Studies of the sodium‐calcium exchanger in bull‐frog atrial myocytes.

Abstract

1. Experimental measurements and computer simulations have been used in attempts to identify an exchanger current (Iex) generated by an electrogenic Na+‐Ca2+ exchanger in single cells from bull‐frog atrium. 2. Voltage clamp measurements of an inward 'slow tail' current observed upon repolarization after depolarizing clamp pulses that elicit net inward Ca2+ currents (ICa) (see Campbell, Giles & Shibata, 1988c), show that these slow tails have a cationic dependence different from ICa. Slow tails are large and prominent in normal [Na+]o solutions containing either Ca2+ or Sr2+, but they are markedly reduced or absent in Ba2+, Ca2+‐free, and Na+‐free solutions. 3. Kinetic measurements on the slow tails show that they are not generated by deactivation of ICa, and suggest that they may be due to activation of Iex at negative potentials (‐70 to ‐100 mV). 4. Computer simulations of the influx, buffering, and extrusion of Ca2+ provide further indirect evidence that the slow tails correspond to Iex. In addition, these calculations give insights into one plausible mechanism of Ca2+ homeostasis in frog atrium. When the Na+‐Ca2+ exchanger formalism of Mullins (1979, 1981), as modified by DiFrancesco & Nobel (1985), is combined with equations for intracellular Ca2+ buffering by myoplasmic proteins (cf. Robertson, Johnson & Potter, 1981), slow inward tails are produced which are qualitatively similar to those recorded experimentally. 5. Comparisons of the size and time course of ICa with those of Iex suggest that Iex does not generate a physiologically significant current (or membrane potential change) during the plateau of the action potential. However, at potentials near the resting potential the inward current due to Iex may be significant. 6. Our theoretical results suggest that in the intact single atrial cell myoplasmic Ca2+‐binding proteins (e.g. calmodulin and troponin) could be physiologically important modulators of the amplitude, polarity and kinetics of Iex. Hence, the specificity, capacity and kinetics of intracellular Ca2+ binding are essential components of any quantitative treatment of Iex in excitable tissue.

A contraction‐related component of slow inward current in dog ventricular muscle and its relation to Na(+)‐Ca2+ exchange.

“…The delayed inwardly directed tail currents similar to the current investigated in this work have so far been identified in isolated cells of various species including rat (Mitchell et al 1984), guinea-pig (Fedida et al 1987;Beuckelmann & Wier, 1988), frog (Hume, 1987;Campbell et al 1988) and rabbit (Giles & Shimoni, 1989b). The relation between delayed tail current and contraction was studied by Fedida et al (1987) in guinea-pig ventricular myocytes.…”

“…This conclusion concerns both the currents measured during depolarization as well as the corresponding tail currents (Brown et al 1986;Fedida et al 1987;Hume, 1987;Campbell et al 1988;Giles & Shimoni 1989 a), and the transient inward current Iti (Mechmann & Pott, 1986;Brown et al 1986). …”

“…In Sr21 media Isic disappeared in response to depolarization similarly to the tail current after repolarization. On the other hand, in isolated atrial myocytes of the bullfrog (Campbell et al 1988) and in ventricular myocytes of the rabbit (Giles & Shimoni, 1989 a), the slow tail current was abolished by Ba2' substitution but remained unchanged after Sr2+ substitution. This discrepancy is so far difficult to explain.…”

“…These currents were recorded in response to the offset of depolarizing pulses as slow tail currents, clearly different from ICa deactivation (Mitchell, Powell, Terrar & Twist, 1984;Fedida, Noble, Shimoni & Spindler, 1987;Hume, 1987;Campbell, Giles, Robinson & Shibata, 1988;Giles & Shimoni, 1989a, b;Egan, Noble, Noble, Powell, Spindler & Twist, 1989), or in response to the onset of depolarizing pulses (Lee, Lee, Noble & Spindler, 1983;Brown, Noble, Noble & Taupignon, 1986;Boyett, Kirby & Orchard, 1988).…”

The giant cardiac membrane patch method: stimulation of outward Na(+)‐Ca2+ exchange current by MgATP.

“…The Na+-Ca2+ exchange process in heart has been extensively studied in sarcolemmal vesicles (for review, see Reeves & Philipson, 1989), as well as in intact tissue (Hilgemann, 1986) and in isolated cardiac myocytes using whole-cell voltage clamp (Kimura, Noma & Irisawa, 1986;Kimura, Miyamae & Noma, 1987;Fedida, Noble & Shimoni, 1987;Campbell, Giles & Robinson, 1988;Egan, Noble & Noble, 1989;Earm, Ho & So, 1989) and optical methods (Barcenas-Ruiz, Beuckelmann & Wier, 1987;Crespo, Grantham & Cannell, 1990). Many properties of the exchanger are consistent among these different experimental approaches; for example, rates of sodium-dependent calcium efflux during repolarization and the accompanying action potential changes were well explained by the current densities of the inward exchange current found in myocytes (Hilgemann & Noble, 1987).…”