbamylcholine and d-tubocurarine.

1. The responsiveness of dystrophic avian muscle to acetylcholine may be altered due to reported elevated acetylcholinesterase activity. 2. To test this hypothesis, the responsiveness of normal and dystrophic muscle in vivo to intra-arterial injection of acetylcholine, carbamylcholine and d-tubocurarine was compared. 3. Results showed that dystrophic muscle was less responsive to acetylcholine, more responsive d-tubocurarine and equally responsive to carbamylcholine when compared to normal suggesting enhanced acetylcholine hydrolysis occurs in vivo in dystrophic avian muscle.

Acetylcholine↗

PubMed1988

[3H]-nitrendipine binding sites in normal and cardiomyopathic hamsters: absence of a selective increase in putative calcium channels in cardiomyopathic hearts.

The number of putative calcium channels in cardiac muscle from young adult hamsters (60 days old) was compared in normal (F1B) hamsters and two different mutant strains (CHF 146 and Bio 14.6) which express cardiomyopathy and muscular dystrophy. Equilibrium binding assays of high affinity sites for [3H]-nitrendipine in ventricular homogenate preparations showed that the maximum number of [3H]-nitrendipine binding sites (Bmax), which corresponds to the number of putative calcium channels, was not significantly different in normal and cardiomyopathic hearts: 79(SEM 9), 64(14) and 69(10) fmol.mg-1 protein in 4-6 hearts from F1B, Bio 14.6 and CHF 146 hamster strains, respectively. Similar results were obtained with binding data after partial purification of the preparation. These data are in agreement with earlier studies comparing two normal strains (CHF 148 and random bred Syrian hamsters) with cardiomyopathic (CHF 146) hamsters, and conflict with other studies comparing normal and cardiomyopathic hamsters. Comparisons with the conflicting data suggest (a) that change in the number of high affinity [3H]-nitrendipine binding sites is not responsible for calcium overload and cell necrosis in cardiomyopathy, and (b) that increased numbers of low affinity [3H]-nitrendipine binding sites may emerge in cardiomyopathic hearts.

Animals↗

PubMed1987

Calcium channels in normal and dystrophic hamster cardiac muscle. [3H]nitrendipine binding studies.

Progressive cardiac cell necrosis in the dystrophic hamster may be related to intracellular calcium overload, particularly as necrosis is prevented by treatment with calcium channel antagonists. Calcium overload could arise as a consequence of an imbalance in calcium influx, efflux and/or sequestration. The possibility that increased numbers of calcium channels in myopathic cells leads to excessive calcium influx has been studied by assaying the number of [3H]nitrendipine [( 3H]NTP) binding sites in cardiac muscle preparations. Crude homogenate and partially-purified ventricular muscle preparations from 60-day-old normal and genetically dystrophic hamsters were compared in this study. The results of equilibrium binding studies showed that, in both crude and partially-purified membrane preparations, the affinity and the maximum number of [3H]NTP binding sites in normal muscle were not significantly different from those measured in dystrophic muscle. For the homogenate preparation, the KD values were 0.07 +/- 0.01 and 0.08 +/- 0.01 nM for normal and dystrophic tissues, respectively, and the Bmax values were 62 +/- 6 and 73 +/- 6 fmol/mg protein for normal and dystrophic preparations respectively. These data show that a simple increase in the number of [3H]NTP binding sites is unlikely to account for calcium overload in the cardiomyopathic hamster.

Animals↗