Elective cardiac arrest with a hyperpolarizing adenosine triphosphate-sensitive potassium channel opener. A novel form of myocardial protection?

BACKGROUND: Hyperkalemic depolarized cardiac arrest has been the cornerstone of myocardial protection during cardiac surgery for more than 30 years. Many of the advances in myocardial protection seek to minimize the cellular damage and to reduce the ongoing metabolic processes occurring as a direct consequence of the depolarized state. Ideally, cardiac arrest at hyperpolarized cellular membrane potentials--the natural resting state of the heart--will meet all the requirements of modern cardioplegia, namely, electromechanical asystole and cardiac relaxation, while preserving the vital integrity of the heart itself. METHODS AND RESULTS: To determine whether activation of adenosine triphosphate-sensitive potassium channels by pharmacologic agents could produce hyperpolarized cardiac arrest, we tested the ability of aprikalim, a known adenosine triphosphate-sensitive potassium channel opener, to arrest the intact beating heart. In a normothermic (37 degrees C) isolated rabbit heart preparation, aprikalim was found to rapidly shorten the action potential duration and produce cardiac asystole that was maintained during 20 minutes of "no-flow" global ischemia without a rise in end-diastolic pressure. Cardiac rhythm and function were fully restored by reperfusion alone (developed pressure was 100.6% +/- 7.9% of prearrest value after 30 minutes of reperfusion). In contrast, 20 minutes of unprotected normothermic global ischemia resulted in a 2.7 +/- 0.55 mmHg rise in end-diastolic pressure and only 58.2% +/- 3.8% recovery of developed pressure after 30 minutes of reperfusion. By way of comparison, 20 minutes of standard hyperkalemic depolarized normothermic rest was accompanied by a 1.2 +/- 0.6 mmHg rise in end-diastolic pressure and only 80.8% +/- 2.6% recovery of developed pressure after 30 minutes of reperfusion. To directly compare hyperkalemic depolarized cardiac arrest to hyperpolarized cardiac arrest induced by potassium channel openers and to better define the characteristics of such hyperpolarized arrest, we studied a fixed (4 mmHg rise in end-diastolic pressure--contracture) ischemic injury model. The time to development of the contracture was prolonged by hyperkalemic arrest (35.8 +/- 1.7 minutes) and significantly more so by hyperpolarized arrest (47.0 +/- 3.3 minutes) when compared with that of unprotected hearts (24.0 +/- 1.2 minutes). Moreover, aprikalim resulted in significantly better postischemic recovery of function (developed pressure was 69.0% +/- 6.7% of prearrest value after 30 minutes of reperfusion) than after no cardioplegia (45.4% +/- 7.5%) or standard hyperkalemic cardioplegia (44.3% +/- 5.7%). CONCLUSIONS: Pharmacologic activation of adenosine triphosphate-sensitive potassium channels can result in predictable and sustainable hyperpolarized cardiac arrest that is reversible by reperfusion. This method of myocardial protection was found to fully preserve cardiac electromechanical function after a 20-minute period of global normothermic ischemia. Furthermore, hyperpolarized arrest induced by potassium channel openers significantly prolonged the period to the development of contracture and afforded a significantly better postischemic recovery of function than obtained in either hearts protected with hyperkalemic depolarized arrest or those not protected by any form of cardioplegia.

Hyperpolarized cardiac arrest with a potassium-channel opener, aprikalim.

Cardioplegic solutions that arrest the heart at or near the resting membrane potential may provide better myocardial protection than standard depolarizing hyperkalemic cardioplegia by reducing both metabolic demand and harmful transmembrane ion fluxes. This hypothesis was investigated in an isolated, blood-perfused, rabbit heart Langendorff model during 30 minutes of normothermic global ischemia. Hyperpolarized cardiac arrest induced by aprikalim, an opener of adenosine triphosphate-dependent potassium channels, was compared with hyperkalemic depolarized arrest and with unprotected global ischemia. Left ventricular pressure was recorded over a wide range of balloon volumes before ischemia and 30 minutes after reperfusion. End-diastolic pressure versus balloon volume data were fitted to a two-coefficient exponential relationship. Changes in the diastolic compliance of the left ventricle were assessed by comparison of preischemic and postischemic coefficients within each cardioplegia group. Postischemic recovery of developed pressure was used to assess changes in left ventricular systolic function. The tissue water content of each heart was also determined. Myocardial protection with aprikalim resulted in better postischemic recovery of developed pressure (90% +/- 9%) than either protection with hyperkalemic cardioplegia (73% +/- 11%) or no protection (62% +/- 9%). Myocardial tissue water content in hearts protected with hyperkalemic cardioplegia (77.4% +/- 1.4%) was less than the tissue water content of either unprotected hearts (79.4% +/- 1.2%) or hearts protected with aprikalim (78.7% +/- 0.9%). Despite these differences, neither hyperkalemic cardioplegia (p = 0.15) nor aprikalim cardioplegia (p = 0.30) was associated with a significant postischemic decrease in ventricular compliance. By contrast, unprotected global ischemia was associated with a significant decrease in ventricular compliance (p < 0.001).

Excitation-contraction coupling in heart cells. Roles of the sodium-calcium exchange, the calcium current, and the sarcoplasmic reticulum.

Experimental data do not support the idea that excitation-contraction coupling in heart muscle can be explained by a simple calcium-induced calcium release mechanism alone or a simple voltage-dependent calcium release mechanism alone. Our data on excitation-contraction coupling combined with the results of others suggest the need to either develop more complex and more sophisticated single-mechanism models, or to establish dual-control models.

Is there an alternative to potassium arrest?

BACKGROUND: Mounting clinical and experimental evidence suggests that postoperative myocardial dysfunction is a frequent consequence of surgical global ischemia and reperfusion, despite our modern techniques of myocardial protection. The ubiquitous use of hyperkalemic depolarizing solutions in all forms of cardioplegia may be partly responsible for this phenomenon because of the known ongoing metabolic processes and damaging transmembrane ionic fluxes that occur at depolarized membrane potentials. Cardiac arrest at hyperpolarized potentials, the natural resting state of the heart, may avoid the shortcomings of depolarized arrest and provide an alternative means of myocardial protection. METHODS: An adenosine triphosphate-sensitive potassium channel opener, aprikalim, was used to induce hyperpolarized arrest. Aprikalim was able to produce sustained and reproducible electromechanical arrest that was reversible by reperfusion. RESULTS: In isolated heart models, when compared with depolarized hyperkalemic arrest, hyperpolarized arrest afforded better protection from global normothermic ischemia and resulted in better postischemic recovery of function upon reperfusion. Preliminary studies in a porcine cardiopulmonary bypass model also have revealed that hyperpolarized arrest can be achieved in a model more closely approximating the clinical setting, and can effectively protect the heart during normothermic surgical global ischemia. CONCLUSIONS: Hyperpolarized cardiac arrest may offer an effective alternative to traditional potassium arrest.

Electrophysiologic consequences of hyperkalemic cardioplegia during surgical ischemia.

Myocardial protection strategies use cardioplegic solutions to reduce the injury induced by surgical ischemia and reperfusion. However, there is a high incidence of electrophysiologic abnormalities after cardioplegic arrest. A computerized epicardial mapping system in a porcine cardiopulmonary bypass model was used to measure the electrophysiologic consequences of different myocardial protection techniques. Both warm and cold, crystalloid and blood cardioplegic solutions were compared. The effects of hypothermia and prolonged cardiopulmonary bypass were examined in a control group that underwent a 2-hour period of hypothermia without cardioplegia or aortic cross-clamping, followed by 2 hours of normothermic reperfusion. Isochronous activation maps, unipolar electrograms, ventricular refractory periods, and pacing thresholds were measured before cardioplegic arrest and during reperfusion. Compared with the control group, crystalloid cardioplegia, but not blood cardioplegia, was accompanied by large changes in the pattern of ventricular activation and by persistent (> 2 hours) and significant slowing of the time required for complete ventricular activation. This was not the result of hypoxia. Moreover, the effective refractory period and the pacing threshold were unchanged by any cardioplegia. Our data suggest that crystalloid cardioplegia increases myocardial resistance to current flow leading to a derangement of electrical impulse propagation that may underlie arrhythmogenesis.

Autonomous control of phosphatidylinositol turnover by histamine and acetylcholine receptors in the N1E-115 neuron-like cell line.

Histamine was found to stimulate the turnover of phosphatidylinositol (PI) in cultures of neuron-like NE-115 cells. Turnover was measured by increased production of [3H]inositol phosphates (breakdown) and by accelerated incorporation of 32P into PI (resynthesis). Data were consistent with hydrolysis of polyphosphoinositides being the initial event in receptor-stimulated PI turnover. This response to histamine desensitized within 10 min. Receptor systems for histamine and acetylcholine were tested for possible interactions: PI turnover in response to dual stimulation was approximately equal to the sum of the individual responses while prior desensitization of the acetylcholine receptor system had no effect on subsequent stimulation of the histamine receptor system. These results are consistent with the hypothesis that components of acetylcholine and histamine receptor systems responsible for PI turnover are autonomously organized and regulated. and regulated.

Benign metastasizing leiomyoma.

Benign metastasizing leiomyoma is a rare clinical entity that has been described in several previous reports. Although the exact pathophysiology of the disease is unknown, two predominant theories exist: (1) metastasis from an existing leiomyoma (commonly seen with uterine leiomyoma) or (2) multicentric leiomyomatous growths rather than actual metastases. We present an interesting case in which several elements of the patient's history complicated the differential diagnosis.

Intrathoracic herniation and perforation 18 years after open Nissen fundoplication.

Nissen fundoplication is the most commonly performed surgical procedure in the management of gastroesophageal reflux disease. Esophageal and gastric perforations most commonly occur in the perioperative period and carry significant morbidity. We describe a unique case of intrathoracic gastric wrap perforation and its suspected pathophysiology almost two decades after the original procedure.

Hyperpolarized arrest attenuates myocardial stunning following global surgical ischemia: an alternative to traditional hyperkalemic cardioplegia?

There is clinical evidence that myocardial stunning is a frequent sequela of surgical global ischemia, despite our modern techniques of myocardial protection. The ubiquitous usage of hyperkalemic depolarizing solutions in all forms of cardioplegia may be partly responsible for this phenomenon because of the known ongoing metabolic requirements and damaging transmembrane ionic fluxes that occur at depolarized membrane potentials. Cardiac arrest at hyperpolarized potentials, the natural resting state of the heart, may avoid the shortcomings of depolarized arrest and provide an alternative means of myocardial protection. To test this hypothesis, a potassium channel opener, aprikalim, was used to induce hyperpolarized arrest in an isolated rabbit heart model. Aprikalim was able to produce sustained and reproducible electromechanical arrest that was reversible by reperfusion. When compared with depolarized hyperkalemic arrest, hyperpolarized arrest afforded better protection after short 20-minute periods of global ischemia and resulted in less myocardial stunning. Moreover, aprikalim was able to significantly prolong the time to ischemic contracture and improve functional recovery after the onset of ischemic contracture when compared with either traditional hyperkalemic cardioplegia or no cardioplegia at all. There was a dose dependence to the protective effect of aprikalim. Preliminary studies in the intact porcine cardiopulmonary bypass model also have revealed that hyperpolarized arrest can effectively protect the heart during surgical global ischemia.

Calcium current in isolated neonatal rat ventricular myocytes.

1. Calcium currents (ICa) from neonatal rat ventricular heart muscle cells grown in primary culture were examined using the 'whole-cell' voltage-clamp technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981). Examination of ICa was limited to one calcium channel type, 'L' type (Nilius, Hess, Lansman & Tsien, 1985), by appropriate voltage protocols. 2. We measured transient and steady-state components of ICa, and could generally describe ICa in terms of the steady-state activation (d infinity) and inactivation (f infinity) parameters. 3. We observed that the reduction of ICa by the calcium channel antagonist D600 can be explained by both a shift of d infinity to more positive potentials as well as a slight reduction of ICa conductance. D600 did not significantly alter either the rate of inactivation of ICa or the voltage dependence of f infinity. 4. The calcium channel modulator BAY K8644 shifted both d infinity and f infinity to more negative potentials. Additionally, BAY K8644 increased the rate of inactivation at potentials between +5 and +55 mV. Furthermore, BAY K8644 also increased ICa conductance, a change consistent with a promotion of 'mode 2' calcium channel activity (Hess, Lansman & Tsien, 1984). 5. We conclude that, as predicted by d infinity and f infinity, there is a significant steady-state component of ICa ('window current') at plateau potentials in neonatal rat heart cells. Modulation of the steady-state and transient components of ICa by various agents can be attributed both to specific alterations in d infinity and f infinity and to more complicated alterations in the mode of calcium channel activity.

Calcium current in single human cardiac myocytes.

INTRODUCTION: Significant species-, tissue-, and age-dependent differences have been described for the L-type calcium current (ICa). Therefore, extrapolation of data obtained from the many animal models to human cardiac physiology is difficult. In this study, we have characterized the voltage-dependent properties of ICa from pediatric and adult, atrial and ventricular human heart tissue. METHODS AND RESULTS: ICa was measured in single human heart muscle cells using the "whole cell," voltage clamp method. Single myocytes were isolated from myocardial specimens obtained intraoperatively from both pediatric and adult patients (ages 3 months to 75 years) undergoing cardiac surgery. Cells obtained for these experiments appeared to be healthy; the resting potential was between -80 and -85 mV. The action potential shape and duration and current-voltage relationship for ICa were similar to that reported by others for human heart cells. The steady-state activation variable, d infinity, was found to be similar in both pediatric atrial and ventricular cells but shifted approximately 5 mV negative in the adult atrial and ventricular cells. ICa of all cells displayed biexponential inactivation and steady-state inactivation was incomplete at positive potentials (steady-state inactivation curves turned up at positive potentials) consistent with inactivation arising from voltage-dependent and calcium-dependent processes as reported in heart cells from many species. The potential of maximal inactivation was more negative for adult cells (around -10 mV) than pediatric cells (around 0 mV). Estimates of the calcium "window" current, using a modified Hodgkin-Huxley model, could explain measured differences in action potential shape and duration. CONCLUSION: Human cardiac ICa can be investigated using whole cell, voltage clamp methods and a modified Hodgkin-Huxley model. Quantitative characterization of many of the properties of ICa in human heart tissue suggests that important species differences do exist and that further investigations are required to characterize the dependence of inactivation on [Ca2+]i in human heart cells. Since the array of characteristics of ICa in different species varies, the study of human myocardial cells per se continues to be important when examining human cardiac physiology.

Changes in the calcium current of rat heart ventricular myocytes during development.

1. Calcium current (ICa) was recorded in single rat heart cells at two periods during development: (1) at 2-7 days post-partum (neonatal), and (2) at 6-8 weeks (adult). 2. We measured both transient and steady-state components of ICa and could describe ICa in terms of the steady-state activation (d infinity) and inactivation (f infinity) parameters, the channel reversal potential (Echannel) and a relative conductance parameter, gr. 3. In adult single cells, the application of ryanodine (10 microM), an agent known to alter the function of the sarcoplasmic reticulum (SR), abolished contraction rapidly and increased ICa. Ryanodine also produced a 13 mV shift in f infinity towards more positive potentials and altered its slope, while producing a small increase in gr but no effect on d infinity. In neonatal single cells, ryanodine (10 microM) had no significant effect on contraction, ICa, d infinity, f infinity, or gr. Caffeine (10 mM), a less specific agent widely used to investigate sarcoplasmic reticulum function, had actions similar to those of ryanodine. 4. In adult myocytes, when EGTA (10 or 20 mM) or bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM) were included in the pipette solution, contractions were rapidly abolished, while a small (4 mV) shift of f infinity to more positive potentials was seen. A large additional shift of f infinity was observed when ryanodine (10 microM) was added to the superfusion solution in the continued presence of EGTA or BAPTA. The alterations of ICa in EGTA (or BAPTA) plus ryanodine were the same as those seen in ryanodine alone. In neonatal cells, in contrast, when EGTA or BAPTA were included in the pipette solution we observed only a small effect on f infinity and the application of ryanodine had no effect. 5. Electron micrographs of our preparations show that the dissociated adult cells have sharp sarcolemmal borders, fully developed sarcomeres with T-tubules and sarcoplasmic reticulum membranes. In contrast, the neonatal cells that we use have few of these intracellular structures. Our observations in these preparations are consistent with the work of others (e.g. Penefsky, 1974; Hirakow & Gotoh, 1975; Ishikawa & Yamada, 1975; Legato, 1975; Hoerter, Mazet & Vassort, 1981). 6. Our data suggest that fully developed sarcoplasmic reticulum in rat heart cells can affect ICa.(ABSTRACT TRUNCATED AT 400 WORDS)

Proteins and insulin release: a dual role of amino-acids and intestinal hormones.

In two subjects concurrent infusion of amino-acids and the hormones secretin and pancreozymin provoked much higher plasma insulin levels than did administration of amino-acids or hormones individually. It is suggested that this may be a physiological phenomenon, augmenting the release of insulin from the pancreas after a meal containing protein.

Cross-sectional-type weight reference values for village children under five years in Lesotho.

Cross-sectional-type reference values for weight attained are described for village children under five years in rural Lesotho (formerly Basutoland). Weight measurements derive from observations on 1317 children attending an Under-Fives clinic; it is estimated that 60-70% of the children under five in the catchment area were represented. 4585 weighings on boys and 4826 weighings for girls are included in the analysis. Figures of weight-for-age of boys and girls are given separately as centile distributions suitable for use on Growth Charts. Lesotho 50 centile approximates to 3 centile of British children and slightly exceeds 80% Harvard standard. Weight attained for age is similar, in both sexes, to reports from other less-priviledged urban and rural areas, emphasizing the relative importance of environmental as compared to genetic influences in determining weight-for-age in early childhood. It is suggested that the construction of locally-derived growth reference values is both appropriate and practicable.

Phorbol ester increases calcium current and simulates the effects of angiotensin II on cultured neonatal rat heart myocytes.

The effects of increased protein kinase C activity were studied in neonatal rat myocytes grown in primary culture. The changes in mechanical and electrical behavior, as well as protein phosphorylation, that followed the apparent activation of protein kinase C by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) were examined. As spontaneous beating frequency was increased minimally by 10 nM TPA and by 100% with 85 nM TPA, shortening amplitude, shortening velocity, and relaxation velocity decreased concomitantly. In contrast, 4-alpha-phorbol-12,13-didecanoate (alpha-PDD), which does not activate protein kinase C, had no effect on beating behavior at 800 nM. In voltage-clamped single myocytes, both steady-state and transient components of the cadmium-sensitive calcium current were increased by the addition of TPA (65 nM). Neither the time constant for the inactivation of the transient component of this calcium current nor the reversal potential was altered by TPA. The phosphorylation state of a discrete set of proteins, with apparent molecular weights of 32 and 83 kDa, was enhanced when TPA was added to intact myocytes. Angiotensin II enhances the phosphorylation state of the same set of proteins as observed with TPA. We conclude that activation of protein kinase C can modify mechanical behavior and increase L-type Ca2+ channel activity in cultured neonatal rat ventricular myocytes. The remarkable similarity in mechanical, electrical, and protein phosphorylation responses of cultured neonatal myocytes following TPA or angiotensin II application indicate that protein kinase C may mediate the action of angiotensin II.

Stretch-activated channel blockers modulate cell volume in cardiac ventricular myocytes.

Stretch-activated channels (SAC) are postulated to regulate cell volume. While this hypothesis is appealing, direct evidence is lacking. Using digital video microscopy, we found that pharmacological blockade of SACs alters the cell volume of isolated rabbit ventricular myocytes during hypoosmotic stress. Under control conditions, relative cell volume increased from 1.0 to 1.311 +/- 0.019 after 10 min in 195 mosmol/l solution. The cation SAC blocker gadolinium (Gd3+; 10 microM) reduced the amount of swelling in hypoosmotic solution by 24% and induced a regulatory volume decrease otherwise not observed. In contrast, the anion SAC blocker 9-anthracene carboxylic acid (9-AC; 1 mM) increased swelling by 44% under the same conditions. Based on the direction of SAC currents, Gd3+ and 9-AC are expected to have opposite effects on cell volume. Furthermore, Gd3+ and 9-AC changed cell volume by only approximately 2% in isosmotic solutions when SACs are expected to be closed. This supports the idea that Gd3+ and 9-AC affect stretch-activated transport processes. In contrast, omitting bath Ca2+ did not alter cell volume under iso- or hypoosmotic conditions suggesting stretch-activated Ca2+ influx is not important in setting cell volume. Not all channels can affect cell volume. Opening ATP-sensitive K+ channels with aprikalim (100 microM) or blocking them with glibenclamide (1 microM) did not alter cell volume under isosmotic or hypoosmotic conditions. These data support the idea that SACs are involved in cardiac cell volume regulation.

Excitation-contraction coupling in heart muscle.

We have investigated the links between electrical excitation and contraction in mammalian heart muscle. Using isolated single cells from adult rat ventricle, a whole-cell voltage-clamp technique and quantitative fluorescence microscopy, we have measured simultaneously calcium current (ICa) and [Ca2+]i (with fura-2). We find that the voltage-dependence of ICa and the [Ca2+]i-transient and the dependence of [Ca2+]i-transient on depolarization-duration cannot both be readily explained by a simple calcium-induced Ca-release ('CICR') mechanism. Additionally, we find that when [Ca2+]i and [Na+]i are at their diastolic levels, activation of the Na-Ca exchange mechanism by depolarization does not measurably trigger the release of Ca2+i. Finally, measuring ICa in adult and neonatal rat heart cells and using the alkaloid ryanodine, we have carried out complementary experiments. These experiments show that there may be an action of ryanodine on ICa that is independent of [Ca2+]i and independent of a direct action of the alkaloid on the calcium channel itself. Along with experiments of others showing that ryanodine binds to the sarcoplasmic reticulum calcium-release channel/spanning protein complex, our data suggests a model to explain our findings. The model links the calcium channels responsible for ICa to the sarcoplasmic reticulum by means of one or more of the spanning protein(s). Information from the calcium channel can be communitated to the sarcoplasmic reticulum by this route and, presumably, information can move in the opposite direction from the sarcoplasmic reticulum to the calcium channel.