Length-dependent changes in the contractility of cardiac muscle arise as a result of mechanisms that can alter intracellular ion concentrations. The contractile response of the heart to stretch is biphasic. Immediately following stretch there is an increase in the sensitivity of the myofilaments to calcium which changes the timecourse of the intracellular calcium transient. This is followed by a secondary slow force response where contractility increases alongside an increase in the amplitude of the calcium transient, and probably an increase in the intracellular sodium concentration.

Modulation of the intracellular concentration of ions such as sodium and calcium will have consequences for the activity of ion channels and electrogenic exchangers sensitive to these ions. This represents a means, additional to the direct activation of stretch-activated channels, for the mechanical modulation of electrical activity. In this review we will detail the experimental evidence for mechanical modulation of intracellular calcium and sodium, the mechanisms that are thought to cause these changes, and discuss the implications for electrical activity in the heart.

We will also discuss how this information must be viewed in the context of the electrical and mechanical inhomogeneity that exist within the various regions of the left ventricle, and the recent observations that stretch may provoke changes in intracellular ion concentrations within specific areas of single cardiac myocytes, such as the sub-sarcolemmal region.

Introduction

At the turn of the 20^th century, Frank and Starling identified what is perhaps the most famous response of the myocardium to mechanical stimulation – an increase in force production. Their work formed the basis of the Frank-Starling law of the heart, which states that the output of the heart is equal to, and determined by, the amount of blood flowing into the heart [29, 69]. The importance of this response is evident when one considers the increase in stroke volume that occurs during exercise, when there is increased ventricular filling and thus dilation, or stretch, of the ventricles. There is an immediate increased contractile response to increased diastolic filling, followed by another, slower increase in cardiac performance that leads to a decline in end-diastolic volume towards its initial value. This was first observed by von Anrep in 1912 [86]. Although von Anrep suggested that the effect was dependent on catecholamine release from the adrenal glands, it has since been established that the mechanism for the slow response is intrinsic to cardiac muscle.

In this chapter we will begin by describing that changes that occur at the cellular level during the biphasic response to stretch, with particular reference to the modulation of intracellular concentrations of Na⁺ ([Na⁺]_i) and Ca²⁺ ([Ca²⁺]_i). Our focus will be the secondary slow response, as this is one of our primary areas of research interest. Changes in intracellular Na⁺ and Ca²⁺ have implications not only for contractility, but also for the activity of ion channels and exchangers that are sensitive to these ions. We will discuss the consequences that stretch-induced changes in [Na⁺]_i and [Ca²⁺]_i have for the electrical activity of the heart, and describe the influence that mechanical and electrical inhomogeneity, a property intrinsic to normal hearts, may have on these feedback processes.

The rapid response to stretch

When cardiac muscle is stretched there is an immediate increase in both passive and active force. Active force is maximal at a length referred to as L_max which equates to a sarcomere spacing of 2.2 – 2.3 μm. Allen & Kurihara [3] have shown that the increase in force seen immediately after stretch is not associated with an increase in the magnitude of the [Ca²⁺]_i transient (see Fig. 1). Instead it is due to an increase in the sensitivity of the myofilaments for Ca²⁺, as demonstrated by a length-dependant leftward shift in the force-Ca²⁺ curve and a decrease in the [Ca²⁺] producing half-maximal activation (e.g. [37]). This increase in myofilament Ca²⁺ sensitivity is reflected in an increase in the affinity of troponin C (TnC) for Ca²⁺ [38], and evidence suggests that these changes are mediated by the stretch-dependent increase in force rather than length (for reviews see [2, 16]). This dependence on force rather than length reveals the central role of the force-producing cross-bridge in the Frank-Starling mechanism. In essence the length-tension relationship arises because of an increase in the probability of strong binding cross-bridge formation, and it is further enhanced by positive co-operativity whereby strong binding cross-bridges promote the formation of additional cross-bridges [60]. It has been suggested that the strong-binding cross-bridge actually increases the affinity of TnC for Ca²⁺ through protein-protein interactions within the thin filament [51].

Figure 1

Rapid changes in the time course of the [Ca²⁺]i transient by stretch A. [Ca²⁺]_i transient (upper traces, measured with aequorin) and active tension (lower traces) in an intact rat trabecula at 100% (L) and 81% (S) of L_max. Stretch increases tension; there (more...)

Several mechanisms are thought to contribute to the increased probability of cross-bridge formation with increased muscle length. When muscle is stretched, the degree of longitudinal thick and thin filament overlap changes and clearly this has implications for cross-bridge availability (see [33]). Another aspect of muscle geometry that changes with stretch is the lateral spacing of the myofilaments. Evidence has shown that a decrease in radial lattice spacing (which will increase the proximity of the myosin head to the actin filament) increases the sensitivity of the myofilaments to Ca²⁺ [31, 65]. Although there is no question that there is an inverse linear relationship between lattice spacing and Ca²⁺ sensitivity (see [32]), doubts have been raised as to its relevance in the physiological range of the length-tension relationship in cardiac muscle [51]. Another candidate mechanism for the length-dependent modulation of Ca²⁺ sensitivity in the heart involves the giant sarcomeric protein titin [18]. Titin forms a structural link between the thick myosin filament and the Z-line, and also binds the thin filament at the Z-line [84]. Although titin controls lattice spacing by exerting radial effects that bring the myofilaments closer together, titin strain also increases the disorder of the myosin heads within the thick filament; an effect which increases the probability of cross-bridge formation [19].

Whilst the increase in myofilament sensitivity to Ca²⁺ seen during the Frank Starling mechanism ensures that increased force is produced without a requirement for an increase in [Ca²⁺]_i transient amplitude, the change in TnC affinity for Ca²⁺ does have consequences for the kinetics of the transient. This has been demonstrated by simultaneous recording of the [Ca²⁺]_i transient and active force in rat trabeculae [3]. Figure 1 illustrates the abbreviation of the time course of the transient seen immediately following an increase in muscle length. Conversely when the preparation was allowed to shorten from a long length, the [Ca²⁺]_i transient was prolonged. These effects were thought to be caused by extra binding (during lengthening) or release (during shortening) of Ca²⁺ from the myofilaments as TnC affinity for Ca²⁺ changed.

The slow response to stretch

The slow contractile response to stretch (the Anrep effect) is present in the isolated heart [75], in isolated cardiac muscle [68], and in the single cardiac myocyte [89]. Thus the mechanism underlying the slow force response to stretch is intrinsic to the cardiac cell itself, although it may be modulated by paracrine influences of fibroblasts and endothelial cells in intact myocardium. In recent years it has been established that changes in both [Na⁺]_i and [Ca²⁺]_i play a vital role in the slow response, although the mechanisms that underlie these changes are still under debate.

Changes in intracellular ion concentrations during the slow response

Changes in [Ca²⁺]_i

By contrast to the rapid response, the slow increase in contractility upon stretch has been shown to be caused by a corresponding slow increase in the amplitude of the [Ca²⁺]_i transient. This has been demonstrated in intact ventricular muscle and in the stretched ventricular myocyte [3, 17] (see Fig. 2).

Figure 2

Slow changes in [Ca²⁺]i and force following stretch in intact ventricular muscle and the single ventricular myocyte. A - [Ca²⁺]_i (measured using aequorin) and force in a feline papillary muscle held at 82% L_max and 100% L_max. Upper traces show slow timebase recordings, (more...)

In atrial muscle, the [Ca²⁺]_i transient also increases during the slow response, although the time course of this is more rapid than in ventricular muscle [82]. Pivotally, it has been shown that changes in [Ca²⁺]_i quantitatively account for the magnitude of the slow response, with no further time-dependent changes in myofilament sensitivity occurring after the rapid response [46]. Consistent with this, no change in the time course of contraction or [Ca²⁺]_i is observed during the slow response to stretch in muscle or cells [40, 39]. Changes in diastolic [Ca²⁺]_i during the slow response appear to be species-dependent. Increased diastolic [Ca²⁺]_i has been seen following stretch in muscle and myocytes from the guinea pig [78, 89], but not the rat [4, 17, 39, 46].

Changes in [Na⁺]_i

In intact cardiac muscle from the rat and cat, increased global [Na⁺]_i has been seen on a similar time scale to slow changes in contractility after stretch [4, 70]. However, for a number of years it was considered that the slow response seen at the level of the myocyte was Na⁺-independent, as no corresponding increase in [Na⁺]_i was detected in single rat cardiac myocytes [39]. At the time, this differential effect of stretch on [Na⁺]_i in the two preparations was reconciled by the idea that paracrine signalling was responsible for modulation of [Na⁺]_i in muscle (see Section 3.3). However, using sensitive detection methods with high spatial resolution (sodium-green fluorescence and electron probe microanalysis), [Na⁺]_i has been shown to increase at 2–4 min following stretch in single murine ventricular myocytes [41]. The observed increase in [Na⁺]_i was spatially heterogenous, with most Na⁺ hotspots localised close to the surface membrane. It is possible that increases in sub-sarcolemmal [Na⁺]_i following stretch may not be detected in single cells as a change in bulk cytosolic [Na⁺]_i.

Mechanisms underlying the slow response

The temporal and spatial changes in [Ca²⁺]_i and [Na⁺]_i during the slow response may give some clue as to the mechanisms which are responsible. Early work focused on the mechanisms that increase [Ca²⁺]_i; more recently those that increase [Na⁺]_i have also been considered.

Ca²⁺ and Na⁺ entry

I_Ca,L

In the cardiac cell, the major source of Ca²⁺ influx is via the L-type Ca²⁺ current (I_Ca,L). However, there is no evidence that I_Ca,L contributes to the slow response. In cardiac muscle, Ca²⁺ channel antagonists do not abolish the slow response [21, 88], and in axially stretched single myocytes, no increase in I_Ca,L has been recorded [10, 39, 43, 76]. It has not proved possible to correlate directly changes in I_Ca,L with contractility during the slow response [39] (see Section 4).

I_SAC

Another potential source of Ca²⁺ entry during the slow response is via the non-selective cationic stretch-activated channel (SAC). Na⁺ entry via SACs could also increase [Ca²⁺]_i indirectly via the Na⁺-Ca²⁺ exchanger (NCX) (see Section 4). The identity of the SAC channel in vertebrates is unknown but it has recently been suggested that it may be the transient receptor potential channel 1 (TRPC-1) [63]. Non-selective cationic SACs show little time-dependent inactivation and have a high conductance (e.g. [43]), therefore maintained stretch could allow sufficient influx of Ca²⁺ or Na⁺ to account for the slow response. Using a mathematical model of the atrial myocyte, Tavi et al. [82] have shown that the slow increase in [Ca²⁺]_i seen upon stretch of rat atrial muscle can be mimicked by introducing a non-selective cationic SAC conductance in parallel with increased TnC affinity for Ca²⁺. Stretch of human atrial myocytes produces a current consistent with I_SAC [44]. In single channel recordings from rat atrial myocytes, however, only K⁺-selective SACs have been seen; clearly K⁺-selective SACs would have very different implications for electrical activity and intracellular ion concentrations to the non-selective cationic SAC. The K⁺-selective SACs have been equated with the mechanosensitive twin pore domain channel TREK 1 [66] (see [20] for a review).

Using ventricular tissue, several studies have relied on streptomycin as a blocker of non-selective cationic SACs (see [9, 10]) to determine the role of these channels in the response to stretch. In murine myocytes, streptomycin abolishes the inward current seen following stretch, as well as the peripheral increase in [Na⁺]_i detected using electron probe microanalysis [49]. Some have found streptomycin to be effective in attenuating the slow force response; others have not. In cardiac muscle and single myocytes from the rat, we have shown that the magnitude of the slow response is significantly reduced by streptomycin ([17], see Fig. 3). Others, however, have found streptomycin to be without significant effect on the slow response in rat and failing human cardiac muscle [56, 88].

Figure 3

The stretch activated channel (SAC) contributes to the slow response to stretch in papillary muscle and ventricular myocytes from the rat. A. Streptomycin (STREP; 80 μM) significantly reduces the amplitude of the slow force response in muscles (more...)

As changes in diastolic [Ca²⁺]_i are not seen in rat cardiac muscle during the slow response (Section 3.1), the implication is that increased Ca²⁺ loading takes place during systole in the rat. Inhibition of reverse-mode NCX has been shown to attenuate the magnitude of the slow force response to stretch in other species (cat, rabbit and failing human myocardium; [70, 87, 88]. A possible scenario that could explain the temporal characteristics of the slow response (see [11]) is that Ca²⁺ entry occurs secondary to Na⁺ loading through SACs. Although both ions can permeate the channel, data from human atrial and guinea pig ventricular myocytes suggest that the permeability is less for Ca²⁺ than Na⁺ [41, 44].

Na⁺-H⁺ exchanger

A body of evidence suggests another source of increased [Na⁺]_i during the slow response, besides I_SAC. This is via the Na⁺-H⁺ exchanger (NHE). Several groups have shown that NHE inhibition reduces the magnitude of the slow response [17, 70, 87]. We recently demonstrated for the first time that NHE inhibition is effective in both multicellular preparations and in the single cardiac myocyte from the rat [17].

In stretched murine myocytes, global increments in [Ca²⁺]_i (measured by electron probe microanalysis during diastole) were reduced by the NHE inhibitor cariporide [50]. However, these data are not consistent with cariporide acting through NHE inhibition, as increased peripheral [Na⁺]_i (which should act as the stimulus for raised [Ca²⁺]_i ) was not sensitive to cariporide [50].

It is possible that both SACs and the NHE play a role in the slow response, and that the balance of these two mechanisms depends on species and/or the stretch stimulus. Han et al. [35] have shown, using computer modelling, that activation of non-selective cationic SACs and stimulation of NHE have qualitatively similar effects on Ca²⁺ handling in cardiac cells.

Ca²⁺ release

In heart muscle the sarcoplasmic reticulum (SR) is the major site of [Ca²⁺]_i release yet, interestingly, most evidence suggests that Ca²⁺ release from the SR does not underlie the slow response. Inhibition of SR Ca²⁺ release does not change the relative magnitude of the slow response in rat, rabbit or ferret papillary muscle [11, 39, 45, 46], although it has been shown to reduce the slow response in the intact canine heart and failing human myocardium [88]. However, under conditions when a functional SR is not obligatory for the slow response, the SR does appear to be involved in the handling of the extra [Ca²⁺]_i [11, 46].

In 2001, Vila Petroff et al. [85] proposed a novel mechanism for the slow response which assigned the SR a primary role in this phenomenon. Experiments were performed with rat ventricular myocytes stretched within an agarose gel, and rat trabeculae. These workers proposed that the slow increase in [Ca²⁺]_i transient following stretch was due to activation of the phosphatidyl inositol-3-OH kinase (Ptd-Ins-3-OH kinase) -Akt - endothelial nitric oxide synthase (eNOS) axis which increases ryanodine receptor (RyR) sensitivity via s-nitrosylation. However, in our hands, the slow response is not attenuated by Ptd-Ins-3-OH kinase inhibition in cells or muscle from the rat heart [17], and several studies have shown no reduction of slow response magnitude when NOS is inhibited in muscle from rabbits, rats and patients with heart failure [17, 87, 88]. A small reduction in slow response magnitude with NOS inhibition has been reported by Bardswell & Kentish [7] in rat trabeculae, one of the preparations used by Vila-Petroff et al. [85].

It has proved difficult to reconcile the data of Vila-Petroff et al. with the existing views of the slow response. This is due in part to the large body of evidence that fails to demonstrate a pivotal role of the SR in the slow response. However, another concern is that enhanced RyR activity only results in transient changes in [Ca²⁺]_i and contraction [27]. Thus for the temporal characteristics of the slow response to be fulfilled, there would have to be a concurrent increase in SR Ca²⁺ loading to compensate for increased SR Ca²⁺ release.

The effect of changes in ion concentration on electrical activity

In previous sections we have described how changes in TnC affinity for Ca²⁺ immediately following stretch can modulate the time course of the [Ca²⁺]_i transient, and how the slow response is associated with an increase in the amplitude of the [Ca²⁺]_i transient and the level of [Na⁺]_i. Modulation of [Ca²⁺]_i and [Na⁺]_i can influence the electrical activity of the heart through effects upon dependant channels and exchangers. This can be via effects on ion equilibrium potentials, or through activation (e.g. of I_K [83]) or inactivation (e.g. of I_CaL [58]) of sensitive channels. The study by Du Bell et al [26] shows the impact of [Ca²⁺]_i on electrical activity by describing the modulation of action potential configuration by the amplitude and time course of the [Ca²⁺]_i transient.

Of the channels and exchangers (other than SACs) that have been implicated in the contractile response of cardiac muscle to stretch, the NCX has perhaps the major role. The exchanger can work in forward (Ca²⁺ extrusion) or reverse (Ca²⁺ influx) mode. It is generally accepted that the stoichiometry of NCX is 3:1 (Na⁺:Ca²⁺) (e.g. [72]), therefore the exchanger is electrogenic. The direction of Na⁺, and hence current, flow depends on the reversal potential of the exchanger (E_NaCa = 3E_Na - 2E_Ca) and the membrane potential (E_m). When E_m is negative to E_NaCa, Ca²⁺ is extruded and an inward depolarising current is generated. As both [Ca²⁺]_i and membrane potential change appreciably during the action potential, it is clear that the direction and amplitude of the NCX current will also be subject to change.

Lab et al. [55] showed that the prolongation of the [Ca²⁺]_i transient in response to a quick release from long length was mirrored by a prolongation of the action potential duration (APD). The effect was most prominent when the release was late, rather than early, in the action potential. The explanation offered by the authors was that Ca²⁺, suddenly released from the myofilaments, as a result of decreased TnC affinity, accumulated in the cytoplasm and was extruded from the cell via NCX, generating an inward current. This interpretation was consistent with the observation that the increase in [Ca²⁺]_i preceded the effect on membrane potential. Computer modelling [42] also supported this explanation of the experimental observation. When considering these findings it is worth noting that the change in length that provoked this alteration in the [Ca²⁺]_i transient and action potential duration was quite large, (`isotonic' shortening from L_max). Other modelling studies have described how stretch-induced changes in myofilament Ca²⁺ sensitivity can modulate the [Ca²⁺]_i transient and thereby the profile of the action potential [48]. Experimentally, further evidence that Ca²⁺ may influence the electrical response to stretch has been provided by Han et al. [34] who showed that stretch prolongation of APD in rat atrial preparations was abolished by reducing [Ca²⁺]_i transient magnitude by SR inhibition.

However, some evidence argues against a powerful role for myofilament modulation of Ca²⁺ binding in the electrical response to stretch. It was observed [30, 36] that, in the canine heart, changing from isovolumic to an ejecting mode of contraction (i.e. allowing a greater degree of shortening and subsequent re-lengthening) depressed the late depolarisations of the ventricular monophasic action potential. These studies suggest that muscle shortening associated with ejection causes the inactivation of depolarising cationic SACs rather than the activation of depolarising Ca²⁺-dependant currents.

One technical difficulty that confounds the assessment of the effect of Ca²⁺ on stretch-activated electrical events is the use of Ca²⁺ buffers in the recording pipettes of whole-cell patch-clamp studies. These buffers are often required to stabilise recordings under the difficult conditions imposed by stretching. We attempted to circumvent this problem by using the carbon fibre technique to stretch single guinea pig ventricular myocytes whilst simultaneously recording electrical activity using microelectrodes (Fig. 4, and [10]). One carbon fibre was used to protect the site of the microelectrode impalement. The use of sharp microelectrodes avoided the necessity for Ca²⁺ buffers in the electrode solution, however, as the resistance of these electrodes is too high to use the conventional continuous voltage clamp technique, discontinuous `switch' voltage clamp was used.

Figure 4

Simultaneous stretching of a guinea pig ventricular myocyte and recording of electrical activity. The microelectrode is used to record membrane potential or currents. It is placed behind the stiff fibre to protect the site of impalement during stretch. (more...)

Recordings were made within 2 min of stretch and we observed that stretch prolonged the APD. The increase in APD with stretch was abolished by application of the cationic SAC blocker streptomycin at 40 μM. Although streptomycin can reduce other currents such as I_Ca,L at higher concentrations [9], we have shown that 50 μM streptomycin does not alter the APD or [Ca²⁺]_i transients of unstretched myocytes [10]. The effect of streptomycin on stretch-induced prolongation of the action potential suggests a role for SACs in the response to stretch. Indeed under action potential clamp (where the cell is voltage clamped with its own action potential waveform [25], stretch evoked a streptomycin-sensitive current consistent with the activation of non-selective cationic SACs (Fig. 5).

Figure 5

Stretch-activated membrane currents in guinea pig ventricular myocytes recorded by action potential clamp. A. Mean current–voltage relationship for the stretch-activated current from 10 cells, having a current of greater than 5 pA at +30 mV. Stretch increased (more...)

However the effect of stretch on the action potential was also abolished by exposure to 5 μM BAPTA-AM, a cell permeant Ca²⁺ buffer that reduces the [Ca²⁺]_i transient to such an extent that contraction is abolished (Fig. 6).

Figure 6

Comparison of the effects of stretch on the action potential of guinea pig ventricular myocytes in the presence and absence of streptomycin or intracellular calcium buffering with BAPTA. The upper panel shows representative experimental records of action (more...)

These data lead us to conclude that both SACs and [Ca²⁺]_i are important for the response of single myocytes to stretch. Consistent with a role for [Ca²⁺]i in the effects of stretch on the action potential, in studies with single guinea pig ventricular myocytes at 37°C, the consequences of stretch are dependent upon the level of [Ca²⁺]_i buffering used: with 5 mM EGTA, an APD shortening with stretch [61]; with 10 μM EGTA, early shortening and late lengthening of the APD [43]; with no exogenous buffering we saw APD lengthening converted to a non-significant shortening with 5μM BAPTA-AM [10].

However, it is difficult to characterise the effects of stretch across preparations and species, and stretch-dependent mechanisms are still proving elusive. For example, Sung et al. [81] showed a stretch-induced lengthening of the epicardial APD in rabbit Langendorff-perfused hearts, measured by optical mapping with the voltage sensitive dye di-4-ANEPPS. Interestingly, in order to reduce motion errors associated with this technique, the cross-bridge uncoupler BDM was used. BDM has been shown to abolish the release of extra Ca²⁺ associated with the shortening-induced fall in myofilament Ca²⁺ sensitivity [53], suggesting that the influence of this mechanism in the responses seen in [81] was small. Sung et al. also showed that the effect of load on the APD was resistant to streptomycin. These data are quite different to those obtained in stretched myocytes in which APD prolongation was seen but was sensitive to both [Ca²⁺]_i manipulation and streptomycin [10].

The increase in the amplitude of the [Ca²⁺]_i transient that occurs during the slow increase in force might influence electrical activity. In a study performed in feline papillary muscles, Allen [1] observed that stretch caused initial APD shortening, followed by a slow lengthening of APD. Tavi et al. [82] also report a slow increase in pressure and a lengthening of the APD with sustained atrial stretch. Action potential prolongation could allow extra Ca²⁺ to enter via I_Ca,L due to delayed voltage-dependent inactivation. In support of a role for APD in the slow response, Hongo et al. [39] failed to see a slow contractile response to stretch in single rat myocytes that were voltage clamped (and subjected to a fixed duration of depolarisation).

However, there are arguments against a role for increased APD in the slow force response to stretch. Von Lewinski et al. [88] failed to see APD prolongation in rabbit ventricular muscle after stretch. A secondary increase in I_Ca,L during the maintained plateau of the action potential as a mechanism for the slow response is not consistent with the lack of effect of Ca²⁺ channel antagonists in cardiac muscle (see Section 3.2.1). Furthermore, if [Na⁺]_i influx is the trigger for the secondary slow increase in [Ca²⁺]_i, as evidence suggests (Section 3.2.1), the electrical consequences of this will be a shortening of APD due to activation of outward (Na⁺-extruding) NCX current.

The influence of mechanical and electrical inhomogeneity

Sub-cellular inhomogeneities

When considering the effects of ion concentrations upon sarcolemmal ion channels and exchangers, it is now apparent that it is the sub-sarcolemmal concentration of these ions that will influence membrane current and exchanger activity. The concentration of Ca²⁺ and Na⁺ in the sub-sarcolemmal `fuzzy-space' is different to that in the cytosolic compartment [57]. Furthermore, there is evidence that stretch can produce local increases in [Na⁺]_i and [Ca²⁺]_i, for example, sub-sarcolemmal [Ca²⁺]_i in chick cardiac myocytes in response to gentle prodding [77] and peripheral [Na⁺]_i in stretched murine ventricular myocytes [41, 49, 50]. By implication, [Ca²⁺]_i increases in the region of the ryanodine receptor (as evidenced by changes in Ca²⁺ spark properties) in rat ventricular myocytes stretched within a gel matrix [85].

Within the myocyte itself, there is increasing evidence that channels and exchangers are not uniformly distributed in the sarcolemma, but are grouped in specific regions such as the t-tubules [23]. The effect that stretch has on t-tubules is not well understood, and the stimulus presented to them by axial stretch may be different to that in the surface sarcolemma, because of their orientation and their association with the sarcomeric Z-lines [52]. However the observation that atrial myocytes, which lack a highly developed tubular system, respond both electrically and mechanically in a similar way to ventricular myocytes may be an indication that the t-tubule is not a major player in the slow response (although the t-tubule may increase the efficiency of stretch-dependent NCX activation, see Section 3.3).

In line with this hypothesis, we have observed that de-tubulation of ventricular myocytes (using formamide, [15]) has little effect on the magnitude of the response to mechanical stimulation by osmotic swelling, although it does slow the time-course of the response ([62], Fig. 7). The conclusion to this study is that de-tubulation reduces the surface area for water influx, but that the channels responsible for regulating the volume response are not exclusively located in the t-tubules.

Figure 7

Changes in cell volume (mean ± S.E.M) relative to pre-swollen volume in response to hypo-osmotic solution (HYPO) in control (•) and detubulated (○) myocytes. Hypo-osmotic solution (191 ± 1 mosM) was iso-ionic with the control (more...)

Conclusions and perspectives

Mechanical stimulation has many consequences for the heart. In this chapter we have described changes in [Na⁺]_i and [Ca²⁺]_i, in electrical activity, and in force that occur following myocardial stretch. Mechanical stretch can have `direct' effects on electrical activity via stretch-sensitive ion channels, mediated through increased bilayer tension or by cytoskeletal tethering. However, there is a more subtle aspect to mechano-electric feedback which is associated with the length-dependent modulation of [Na⁺]_i and [Ca²⁺]_i. Changes in the intracellular concentration of these ions can regulate the activity of certain ion channels and electrogenic exchangers, with consequences for electrical activity. In order to understand the full implications of these effects, the mechanisms associated with the rapid and slow increase in force, and the degree, timing and cellular location of the changes in ion concentrations that they cause, need to be better understood. Of equal importance is an understanding of why diverse experimental findings are reported from apparently similar studies. At the heart of the problem may be our current inability to measure accurately the stimuli associated with myocardial stretch.

Acknowledgements

The authors thank the British Heart Foundation for funding our work.

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