What is the purpose of the large sarcolemmal calcium flux on each heartbeat?

Abstract

In cardiac muscle, although most of the calcium that activates contraction comes from the sarcoplasmic reticulum (SR), a significant fraction (up to 30%, depending on the species) enters from outside the cell and is then pumped out at the end of systole. Although some of this calcium influx is required to trigger calcium release from the SR, the bulk serves to reload the cell (and thence the SR) with calcium to replace the calcium that is pumped out of the cell. An alternative strategy would be for the heart to have a much smaller calcium influx balancing a smaller efflux. We demonstrate that this would result in a slowing of inotropic responses due to changes of SR calcium content. We conclude that the large sarcolemmal calcium fluxes facilitate rapid changes of contractility.

the calcium that activates cardiac contraction comes from two sources: 1) an entry across the surface membrane, largely via the L-type calcium current and 2) a release from the sarcoplasmic reticulum (SR). The relative proportions of these components vary between species, with rodents having about 10% entering the cell and 90% released from the SR and larger animals having up to 30% entering across the surface membrane (see Ref. 1 for a review). Calcium is released from the SR via the process of calcium-induced calcium release, and, therefore, calcium release from the SR requires calcium entry. However, as originally pointed out by Fabiato (4), the L-type calcium current can be thought of as playing two distinct roles in excitation-contraction coupling: 1) it triggers the release of calcium from the SR by opening the ryanodine receptors, and 2) the calcium entry loads the cell with calcium. The triggering function occurs within a few milliseconds and can therefore be thought of in terms of the peak amplitude of the calcium current with the remainder of the time course of the current being responsible for reloading the cell with calcium.

The loading function of the calcium current is required to balance the calcium pumped out of the cell during systole, largely on sodium-calcium exchange (NCX). As far as the SR calcium content is concerned, the trigger and loading functions have opposite effects: 1) an increase of trigger will increase the calcium release from the SR and thence the amount of calcium pumped out of the cell by NCX, thereby depleting the SR, and 2) an increase of the loading component will increase SR calcium content. It has been shown that the two effects balance each other, thereby allowing changes of calcium current to produce rapid changes of the calcium transient amplitude (6).

An unanswered question is, Why does the cell need such a large calcium influx and efflux? It would be more economical for the cell to have a smaller calcium influx and efflux on each beat. This is the situation in skeletal muscle where the calcium flux across the sarcolemma is only about 0.1 to 3% of that released from the SR (2, 8). The question, then, is, Why does the heart not have a much smaller calcium entry, perhaps achieved with a faster inactivating calcium current that could then be balanced with a less-active NCX, such that the fluxes across the surface membrane are considerably less?

One possible answer is that the L-type calcium current contributes to producing the long duration of the action potential. However, this does not seem to be a complete answer; presumably, a long action potential could be produced even with a faster inactivating calcium current simply by decreasing the outward currents. We would like to suggest that the reason for having a prolonged calcium influx is to facilitate the regulation of the size of the calcium transient.

Consider the inotropic effects of stimulating sarco(endo) plasmic reticulum calcium-ATPase (SERCA), as happens, for example, during β-adrenergic stimulation. Under control conditions, the calcium influx via the L-type calcium current will balance the calcium efflux (largely on NCX). When SERCA is stimulated, the calcium transient will decay more quickly, thereby decreasing the time available for NCX to pump calcium out of the cell, and the net result will be an increase of SR calcium content. It is important to note that the larger the sarcolemmal fluxes, the greater is the change of SR calcium content that can be produced on a single beat. Consider the case of the rabbit where perhaps 30% of the calcium comes across the surface membrane. For simplicity, we assume that 50 μmol of calcium per liter cell is released from the SR and 20 μmol enter from outside. In the steady-state, this 20 μmol/l must be pumped out of the cell on NCX. Halving the duration of the calcium transient will then result in only 10 μmol/l leaving, and the cell will therefore have gained 10 μmol/l calcium on one beat. This calcium will be pumped into the SR, thereby increasing its content by a significant fraction on one beat. Conversely, consider a case where only 0.3% comes across the sarcolemma (i.e., 70 from the SR and 0.2 μmol/l by calcium entry). Halving the duration of the calcium transient will now result in a gain by the cell of 0.1 μmol/l, and to obtain the same total change of SR calcium will require 100 times as many beats. In other words, a possible reason for the large calcium current is that this is balanced by a large NCX flux, therefore allowing rapid changes of SR calcium (Ca_SR).

This argument is made more formally in Fig. 1. The model is based on one published previously (3). Briefly, we assume that the amount of calcium released from the SR (Ca_rel) is given by Ca_rel = [Ca_SR]{[Ca_SR]/([Ca_SR] + K_d)}³. This steep dependence reflects the observed dependence of calcium release on SR calcium content (5, 7).

Fig. 1.

Simulation of the effects of increasing sarco(endo)plasmic reticulum calcium-ATPase (SERCA) activity in the presence of different amounts of sarcolemmal calcium fluxes. The number next to each of the traces is the integrated calcium influx through the L-type calcium channel (in arbitrary units). The amount of sodium-calcium exchange (NCX) was scaled proportionately so that the initial amplitude of the calcium transient was the same. Thus b, the fraction of calcium removed by NCX (as opposed to SERCA), was 0.5, 0.25, 0.12, and 0.05%. In other words the calcium removal from the cytoplasm varies from 50% via NCX (line 14) to only 5% via NCX (line 1.4). In the steady state, the fraction of calcium entering via the L-type calcium current, as opposed to being released from the SR, will also vary from 50% (line 14) to 5% (line 1.4). During the period shown by the “stimulate SERCA” bar, the SERCA activity was increased by a factor of 2, thus decreasing b to 0.25, 0.125, 0.062, and 0.025.

The amplitude of the calcium transient (Ca_sys) is given by the sum of the calcium released from the SR and that entering across the surface membrane (I_Ca). Ca_sys = Ca_rel + I_Ca. The amount of calcium pumped out of the cell (I_NCX) is given by I_NCX = b·Ca_sys, where b depends on the relative rates of NCX and SERCA. Correspondingly, the amount of calcium pumped back into the SR is (1 − b)Ca_sys.

The SR calcium content on the n + 1^th beat ([Ca_SR]_n+1) relates to that on the n^th beat: [Ca_SR]_n+1 = [Ca_SR]_n + I_Ca − I_NCX.

The specimen output of Fig. 1 shows the effect of simulating increasing SERCA by a factor of two for the period shown. This results in a predicted increase of the calcium transient due to an increase of SR calcium content. Simulations are shown for four different values of calcium current. On this model the change of calcium current does not affect the number of ryanodine receptors that open, and, therefore, these changes of calcium current are equivalent to simply changing the loading of the cell with calcium. The value of b has been scaled proportionately to keep the initial value of the systolic calcium transient comparable. It is clear that, as predicted by the qualitative argument above, the effects of increased SERCA develop more quickly the greater the sarcolemmal fluxes. We therefore suggest that one reason for the large sarcolemmal fluxes of calcium is that it allows for rapid changes in the inotropic state of the cell.

GRANTS

This work was supported by the British Heart Foundation.