Showing papers by "Donald M. Bers published in 1999"

Upregulation of Na(+)/Ca(2+) exchanger expression and function in an arrhythmogenic rabbit model of heart failure.

Abstract: Three-dimensional cardiac mapping in rabbits with nonischemic cardiomyopathy has shown that ventricular arrhythmias initiate by a nonreentrant mechanism that may be due to triggered activity from delayed afterdepolarizations. Delayed afterdepolarizations are thought to be due to spontaneous release of Ca(2+) from the sarcoplasmic reticulum (SR) and consequent activation of an inward Na(+)/Ca(2+) exchange (NaCaX) current. The goal of this study was to determine whether there is enhanced NaCaX gene expression and functional activity that may contribute to nonreentrant activation. Heart failure (HF) was induced in rabbits by combined aortic insufficiency and aortic constriction. HF rabbits had left ventricular enlargement (left ventricular end-diastolic dimension increased from 1.43+/-0.03 to 1.97+/-0.05 cm) and severely depressed function (fractional shortening reduced from 37% to 26%, P<0.02). Heart-to-body weight was increased by 79% in HF. Western blots showed a 93% increase in NaCaX protein in HF (P<0.04). NaCaX mRNA (7-kb transcript) was increased by 104% relative to the 18S rRNA in HF. A 14-kb NaCaX transcript was also seen in the HF rabbits, raising total NaCaX mRNA to 2.7-fold compared with controls. The amplitude of caffeine-induced contractures, used to assess SR Ca(2+) load, was not significantly different in HF. Relaxation and [Ca(2+)](i) decline during caffeine-induced contractures is attributable to Ca(2+) transport by NaCaX and was 61% and 45% faster in HF (P<0.05), respectively. NaCaX current measured under controlled voltage clamp conditions was also 2-fold higher in HF cells. SR Ca(2+)-ATPase mRNA and protein levels and Ca(2+) current density were not significantly altered in HF. Twitch amplitudes from HF myocytes were 26% smaller compared with control (P<0.02), but twitch relaxation and [Ca(2+)](i) decline (due largely to SR Ca(2+)-ATPase) were not altered. Thus myocytes and myocardium from HF rabbits exhibit enhanced NaCaX expression and function. The enhanced NaCaX activity may contribute to depressed contractions, increased transient inward current (for a given SR Ca(2+) release), delayed afterdepolarizations, and nonreentrant initiation of ventricular tachycardia in this arrhythmogenic model of HF.

Ca2+ Handling and Sarcoplasmic Reticulum Ca2+ Content in Isolated Failing and Nonfailing Human Myocardium

Abstract: —Disturbed sarcoplasmic reticulum (SR) Ca2+ content may underlie the altered force-frequency and postrest contractile behavior in failing human myocardium. We used rapid cooling contractures (RCCs) to assess SR Ca2+ content in ventricular muscle strips isolated from nonfailing and end-stage failing human hearts. With an increase in rest intervals (1 to 240 s; 37°C), nonfailing human myocardium (n=7) exhibited a parallel increase in postrest twitch force (at 240 s by 121±44%; P<0.05) and RCC amplitude (by 69±53%; P<0.05). In contrast, in failing myocardium (n=30), postrest twitch force decreased at long rest intervals and RCC amplitude declined monotonically with rest (by 25±9% and 53±9%, respectively; P<0.05). With an increase in stimulation frequencies (0.25 to 3 Hz), twitch force increased continuously in nonfailing human myocardium (n=7) by 71±17% (at 3 Hz; P<0.05) and RCC amplitude increased in parallel by 247±55% (P<0.05). In contrast, in failing myocardium (n=26), twitch force declined by 29...

Ca channels in cardiac myocytes: structure and function in Ca influx and intracellular Ca release

Abstract: There are Ca channels in the plasma membrane and also the sarcoplasmic reticulum (SR) membrane in cardiac myocytes. The relationship between channel structure, associated proteins and function of these Ca channels is discussed. The sarcolemmal Ca channels are crucial both to the basic cellular electrophysiological properties and control of cardiac contractility (via excitation-contraction coupling). The intracellular Ca release channels (or ryanodine receptors) respond to triggering events mediated by sarcolemmal ion currents and are largely responsible for releasing Ca which activates the myofilaments to produce contraction. Several possible mechanisms of excitation-contraction coupling are discussed. The Ca released from the SR can also feedback on several sarcolemmal ion currents and alter action potential configuration as well as contribute to arrhythmogenesis.

Subcellular [Ca2+]i gradients during excitation-contraction coupling in newborn rabbit ventricular myocytes.

Abstract: The central role of T-tubule and sarcoplasmic reticulum (SR) diadic junctions in excitation-contraction (EC) coupling in adult (AD) ventricular myocytes suggests that their absence in newborn (NB) cells may manifest as an altered EC coupling phenotype. We used confocal microscopy to compare fluo-3 [Ca2+]i transients in the subsarcolemmal space and cell center of field-stimulated NB and AD rabbit ventricular myocytes. Peak systolic [Ca2+]i occurred sooner and was higher in the subsarcolemmal space compared with the cell center in NB myocytes. In AD myocytes, [Ca2+]i rose and declined with similar profiles at the cell center and subsarcolemmal space. Disabling the SR (10 micromol/L thapsigargin) slowed the rate of rise and decline of Ca2+ in AD myocytes but did not alter Ca2+ transient kinetics in NB myocytes. In contrast to adults, localized SR Ca2+ release events ("Ca2+ sparks") occurred predominantly at the cell periphery of NB myocytes. Immunolabeling experiments demonstrated overlapping distributions of the Na(+)-Ca2+ exchanger and ryanodine receptors (RyR2) in AD myocytes. In contrast, RyR2s were spatially separated from the sarcolemma in NB myocytes. Confocal sarcolemmal imaging of di-8-ANEPPS-treated myocytes confirmed an extensive T-tubule network in AD cells, and that T-tubules are absent in NB myocytes. A mathematical model of subcellular Ca2+ dynamics predicts that Ca2+ flux via the Na(+)-Ca2+ exchanger during an action potential can account for the subsarcolemmal Ca2+ gradients in NB myocytes. Spatial separation of sarcolemmal Ca2+ entry from SR Ca2+ release channels may minimize the role of SR Ca2+ release during normal EC coupling in NB ventricular myocytes.

Ca(2+) influx through Ca(2+) channels in rabbit ventricular myocytes during action potential clamp: influence of temperature.

Abstract: Ca(2+) influx via Ca(2+) current (I(Ca)) during the action potential (AP) was determined at 25 degrees C and 35 degrees C in isolated rabbit ventricular myocytes using AP clamp. Contaminating currents through Na(+) and K(+) channels were eliminated by using Na(+)- and K(+)-free solutions, respectively. DIDS (0.2 mmol/L) was used to block Ca(2+)-activated chloride current (I(Cl(Ca))). When the sarcoplasmic reticulum (SR) was depleted of Ca(2+) by preexposure to 10 mmol/L caffeine, total Ca(2+) entry via I(Ca) during the AP was approximately 12 micromol/L cytosol (at both 25 degrees C and 35 degrees C). Similar Ca(2+) influx at 35 degrees C and 25 degrees C resulted from a combination of higher and faster peak I(Ca), offset by more rapid I(Ca) inactivation at 35 degrees C. During repeated AP clamps, the SR gradually fills with Ca(2+), and consequent SR Ca(2+) release accelerates I(Ca) inactivation during the AP. During APs and contractions in steady state, total Ca(2+) influx via I(Ca) was reduced by approximately 50% but was again unaltered by temperature (5.6+/-0.2 micromol/L cytosol at 25 degrees C, 6.0+/-0.2 micromol/L cytosol at 35 degrees C). Thus, SR Ca(2+) release is responsible for sufficient I(Ca) inactivation to cut total Ca(2+) influx in half. However, because of the kinetic differences in I(Ca), the amount of Ca(2+) influx during the first 10 ms, which presumably triggers SR Ca(2+) release, is much greater at 35 degrees C. I(Ca) during a first pulse, given just after the SR was emptied with caffeine, was subtracted from I(Ca) during each of 9 subsequent pulses, which loaded the SR. These difference currents reflect I(Ca) inactivation due to SR Ca(2+) release and thus indicate the time course of local [Ca(2+)] in the subsarcolemmal space near Ca(2+) channels produced by SR Ca(2+) release (eg, maximal at 20 ms after the AP activation at 35 degrees C). Furthermore, the rate of change of this difference current may reflect the rate of SR Ca(2+) release as sensed by L-type Ca(2+) channels. These results suggest that peak SR Ca(2+) release occurs within 2.5 or 5 ms of AP upstroke at 35 degrees C and 25 degrees C, respectively. I(Cl(Ca)) might also indicate local [Ca(2+)], and at 35 degrees C in the absence of DIDS (when I(Cl(Ca)) is prominent), peak I(Cl(Ca)) also occurred at a time comparable to the peak I(Ca) difference current. We conclude that SR Ca(2+) release decreases the Ca(2+) influx during the AP by approximately 50% (at both 25 degrees C and 35 degrees C) and that changes in I(Ca) (and I(Cl(Ca))), which depend on SR Ca(2+) release, provide information about local subsarcolemmal [Ca(2+)].