Effect of perfusion pressure on force of contraction in thin papillary muscles and trabeculae from rat heart

Exploring the Connection Between Relaxed Myosin States and the Anrep Effect

The Anrep effect is an adaptive response that increases left ventricular contractility following an acute rise in afterload. Although the mechanistic origin remains undefined, recent findings suggest a two-phase activation of resting myosin for contraction, involving strain-sensitive and posttranslational phases. We propose that this mobilization represents a transition among the relaxed states of myosin-specifically, from the super-relaxed (SRX) to the disordered-relaxed (DRX)-with DRX myosin ready to participate in force generation. This hypothesis offers a unified explanation that connects myosin's SRX-DRX equilibrium and the Anrep effect as parts of a singular phenomenon. We underscore the significance of this equilibrium in modulating contractility, primarily studied in the context of hypertrophic cardiomyopathy, the most common inherited cardiomyopathy associated with diastolic dysfunction, hypercontractility, and left ventricular hypertrophy. As we posit that the cellular basis of the Anrep effect relies on a two-phased transition of myosin from the SRX to the contraction-ready DRX configuration, any dysregulation in this equilibrium may result in the pathological manifestation of the Anrep phenomenon. For instance, in hypertrophic cardiomyopathy, hypercontractility is linked to a considerable shift of myosin to the DRX state, implying a persistent activation of the Anrep effect. These valuable insights call for additional research to uncover a clinical Anrep fingerprint in pathological states. Here, we demonstrate through noninvasive echocardiographic pressure-volume measurements that this fingerprint is evident in 12 patients with hypertrophic obstructive cardiomyopathy before septal myocardial ablation. This unique signature is characterized by enhanced contractility, indicated by a leftward shift and steepening of the end-systolic pressure-volume relationship, and a prolonged systolic ejection time adjusted for heart rate, which reverses post-procedure. The clinical application of this concept has potential implications beyond hypertrophic cardiomyopathy, extending to other genetic cardiomyopathies and even noncongenital heart diseases with complex etiologies across a broad spectrum of left ventricular ejection fractions.

CaMKII activity contributes to homeometric autoregulation of the heart: A novel mechanism for the Anrep effect

KEY POINTS

The Anrep effect represents the alteration of left ventricular (LV) contractility to acutely enhanced afterload in a few seconds, thereby preserving stroke volume (SV) at constant preload. As a result of the missing preload stretch in our model, the Anrep effect differs from the slow force response and has a different mechanism. The Anrep effect demonstrated two different phases. First, the sudden increased afterload was momentary equilibrated by the enhanced LV contractility as a result of higher power strokes of strongly-bound myosin cross-bridges. Second, the slightly delayed recovery of SV is perhaps dependent on Ca²⁺ /calmodulin-dependent protein kinase II activation caused by oxidation and myofilament phosphorylation (cardiac myosin-binding protein-C, myosin light chain 2), maximizing the recruitment of available strongly-bound myosin cross-bridges. Short-lived oxidative stress might present a new facet of subcellular signalling with respect to cardiovascular regulation. Relevance for human physiology was demonstrated by echocardiography disclosing the Anrep effect in humans during handgrip exercise.

ABSTRACT

The present study investigated whether oxidative stress and Ca²⁺ /calmodulin-dependent protein kinase II (CaMKII) activity are involved in triggering the Anrep effect. LV pressure-volume (PV) analyses of isolated, preload controlled working hearts were performed at two afterload levels (60 and 100 mmHg) in C57BL/6N wild-type (WT) and CaMKII-double knockout mice (DKO^CaMKII ). In snap-frozen WT hearts, force-pCa relationship, H₂ O₂ generation, CaMKII oxidation and phosphorylation of myofilament and Ca²⁺ handling proteins were assessed. Acutely raised afterload showed significantly increased wall stress, H₂ O₂ generation and LV contractility in the PV diagram with an initial decrease and recovery of stroke volume, whereas end-diastolic pressure and volume, as well as heart rate, remained constant. Afterload induced increase in LV contractility was blunted in DKO^CaMKII -hearts. Force development of single WT cardiomyocytes was greater with elevated afterload at submaximal Ca²⁺ concentration and associated with increases in CaMKII oxidation and phosphorylation of cardiac-myosin binding protein-C, myosin light chain and Ca²⁺ handling proteins. CaMKII activity is involved in the regulation of the Anrep effect and associates with stimulation of oxidative stress, presumably starting a cascade of CaMKII oxidation with downstream phosphorylation of myofilament and Ca²⁺ handling proteins. These mechanisms improve LV inotropy and preserve stroke volume within few seconds.

ATP- and voltage-dependent electro-metabolic signaling regulates blood flow in heart

Local control of blood flow in the heart is important yet poorly understood. Here we show that ATP-sensitive K⁺ channels (K_ATP), hugely abundant in cardiac ventricular myocytes, sense the local myocyte metabolic state and communicate a negative feedback signal-correction upstream electrically. This electro-metabolic voltage signal is transmitted instantaneously to cellular elements in the neighboring microvascular network through gap junctions, where it regulates contractile pericytes and smooth muscle cells and thus blood flow. As myocyte ATP is consumed in excess of production, [ATP]_i decreases to increase the openings of K_ATP channels, which biases the electrically active myocytes in the hyperpolarization (negative) direction. This change leads to relative hyperpolarization of the electrically connected cells that include capillary endothelial cells, pericytes, and vascular smooth muscle cells. Such hyperpolarization decreases pericyte and vascular smooth muscle [Ca²⁺]_i levels, thereby relaxing the contractile cells to increase local blood flow and delivery of nutrients to the local cardiac myocytes and to augment ATP production by their mitochondria. Our findings demonstrate the pivotal roles of local cardiac myocyte metabolism and K_ATP channels and the minor role of inward rectifier K⁺ (Kir2.1) channels in regulating blood flow in the heart. These findings establish a conceptually new framework for understanding the hugely reliable and incredibly robust local electro-metabolic microvascular regulation of blood flow in heart.

Acute interaction between human epicardial adipose tissue and human atrial myocardium induces arrhythmic susceptibility

Epicardial adipose tissue (EAT) deposition has a strong clinical association with atrial arrhythmias; however, whether a direct functional interaction exists between EAT and the myocardium to induce atrial arrhythmias is unknown. Therefore, we aimed to determine whether human EAT can be an acute trigger for arrhythmias in human atrial myocardium. Human trabeculae were obtained from right atrial appendages of patients who have had cardiac surgery (n = 89). The propensity of spontaneous contractions (SCs) in the trabeculae (proxy for arrhythmias) was determined under physiological conditions and during known triggers of SCs (high Ca²⁺, β-adrenergic stimulation). To determine whether EAT could trigger SCs, trabeculae were exposed to superfusate of fresh human EAT, and medium of 24 h-cultured human EAT treated with β_1/2 (isoproterenol) or β₃ (BRL37344) adrenergic agonists. Without exposure to EAT, high Ca²⁺ and β_1/2-adrenergic stimulation acutely triggered SCs in, respectively, 47% and 55% of the trabeculae that previously were not spontaneously active. Acute β₃-adrenergic stimulation did not trigger SCs. Exposure of trabeculae to either superfusate of fresh human EAT or untreated medium of 24 h-cultured human EAT did not induce SCs; however, specific β₃-adrenergic stimulation of EAT did trigger SCs in the trabeculae, either when applied to fresh (31%) or cultured (50%) EAT. Additionally, fresh EAT increased trabecular contraction and relaxation, whereas media of cultured EAT only increased function when treated with the β₃-adrenergic agonist. An acute functional interaction between human EAT and human atrial myocardium exists that increases the propensity for atrial arrhythmias, which depends on β₃-adrenergic rather than β_1/2-adrenergic stimulation of EAT.

A dipeptidyl peptidase-IV inhibitor improves diastolic dysfunction in Dahl salt-sensitive rats

To date, there is no established treatment for heart failure with preserved ejection fraction (HFpEF). Dipeptidyl peptidase-IV (DPP-IV) inhibitors reportedly have improved not only diabetes mellitus but also heart failure with systolic dysfunction in experimental models. We investigated the effects of a DPP-IV inhibitor on HFpEF in rats. Dahl salt-sensitive rats were fed either high-salt (high-salt diet (HSD): 8% NaCl) or low-salt diets (0.3% NaCl) from 6.5 weeks of age. They were then treated with or without a DPP-IV inhibitor, vildagliptin (10 mg/kg/day, orally), from 11 weeks of age for 9 weeks and analyzed at the age of 20 weeks. HSD rats mimicked the pathophysiology of HFpEF. There were no differences in heart rate, blood pressure, left ventricular (LV) systolic function, or the extent of LV hypertrophy between HSD rats with or without vildagliptin. However, vildagliptin decreased LV end-diastolic pressure, the most reliable hemodynamic parameter of HFpEF in HSD rats. Vildagliptin also decreased the LV distensibility index, a sensitive marker of LV diastolic function in HSD rats. Vildagliptin decreased the expression of collagen genes in HSD hearts and attenuated LV interstitial fibrosis (HSD with vehicle and vildagliptin, 2.9% vs. 1.9%; P < 0.05). Furthermore, vildagliptin administration reduced both plasma renin activity and aldosterone concentrations in HSD rats. A DPP-IV inhibitor, vildagliptin, improved the severity of LV fibrosis, and thus, diastolic dysfunction of HFpEF in Dahl salt-sensitive hypertensive rats. DPP-IV inhibitors are promising medicines for treatment of HFpEF in patients with diabetes mellitus.

Cardiac β-adrenergic responsiveness of obese Zucker rats: The role of AMPK

NEW FINDINGS

What is the central question of the study? Is the reduced signalling of AMP-activated protein kinase (AMPK), a key regulator of energy homeostasis in the heart, responsible for the reduced β-adrenergic responsiveness of the heart in obesity? What is the main finding and its importance? Inhibition of AMPK in isolated hearts prevented the reduced cardiac β-adrenergic responsiveness of obese rats, which was accompanied by reduced phosphorylation of AMPK, a proxy of AMPK activity. This suggests a direct functional link between β-adrenergic responsiveness and AMPK signalling in the heart, and it suggests that AMPK might be an important target to restore the β-adrenergic responsiveness in the heart in obesity.

ABSTRACT

The obesity epidemic impacts heavily on cardiovascular health, in part owing to changes in cardiac metabolism. AMP-activated protein kinase (AMPK) is a key regulator of energy homeostasis in the heart and is regulated by β-adrenoceptors (β-ARs) in normal conditions. In obesity, chronic sympathetic overactivation leads to impaired cardiac β-AR responsiveness, although it is unclear whether AMPK signalling, downstream of β-ARs, contributes to this dysfunction. Therefore, we aimed to determine whether reduced AMPK signalling is responsible for the reduced β-AR responsiveness in obesity. In isolated hearts of lean and obese Zucker rats, we tested β-AR responsiveness to the β₁ -AR agonist isoprenaline (ISO, 1 × 10^-10 to 5 × 10^-8  m) in the absence and presence of the AMPK inhibitor, compound C (CC, 10 μm). The β₁ -AR expression and AMPK phosphorylation were assessed by Western blot. β-Adrenergic responsiveness was reduced in the hearts of obese rats (logEC₅₀ of ISO-developed pressure dose-response curves: lean -8.53 ± 0.13 × 10^x  m versus obese -8.35 ± 0.10 × 10^x  m ; P < 0.05 lean versus obese, n = 6 per group). This difference was not apparent after AMPK inhibition (logEC₅₀ of ISO-developed pressure curves: lean CC -8.19 ± 0.12 × 10^x  m versus obese CC 8.17 ± 0.13 × 10^x  m, P < 0.05, n = 6 per group). β₁ -Adrenergic receptor expression and AMPK phosphorylation were reduced in hearts of obese rats (AMPK at Thr¹⁷² : lean 1.73 ± 0.17 a.u. versus lean CC 0.81 ± 0.13 a.u., and obese 1.18 ± 0.09 a.u. versus obese CC 0.81 ± 0.16 a.u., P < 0.05, n = 6 per group). Thus, a direct functional link between β-adrenergic responsiveness and AMPK signalling in the heart exists, and AMPK might be an important target to restore the reduced cardiac β-adrenergic responsiveness in obesity.

Increased Efferent Cardiac Sympathetic Nerve Activity and Defective Intrinsic Heart Rate Regulation in Type 2 Diabetes

Elevated sympathetic nerve activity (SNA) coupled with dysregulated β-adrenoceptor (β-AR) signaling is postulated as a major driving force for cardiac dysfunction in patients with type 2 diabetes; however, cardiac SNA has never been assessed directly in diabetes. Our aim was to measure the sympathetic input to and the β-AR responsiveness of the heart in the type 2 diabetic heart. In vivo recording of SNA of the left efferent cardiac sympathetic branch of the stellate ganglion in Zucker diabetic fatty rats revealed an elevated resting cardiac SNA and doubled firing rate compared with nondiabetic rats. Ex vivo, in isolated denervated hearts, the intrinsic heart rate was markedly reduced. Contractile and relaxation responses to β-AR stimulation with dobutamine were compromised in externally paced diabetic hearts, but not in diabetic hearts allowed to regulate their own heart rate. Protein levels of left ventricular β1-AR and Gs (guanine nucleotide binding protein stimulatory) were reduced, whereas left ventricular and right atrial β2-AR and Gi (guanine nucleotide binding protein inhibitory regulatory) levels were increased. The elevated resting cardiac SNA in type 2 diabetes, combined with the reduced cardiac β-AR responsiveness, suggests that the maintenance of normal cardiovascular function requires elevated cardiac sympathetic input to compensate for changes in the intrinsic properties of the diabetic heart.

Effects of intra-aortic balloon pump counterpulsation on left ventricular mechanoenergetics in a porcine model of acute ischemic heart failure

We investigated the effects of intra-aortic balloon pump (IABP) counterpulsation on left ventricular (LV) contractility, relaxation, and energy consumption and probed the underlying physiologic mechanisms in 12 farm pigs, using an ischemia-reperfusion model of acute heart failure. During both ischemia and reperfusion, IABP support unloaded the LV, decreased LV energy consumption (pressure-volume area, stroke work), and concurrently improved LV mechanical performance (ejection fraction, stroke volume, cardiac output). During reperfusion exclusively, IABP also improved LV relaxation (tau) and contractility (Emax, PRSW). The beneficial effects of IABP support on LV relaxation and contractility correlated with IABP-induced augmentation of coronary blood flow. In conclusion, we find that during both ischemia and reperfusion, IABP support optimizes LV energetic performance (decreases energy consumption and concurrently improves mechanical performance) by LV unloading. During reperfusion exclusively, IABP support also improves LV contractility and active relaxation, possibly due to a synergistic effect of unloading and augmentation of coronary blood flow.

Impaired relaxation despite upregulated calcium-handling protein atrial myocardium from type 2 diabetic patients with preserved ejection fraction

BACKGROUND

Diastolic dysfunction is a key factor in the development and pathology of cardiac dysfunction in diabetes, however the exact underlying mechanism remains unknown, especially in humans. We aimed to measure contraction, relaxation, expression of calcium-handling proteins and fibrosis in myocardium of diabetic patients with preserved systolic function.

METHODS

Right atrial appendages from patients with type 2 diabetes mellitus (DM, n = 20) and non-diabetic patients (non-DM, n = 36), all with preserved ejection fraction and undergoing coronary artery bypass grafting (CABG), were collected. From appendages, small cardiac muscles, trabeculae, were isolated to measure basal and β-adrenergic stimulated myocardial function. Expression levels of calcium-handling proteins, sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) and phospholamban (PLB), and of β1-adrenoreceptors were determined in tissue samples by Western blot. Collagen deposition was determined by picro-sirius red staining.

RESULTS

In trabeculae from diabetic samples, contractile function was preserved, but relaxation was prolonged (Tau: 74 ± 13 ms vs. 93 ± 16 ms, non-DM vs. DM, p = 0.03). The expression of SERCA2a was increased in diabetic myocardial tissue (0.75 ± 0.09 vs. 1.23 ± 0.15, non-DM vs. DM, p = 0.007), whereas its endogenous inhibitor PLB was reduced (2.21 ± 0.45 vs. 0.42 ± 0.11, non-DM vs. DM, p = 0.01). Collagen deposition was increased in diabetic samples. Moreover, trabeculae from diabetic patients were unresponsive to β-adrenergic stimulation, despite no change in β1-adrenoreceptor expression levels.

CONCLUSIONS

Human type 2 diabetic atrial myocardium showed increased fibrosis without systolic dysfunction but with impaired relaxation, especially during β-adrenergic challenge. Interestingly, changes in calcium-handling protein expression suggests accelerated active calcium re-uptake, thus improved relaxation, indicating a compensatory calcium-handling mechanism in diabetes in an attempt to maintain diastolic function at rest despite impaired relaxation in the diabetic fibrotic atrial myocardium. Our study addresses important aspects of the underlying mechanisms of diabetes-associated diastolic dysfunction, which is crucial to developing new therapeutic treatments.

Control of cardiac contractility in the rat working heart-brainstem preparation

A great deal of knowledge exists regarding neural control of myocardial function in the rat. Most of the studies addressing this issue were conducted either under general anaesthesia or in isolated hearts in vitro. Our principal aim was to provide a detailed quantitative description of mechanisms controlling cardiac contractility in the rat, in an anaesthetic-free preparation with a preserved functional brainstem. Furthermore, while vagally mediated negative inotropy is a well-known phenomenon, at present there is no direct evidence for its presence in the rat; we searched for such evidence. To this end, in the arterially perfused working heart-brainstem preparation of the rat, we measured left ventricular pressure (LVP) and computed its first derivative (LVdP/dt). We made the following new observations. (i) Zatebradine (cardiac sodium pacemaker current blocker) caused a bradycardia associated with increases in LVP and LVdP/dt; the latter effect was via a frequency-dependent mechanism. (ii) We confirmed that in the rat, the force-frequency relationship (dependence of contractility on heart rate) is positive over a low range of heart rates, and negative and linear at physiological levels of heart rate, and provided its quantitative description. (iii) The increase in systemic pressure caused a rise in contractility, and vagal blockade or destruction of the central nervous system did not alter this inotropic effect, suggesting that it was mediated by intrinsic cardiac mechanisms. (iv) Vagal stimulation caused complex polyphasic changes in LVdP/dt and LVP in unpaced preparations; during pacing, it caused slowly developing falls in LVdP/dt that could be prevented by atropine. We conclude that control of ventricular contractility in the rat heart differs from that in other mammals not only by its negative frequency dependence, but also in the potent influence of aortic pressure on LVdP/dt. At the level of autonomic neural control, our newly found, vagally mediated negative inotropic effect adds to the accumulating body of data regarding both the presence and the functional importance of parasympathetic innervation of the ventricular myocardium.

Cross-Talk Between Cardiac Muscle and Coronary Vasculature

The cardiac muscle and the coronary vasculature are in close proximity to each other, and a two-way interaction, called cross-talk, exists. Here we focus on the mechanical aspects of cross-talk including the role of the extracellular matrix. Cardiac muscle affects the coronary vasculature. In diastole, the effect of the cardiac muscle on the coronary vasculature depends on the (changes in) muscle length but appears to be small. In systole, coronary artery inflow is impeded, or even reversed, and venous outflow is augmented. These systolic effects are explained by two mechanisms. The waterfall model and the intramyocardial pump model are based on an intramyocardial pressure, assumed to be proportional to ventricular pressure. They explain the global effects of contraction on coronary flow and the effects of contraction in the layers of the heart wall. The varying elastance model, the muscle shortening and thickening model, and the vascular deformation model are based on direct contact between muscles and vessels. They predict global effects as well as differences on flow in layers and flow heterogeneity due to contraction. The relative contributions of these two mechanisms depend on the wall layer (epi- or endocardial) and type of contraction (isovolumic or shortening). Intramyocardial pressure results from (local) muscle contraction and to what extent the interstitial cavity contracts isovolumically. This explains why small arterioles and venules do not collapse in systole. Coronary vasculature affects the cardiac muscle. In diastole, at physiological ventricular volumes, an increase in coronary perfusion pressure increases ventricular stiffness, but the effect is small. In systole, there are two mechanisms by which coronary perfusion affects cardiac contractility. Increased perfusion pressure increases microvascular volume, thereby opening stretch-activated ion channels, resulting in an increased intracellular Ca2+transient, which is followed by an increase in Ca2+sensitivity and higher muscle contractility (Gregg effect). Thickening of the shortening cardiac muscle takes place at the expense of the vascular volume, which causes build-up of intracellular pressure. The intracellular pressure counteracts the tension generated by the contractile apparatus, leading to lower net force. Therefore, cardiac muscle contraction is augmented when vascular emptying is facilitated. During autoregulation, the microvasculature is protected against volume changes, and the Gregg effect is negligible. However, the effect is present in the right ventricle, as well as in pathological conditions with ineffective autoregulation. The beneficial effect of vascular emptying may be reduced in the presence of a stenosis. Thus cardiac contraction affects vascular diameters thereby reducing coronary inflow and enhancing venous outflow. Emptying of the vasculature, however, enhances muscle contraction. The extracellular matrix exerts its effect mainly on cardiac properties rather than on the cross-talk between cardiac muscle and coronary circulation.

The mechanism of sevoflurane-induced cardioprotection is independent of the applied ischaemic stimulus in rat trabeculae

BACKGROUND

Sevoflurane protects the myocardium against ischaemic injury through protein kinase C (PKC) activation, mitochondrial K+ATP-channel (mitoK+ATP) opening and production of reactive oxygen species (ROS). However, it is unclear whether the type of ischaemia determines the involvement of these signalling molecules. We therefore investigated whether hypoxia (HYP) or metabolic inhibition (MI), which differentially inhibit the mitochondrial electron transport chain (ETC), are comparable concerning the relative contribution of PKC, mitoK+ATP and ROS in sevoflurane-induced cardioprotection.

METHODS

Rat right ventricular trabeculae were isolated and isometric contractile force (Fdev) was measured. Trabeculae were subjected to HYP (hypoxic glucose-free buffer; 40 min) or MI (glucose-free buffer, 2 mM cyanide; 30 min), followed by 60 min recovery (60 min). Contractile recovery (Fdev,rec) was determined at the end of the recovery period and expressed as a percentage of Fdev before hypoxia or MI, respectively. Chelerythrine (CHEL; 6 microM), 5-hydroxydecanoic acid sodium (100 microM) and n-(2-mercaptopropionyl)-glycine (MGP; 300 microM) were used to inhibit PKC, mitoK+ATP and ROS, respectively.

RESULTS

Fdev,rec after HYP was reduced to 47 (3)% (P<0.001 vs control; n=5) whereas MI reduced Fdev,rec to 28 (5)% (P<0.001 vs control; n=5). A 15 min period of preconditioning with sevoflurane (3.8%) equally increased contractile recovery after HYP [76 (9)%; P<0.05 vs HYP] and MI [67 (8)%; P<0.01 vs MI]. Chelerythrine, 5-hydroxydecanoate and n-(2-mercaptopropionyl)-glycine abolished the protective effect of sevoflurane in both ischaemic models. Trabeculae subjected to HYP or MI did not demonstrate any increased apoptotic or necrotic markers.

CONCLUSIONS

PKC, mitoK+ATP and ROS are involved in sevoflurane-induced cardioprotection after HYP or MI, suggesting that the means of mitochondrial ETC inhibition does not determine the signal transduction pathway for cardioprotection by anaesthetics.

In vivo hemodynamic responses to thoracic artificial lung attachment

A thoracic artificial lung (TAL) was attached to the pulmonary circulation in a porcine model. Proximal main pulmonary artery (PA) blood flow, in part or whole, was diverted to the TAL, and TAL outlet blood flow was split between the distal main PA and left atrium (LA). The right ventricle (RV) drove blood flow through the combined TAL/natural lung (NL) pulmonary system. Selective banding placed the TAL in parallel with the NLs, in series with the NLs, or in an intermediary hybrid configuration. Parallel TAL attachment lowered pulmonary system impedance, raised cardiac output (CO), and provided the greatest TAL blood flow rate, but reduced the NL blood flow rate which is important for pulmonary embolic clearance and metabolic blood processing. Hybrid or series TAL attachment raised pulmonary system impedance, lowered CO, increased RV oxygen consumption, and reduced RV oxygen supply. Redesign of the PA anastomoses, TAL inlet graft, and TAL entrance and exit would significantly improve hemodynamics and RV function with TAL attachment. Mean LA pressure increased throughout the experiment, which may indicate damage caused by graft attachment to the LA. Pulmonary resistance-flow rate curves may enable clinical prediction of tolerable TAL attachment configurations.

Acute and specific collagen type I degradation increases diastolic and developed tension in perfused rat papillary muscle

Collagen degradation is suggested to be responsible for long-term contractile dysfunction in different cardiomyopathies, but the effects of acute and specific collagen type I removal (main type in the heart muscle) on tension have not been studied. We determined the diastolic and developed tension length relations in isometric contracting perfused rat papillary muscles (perfusion pressure 60 cmH2O) before and after acute and specific removal of small collagen struts with the use of purified collagenase type I. At 95% of the maximal length (95% Lmax), diastolic tension increased 20.4 ± 8.1% ( P < 0.05, n = 6) and developed tension increased 15.0 ± 6.7% after collagenase treatment compared with time controls. Treatment increased the diastolic muscle diameter by 7.1 ± 3.4% at 95% Lmax, whereas the change in diameter due to contraction was not changed. Diastolic coronary flow and normalized coronary arterial flow impediment did not change after collagenase treatment. Electron microscopy revealed that the number of small collagen struts, interconnecting myocytes, and capillaries was reduced to ∼32% after treatment. We conclude that removal of the small collagen struts by acute and specific collagen type I degradation increases diastolic and developed tension in perfused papillary muscle. We suggest that diastolic tension is increased due to edema, whereas developed tension is increased because the removal of the struts poses a lower lateral load on the cardiac myocytes, allowing more myocyte thickening.

The Cardioprotective Effect of Sevoflurane Depends on Protein Kinase C Activation, Opening of Mitochondrial K+ATP Channels, and the Production of Reactive Oxygen Species

Several studies suggest that the cardioprotective effect of sevoflurane depends on protein kinase C (PKC) activation, mitochondrial K(+)(ATP) channel (mitoK(+)(ATP)) opening, and reactive oxygen species (ROS). However, evidence for their involvement was obtained in separate experimental models. Here, we studied the relative roles of PKC, mitoK(+)(ATP), and ROS in sevoflurane-induced cardioprotection in one model. Rat trabeculae were subjected to simulated ischemia by applying metabolic inhibition (MI) through buffer containing NaCN, followed by 60-min reperfusion. Recovery of active force (F(a)) was assessed as percentage of pre-MI force. In time controls, F(a) amounted 60% +/- 5% at the end of the experiment. The recovery of F(a) after MI was reduced to 28% +/- 5% (P = 0.045 versus time control), whereas sevoflurane reversed the detrimental effect of MI (F(a) recovery, 67% +/- 8%; P = 0.01 versus MI). The PKC inhibitor chelerythrine, the mitoK(+)(ATP) inhibitor 5-hydroxy decanoic, and the ROS scavenger N-(2-mercaptopropionyl)-glycine all completely abolished the protective effect of sevoflurane (recovery of F(a), 31% +/- 8%, 33% +/- 8%, and 24% +/- 9% for chelerythrine, 5-hydroxy decanoic, and N-(2-mercaptopropionyl)-glycine, respectively). In conclusion, PKC activation, mitoK(+)(ATP) channel opening, and ROS production are all essential for sevoflurane-induced cardioprotection. These signaling events are arranged in series within a common signaling pathway, rather than in parallel cascades. Our findings implicate that the perioperative use of sevoflurane preserves cardiac function by preventing ischemia-reperfusion injury.

IMPLICATIONS

Protein kinase C, mitochondrial K(+)(ATP) channels and reactive oxygen species act within one downstream signaling pathway in mediating the cardioprotective effect of sevoflurane.

Dual role of endothelin-1 via ETA and ETB receptors in regulation of cardiac contractile function in mice

An increase in coronary perfusion pressure leads to increased cardiac contractility, a phenomenon known as the Gregg effect. Exogenous endothelin (ET)-1 exerts a positive inotropic effect; however, the role of endogenous ET-1 in the contractile response to elevated load is unknown. We characterized here the role of ETA and ETB receptors in regulation of contractility in isolated, perfused mouse hearts subjected to increased coronary flow. Elevation of coronary flow from 2 to 5 ml/min resulted in 80 +/- 10% increase in contractile force (P < 0.001). BQ-788 (ETB receptor antagonist) augmented the load-induced contractile response by 35% (P < 0.05), whereas bosentan (ETA/B receptor antagonist) and BQ-123 (ETA receptor antagonist) attenuated it by 34% and 56%, respectively (P < 0.05). CV-11974 (ANG II type 1 receptor antagonist) did not modify the increase in contractility. These results show that endogenous ET-1 is a key mediator of the Gregg effect in mouse hearts. Moreover, ET-1 has a dual role in the regulation of cardiac contractility: ETA receptor-mediated increase in contractile force is suppressed by ETB receptors.

Fatty acids are important for the Frank-Starling mechanism and Gregg effect but not for catecholamine response in isolated rat hearts

In some pathophysiological conditions myocardial metabolism can switch from mainly long chain fatty acid (LCFA) oxidation to mainly glucose oxidation. Whether the predominant fatty acid or glucose oxidation affects cardiac performance has not been defined. In a buffer perfused isovolumetrically contracting rat heart, oxidation of endogenous pool LCFA was avoided by inhibiting carnitine-palmitoyl-transferase I (CPT-I) with oxfenicine (2 mM). In order to restore fatty acid oxidation, hexanoate (1 mM), which bypasses CPT-I inhibition, was added to the perfusate. Three groups of hearts were subjected to either an increase in left ventricular volume (VV, +25%) or an increase in coronary flow (CF, +50%), or inotropic stimulation with isoproterenol (10(-8) and 10(-6) m). The increase in VV (the Frank-Starling mechanism) increased rate-pressure product (RPP) by 21 +/- 2% under control conditions, but only by 6 +/- 2% during oxfenicine-induced CPT-I inhibition. The contractile response to changes in VV recovered after the addition of hexanoate. Similar results were obtained in hearts, in which an increase in CF was elicited (the Gregg phenomenon). Isoproterenol caused a similar increase in contractility regardless of the presence of oxfenicine or hexanoate. In all groups, a commensurate increase in oxygen consumption accompanied the increase in contractility. The fatty acid oxidation is necessary for an adequate contractile response of the isolated heart to increased pre-load or flow, whereas the inotropic response to adrenergic beta-receptor stimulation is insensitive to changes in substrate availability.

Coronary perfusion and muscle lengthening increase cardiac contraction: different stretch-triggered mechanisms

An increase in coronary perfusion, transversal stretch of the myocardium, increases developed force (F(dev)) (Gregg effect) through activation of stretch-activated ion channels (SACs). Lengthening of the muscle, longitudinal stretch of the myocardium, causes an immediate increase in F(dev) followed by a slow F(dev) increase (Anrep effect). In isometrically contracting perfused papillary muscles of Wistar rats, we investigated whether both effects were based on similar stretch-induced mechanisms by measuring F(dev) and intracellular Ca(2+) concentration ([Ca(2+)](i)) after a muscle length increase from 85% to 95% L(max) (length at which maximal isometric force develops) at low and high coronary perfusion before and after inhibition of SACs with gadolinium (10 micromol/l Gd(3+)). The increase of F(dev) and peak [Ca(2+)](i) by the Gregg effect was of similar magnitude as the Anrep effect (from 3.5 +/- 0.8 to 3.9 +/- 1.2 mN/mm(2) and from 3.0 +/- 0.7% to 3.8 +/- 0.9% normalized [Ca(2+)](i), means +/- SE). SAC blockade completely blunted the increase of F(dev) and peak [Ca(2+)](i) by the Gregg effect; however, it did not affect the Anrep effect. The slow force response, but not the calcium response, was augmented by an increase in coronary perfusion. Therefore, increased coronary perfusion, transversal stretch of the myocardium, and muscle lengthening, longitudinal stretch of the myocardium, increase myocardial contraction in the rat through different stretch-triggered mechanisms.

Role of nitric oxide synthase, cytochrome P-450, and cyclooxygenase in the inotropic and lusitropic cardiac response to increased coronary perfusion

Although studies have reported that increase in coronary perfusion (CP) results in positive inotropic effects, the underlying mechanisms of these actions and possible alterations in myocardial diastolic function are not well defined. Hypothesis was that nitric oxide (NO) and derivatives of cytochrome (CYT) P-450 or cyclooxygenase (COX) might contribute to interplay between coronary and myocardial compartments in these conditions. Using isovolumically contracting, isolated perfused hamster heart model, coronary flow (CF) was increased mechanically, stepwise in the physiologic range (+2 to +10 ml/min), before and after inhibition of NO synthase by NG-nitro-l-arginine methyl ester (l-NAME) (30 microM), CYT P-450 by SKF525A (1 microM), or COX by indomethacin (10 microM). CP pressure, left ventricular systolic pressure (VSP) and ventricular diastolic pressure (VDP), and heart rate (HR) were monitored continuously during the experiments. Mechanical increases in CF resulted in gradual change in CP pressure (+20% to +100%), left VSP (+5% to +40%) and VDP (+2% to +25%), whereas HR was not affected. In presence of l-NAME, the positive inotropic response and negative lusitropic effect of CF changes were similar. Exposure to SKF525A did not modify cardiac response to mechanical increases in CF. In presence of COX inhibitor indomethacin, left VSP rose to a level similar to that observed in control conditions, whereas VDP deteriorated further. These results suggest that mediators originating from NO synthase, CYT P-450, or COX do not contribute to positive inotropic response elicited by increased CP. However, COX derivatives seem to attenuate impairment of myocardial relaxation observed in these conditions. Such findings may have implications in development of therapeutics for patients with myocardial diastolic dysfunction.

Increased coronary perfusion augments cardiac contractility in the rat through stretch-activated ion channels

The role of stretch-activated ion channels (SACs) in coronary perfusion-induced increase in cardiac contractility was investigated in isolated isometrically contracting perfused papillary muscles from Wistar rats. A brief increase in perfusion pressure (3-4 s, perfusion pulse, n = 7), 10 repetitive perfusion pulses (n = 4), or a sustained increase in perfusion pressure (150-200 s, perfusion step, n = 7) increase developed force by 2.7 +/- 1.1, 7.7 +/- 2.2, and 8.3 +/- 2.5 mN/mm(2) (means +/- SE, P < 0.05), respectively. The increase in developed force after a perfusion pulse is transient, whereas developed force during a perfusion step remains increased by 5.1 +/- 2.5 mN/mm(2) (P < 0.05) in the steady state. Inhibition of SACs by addition of gadolinium (10 micromol/l) or streptomycin (40 and 100 micromol/l) blunts the perfusion-induced increase in developed force. Incubation with 100 micromol/l N(omega)-nitro-L-arginine [nitric oxide (NO) synthase inhibition], 10 micromol/l sodium nitroprusside (NO donation) and 0.1 micromol/l verapamil (L-type Ca(2+) channel blockade) are without effect on the perfusion-induced increase of developed force. We conclude that brief, repetitive, or sustained increases in coronary perfusion augment cardiac contractility through activation of stretch-activated ion channels, whereas endothelial NO release and L-type Ca(2+) channels are not involved.

Microvascular pressure measurement reveals a coronary vascular waterfall in arterioles larger than 110 microm

Pressure-flow relationships at the entrance of the coronary circulation in the diastolic myocardium exhibit a zero-flow pressure intercept (P(int)). We tested whether this intercept is the same throughout the vascular bed. Microvascular pressure-flow relationships were therefore measured in vessels of various sizes of the maximally dilated vasculature of perfused unstimulated papillary muscle using the servo-null technique. From these relationships, P(int) were calculated with nonlinear regression. The P(int) at the level of the septal artery (diameter, 150-250 microm) was 23.2 +/- 4.4 cmH2O (n = 12). In arterioles with a diameter range between 24 and 110 microm, P(int) was 1.7 +/- 0.5 cmH2O (n = 6, P < 0.01), significantly lower than in the septal artery but significantly higher than zero, and not dependent on vessel size. In venules with the same diameters, P(int) was 1.1 +/- 1.1 cmH2O (n = 4), which was not different from zero. We conclude that, in the dilated vascular bed of the papillary muscle, two vascular waterfalls are found. The first waterfall is located in arterioles between 150 and 110 microm. The second waterfall is probably located in the small postcapillary venules.

Decrease in coronary vascular volume in systole augments cardiac contraction

Coronary arterial inflow is impeded and venous outflow is increased as a result of the decrease in coronary vascular volume due to cardiac contraction. We evaluated whether cardiac contraction is influenced by interfering with the changes of the coronary vascular volume over the heart cycle. Length-tension relationships were determined in Tyrode-perfused rat papillary muscle and when coronary vascular volume changes were partly inhibited by filling it with congealed gelatin or perfusing it with a high viscosity dextran buffer. Also, myocyte thickening during contraction was reduced by placing a silicon tube around the muscle. Increasing perfusion pressure from 8 to 80 cmH2O, increased developed tension by approximately 40%. When compared with the low perfusion state, developed tension of the gelatin-filled vasculature was reduced to 43 +/- 6% at the muscle length where the muscle generates the largest developed tension (n = 5, means +/- SE). Dextran reduced developed tension to 73 +/- 6% (n = 6). The silicon tube, in low perfusion state, reduced the developed tension to 83 +/- 7% (n = 4) of control. Time-control and oxygen-lowering experiments show that the findings are based on mechanical effects. Thus interventions to prevent myocyte thickening reduce developed tension. We hypothesize that when myocyte thickening is prevented, intracellular pressure increases and counteracts the force produced by the contractile apparatus. We conclude that emptying of the coronary vasculature serves a physiological purpose by facilitating cardiomyocyte thickening thereby augmenting force development.

Response of the intact canine left ventricle to increased afterload and increased coronary perfusion pressure in the presence of coronary flow autoregulation

BACKGROUND

Increased left ventricular (LV) contractile force or oxygen consumption has been documented with increased coronary arterial pressure (CAP) and flow (Gregg phenomenon). We investigated whether the increase in contractile force with increased LV afterload might be mediated by the concomitant increase in CAP when coronary autoregulation is intact.

METHODS AND RESULTS

The LV of 6 autonomically blocked open-chest dogs was perfused through the left main coronary artery by a cannula with a side gate to the aortic root. With the gate open, CAP increased from 77+/-20 to 93+/-20 mm Hg (P<0.05) with aortic constriction (AC). With the gate closed, CAP was maintained at a constant level of 100 mm Hg. A small reduction in the slope of the preload recruitable stroke work (PRSW) relationship was observed with AC, but this response was not altered by the coronary perfusion gate position. The end-systolic pressure-volume (ESPV) relationship shifted upward significantly with AC (P<0.001), but this shift was not greater with open-gate perfusion than with closed-gate perfusion. Furthermore, with coronary autoregulation intact, wide changes in CAP (between 60 and 180 mm Hg, n=5) did not alter either the PRSW or ESPV relationship. In contrast, when autoregulation was abolished with intracoronary adenosine (n=6), both indexes of contractility increased progressively with increased CAP.

CONCLUSIONS

The concomitant increase in CAP with increased afterload in the intact canine LV does not contribute to the afterload-induced increase in contractile force. Coronary perfusion pressure per se does not influence LV contractile function. Coronary perfusion pressure influences contractility only when coronary flow changes.

The left ventricular contractility of the rat heart is modulated by changes in flow and alpha 1-adrenoceptor stimulation

Myocardial contractility depends on several mechanisms such as coronary perfusion pressure (CPP) and flow as well as on alpha 1-adrenoceptor stimulation. Both effects occur during the sympathetic stimulation mediated by norepinephrine. Norepinephrine increases force development in the heart and produces vasoconstriction increasing arterial pressure and, in turn, CPP. The contribution of each of these factors to the increase in myocardial performance needs to be clarified. Thus, in the present study we used two protocols: in the first we measured mean arterial pressure, left ventricular pressure and rate of rise of left ventricular pressure development in anesthetized rats (N = 10) submitted to phenylephrine (PE) stimulation before and after propranolol plus atropine treatment. These observations showed that in vivo alpha 1-adrenergic stimulation increases left ventricular developed pressure (P < 0.05) together with arterial blood pressure (P < 0.05). In the second protocol, we measured left ventricular isovolumic systolic pressure (ISP) and CPP in Langendorff constant flow-perfused hearts. The hearts (N = 7) were perfused with increasing flow rates under control conditions and PE or PE + nitroprusside (NP). Both CPP and ISP increased (P < 0.01) as a function of flow. CPP changes were not affected by drug treatment but ISP increased (P < 0.01). The largest ISP increase was obtained with PE + NP treatment (P < 0.01). The results suggest that both mechanisms, i.e., direct stimulation of myocardial alpha 1-adrenoceptors and increased flow, increased cardiac performance acting simultaneously and synergistically.

Perfusion-induced changes in cardiac contractility depend on capillary perfusion

The perfusion-induced increase in cardiac contractility (Gregg phenomenon) is especially found in heart preparations that lack adequate coronary autoregulation and thus protection of changes in capillary pressure. We determined in the isolated perfused papillary muscle of the rat whether cardiac muscle contractility is related to capillary perfusion. Oxygen availability of this muscle is independent of internal perfusion, and perfusion may be varied or even stopped without loss of function. Muscles contracted isometrically at 27 degrees C (n = 7). During the control state stepwise increases in perfusion pressure resulted in all muscles in a significant increase in active tension. Muscle diameter always increased with increased perfusion pressure, but muscle segment length was unaffected. Capillary perfusion was then obstructed by plastic microspheres (15 microns). Flow, at a perfusion pressure of 66.6 +/- 26.2 cmH2O, reduced from 17.6 +/- 5.4 microliters/min in the control state to 3.2 +/- 1.3 microliters/min after microspheres. Active tension developed by the muscle in the unperfused condition before microspheres and after microspheres did not differ significantly (-12.8 +/- 29.4% change). After microspheres similar perfusion pressure steps as in control never resulted in an increase in active tension. Even at the two highest perfusion pressures (89.1 +/- 28.4 and 106.5 +/- 31.7 cmH2O) that were applied a significant decrease in active tension was found. We conclude that the Gregg phenomenon is related to capillary perfusion.

How cardiac contraction affects the coronary vasculature

We modeled the influence of cardiac contraction on maximally dilated coronary blood vessels, whether single or in juxtaposition, taking into account the nonlinear material properties of both the vascular wall and the myocardium. We calculated pressure-area relations of single, embedded coronary blood vessels, and used these relations to calculate diastolic and systolic coronary pressure-flow relations in a model of the coronary vasculature. The model shows that the change in myocardial material properties during contraction can explain the decrease in coronary vessel area and coronary flow generally observed in experiments. The model also shows that arterioles can be protected from the compressive action of the cardiac muscle by the presence of accompanying venules, which is favorable for coronary blood flow.

Echocardiographic contrast agents and left ventricular contractility: evaluation using an isolated rabbit heart model

The effects of Albunex (Molecular Biosystems, Inc., San Diego, Calif.) and a second generation contrast agent, FS069, on left ventricular (LV) contractility were evaluated using an isolated rabbit heart model under constant loading conditions and heart rate. Contrast injections (2 ml total volume) were performed in two separate protocols (N1 = 6, N2 = 6). In protocol 1, various doses of Albunex (0.1 to 2.0 ml in saline solution) were used, and paired control injections of a matched dose of 5% solution of human albumin in saline solution were administered. In protocol 2, LV contractility was assessed during injections of the following solutions: (1) 1:250 suspension of FS069 in saline solution, which caused optimal myocardial contrast enhancement; (2) a 1:25 suspension of FS069; (3) a 1:25 suspension of FS069 prefiltered using an 8 microns pore filter; and (4) 2 ml saline solution as a control. Instantaneous LV pressure was analyzed for variations in peak systolic pressure (peak P) and maximum pressure derivative (peak P'), both indices of LV contractility under conditions of fixed heart rate and chamber volume. Albumin alone caused a transient, dose-dependent depression of LV contractility, reflected by decreases in both peak P and peak P' values. These decreases presumably were caused by the decreasing availability of ionized calcium as a result of calcium binding. No further decrease in contractility was noted when Albunex microspheres were present in the solution. Saline injections caused a transient minor increase in LV contractility, reflected by increases of 4.5% +/- 1.1% and 10.6% +/- 3.8% in peak P and peak P' values, respectively. These levels returned to baseline levels within 2 minutes. A similar response was observed when a 1:250 suspension of FS069 was used. The 1:25 suspension of FS069 caused a bimodal response, with initial rises in peak P and peak P' levels (5.2% +/- 3.6% and 12.8% +/- 6.5%, respectively), followed by minor reductions in contractility (2.0% +/- 2.4% and 1.7% +/- 2.1%, respectively). The latter decrease in contractility caused by the 1:25 suspension of FS069 was eliminated by filtering. The isolated rabbit heart model is a highly sensitive tool that allows accurate and direct assessment of possible adverse effects of intravascular contrast agents on LV contractility. Using this model, we showed that neither Albunex microspheres nor FS069 microspheres impaired myocardial contractility.

Heart rate affects the dependency of myocardial oxygen consumption on flow in goats

The effect of flow steps in coronary arterial flow (Qa) on myocardial oxygen consumption (MVo2) was investigated at different heart rates (HR) to further elucidate the dependency of myocardial oxygen consumption on perfusion. In six anesthetized goats the left main coronary artery and the great cardiac vein were cannulated. The hearts were paced alternately at 60 and 130 beats per min. Flow steps were applied at both HR during control and maximal vasodilation by adenosine. MVo2, in steady state before and after the flow step, was calculated by multiplication of Qa and arterio-venous oxygen content difference (Fick's law). Heart rate affected the MVo2 dependency on flow during control as well as during maximal vasodilation. With vascular tone present, the MVo2 dependency on flow (DeltaMVo2/DeltaQa), in mu l O2/ml, was 16.0 +/- 3.6 at HR 60 and 21.7 +/- 3.9 at HR 130. During maximal vasodilation, these values were 9.5 +/- 2.9 and 17.0 +/- 5.3 at HR 60 and 130, respectively. The higher MVo2 dependency on flow at high HR may be explained via a dependency of MVo2 on microvascular pressure. The pressure change in the microvessels induced by a flow step is probably larger at high HR than at low HR because of increased venous resistance at high HR, due to increased compression by the heart contraction.

Oxygen exchange in the isolated, arrested guinea pig heart: theoretical and experimental observations

A model of oxygen transport in perfused myocardial tissue is presented. Steady-state conditions are assumed in order to mimic the metabolic rate of the arrested heart. The model incorporates Michaelis-Menten dependence of mitochondrial oxygen consumption, oxymyoglobin saturation and oxyhemoglobin saturation on oxygen partial pressure (PO2). The transport equations model both the advective supply of oxygen via the coronary circulation and the diffusive exchange of oxygen between tissues and environment across the epicardial and endocardial surfaces. The left ventricle is approximated by an axisymmetric prolate spheroid and the transport equations solved numerically using finite element techniques. Solution yields the PO2 profile across the heart wall. Integration of this profile yields the simulated rate of metabolic oxygen uptake determined according to the Fick principle. Correction for the diffusive flux of oxygen across the surfaces yields the simulated true metabolic rate of oxygen consumption. Simulated values of oxygen uptake are compared with those measured experimentally according to the Fick principle, using saline-perfused, Langendorff-circulated, K(+)-arrested, guinea pig hearts. Four perfusion variables were manipulated: arterial PO2, environmental PO2, coronary flow and perfusion pressure. In each case agreement between simulated and experimentally determined rates of oxygen consumption gives confidence that the model adequately describes the advective and diffusive transport of oxygen in the isolated, arrested, saline-perfused heart.

The metabolic consequences of an increase in the frequency of stimulation in isolated ferret hearts

1. The metabolic consequences of an increase in the frequency of stimulation were examined in isolated ferret hearts. Intracellular pH (pHi) and the intracellular concentrations of phosphocreatine ([PCr]i), inorganic phosphate ([Pi]i) and ATP were measured by 31P nuclear magnetic resonance (NMR) spectroscopy. 2. Increasing the stimulus rate from 0.1-0.7 to 2 Hz caused an increase in [Pi]i and a decrease recovery of both [PCr]i and [Pi]i during continued stimulation. There was no change in [ATP]i during stimulation at 2 Hz. Increasing the stimulus rate caused an intracellular acidosis of around 0.1 pH units. 3. Increasing the stimulus rate generally caused an initial increase in developed pressure, followed by a decrease over 1-2 min to a steady level slightly lower than developed pressure at the low (control) stimulus rate. The increase in stimulus rate caused a 4- to 6-fold increase in time-averaged muscle activity. 4. Both oxygen uptake and production of lactate increased on 2 Hz stimulation. Lactate production accounted for less than 5% of ATP production at low or high stimulus rates, suggesting that significant anoxia was not occurring during stimulation. The observed lactate production was, however, sufficient to explain most of the intracellular acidosis observed when the stimulus rate was raised. When glycolysis was prevented by removal of glucose and depletion of glycogen stores, 2 Hz stimulation was accompanied by an intracellular alkalosis rather than an acidosis, suggesting that lactate production by glycolysis was the cause of the intracellular acidosis. 5. Reducing the rate of glycolysis increased the size of changes in [PCr]i and [Pi]i evoked by stimulation at 2 Hz. Furthermore, there was now no partial reversal of the changes in [PCr]i and [Pi]i during 2 Hz stimulation. 6. When oxidative phosphorylation was inhibited by replacing O2 with N2, increasing the rate of stimulation from 0.1-0.7 to 1-2 Hz caused an initial increase followed by a large fall in developed pressure, which declined to a level well below that at the control stimulus rate. The increase in stimulus rate was accompanied by a large fall in [PCr]i, an increase in [Pi]i, and an intracellular acidosis of 0.1-0.3 pH units. The fall in developed pressure was consistent with the known effects of the rise in [Pi]i and the fall in pHi on the contractile apparatus.