Showing papers by "David A. Eisner published in 1996"

Variability of Spontaneous Ca2+ Release Between Different Rat Ventricular Myocytes Is Correlated with Na+-Ca2+ Exchange and [Na+]i

Abstract: We have studied the factors responsible for the variation of the frequency of "waves" caused by spontaneous Ca2+ release in rat ventricular myocytes. The experiments were performed in isolated myocytes using the fluorescent indicators Indo-1 (to measure [Ca2+]i) and SBFI (to measure [Na+]i). After electrical stimulation (either with action potentials or voltage-clamp pulses), some cells showed spontaneous Ca2+ release. The frequency of this release, where present, was variable. The Ca2+ content of the sarcoplasmic reticulum (SR) was measured by applying caffeine (10 mmol/L). The resulting increase of [Ca2+]i activated the electrogenic Na(+)-Ca2+ exchange, and the integral of this current was used to estimate the Ca2+ content of the SR. The SR Ca2+ content was significantly higher in cells that oscillated at high rates ( > 10 . min-1) than in those that were quiescent. The rate of removal of Ca2+ from the cytoplasm by non-SR mechanisms was measured by adding caffeine (10 mmol/L) and measuring the rate constant of decay of the resulting increase of [Ca2+]i. Cells that had a high rate constant of decay of [Ca2+]i had a low frequency of oscillations. Measurements of [Na+]i showed a positive correlation between the frequency of spontaneous SR Ca2+ release and [Na+]i. After cessation of stimulation, there was a gradual decrease of [Na+]i, which was correlated with a parallel decrease of the frequency of oscillation rate. We conclude that the variability of frequency of spontaneous SR Ca2+ release is due to variations of the rate of Ca2+ removal from the cell, which are probably due to Na(+)-Ca2+ exchange. The variability of Na(+)- Ca2+ exchange rate, in turn, is likely to result from variations of [Na+]i.

The sarcolemmal mechanisms involved in the control of diastolic intracellular calcium in isolated rat cardiac trabeculae

Abstract: We performed experiments using the calcium indicator Indo-1 to determine the relative roles of the sarcolemmal mechanisms involved in the regulation of diastolic intracellular calcium concentration ([Ca2+]i) in trabeculae from the rat heart. Ryanodine was used to eliminate sarcoplasmic reticulum (SR) function. In the functional absence of the SR, 76.8 ± 3.9% of the calcium was extruded by the Na-Ca exchange carrier in the [Ca2+]i range of diastolic concentration ± 200–400 nM. This was assessed by measuring the recovery of [Ca2+]i from small perturbations in the presence and absence of extracellular sodium. The steady-state relationship between [Ca2+]o and [Ca2+]i was linear over the range of 1–40 mM, a 20-fold increase of [Ca2+]o produced a 1.97-fold ± 0.13-fold increase in [Ca2+]i (n = 5). In the absence of extracellular sodium raising [Ca2+]o had a variable effect. In some preparations there was little change of [Ca2+]i while in others the response was almost as large as in control conditions. We conclude that the NaCa exchanger contributes ≈ 77% of sarcolemmal calcium extrusion following small perturbations in [Ca2+]i and that this fraction does not diminish as the [Ca2+]i declines. In addition we have shown a sodium-independent entry of calcium into quiescent cardiac muscle under resting conditions.

A sideways look at sparks, quarks, puffs and blips.

Abstract: An important advance in our understanding of excitation-contraction coupling was provided by the demonstration of calcium sparks attributed to the release of calcium ions from single sarcoplasmic reticulum (SR) release sites (Cheng, Lederer & Cannell, 1993). Subsequent work has shown that these sparks summate to produce the normal systolic Ca2P transient in cardiac muscle (L6pez-Lopez, Shacklock, Balke & Wier, 1994). Sparks have also been found in both skeletal and smooth muscle (Nelson et al. 1995; Klein, Cheng, Santana, Jiang, Lederer & Schneider, 1996). In cardiac and skeletal muscle the sparks activate contraction whereas in smooth muscle they appear preferentially to activate Ca2+-activated K+ currents and thereby promote relaxation (Nelson et al. 1995). In smooth muscle the sparks may be the explanation for the spontaneous transient outward currents previously observed (Benham & Bolton, 1986). The sparks do not appear to be restricted to muscle, or indeed to systems using the ryanodine receptor channel (RyR), as closely related phenomena (puffs) have been found in other cells such as the Xei7 opus oocyte where Ca2+ release occurs through the InsP3 receptor channel (Yao, Choi & Parker, 1995). These puffs may underlie the spontaneous outward currents seen in 2ultured neurones (Satin & Adams, 1987).

Comparison of “Near Membrane” and Bulk Cytoplasmic Calcium Concentration in Single Cardiac Ventricular Myocytes During Spontaneous Calcium Waves

Abstract: In this paper we discuss differences in the time course of changes of intracellular calcium concentration ([Ca2+]i) occurring in the bulk cytoplasm and adjacent to the surface membrane (subsarcolemmal or ‘fuzzy’ space) during spontaneous oscillatory release of Ca2+ from the sarcoplasmic reticulum (SR) Sarcolemmal Na-Ca exchange current and [Ca2+]i were measured in single voltage clamped rat ventricular myocytes

Intracellular pH Is Insensitive to Changes in Intracellular Calcium Concentration in Isolated Rat Ventricular Myocytesa

Abstract: Studies using multicellular preparations of cardiac muscle have found a close relationship between the control of intracellular calcium ([Caz+Ii) and pH (pHi)l such that an increase in [CaztIi causes an intracellular acidifi~ation.~,~ Two hypotheses have been proposed for this acidosis, i) displacement of Hf from intracellular buffers by Ca2+ and ii) accumulation of lactate influencing pHi. We have investigated this further in single cells by measuring pHi using the fluorescent indicator SNARF and increasing [Ca2+Ii by a) electrical stimulation, b) increased extracellular calcium concentration ([Ca?'],) and c) increased [CaZ'], plus 50 pM ouabain, an inhibitor of the membrane NaiK pump, which causes an increase in [Na+li resulting in an increased [Ca2+Ii via the action of Na-Ca e ~ c h a n g e . ~ Transient increases in [Ca2'Ii produced by electrical stimulation did not produce any detectable change of pHi. Ouabain caused an acidification, presumably by increased [Na+Ii inhibiting Na-H exchange. The additional application of 10 rnM Ca2+ in the presence of ouabain caused no further intracellular acidification. Measurements of [Ca2+Ii using indo-l showed that ouabain and high [Cat+], caused an increase similar in magnitude to peak systolic [Ca2+Ii which was considerably greater than that produced by ouabain alone. We have also measured [Ca2+li while changing pHi by the extracellular application of the weak base NH4C1. On increasing pHi from 7.15 to 7.20 no change in [Ca2+Ii was observed. Preliminary experiments in which [Cali and pHi were measured simultaneously using the above indicators support these findings (FIG. 1). The fact that elevating [Ca2'Ii decreases pHi in multicellular but not single cell preparations could be explained if the acidification is due to extracellular accumulation of lactate influencing pHi in multicellular preparations, whereas in single cells any lactate produced may diffuse away freely. Previous studies in which the application of cyanide produces an acidification in whole heart but not in isolated cells support this hyp~ thes i s .~