Cardiac Mineralocorticoid Receptor and the Na+/H+ Exchanger: Spilling the Beans

Current evidence reveals that cardiac mineralocorticoid receptor (MR) activation following myocardial stretch plays an important physiological role in adapting developed force to sudden changes in hemodynamic conditions. Its underlying mechanism involves a previously unknown nongenomic effect of the MR that triggers redox-mediated Na⁺/H⁺ exchanger (NHE1) activation, intracellular Na⁺ accumulation, and a consequent increase in Ca²⁺ transient amplitude through reverse Na⁺/Ca²⁺ exchange. However, clinical evidence assigns a detrimental role to MR activation in the pathogenesis of severe cardiac diseases such as congestive heart failure. This mini review is meant to present and briefly discuss some recent discoveries about locally triggered cardiac MR signals with the objective of shedding some light on its physiological but potentially pathological consequences in the heart.

Mechanosignaling in the vasculature: emerging concepts in sensing, transduction and physiological responses

Cells are constantly exposed to mechanical forces that play a role in modulating cellular structure and function. The cardiovascular system experiences physical forces in the form of shear stress and stretch associated with blood flow and contraction, respectively. These forces are sensed by endothelial cells and cardiomyocytes and lead to responses that control vascular and cardiac homeostasis. This was highlighted at the Pan American Physiological Society meeting at Iguassu Falls, Brazil, in a symposium titled "Mechanosignaling in the Vasculature." This symposium presented recent research that showed the existence of a vital link between mechanosensing and downstream redox sensitive signaling cascades. This link helps to transduce and transmit the physical force into an observable physiological response. The speakers showcased how mechanosensors such as ion channels, membrane receptor kinases, adhesion molecules, and other cellular components transduce the force via redox signals (such as reactive oxygen species and nitric oxide) to receptors (transcription factors, growth factors, etc.). Receptor activated pathways then lead to cellular responses including cellular proliferation, contraction, and remodeling. These responses have major relevance to the physiology and pathophysiology of various cardiovascular diseases. Thus an understanding of the complex series of events, from the initial sensing through the final response, is essential for progress in this field. Overall, this symposium addressed some important emerging concepts in the field of mechanosignaling and the eventual pathophysiological responses.

Mitochondrial reactive oxygen species (ROS) as signaling molecules of intracellular pathways triggered by the cardiac renin-angiotensin II-aldosterone system (RAAS)

Mitochondria represent major sources of basal reactive oxygen species (ROS) production of the cardiomyocyte. The role of ROS as signaling molecules that mediate different intracellular pathways has gained increasing interest among physiologists in the last years. In our lab, we have been studying the participation of mitochondrial ROS in the intracellular pathways triggered by the renin-angiotensin II-aldosterone system (RAAS) in the myocardium during the past few years. We have demonstrated that acute activation of cardiac RAAS induces mitochondrial ATP-dependent potassium channel (mitoK_ATP) opening with the consequent enhanced production of mitochondrial ROS. These oxidant molecules, in turn, activate membrane transporters, as sodium/hydrogen exchanger (NHE-1) and sodium/bicarbonate cotransporter (NBC) via the stimulation of the ROS-sensitive MAPK cascade. The stimulation of such effectors leads to an increase in cardiac contractility. In addition, it is feasible to suggest that a sustained enhanced production of mitochondrial ROS induced by chronic cardiac RAAS, and hence, chronic NHE-1 and NBC stimulation, would also result in the development of cardiac hypertrophy.

Echocardiographic assessment of left ventricular midwall mechanics in spontaneously hypertensive rats

AIMS

The present study was attempted to determine whether LV midwall mechanics yielded different conclusions about LV systolic function than the assessment of endocardial LV mechanics by echocardiography in spontaneously hypertensive rats (SHR).

METHODS AND RESULTS

Thirty-six (18 Wistar normotensive (W), 18 [SHR]) anesthetized rats were studied with two-dimensional directed M-mode echocardiogram to analyze LV structure (LV diameter, left ventricular wall thickness and LV mass [LVM]) and LV function (endocardial shortening [ES] and midwall shortening [MS]). Measurements were made from three consecutive cardiac cycles on the M-mode tracings. There was no significant difference in LV dimension. LVM was higher in SHR (SHR: 595 +/- 111 mg, W: 413 +/- 83 mg--p < 0.01). ES was higher in SHR (SHR: 64.1 +/- 6%, w: 58.2 +/- 4%--p < 0.01), whereas no significant difference was found in MS (SHR: 24 +/- 4%, W: 27.6 +/- 4%--ns). Twelve of 18 (66%) SHR showed endocardial shortening higher than normally predicted, compared with 3/18 (16%) with observed enhanced MS (p < 0.01).

CONCLUSION

These results suggest that the analysis of midwall mechanics by echo allows us to better understand the LV performance in SHR and that the exaggerated endocardial motion could not represent a really supernormal systolic performance.

An autocrine/paracrine mechanism triggered by myocardial stretch induces changes in contractility

An autocrine/paracrine mechanism is triggered by stretching the myocardium. This mechanism involves release of angiotensin II, release/increased formation of endothelin, activation of the Na(+)/H(+) exchanger, increase in intracellular Na(+), and the increase in the Ca(2+) transient that underlies the slow force response to stretch. The autocrine/paracrine mechanism could explain how changes in afterload alter cardiac contractility.

Reverse mode of the Na+-Ca2+ exchange after myocardial stretch: underlying mechanism of the slow force response

This study was designed to gain additional insight into the mechanism of the slow force response (SFR) to stretch of cardiac muscle. SFR and changes in intracellular Na(+) concentration ([Na(+)](i)) were assessed in cat papillary muscles stretched from 92% to approximately 98% of L(max). The SFR was 120+/-0.6% (n=5) of the rapid initial phase and coincided with an increase in [Na(+)](i). The SFR was markedly depressed by Na(+)-H(+) exchanger inhibition, AT(1) receptor blockade, nonselective endothelin-receptor blockade and selective ET(A)-receptor blockade, extracellular Na(+) removal, and inhibition of the reverse mode of the Na(+)-Ca(2+) exchange by KB-R7943. KB-R7943 prevented the SFR but not the increase in [Na(+)](i). Inhibition of endothelin-converting enzyme activity by phosphoramidon suppressed both the SFR and the increase in [Na(+)](i). The SFR and the increase in [Na(+)](i) after stretch were both present in muscles with their endothelium (vascular and endocardial) made functionally inactive by Triton X-100. In these muscles, phosphoramidon also suppressed the SFR and the increase in [Na(+)](i). The data provide evidence that the last step of the autocrine-paracrine mechanism leading to the SFR to stretch is Ca(2+) entry through the reverse mode of Na(+)-Ca(2+) exchange.

Mechanisms underlying the increase in force and Ca(2+) transient that follow stretch of cardiac muscle: a possible explanation of the Anrep effect

Myocardial stretch produces an increase in developed force (DF) that occurs in two phases: the first (rapidly occurring) is generally attributed to an increase in myofilament calcium responsiveness and the second (gradually developing) to an increase in [Ca(2+)](i). Rat ventricular trabeculae were stretched from approximately 88% to approximately 98% of L(max), and the second force phase was analyzed. Intracellular pH, [Na(+)](i), and Ca(2+) transients were measured by epifluorescence with BCECF-AM, SBFI-AM, and fura-2, respectively. After stretch, DF increased by 1.94+/-0.2 g/mm(2) (P<0.01, n = 4), with the second phase accounting for 28+/-2% of the total increase (P<0.001, n = 4). During this phase, SBFI(340/380) ratio increased from 0.73+/-0.01 to 0.76+/-0.01 (P<0.05, n = 5) with an estimated [Na(+)](i) rise of approximately 6 mmol/L. [Ca(2+)](i) transient, expressed as fura-2(340/380) ratio, increased by 9.2+/-3.6% (P<0.05, n = 5). The increase in [Na(+)](i) was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA). The second phase in force and the increases in [Na(+)](i) and [Ca(2+)](i) transient were blunted by AT(1) or ET(A) blockade. Our data indicate that the second force phase and the increase in [Ca(2+)](i) transient after stretch result from activation of the Na(+)/H(+) exchanger (NHE) increasing [Na(+)](i) and leading to a secondary increase in [Ca(2+)](i) transient. This reflects an autocrine-paracrine mechanism whereby stretch triggers the release of angiotensin II, which in turn releases endothelin and activates the NHE through ET(A) receptors.

Preservation of myofilament calcium responsiveness underlies protection against myocardial stunning by ischemic preconditioning

OBJECTIVE

Whereas diminution of infarct size by ischemic preconditioning (IP) is well-accepted, protection against stunning is controversial. Since stunning is characterized by decreased myofilament Ca2+ responsiveness, we investigated whether IP would preserve myofilament responsiveness in a model of stunning.

METHODS

Rat hearts were retrogradely perfused with Krebs-Henseleit (K-H) solution for 20 min and then subjected to 20 min of no-flow global ischemia, followed by 20 min of reperfusion in the absence (stunning) or in the presence (IP) of a previous 5-min period of ischemia followed by 15 min of reperfusion. A group of hearts perfused under non-ischemic conditions served as control. Thin ventricular trabeculae were dissected from each of the experimental groups and loaded with fura-2 to measure intracellular calcium concentration ([Ca2+]i) and developed force.

RESULTS

After 20 min of reperfusion, left ventricular developed pressure decreased in stunned hearts to 61 +/- 5% of control (P < 0.01), whereas recovery was complete in the IP hearts (97 +/- 4%). Steady-state [Ca2+]i-force relationships revealed a decreased maximal Ca(2+)-activated force in stunned hearts relative to control, but no change in the IP group. The Ca2+ required for 50% activation increased in stunning but not in IP.

CONCLUSIONS

These results show that the decrease in myofilament responsiveness that characterizes stunning is prevented by ischemic preconditioning.

Origin of contractile dysfunction in heart failure: calcium cycling versus myofilaments

BACKGROUND

Chronic congestive heart failure is a common, often lethal disorder of cardiac contractility. The fundamental pathophysiology of the contractile failure remains unclear, the focus being on abnormal Ca2+ cycling despite emerging evidence for depressed myofilament function.

METHODS AND RESULTS

We measured intracellular Ca2+ concentration ([Ca2+]i) and contractile force in intact ventricular muscle from SHHF rats with spontaneous heart failure and from age-matched controls. At physiological concentrations of extracellular Ca2+ ([Ca2+]o), [Ca2+]i transients were equal in amplitude in the 2 groups, but [Ca2+]i peaked later in SHHF muscles. Twitch force peaked slowly and was equivalent or modestly decreased in amplitude relative to controls. Steady-state analysis revealed a much greater (53%) depression of maximal Ca2+-activated force in SHHF muscles, which, had other factors been equal, would have produced an equivalent suppression of twitch force. Phase-plane analysis reveals that the slowing of Ca2+ cycling prolongs the time available for Ca2+ to activate the myofilaments in failing muscle, partially compensating for the marked dysfunction of the contractile machinery.

CONCLUSIONS

Our results indicate that myofilament activation is severely blunted in heart failure, but concomitant changes in [Ca2+]i kinetics minimize the contractile depression. These results challenge prevailing concepts regarding the pathophysiology of heart failure: the myofilaments emerge as central players, whereas changes in Ca2+ cycling are reinterpreted as compensatory rather than causative.

Altered cardiac excitation-contraction coupling in mutant mice with familial hypertrophic cardiomyopathy

Excitation-contraction coupling in cardiac muscle of familial hypertrophic cardiomyopathy (FHC) remains poorly understood, despite the fact that the genetic alterations are well defined. We characterized calcium cycling and contractile activation in trabeculae from a mutant mouse model of FHC (Arg403Gln knockin, alpha-myosin heavy chain). Wild-type mice of the same strain and age ( approximately 20 weeks old) served as controls. During twitch contractions, peak intracellular Ca2+ ([Ca2+]i) was higher in mutant muscles than in the wild-type (P < 0.05), but force development was equivalent in the two groups. Ca2+ transient amplitude increased dramatically in both groups as stimulation rate increased from 0.2 to 4 Hz. Nevertheless, developed force fell at the higher stimulation rates in the mutants but not in controls (P < 0.05). The steady-state force-[Ca2+]i relationship was less steep in mutants (Hill coefficient, 2.94 +/- 0.27 vs. 5.28 +/- 0.64; P > 0.003), with no changes in the [Ca2+]i required for 50% activation or maximal Ca2+-activated force. Thus, calcium cycling and myofilament properties are both altered in FHC mutant mice: more Ca2+ is mobilized to generate force, but this does not suffice to maintain contractility at high stimulation rates.

Novel myofilament Ca2+-sensitizing property of xanthine oxidase inhibitors

Antioxidants are known to mitigate the cardiac contractile dysfunction that follows brief periods of ischemia ("myocardial stunning"). Stunning decreases contractility at the level of the contractile proteins; therefore, we asked whether antioxidant treatment preserves myofilament Ca2+ responsiveness after global ischemia and reflow. Right ventricular trabeculae were dissected from rat hearts subjected either to 20 minutes ischemia and reperfusion in the absence of drugs (stunned group) or to the same protocol in the presence of allopurinol, an inhibitor of xanthine oxidase (XO), and mercaptopropionylglycine (MPG), a hydroxyl radical scavenger (antioxidant group). At 20 minutes of reflow, isovolumic developed pressure recovered completely in the antioxidant group, but in the stunned group it recovered by only 57%. [Ca2+]i and contractile force measurements in trabeculae revealed the expected depression of myofilament function in the stunned group, with no change in Ca2+ transients relative to nonischemic controls. In contrast, Ca2+ transients were smaller, but force was greater, in the antioxidant group relative to both the stunned group and to nonischemic controls. Steady-state [Ca2+]i-force relationships revealed a striking increase of maximal force and a modest shift of activation to a lower range of [Ca2+]i. The increase in maximal force was reproduced by allopurinol+MPG or by allopurinol alone under nonischemic conditions and also by oxypurinol (100 micromol/L), a potent inhibitor of XO. We conclude that allopurinol and oxypurinol sensitize the cardiac myofilaments to Ca2+. This Ca2+-sensitizing effect underlies the preservation of contractility observed with an allopurinol+MPG antioxidant cocktail in a model of stunned myocardium. These serendipitous findings identify allopurinol and oxypurinol as the lead compounds of a novel class of inotropic agents.

Angiotensin II activates Na+-independent Cl--HCO3- exchange in ventricular myocardium

The effect of angiotensin II (Ang II) on the activity of the cardiac Na+-independent Cl--HCO3- exchanger (anionic exchanger [AE]) was explored in cat papillary muscles. pHi was measured by epifluorescence with BCECF-AM. Ang II (500 nmol/L) induced a 5-(N-ethyl-N-isopropyl)amiloride-sensitive increase in pHi in the absence of external HCO3- (HEPES buffer), consistent with its stimulatory action on Na+-H+ exchange (NHE). This alkalinizing effect was not detected in the presence of a CO2-HCO3- buffer (pHi 7.07+/-0.02 and 7.08+/-0.02 before and after Ang II, respectively; n=17). Moreover, in Na+-free HCO3--buffered medium, in which neither NHE nor Na+-HCO3- cotransport are acting, Ang II decreased pHi, and this effect was canceled by previous treatment with SITS. These findings suggested that the Ang II-induced activation of NHE was masked, in the presence of the physiological buffer, by a HCO3--dependent acidifying mechanism, probably the AE. This hypothesis was confirmed on papillary muscles bathed with HCO3- buffer that were first exposed to 1 micromol/L S20787, a specific inhibitor of AE activity in cardiac tissue, and then to 500 nmol/L Ang II (n=4). Under this condition, Ang II increased pHi from 7.05+/-0.05 to 7.22+/-0.05 (P<.05). The effect of Ang II on AE activity was further explored by measuring the velocity of myocardial pHi recovery after the imposition of an intracellular alkali load in a HCO3--containing solution either with or without Ang II. The rate of myocardial pHi recovery was doubled in the presence of Ang II, suggesting a stimulatory effect on AE. The enhancement of the activity of this exchanger by Ang II was also detected when the AE activity was reversed by the removal of extracellular Cl- in a Na+-free solution. Under this condition, the rate of intracellular alkalinization increased from 0.053+/-0.016 to 0.108+/-0.026 pH unit/min (n=6, P<.05) in the presence of Ang II. This effect was canceled either by the presence of the AT1 receptor antagonist, losartan, or by the previous inhibition of protein kinase C with chelerythrine or calphostin C. The above results allow us to conclude that Ang II, in addition to its stimulatory effect on alkaline loading mechanisms, activates the AE in ventricular myocardium and that the latter effect is mediated by a protein kinase C-dependent regulatory pathway linked to the AT1 receptors.

Role of an electrogenic Na(+)-HCO3- cotransport in determining myocardial pHi after an increase in heart rate

The contribution of electrogenic Na(+)-HCO3- cotransport to pHi regulation during changes in heart rate was explored in cat papillary muscles loaded with BCECF-AM in bicarbonate-free (HEPES) medium and in CO2/HCO3(-)-buffered medium. Stepwise increments in the frequency of contraction from 15 to 100 bpm induced a reversible increase in the pHi from 7.13 +/- 0.03 to 7.36 +/- 0.03 (P < .05, n = 5) in the presence of CO2/ HCO3- buffer. The same increase in the frequency of stimulation, however, decreased pHi from 7.10 +/- 0.02 to 6.91 +/- 0.06 (P < .05, n = 5), in the absence of bicarbonate. Moreover, in CO2/HCO3(-)-superfused muscles pretreated with SITS (0.1 mmol/L), this effect of increasing the contraction frequency was reversed, and a decrease of pHi from 7.03 +/- 0.04 to 6.88 +/- 0.06 (P < .05, n = 4) was observed when the pacing rate was increased stepwise from 15 to 100 bpm. High [K+]o-induced depolarization of cell membrane alkalinized myocardial cells in the presence of HCO3- ions, whereas acidification was observed as a consequence of hyperpolarization induced by low external [K+]o. Myocardial resting membrane potential became hyperpolarized upon exposure to HCO3(-)-buffered media. This HCO3(-)-induced hyperpolarization was not blocked by the inhibition of Na+,K(+)-ATPase activity by ouabain (0.5 mumol/L) but was prevented by SITS. The results suggested that membrane depolarization during cardiac action potential causes an increase in electrogenic Na(+)-HCO3- cotransport. Such depolarizations occurring as a consequence of increases in heart rate would thus, by means of elevated bicarbonate influxes, substantially increase the myocardial cell's ability to recover from an enhanced proton production.

pHi regulation in myocardium of the spontaneously hypertensive rat. Compensated enhanced activity of the Na(+)-H+ exchanger

To elucidate the mechanisms controlling pHi in myocardium of the spontaneously hypertensive rat (SHR), experiments were performed in papillary muscles (isometrically contracting at 0.2 Hz) from SHR and age-matched normotensive Wistar-Kyoto (WKY) rats loaded with the pH-sensitive fluorescent probe BCECF-AM. An enhanced activity of the Na(+)-H+ exchanger was detected in the hypertrophic myocardium of SHR. This conclusion was based on the following: (1) The myocardial pHi was more alkaline in SHR (7.23 +/- 0.03) than in WKY rats (7.10 +/- 0.03) (P < .05) in HEPES buffer. (2) SITS (0.1 mmol/L in HEPES buffer) did not alter pHi in the SHR (pHi 7.26 +/- 0.03 and 7.28 +/- 0.03 before and after SITS, respectively). (3) The fall in pHi observed after 20 minutes of Na(+)-H+ exchanger inhibition [5 mumol/L 5-(N-ethyl-N-isopropyl)amiloride (EIPA)] was greater in SHR (-0.16 +/- 0.01) than in WKY rats (-0.09 +/- 0.02, P < 0.05). (4) The velocity of pHi recovery from an intracellular acid load was faster in SHR than in WKY rats (0.068 +/- 0.02 versus 0.014 +/- 0.002 pH units/min at pHi 6.99, P < .05). (5) After EIPA inhibition, the rate of pHi recovery from the same acid load decreased to a similar value in both rat strains (0.0032 +/- 0.002 pH units/min in SHR and 0.0032 +/- 0.002 pH units/min in WKY rats). Under the more physiological HCO3(-)-CO2 buffer, no significant difference in steady state myocardial pHi was detected between rat strains (7.15 +/- 0.03 in SHR and 7.11 +/- 0.05 in WKY rats).(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial contractility recovery during hypercapnic acidosis: its dissociation from recovery in pHi by ryanodine

Myocardial contractility falls quickly during respiratory acidosis but if acidosis is maintained a slow gradual return towards control state is detected. In cat papillary muscle, changes in developed tension (DT) during isometric contractions (pacing rate 0.2 Hz) and intracellular pH (pHi) were continuously monitored before and during hypercapnia to study the contribution of pHi recovery to the recovery of contractility. On exposure to hypercapnia (extracellular pH [pHo] = 6.90) DT fell to 50.33 +/- 2.20% of control and pHi decreased from 7.21 +/- 0.05 to 6.90 +/- 0.02. After 30 mins of hypercapnia DT recovered to 64.66 +/- 4.05% of control, but no significant recovery in pHi was detected. Intracellular sodium concentration slowly rose to 61.05 +/- 23.79% over basal level 10 mins after the onset of hypercapnia and it remained elevated for 10 mins before gradually returning to control levels. When pHo was kept at 7.40 during hypercapnia by increasing sodium bicarbonate concentration, DT recovered to 79.11 +/- 6.94% of control after 30 mins of hypercapnia, while a significant recovery of pHi (0.12 +/- 0.02 pH units) was detected. Low extracellular sodium concentration diminished contractility recovery during hypercapnia without changing the initial decrease in DT. 5-[N-ethyl-N-isopropyl] amiloride (EIPA) (5 microM) increased the initial fall in DT to 34.33 +/- 8.68% of control and abolished the recovery. Sarcoplasmic reticulum (SR) inhibition by ryanodine (0.5 microM) markedly reduced the recovery of contractility without altering the recovery in pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

An electrogenic sodium-bicarbonate cotransport in the regulation of myocardial intracellular pH

Experiments were performed in cat papillary muscles in order to explore the possible existence of an electrogenic Na+/HCO3- cotransport. Developed tension (DT), intracellular pH (pHi) with the pH-sensitive dye 2'-7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) and resting membrane potential (Vm) with 3M KCl filled glass microelectrodes were measured. A change from HEPES to HCO3(-)-buffered superfusate induced an immediate decrease in pHi and DT followed by a recovery in which pHi and DT stabilized at values slightly higher than in HEPES buffer. Introduction of HCO3- hyperpolarized Vm by 8 +/- 2.3 mV (P < 0.05). SITS (0.1 mM) completely abolished the hyperpolarization and attenuated the recovery of both pHi and DT. Under steady-state conditions in HCO3- buffered media, SITS induced a depolarization compatible with the suppression of the entry of negative charges. Depolarization by high Ko+ (45 mM) elicited a rise in pHi of 0.07 +/- 0.02 (P < 0.05), that was reversed by returning Ko+ to normal. The depolarization-induced rise in pHi proved to be Na(+)-dependent, SITS sensitive and still occurred after EIPA (microM) blockade. All the evidence strongly supports the existence of an electrogenic Na+/HCO3- cotransport mechanism that participates in the regulation of myocardial pHi. At pHi of 6.94 this mechanism seems to contribute almost equally to the Na+/H+ exchanger to pHi regulation. However, acid equivalent extrusion is potentiated when both the Na+/H+ exchanger and the HCO3-dependent mechanism are simultaneously regulating pHi.

Lusitropic changes induced by acid base alterations in cat papillary muscles

The present work investigates the effects of acid-base alterations upon myocardial relaxation. Experiments were performed in cat papillary muscles contracting isometrically at constant frequency (0.2 Hz) and temperature (29 degrees C). To induce intracellular alkalosis at constant pH0, 20 mM NH4Cl were added to the perfusate. Alkalosis at variable pH0 was induced by switching from the control solution (5% CO2-95% O2, pH0 7.40) to a solution identical to the control one, equilibrated with 3% CO2-97% O2. Acidosis was induced by switching the control perfusate to a solution equilibrated with 12% CO2-88% O2 in which pH0 was either allowed to change or kept constant by manipulation of bicarbonate concentration. Alkalosis produced a negative lusitropic effect either when pH0 was kept constant or when it was allowed to increase. For an increase in myocardial contractility of 30%, half relaxation tme (T50) and time to peak tension (TTP) were prolonged 9.4 +/- 5% and 5.4 +/- 2% respectively at constant pH0 and 6.8 +/- 0.8 and 4.7 +/- 1% respectively at variable pHo. It is suggested that this negative lusitropic effect of alkalosis can be attributed to an increase in myofilament sensitivity to calcium. Either at constant or at variable pHo acidosis decreased myocardial contractility by approximately 50%. This decrease in contractility was accompanied by a positive lusitropic action only when pHo was allowed to decrease, or when acidosis at constant pHo was evoked in the presence of EIPA, a specific inhibitor of the Na+/H+ exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)