Nickel inhibits endothelin-induced contractions of vascular smooth muscle

In Vivo Comparative Study of Two Lactobionate Based Solutions for Prolonged Heart Preservation

The duration of safe heart preservation must be improved. Using a heterotopic heart transplantation model, we compared in vivo the recovery of rabbits hearts preserved with a K+ Lactobionate based fluid (UW: University of Wisconsin solution) or with a Na+Lactobionate based fluid. In the “preservation” group, hearts were cold stored (4°C) for 6 hours with UW (n=9) or Na+Lactobionate solution (n=9). In the “transplantation” group, cold storage was followed by 3 hours of reperfusion (UW: n=8, Na+Lactobionate solution: n=7). Functional recovery, adenine nucleotide pool, circulating blood cardiac enzymes, circulating blood and tissue malondialdehyde (MDA) were measured. Left ventricular enddiastolic and developed pressures at different preload levels were better after preservation with UW than with Na+Lactobionate solution (p<0.05). Also with UW, adenosine diphosphate and total adenine nucleotide content were significantly higher than with Na+Lactobionate solution (p<0.05) whereas adenosine triphosphate, monophosphate and energy charges were similar. Cardiac enzymes and tissue MDA were similar with UW and Na+Lactobionate solution. In circulating blood, MDA was not detected. These results enhance the superiority of UW solution over a Na+Lactobionate based solution for long term heart preservation.

Involvement of L-type Ca²⁺ channel and toll-like receptor-4 in nickel-induced interleukin-8 gene expression

The metal nickel (Ni(2+)) is found everywhere in our daily lives, including coins, costume jewelry, and even nuts and chocolates. Nickel poisoning can cause inflammatory reactions, respiratory diseases, and allergic contact dermatitis. To clarify the mechanism by which nickel induces mediators of inflammation, we used the human acute monocytic leukemia THP-1 cell line as a model. Interleukin (IL)-8 promoter activity as well as gene expression were tested by luciferase assay and real-time polymerase chain reaction. The underlying mechanisms of nickel-induced IL-8 were investigated. We found that nickel induced IL-8 gene expression via the L-type Ca(2+) channel, Toll-like receptor-4 (TRL-4) and nuclear factor NF-κB signal transduction pathways. Nickel activated NF-κB expression through extracellular signal-regulated kinase 1/2 phosphorylation and then increased IL-8 expression. Thus, the L-type Ca(2+) channel and TRL-4 play important roles in nickel-induced inflammatory gene expressions.

Epicardial measurement of alterations in extracellular pH and electrolytes during ischemia and reperfusion in cardiac surgery

Simultaneous measurements of extracellular pH, potassium (K(+)), and calcium (Ca(2+)) activity might be indicative of myocardium vitality or ischemia. Ten consecutive patients undergoing elective coronary artery bypass grafting were studied. Epicardial extracellular pH, potassium, and calcium were measured by a miniaturized disposable multi-sensor probe. Blood gases and electrolytes were derived with measurements of arterial and mixed venous blood samples at intervals during surgery. The mean epicardial baseline levels for pH in all patients were 8.04+/-0.22 arbitrary units (AU) for the right ventricle (RV) and 8.03+/-0.21 AU for the left ventricle (LV); for Ca(2+) 0.23+/-0.07 mmol/l (RV) and 0.20+/-0.10 mmol/l (LV); and for K(+) 4.54+/-1.51 mmol/l (RV) and 4.38+/-0.57 mmol/l (LV). Before ischemia, epicardial pH was moderately (p<0.05), and K(+), and Ca(2+) were closely correlated (p<0.001) with blood values. During reperfusion, epicardial measurements were weakly correlated (p<0.001) with blood values for pH, venous K(+) and Ca(2+), but moderately correlated with arterial K(+) and Ca(2+) (p<0.01). The measurements indicated intraoperative episodes of ischemia and reperfusion with reproducible trends of extracellular pH, K(+), and Ca(2+), which results in electrolyte patterns applicable for detecting inadequate myocardial protection during cardiac surgery in patients.

Attentuation of cardiac stunning by losartan in a cellular model of ischemia and reperfusion is accompanied by increased sarcoplasmic reticulum Ca2+ stores and prevention of cytosolic Ca2+ elevation

This study investigates whether protective effects of an angiotensin II type 1 receptor antagonist (losartan) in ischemia and reperfusion are mediated by actions on Ca(2+) cycling. Effects of exposure to losartan (10 microM) in ischemia were evaluated in isolated guinea pig ventricular myocytes exposed to simulated ischemia and reperfusion at 37 degrees C. Field-stimulated myocytes were exposed to 30 min of simulated ischemia (hypoxia, acidosis, lactate, hyperkalemia, and glucose-free) and reperfusion with Tyrode's solution for 40 min. Cell shortening was measured with a video edge detector, and Ca(2+) concentration was measured with fura-2. Field-stimulated myocytes exhibited stunning in reperfusion, which was abolished in cells exposed to losartan. In microelectrode studies, losartan did not alter the responses of resting potentials or action potentials to ischemia and reperfusion. In the absence of losartan, diastolic Ca(2+) increased in ischemia, and Ca(2+) transients exhibited a rebound overshoot in early reperfusion. Losartan did not affect amplitudes of Ca(2+) transients in ischemia but prevented elevations in diastolic Ca(2+) in ischemia. Furthermore, losartan prevented the overshoot of Ca(2+) transients in early reperfusion and increased the magnitude of Ca(2+) transients in late reperfusion. Sarcoplasmic reticulum (SR) Ca(2+) stores, determined as Ca(2+) released by rapid application of 10 mM caffeine, were not altered in ischemia and reperfusion. However, losartan increased SR Ca(2+) stores in late reperfusion, even in cells that were not exposed to simulated ischemia. We conclude that losartan abolishes stunning in reperfusion by preserving normal diastolic Ca(2+) in ischemia and by increasing Ca(2+) transients through elevation of releasable SR Ca(2+).

Gender influences [Ca(2+)](i) during metabolic inhibition in myocytes overexpressing the Na(+)-Ca(2+) exchanger

BACKGROUND

The Na(+)-Ca(2+) exchanger (NCX) may contribute to Ca(2+) overload and injury in ischemic cardiomyocytes. Recently, NCX overexpression was reported to increase ischemia/reperfusion injury in male and oophorectomized female but not in female mice. We therefore measured the effects of gender and estrogen on [Ca(2+)](i) and [Na(+)](i) during metabolic inhibition (MI) in myocytes from wild-type (WT) and transgenic (TG) mice overexpressing NCX.

METHODS AND RESULTS

Flow cytometry was used with fluo 3 for [Ca(2+)](i) and sodium green for [Na(+)](i) measurements. Male TG mouse myocytes had higher [Ca(2+)](i) after 30 minutes of MI (1086+/-160 nmol/L, n=8) than male WT (688+/-104 nmol/L, n=9, P=0.01). The increase in [Ca(2+)](i) during MI induced by NCX overexpression in female myocytes was not significant, however (TG 552+/-62 nmol/L, n=9; WT 426+/-44 nmol/L, n=7). The magnitude of rise in [Ca(2+)](i) during MI was greater in male than female myocytes. KB-R7943, an NCX inhibitor, abolished the effect of NCX overexpression but did not totally eliminate the effect of gender on [Ca(2+)](i) during MI. NCX current density and basal Na(+) pump function were not influenced by gender. The rise in [Na(+)](i) during MI was greater in male than in female myocytes. Estrogen attenuated the increase in [Ca(2+)](i) and [Na(+)](i) in male myocytes during MI and abolished the gender difference in [Na(+)](i) during MI.

CONCLUSIONS

Increased expression of NCX results in a more marked rise in [Ca(2+)](i) during MI in male than in female mouse myocytes. This gender difference appears to be mediated in part by an inhibitory effect of estrogen on the rise in [Na(+)](i), an NCX modifier, during MI.

L-NAME inhibits Mg(2+)-induced rat aortic relaxation in the absence of endothelium

1 L-NG-nitro-arginine methyl ester (L-NAME; 100 microM), a nitric oxide synthase (NOS) inhibitor, reversed the relaxation induced by 3 microM acetylcholine (ACh) and 2-10 mM Mg2+ in endothelium-intact (+E) rat aortic rings precontracted with 1 microM phenylephrine (PE). In PE-precontracted endothelium-denuded (-E) rat aorta, 3 microM ACh did not, but Mg2+ caused relaxation which was reversed by L-NAME, but not by D-NAME. 2 The concentration response profiles of L-NAME in reversing the equipotent relaxation induced by 5 mM Mg2+ and 0.2 microM ACh were not significantly different. 3 L-NAME (100 microM) also reversed Mg(2+)-relaxation of -E aorta pre-contracted with 20 mM KCl or 10 microM prostaglandin F2alpha (PGF2alpha). L-NG-monomethyl-arginine (L-NMMA; 100 microM) was also effective in reversing the Mg(2+)-relaxation. 4 Addition of 0.2 mM Ni2+, like Mg2+, caused relaxation of PE-pre-contracted -E aorta, which was subsequently reversed by 100 microM L-NAME. 5 Reversal of the Mg(2+)-relaxation by 100 microM L-NAME in PE-precontracted -E aorta persisted following pre-incubation with 1 microM dexamethasone or 300 microM aminoguanidine (to inhibit the inducible form of NOS, iNOS). 6 Pretreatment of either +E or -E aortic rings with 100 microM L-NAME caused elevation of contractile responses to Ca2+ in the presence of 1 microM PE. 7 Our results suggest that L-NAME exerts a direct action on, as yet, unidentified vascular smooth muscle plasma membrane protein(s), thus affecting its reactivity to divalent cations leading to the reversal of relaxation. Such an effect of L-NAME is unrelated to the inhibition of endothelial NOS or the inducible NOS.

Overexpression of the cardiac Na+/Ca2+ exchanger increases susceptibility to ischemia/reperfusion injury in male, but not female, transgenic mice

Influx of Ca2+ into myocytes via Na+/Ca2+ exchange may be stimulated by the high levels of intracellular Na+ and the changes in membrane potential known to occur during ischemia/reperfusion. This increased influx could, in turn, lead to Ca2+ overload and injury. Overexpression of the cardiac Na+/Ca2+ exchanger therefore may increase susceptibility to ischemia/reperfusion injury. To test this hypothesis, the hearts of male and female transgenic mice, overexpressing the Na+/Ca2+ exchange protein, and hearts of their wild-type littermates, were perfused with Krebs-Henseleit buffer and subjected to 20 minutes of ischemia and 40 minutes of reperfusion. Preischemic left ventricular developed pressures and +dP/dtmax, as well as -dP/dtmin, were higher in the male transgenic hearts compared with wild-type, implying a role for Na+/Ca2+ exchange in the contraction, as well as the relaxation, phases of the cardiac beat. Postischemic function was lower in male transgenic than in male wild-type hearts (7+/-2% versus 32+/-6% of preischemic function), but there was no difference between female transgenic and female wild-type hearts, both at approximately 30% of preischemic function. To assess whether this male/female difference was due to female-specific hormones such as estrogen, the hearts of bilaterally ovariectomized and sham-operated transgenic females were subjected to the same protocol. The functional recoveries of ovariectomized female transgenic hearts were lower (17+/-3% of preischemic function) than those of wild-type and sham-operated transgenic females. The lower postischemic functional recovery in the male transgenic and female ovariectomized transgenic hearts correlated with lower recoveries of the energy metabolites, ATP and phosphocreatine, as measured by 31P nuclear magnetic resonance spectroscopy. Alternans were observed during reperfusion in male transgenic and female ovariectomized transgenic hearts only, consistent with intracellular Ca2+ overload. Western analyses showed that alterations in the expression of the Na+/Ca2+ exchange or L-type Ca2+ channel proteins were not responsible for the protection observed in the female transgenic hearts. In conclusion, in males, overexpression of the Na+/Ca2+ exchanger reduced postischemic recovery of both contractile function and energy metabolites, indicating that the Na+/Ca2+ exchanger may play a role in ischemia/reperfusion injury. From the studies of females, however, it appears that this exacerbation of ischemia/reperfusion injury by overexpression of the Na+/Ca2+ exchanger can be overcome partially by female-specific hormones such as estrogen.

Pharmacologic preconditioning induced by beta-adrenergic stimulation is mediated by activation of protein kinase C

Ischemic preconditioning (I-PC) occurs via activation of protein kinase C (PKC). This study was undertaken to determine whether pharmacologic preconditioning by beta-adrenergic stimulation (beta-PC) is mediated by PKC activation. Isolated rat hearts were subjected to 40-min ischemia and 30-min reperfusion. Beta-PC was induced by 0.25 microM isoproterenol pretreatment for 2 min followed by 10-min normoxic perfusion. Beta-PC enhanced the recovery of rate-pressure product of the ischemic/reperfused heart (79.1 +/- 8.4% vs. 12.4 +/- 1.6% of initial for Non-PC group, n = 6) and attenuated the release of creatine kinase during 30-min reperfusion (30.2 +/- 2.2 vs. 59.8 +/- 6.1 nmol/min/g wet wt for Non-PC group, n = 6), similar to an I-PC stimulus of 5-min ischemia and 5-min reperfusion. Treatment with 50 microM polymyxin B, a PKC inhibitor, abolished the cardioprotection of both beta-PC and I-PC. Furthermore, similar changes in subcellular distribution of PKC were induced by both beta-PC and I-PC. The changes in subcellular distribution of PKC-delta suggested its translocation from cytosol to membrane fraction, a marker of PKC activation. These results suggest that the cardioprotection induced by beta-PC, like I-PC, is mediated by PKC activation.

Pharmacological characterization of endothelin-induced contraction in the guinea-pig oesophageal muscularis mucosae

1. In the oesophageal muscularis mucosae, we examined the effects of endothelin-1 (ET-1), endothelin-2 (ET-2), endothelin-3 (ET-3) and sarafotoxin S6c (SX6c) as agonists, and FR139317, BQ-123 and RES-701-1 as endothelin receptor antagonists. 2. All of the endothelins produced tonic contractions which were frequently superimposed on rhythmic motility in a concentration-dependent manner. The order of potency (-log EC50) was ET-1 (8.61)=SX6c (8.65)>ET-2 (8.40)>ET-3 (8.18). 3. FR139317 (1-3 microM) and BQ-123 (1 microM) caused parallel rightward shifts of the concentration-response curve to ET-1, but at higher concentrations caused no further shift. RES-701-1 (3 microM) caused a rightward shift of the concentration-response curve to ET-1, while RES-701-1 (10 microM) had no additional effect. RES-701-1 (0.1-1 microM) concentration-dependently caused a rightward shift of the concentration-response curve to SX6c. The contraction to ET-1 (10 nM) in preparations desensitized to the actions of SX6c was greatly inhibited by pretreatment with FR139317 (10 microM). 4. Modulation of the Ca2+ concentration in the Krebs solution caused the concentration-response curve to ET-1 or SX6c to shift to the right and downward as external Ca2+ concentrations decreased. Verapamil (30 microM) abolished rhythmic motility induced by ET-1 or SX6c. Ni2+ (0.1 mM) weakly inhibited ET-1- or SX6c-induced tonic contraction. SK&F 96365 (60 microM) completely inhibited ET-1-induced contractions. 5. We conclude that there are two types of ET-receptors, excitatory ET(A)- and ET(B)-receptors in the oesophageal muscularis mucosae. These receptors mediate tonic contractions predominantly by opening receptor-operated Ca2+ channels (ROCs) and partly by opening T-type Ca2+ channels, and mediate rhythmic motility by opening L-type Ca2+ channels.

Actions of endothelin-1, 2, and 3 in the microvasculature of bone

Endothelins have recently been associated with hypoxia-related vascular smooth-muscle constriction and with the so-called no-reflow phenomenon following reperfusion. Their action is tissue dependent. The role of endothelins on vascular smooth muscle in bone is unknown. An ex vivo perfusion model was used to investigate the effects of the three different endothelins on the vascular resistance in the canine tibia. Endothelin-1 and endothelin-2 had molar potencies similar to that of norepinephrine, whereas endothelin-3 was less potent. Tachyphylaxis to the same dose of endothelin-1 did not occur. The calcium channel blocker, diltiazem, attenuated (45% reduction) the vasoconstrictor responses to norepinephrine (p < 0.005) but had a smaller effect (24% reduction) on the responses to endothelin (p < 0.025). Vascular smooth muscle in bone appears to have endothelin receptors that are similar to those in other organs.

Protective effect of amiloride against reperfusion damage as evidenced by inhibition of accumulation of free fatty acids in working rat hearts

To examine whether amiloride protects against ischemia-induced or reperfusion-induced damage to the heart, mechanical and metabolic studies were performed in the isolated, working rat heart. Ischemia decreased both mechanical function and the tissue levels of high-energy phosphates and increased the tissue levels of free fatty acids (FFAs). Reperfusion restored the levels of high-energy phosphates but further increased FFA accumulation. For this reason, accumulation of FFAs was used as an indicator of both ischemia-induced and reperfusion-induced damage. Drugs were added to the perfusion solution 5 min before ischemia until the end of ischemia (pre) or until 10 min after reperfusion (pre + post). Diltiazem (1 or 5 mumol/L pre) decreased the mechanical function of the non-ischemic heart and attenuated both ischemia-induced and reperfusion-induced accumulation of FFAs. Amiloride (50 mumol/L pre) did not affect the mechanical function of the non-ischemic heart or attenuate ischemia-induced or reperfusion-induced FFA accumulation effectively. However, amiloride (50 mumol/L pre + post) did markedly attenuate the reperfusion-induced accumulation of FFAs. In conclusion, diltiazem attenuates both ischemia-induced and reperfusion-induced myocardial damage, probably through its energy-sparing effect as a result of a decrease in mechanical function before ischemia. In contrast, amiloride attenuates only the reperfusion-induced myocardial damage through mechanisms other than the energy-sparing effect.

Endothelin receptor-mediated Ca2+ mobilization and contraction in bovine oviductal arteries: comparison with noradrenaline and potassium

1. The effects of endothelin-1 (ET-1) were studied in bovine oviductal arteries and compared to those of noradrenaline (NA) and high K+ (K+). The influence of endothelium, the receptor subtypes involved, and the mechanisms of Ca2+ mobilization were assessed. 2. ET-1 (0.1-300 nM) induced concentration-dependent contractions with a potency of 10(3) and 10(2) times higher than NA (0.1 microM-0.1 mM) and K+ (9.5-119 mM), respectively. Removal of endothelium or NG-nitro-L-arginine (L-NOARG, 0.1 mM) pretreatment did not affect responses to either ET-1 or K+, whereas the NA response was significantly increased. Indomethacin (1 microM) had no effect on either of these agonists. 3. The rank order of potency for the ET isopeptides was: ET-1 = ET-2 > ET-3. The ETA receptor-selective agonist, sarafotoxin 6c (S6c), had no effect. The ETA receptor-selective antagonist, BQ-123, showed a competitive antagonism on the ET-1 response (pA2 value of 6.58 +/- 0.01), whereas contractions to ET-3 were completely abolished by BQ-123 at 0.1 microM. 4. Concentration-response curves to both ET-1 and NA were shifted to the right and their maximum response reduced to approximately 56% and 65% of controls, respectively, under 30 min of incubation in Ca(2+)-free solution, whereas responses to K+ were almost abolished by this treatment. Contractions to both NA (30 microM) and ET-1 (30 nM) were maximally inhibited after 10 min of extracellular Ca2+ deprivation. 5. Contractions to ET-1 were more potently inhibited by nickel (Ni2+, 0.3 mM), whereas nifedipine (1 microM) and cadmium (Cd2+, 0.1 mM) induced only a slight effect. In contrast, opposite effects were found for both NA and K+. 6. Treatment with ryanodine (100 microM) and caffeine (10 mM) in Ca(2+)-free solution reduced the tension measured 5 min after NA (30 microM) and ET-1 (30 nM) addition, but the sustained response (tension at 25 min) remained unaffected. 7. Calphostin C (1 microM), a specific protein kinase C (PKC) inhibitor, reduced the maximum contractile response to ET-1 by about 50% without significantly affecting its pD2 value. 8. These results suggest that ET-1 acts in bovine oviductal arteries by directly activating a homogenous population of ETA receptors in smooth muscle, without endothelial modulation. Several Ca2+ activation mechanisms seem to be involved in the contractile action of the peptide, including: (1) extracellular Ca2+ entrance through Ni(2+)-sensitive and L-type Ca2+ channels; (2) intracellular Ca2+ release from a ryanodine-sensitive Ca2+ store; and (3) sensitization of the contractile machinery to Ca2+ via PKC.

Role of endothelin receptors, calcium and nitric oxide in the potentiation by endothelin-1 of the sympathetic contraction of rabbit ear artery during cooling

1. To examine further the potentiation by endothelin-1 on the vascular response to sympathetic stimulation, we studied the isometric response of isolated segments, 2 mm long, from the rabbit central ear artery to electrical field stimulation (1-8 Hz), under different conditions, at 37 degrees C and during cooling (30 degrees C). 2. Electrical stimulation produced frequency-dependent contraction, which was reduced (about 63% for 8 Hz) during cooling. At 30 degrees C, but not at 37 degrees C, endothelin-1 (1, 3 and 10 nM) potentiated the contraction to electrical stimulation in a dose-dependent way (from 43 +/- 7% to 190 +/- 25% for 8 Hz). 3. This potentiation by endothelin-1 was reduced by the antagonist for endothelin ETA receptors BQ-123 (10 microM) but not by the antagonist for endothelin ETB receptors BQ-788 (10 microM). The agonist for endothelin ETB receptors IRL-1620 (0.1 microM) did not modify the contraction to electrical stimulation. 4. The blocker of L-type Ca2+ channels verapamil (10 microM l-1) reduced (about 72% for 8 Hz) and the unspecific blocker of Ca(2+)-channels NiCl2 (1 mM) practically abolished (about 98%), the potentiating effects of endothelin-1 found at 30 degrees C. 5. Inhibition of nitric oxide synthesis with NG-nitro-L-arginine (L-NOARG, 0.1 mM) increased the contraction to electrical stimulation at 30 degrees C more than at 37 degrees C (for 8 Hz, this increment was 297 +/- 118% at 30 degrees C, and 66 +/- 15% at 37 degrees C). Endothelium removal increased the contraction to electrical stimulation at 30 degrees C (about 91% for 8 Hz) but not at 37 degrees C. Both L-NOARG and endothelium removal abolished the potentiating effects of endothelin-1 on the response to electrical stimulation found at 30 degrees C. 6. These results in the rabbit ear artery suggest that during cooling, endothelin-1 potentiates the contraction to sympathetic stimulation, which could be mediated at least in part by increasing Ca2+ entry after activation of endothelin ETA receptors. This potentiating effect of endothelin-1 may require the presence of an inhibitory tone due to endothelial nitric oxide.

Role of extracellular Ca2+ in subarachnoid hemorrhage-induced spasm of the rabbit basilar artery

BACKGROUND AND PURPOSE

The role of extracellular Ca2+ in the maintenance of chronic vasospasm after subarachnoid hemorrhage (SAH) is largely unknown. Indeed, studies thus far have been limited to demonstrations that L-type Ca(2+)-channel antagonists were unable to reverse the spasm. This study tested whether SAH-induced vasospasm is maintained, at least in part. through the influx of extracellular Ca2+ and whether the influx of extracellular Ca2+ occurs through L-type Ca2+ channels and possibly, in addition, through store operated channels (SOCs). Furthermore, as there is considerable evidence in the literature to suggest that the spasm is mediated through endothelin-1 (ET-1) release, we tested whether the Ca2+ dependency of the spasm was consistent with the mediation of the spasm by ET-1.

METHODS

Chronic spasm of the basilar artery was induced in a double SAH rabbit model. Relaxation of SAH-, ET-1-, serotonin-, and KC1-constricted basilar artery in response to Ca(2+)-free solution, verapamil, and Ni2+ was measured in situ with the use of a cranial window.

RESULTS

SAH induced 23% constriction of the basilar artery. Ca(2+)-free solution and 1 mumol/L verapamil reversed the constriction of SAH vessels by 60% and 17%, respectively. In contrast, control vessels challenged with 40 to 50 mmol/L KCl, which induced 34% constriction, relaxed in response to Ca(2+)-free solution and verapamil by 98% and 89%, respectively. In SAH vessels, verapamil followed by 0.1 mmol/L Ni2+, which is known to block SOCs, induced a combined relaxation of 67%. Control vessels challenged with 3 nmol/L ET-1, which induced a magnitude of constriction similar to that of SAH (29%), relaxed in response to Ca(2+)-free solution, verapamil, and verapamil plus Ni2+ by 69%, 20%, and 50%, respectively (P > .05) versus respective values in SAH vessels). In contrast, control vessels challenged with 2 to 8 mumol/L serotonin, which induced a magnitude of constriction similar to those of SAH and ET-1 (22%), completely relaxed in response to Ca(2+)-free solution and verapamil.

CONCLUSIONS

These results demonstrate that the maintenance of chronic spasm in the two-hemorrhage rabbit model after SAH is due to smooth muscle cell contractile mechanisms partly dependent on the influx of extracellular Ca2+. The influx of extracellular Ca2+ results from the opening of L-type Ca2+ channels and an additional channel or channels. We speculate that the L-type Ca2+ channel-independent influx of extracellular Ca2+ results from the opening of SOCs. The Ca(2+)-dependent characteristics of the spasm likely reflect the mediation of the spasm by ET-1.

Endothelin-induced electrical activity and calcium dynamics in vascular smooth muscle cells: a model study

A model is proposed to describe the electrical activity and intracellular calcium dynamics of vascular smooth muscle cells (SMC) induced by endothelin (ET1). The conductance of the nonselective channels (NSCs), proportional to the ET1-receptor complex (ET . R), is intracellular calcium dependent. Inositol (1,4,5)-trisphosphate (IP3) produced by ET1 releases Ca2+ from the IP3-sensitive Ca2+ store. The transient increase of intracellular Ca2+ triggers the release of Ca2+ from the Ca(2+)-sensitive store by a Ca(2+)-induced Ca2+ (CICR) mechanism and activates the Ca(2+)-activated K+ current (IK,Ca). The inward current (Iin) via the NSC can depolarize the cell to a level at which the L-type Ca2+ current becomes activated (ICa). The level of depolarization is determined by the relative amplitude of (Iin + ICa + IK,Ca) and the voltage- and time-dependent K+ current. The model simulations show that (a) in cells without a CICR mechanism, short-lasting stimulation by ET1 elicits higher membrane potential and Ca2+ than long-lasting stimulation; (b) in cells with or without a CICR mechanism, a reduction of normal membrane capacitance (1 muf/cm2) results in either significant and sustaining or oscillatory membrane potential and intracellular calcium concentration. The applicability of the model to the study of electrical activity and calcium dynamics associated with hypercholesterolemia is discussed.

Myocardial recovery during post-ischemic reperfusion: optimal concentrations of Na+ and Ca2+ in the reperfusate and protective effects of amiloride added to cardioplegic solution

The effects of Na+ and Ca2+ concentrations in the reperfusate on post-ischemic myocardial recovery were examined. Also, the myocardial protective effects of amiloride, an inhibitor of the Na+/Ca2+ and Na+/H+ exchange systems, added to cardioplegic solutions were assessed, using an isolated working rat heart perfusion system. Global myocardial ischemia was induced by 30-min normothermic cardioplegic arrest, using St. Thomas' solution. The concentration of Na+ in the reperfusate varied, stepwise, from 75 to 145 mM/l, and that of Ca2+, from 0.1 to 2.5 mM/l. In this study post-ischemic functional recovery was best at 110mM/l Na+ and 1.2-1.8 mM/l Ca2+ in the reperfusate. A significantly greater post-ischemic functional recovery and a lower creatine kinase release were observed when amiloride was added to the cardioplegic solution. Ca2+ overload via Na+/Ca2+ and Na+/H+ exchange systems would, thus, appear to be due, at least in part, to post-ischemic reperfusion injury.

Endothelin-induced changes in intracellular pH and Ca2+ in coronary smooth muscle: role of Na(+)-H+ exchange

The relationship between endothelin-1 (ET-1)-induced stimulation of Na(+)-H+ exchange and intracellular free Ca2+ ([Ca2+]i) was examined in primary cultures of porcine coronary artery smooth muscle cells. Intracellular pH (pHi) and [Ca2+]i were measured using 2,7-bis-carboxyethyl-5(6)-carboxyfluorescein and the acetoxymethyl ester of fura-2 respectively. In HCO3(-)-free buffer (pH = 7.4), ET-1 (0.1-50 nM) induced a sustained, dose-dependent increase in pHi. ET-1 (10 nM) increased pHi from 6.83 +/- 0.01 to 6.93 +/- 0.02 (P < 0.01). The alkalinization was blocked by the Na(+)-H+ exchange inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA, 3 microM) or by substitution of Na+ with N-methylglucamine or choline chloride (P < 0.05). Recovery of pHi in response to acidification, induced by washout of a 20 mM NH4Cl prepulse, was > 90% inhibited by EIPA (3 microM), confirming the presence of an ET-1-responsive Na(+)-H+ exchanger. Coronary smooth muscle cells responded to ET-1 with a dose-dependent, biphasic increase in [Ca2+]i which was not inhibited by manipulations (EIPA pretreatment or Na(+)-free media) shown to block the Na(+)-H+ exchanger. The ET-1-mediated alkalinization was not inhibited by removal of extracellular Ca2+ ([Ca2+]o). However, complete blockade of the ET-1-mediated [Ca2+]i response using the intracellular Ca(2+)-chelator, [bis-(2-amino-5-methylphenoxy)ethane-NNN'N'-tetraacetic acid tetraacetoxymethyl ester] (MAPTAM), in [Ca2+]o-free media, demonstrated that an increment in [Ca2+]i is required for activation of the Na(+)-H+ exchanger by ET-1. The ET-1-induced rise in [Ca2+]i was not associated with a rise in pHi in the presence of CO2/HCO3-. We conclude that: (1) activation of Na(+)-H+ exchange by ET-1 requires an increment in [Ca2+]i; and (2) ET-1 stimulates EIPA-sensitive Na(+)-H+ exchange, but this stimulation does not modulate ET-1-induced changes in [Ca2+]i.

Potentiation by endothelin-1 of Ca2+ sensitivity of contractile elements depends on Ca2+ influx through L-type Ca2+ channels in the canine cerebral artery

1. Endothelin-1 (ET-1) contracted canine cerebral artery in a concentration-dependent manner with an increase in intracellular Ca2+ concentration ([Ca2+]i), and at higher concentrations it produced a greater contraction with a smaller increase in [Ca2+]i. 2. Ca2+ channel antagonist such as d-cis-diltiazem inhibited the tension more effectively than the [Ca2+]i increased by ET-1. 3. In Ca(2+)-free solution containing 0.2 mM EGTA, ET-1 elicited a transient increase in [Ca2+]i and tension. 4. In the Staphylococcus aureus alpha-toxin-permeabilized artery, ET-1 shifted the pCa-tension relationship leftwards in the presence of GTP. 5. These findings suggest that ET-1 contracts the canine cerebral artery by increasing not only the Ca2+ influx through L-type Ca2+ channels, but also Ca2+ release from the intracellular storage sites, and also Ca2+ sensitivity of contractile elements. The degree of Ca2+ sensitivity is strongly affected by [Ca2+]i which is increased by the Ca2+ influx through L-type Ca2+ channels.

New approaches to blood cardioplegic delivery to reduce hemodilution and cardioplegic overdose

These studies present modifications of the cardioplegic components of a standard cardioplegic formulation, delivered previously at 4:1, to allow an 8:1 blood:cardioplegic mixture, thereby limiting hemodilution. Data are included to show that the hyperkalemic (20 mEq/L) glutamate/aspartate enriched solution used previously for warm induction can be used also for warm reperfusion (instead of the 10 mEq/L KCl solution), thereby avoiding the need to formulate two different solutions that vary only in potassium concentration. Studies of cold (4 degrees C) retrograde noncardioplegic blood infusion after arrest with hyperkalemic blood cardioplegia document the safety of maintaining arrest with continuous retrograde infusion of cold normokalemic blood, thereby indicating that ischemia and cardioplegia are unnecessary during aspects of cardiac operations where perfusion does not impair visualization (i.e., construction of proximal anastomoses). A new cardioplegic delivery system containing a shunt line is presented to expedite conversion from blood cardioplegia to blood infusion when continuous infusion is used.

Role of catalase in myocardial protection against ischemia in heat shocked rats

It was recently reported that in rats exposure to heat shock leads to appearance of a myocardial heat shock protein (HSP 70) and to an increase in myocardial catalase activity. This correlated with an improvement in post-ischemic function either in Langendorff-perfused hearts after low-flow ischemia or in working hearts after short-term, no-flow ischemia. We investigated the effect of the same hyperthermic treatment on functional recovery from no-flow ischemia of various durations in isolated working rat hearts performing at high or low external workloads. Rats were heated to core temperature of 42 degrees C for 15 min. No significant protein oxidation (% oxidized methionine) was observed 2.5 hr after treatment. A protein with migration characteristics similar to HSP 70 was observed in hearts of heat shocked rats 24 hr after this treatment while their myocardial catalase activity was not increased. Hearts of similarly treated rats were excised 24 hr after hyperthermia and perfused in a working mode with Krebs-Henseleit buffer (1.25 mM Ca2+, 11 mM glucose). At 15 cm H2O preload and 100 cm H2O afterload after 30 min no-flow ischemia, control hearts recovered to 36.9%, 2%, 47.6%, and 21.5% of the preischemic values of heart rate-peak systolic pressure product (RPP), aortic output, coronary flow, and cardiac output, respectively. After only 25 min of ischemia the respective recovered values were 61.6%, 11.5%, 58.7%, and 33.5%. Throughout the recovery period these hemodynamic values were consistently higher in hearts of heat shocked animals than in those of control hearts but the differences were not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade by nickel ions of phasic contraction to K+ and high affinity calcium of ileal longitudinal muscle of guinea-pig

1. Preincubation with 1 or 2 mM Ni2+ inhibited dose-dependently the ileal phasic response to K+ (60 mM) without appreciable effects on the tonic response. Ni2+ above 3 mM inhibited the tonic response. 2. Ni2+ inhibited the high affinity Ca2+ sites than the low affinity sites during K+ contraction. 3. After treatment with Ni2+, the K+ response was fairly restored by a wash with normal medium. The nickel bound to the ileal cells was almost eliminated with the washing. 4. This probably indicates that Ni2+ mainly inhibited the K(+)-induced phasic tension by reducing Ca2+ release rather than Ca2+ influx.

The role of endothelin in the vessel wall

Endothelins (ETs) are a family of vasoactive peptides occurring in three isoforms (ET-1, ET-2, ET-3) encoded by three distinct genes in the human genome. ETs arise from precursor peptides (big-ETs) that are cleaved and released by an endothelin-converting enzyme. ET-1 secretion, which can be stimulated by various agents, is preferentially directed towards the abluminal site of endothelial cells, suggesting a local paracrine action of the peptide. ETs exert their actions through the activation of at least two receptor subtypes: ET-A receptors, which mediate the proliferative and vasoconstrictive effects, and ET-B receptors, which mediate vasorelaxation. Although, the potential roles of ETs are mostly hypothetical, considering their potent cardiovascular effects, it has been suggested that maintenance of a basal vascular tone and regulation of vascular growth and haemostasis may well represent the biological functions of this family of peptides. The recent discovery of specific receptor antagonists will provide a means to assess their physiological and pathophysiological roles.

Endothelin-1-induced phosphoinositide hydrolysis and contraction in isolated rabbit detrusor and urethral smooth muscle

1. Endothelin-1 (ET-1) caused a concentration-dependent increase in the formation of inositol phosphates (IPs) in isolated rabbit detrusor and urethral smooth muscle preparations prelabelled with myo-[3H]inositol. 2. The increase in accumulation of IPs was slow in onset in both detrusor and urethra, with no significant accumulation demonstrable during the first 30 min. The increase in IPs accumulation found after exposure of detrusor tissue to ET-1 (10(-7) M) for 2 hr (250 +/- 38%, n = 7) was not significantly different from that found in the urethra (279 +/- 40%, n = 6), when expressed as per cent of corresponding control values. 3. Pretreatment with nifedipine (10(-6) M) did not reduce IPs formation. In contrast, no increase in IPs formation was demonstrated in Ca(2+)-free medium. 4. ET-1 (10(-11)-10(-7) M) produced concentration-dependent, slowly developing contractions in both detrusor and urethral preparations. Pretreatment with H-7 (3 x 10(-5) M) for 30 min before ET-1 application resulted in a non-parallel shift of the ET-1 concentration-response curve with significant reductions in maximal responses in both tissues. 5. ET-1-induced contractions in urethral preparations were markedly inhibited by Ni2+ (3 x 10(-4) M), whereas the effect of Ni2+ in the detrusor was less pronounced. 6. The results suggest that ET-1 stimulates phosphoinositide hydrolysis in the rabbit detrusor and urethra. Both IPs formation and contractile activation evoked by ET-1 are dependent on extracellular Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of acetylcholine- and endothelin-induced calcium entry in cultured human ciliary muscle cells

We characterized the effects of acetylcholine and endothelin on cultured human ciliary muscle cells, using the calcium-sensitive dye fura-2 to measure intracellular calcium and intracellular microelectrodes to measure the membrane potential. Both agonists, endothelin and acetylcholine, had a typical biphasic effect on the intracellular calcium concentration. Calcium peaked initially, because of its release from intracellular stores, and then reached a plateau, owing to entry of extracellular calcium. Endothelin-induced calcium entry was almost completely blocked by addition of extracellular La3+ (50 mumol/l) and Ni2+ (1 mmol/l). Acetylcholine-induced calcium entry was likewise almost completely abolished by La3+ and Ni2+. Both endothelin and acetylcholine led to an initial transient hyperpolarization with a subsequent depolarization. The hyperpolarization of the membrane potential had a time course similar to the initial calcium peak, while the depolarization occurred parallel to the calcium plateau. The depolarization induced by both agonists was reduced in the presence of La3+ and Ni2+. Verapamil (10 mumol/l) had no effect on either the calcium entry or the depolarization. Acetylcholine did not induce a [Ca2+]i peak when it was applied during the endothelin-induced [Ca2+]i plateau and vice versa. The [Ca2+]i plateau was not higher with concomitant than with single application of acetylcholine or endothelin. Thus, calcium entry and membrane depolarization induced by acetylcholine and endothelin seem to be mediated by a common La(3+)- and Ni(2+)-sensitive but verapamil-insensitive mechanism.

The vasoinhibitory activity of bovine chromogranin A fragment (vasostatin) and its independence of extracellular calcium in isolated segments of human blood vessels

Endothelium-independent vasoconstrictor responses in isolated segments of human internal thoracic artery (ITA) and saphenous vein (SV) were used as a bioassay system for the vasoinhibitory activity of bovine chromogranin A (CGA). Preincubation with vasostatin (0.8 micrograms/ml), containing the N-terminal domain of CGA, (CGA1-76, CGA1-113 and CGA1-143ff), inhibited the contractile responses evoked by 80 mM K+, 2.6 microM noradrenaline (NA), or 65 nM endothelin-1 (ET-1) in Ca(2+)-free solution in SV but not in ITA. The results demonstrate a vasoinhibitory activity in vasostatin and show that there is a marked difference between the arterial and venous segments in the Ca2+ independent component of the inhibitory response. A vascular role for the N-terminal domain of CGA is indicated, presumably by inhibiting Ca2+ release from intracellular stores in the human vein but not the artery.

Effects of the divalent cations nickel and cadmium on contractions of rat aorta to endothelin-1

1. The effects of inorganic and organic calcium channel antagonists on the contractile responses of rat isolated aortic rings to endothelin-1 (ET-1) were studied. 2. ET-1 (0.1-100 nM) evoked concentration-related contractile responses of the rat aorta (EC50 1.65 +/- 0.22 nM, Emax 125.8 +/- 4.5 %KClmax, n = 20). In Ca(2+)-free modified Krebs solution (containing 1 mM EGTA) aortic rings failed to contract to ET-1 (0.1-30 nM). 3. Nickel chloride (0.2-0.8 mM) attenuated the ET-1 (1-100 nM)-induced contraction of rat aorta (response (%KClmax) to 10 nM ET-1: control 132.0 +/- 8.7 and after 0.2 mM Ni2+ 90.3 +/- 14.8, 0.4 mM Ni2+ 54.7 +/- 12.3, 0.8 mM Ni2+ 10.3 +/- 4.4, n = 6/group). Cadmium chloride (10-30 microM) depressed the maxima of the concentration-response curves to ET-1 with an IC50 of 15.4 +/- 1.5 microM (n = 6). 4. The ET-1 evoked contractile responses were not modified by the dihydropyridine calcium channel antagonist, nicardipine (0.1 microM), or by omega-conotoxin (1.0 microM). Cinnarizine (10 microM), however, significantly attenuated the maximum response to ET-1 (96.9 +/- 6.0 vs 128.0 +/- 5.8 %KClmax for control), but failed to modify the EC50 value. 5. Amiloride, a Na(+)-Ca2+ exchange inhibitor, also depressed the maxima of the concentration-response curves to ET-1 with an IC50 of 0.45 +/- 0.05 mM (n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin-1-induced contractions of isolated pig detrusor and vesical arterial smooth muscle: calcium dependence and phosphoinositide hydrolysis

1. In isolated pig detrusor and vesical arterial smooth muscle preparations, endothelin-1 (ET-1) caused concentration-dependent contractions. Nifedipine (10(-6) M) did not significantly affect the action of ET-1 in the vessels, but almost abolished its effect in the detrusor. Incubation for 30 min in Ca(2+)-free solution markedly reduced the ET-1-induced contractions in both detrusor and vesical arteries. 2. The protein kinase C inhibitor H-7 (3 x 10(-5) M), reduced the response to ET-1 in detrusor muscle as well as in vessels, and abolished the contractions evoked by ET-1 in Ca(2+)-free solution. 3. ET-1 caused an increase in the accumulation of inositol phosphates (IPs) in preparations prelabelled with myo-[3H]inositol. After exposure to ET-1 (10(-7) M) for 60 min, an approx. 4-fold increase in IPs levels were demonstrated, compared to untreated controls, in both detrusor and vessel preparations. Pretreatment with nifedipine (10(-6) M) did not reduce IPs formation. In contrast, no increase in IPs formation was demonstrated in Ca(2+)-free medium. 4. The increase in accumulation of IPs was slow in onset in both detrusor and vesical arteries, with no significant accumulation demonstrable during the first 30 min. Time-course studies of tension development for ET-1 revealed that maximum tension was reached before significant levels of IPs could be detected.

Activation of endothelin receptors by sarafotoxin regulates Ca2+ homeostasis in cerebellar astrocytes

We carried out experiments designed to investigate the effects of sarafotoxin-6B (SFTx) on [Ca2+]i in cerebellar astrocytes using the Ca2+ indicator fura-2. Both endothelin-1 and sarafotoxin-6B increased [Ca2+]i in individual cerebellar astrocytes in cell culture. The shape of the response was variable but usually consisted of an initial peak of [Ca2+]i followed by an extended plateau increase in [Ca2+]i. In Ca(2+)-free medium only the initial peak was observed. If Ca2+ was subsequently readmitted to the external medium a plateau was now formed. When external Ca2+ was removed during a plateau, [Ca2+]i rapidly declined; replacing the external Ca2+ reversed this decline. The plateau was also reversibly reduced by addition of Ni2+ (5 mM) to the external medium. Addition of 50 mM K+ produced a small increase in [Ca2+]i in most cells. This response was blocked by nimodipine. However, nimodipine only slightly blocked the plateau increase in [Ca2+]i that was formed following activation of endothelin receptors. Furthermore, perfusion of cells with 50 mM K+ during the plateau portion of a response to SFTx reduced [Ca2+]i. In some cells addition of a phorbol ester produced a sustained increase in [Ca2+]i that was blocked by nimodipine. In conclusion, activation of endothelin receptors by SFTx in cerebellar astrocytes produces both Ca2+ mobilization and Ca2+ influx. The pathway for Ca2+ influx is predominantly a non-voltage-dependent one, although some entry through a dihydropyridine-sensitive pathway also appears to occur. Furthermore, activation of protein kinase C in cerebellar astrocytes activates voltage-sensitive Ca2+ channels.

Contraction of rat thoracic aorta strips by endothelin-1 in the absence of extracellular Ca2+

1. Endothelin-1 (ET-1) caused a concentration-dependent contraction of helical strips from rat thoracic aorta in the absence of extracellular Ca2+. The Ca(2+)-depleted muscle strips, prepared by three repeated applications of 10(-2) M caffeine or 10(-6) M noradrenaline in Ca(2+)-free buffer, were contracted by 10(-8) M ET-1 in the same manner as non-treated strips. 2. In the absence of extracellular Ca2+, 10(-7) M phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, induced a small but sustained contraction of the rat thoracic aorta strips within 60 min. Preincubation of the strips with 10(-7) M PMA for 60 min in Ca(2+)-free buffer, did not affect the 10(-8) M ET-1-induced contraction, but decreased the 5 x 10(-8) M phorbol 12,13-dibutyrate (PDB)-, or the 10(-7) M PMA-induced contraction, and potentiated the contraction induced by 10(-8) M urotensin II. Preincubation with 10(-8) M ET-1 (which induced maximum contraction) for 25 min in Ca(2+)-free buffer did not change the subsequent contraction induced by PMA (10(-7) M) or urotensin II (10(-8) M) but gave a somewhat lower maximum tension than in non-treated strips. 3. Calyculin-A, a potent inhibitor of phosphatase, also induced a contraction of the Ca(2+)-depleted muscle strips in Ca(2+)-free buffer. Preincubation of the strips with ET-1 (10(-8) M) or PMA (10(-7) M) decreased the calyculin-A (3 x 10(-8) M)-induced contraction.4. These results suggest that ET-1 may induce phosphorylation of an unknown protein either without an increase in myoplasmic Ca2 + concentration or, alternatively, with mobilization of intracellular Ca2+ from noradrenaline- and caffeine-insensitive Ca2 + sources, through a mechanism different from that of phorbol ester.

Endothelin-evoked contractions in bovine ciliary muscle and trabecular meshwork: interaction with calcium, nifedipine and nickel

In the present study, we compared contractile responses of isolated bovine ciliary muscle and trabecular meshwork strips to endothelin-1 and carbachol. 1. Endothelin-1 is a potent contracting agent for ciliary muscle and trabecular meshwork. The EC50 was 5 x 10(-9) mol/l for both tissues. The maximal force evoked by endothelin was 73% of the maximal carbachol response in trabecular meshwork and 52% in ciliary muscle. 2. Carbachol contracted both tissues with an EC50 of 2 x 10(-7) mol/l. 3. In ciliary muscle, the tension was completely dependent on extracellular calcium. 4. In trabecular meshwork, 23 +/- 4% of the endothelin- and 42 +/- 10% of the carbachol-induced force response remained after removal of extracellular calcium. 5. Nifedipine (10(-5) mol/l) had only a slight relaxing effect in both tissues. 6. Nickel (10(3) mol/l) inhibited the development of force in both tissues. The relaxation induced by nickel was more pronounced in endothelin- than in carbachol-induced contractions. Different intracellular mechanisms mediating the action of endothelin and carbachol can be described: i) a calcium- and a nickel-sensitive pathway in both tissues and ii) an additional mechanism independent of external calcium in trabecular meshwork. These results indicate functional differences between the contractile region of trabecular meshwork and ciliary muscle. Endothelin may participate in accommodation and regulation of the intraocular pressure.

Endothelin induces a nonselective cation current in vascular smooth muscle cells

The potent vasoconstrictor endothelin leads to smooth muscle cell depolarization and increases in intracellular Ca2+. Although effects of endothelin on calcium channels have been described, it also has been speculated that endothelim may activate additional ion channels. The purpose of the present study was to identify an alternative ion current that could play a role in depolarizing cells in response to vasoconstrictors like endothelin and vasopressin. The effects of endothelin, vasopressin, sarafotoxin S6b, and phenylephrine were assessed using whole-cell patch-clamp recordings from primary dissociated rat aortic or mesenteric arterial smooth muscle cells cultured for 24-72 hours. From the usual resting potentials of these cells of -50 to -60 mV, endothelin (1-100 nM) induced a depolarization via an increase in membrane conductance. This depolarization was phasic, oscillating repeatedly from the resting potential to a relatively depolarized level and back to the resting potential. From a holding potential of -60 mV, endothelin-1, endothelin-3, vasopressin, or sarafotoxin S6b (but not phenylephrine) induced transient inward currents that also could be phasic. In external sodium, lithium, or cesium (but not Tris) and in internal potassium or cesium, these currents reversed near 0 mV. Although nifedipine-insensitive, the inward currents were absent in zero calcium, barium, or strontium, or in the presence of cobalt or nickel. These results represent the first report of a nonselective cation current in primary vascular smooth muscle cells that is calcium dependent and that could be responsible for the depolarizations induced from the resting potential by vasoconstrictors such as endothelin.

Participation of calcium ions in the molecular mechanism of cardioprotective action of exogenous phosphocreatine

The purpose of this study was to find out whether Ca2+ is necessary for the protective effect of phosphocreatine (PCr) on ischemic myocardium. Isolated Langendorff-perfused rat hearts were used in the study. When ischemic arrest was induced in Ca(2+)-free buffer, PCr did not exert a protective effect on ischemic myocardium. PCr improved postischemic contractile recovery of hearts subjected to ischemia in perfusion media containing 0.5 and 1.2 mmol/l Ca2+. Phosphoarginine, a structural analogue of PCr which possesses Ca(2+)-binding property similar to that of PCr did not exert any protective effect on ischemic myocardium. The effects of PCr and Ca2+ on lipid order of sarcolemmal vesicles from canine heart were studied by using ESR spectroscopy. PCr made membrane phospholipids more tightly packed at mildly acidic and neutral pH, but did not at pH 8.5. Although Ca2+ itself did not influence the membrane structure at pH 5.5, it potentiated the effect of PCr on sarcolemmal phospholipids. Thus, the protective effect of PCr on ischemic myocardium is not attributed to its Ca2+ binding properly, but Ca2+ is a necessary component of the mechanism of protective effect of PCr on ischemic myocardium.

Deleterious effects of digitalis on reperfusion-induced arrhythmias and myocardial injury in ischemic rat hearts: possible involvements of myocardial Na+ and Ca2+ imbalance

Isolated rat hearts were made ischemic for 25 min after an initial recirculating perfusion, followed by 30 min of reperfusion. In some hearts, interventions including administration of ouabain and/or high [K+] in the buffer were performed during the first 10 min of reperfusion. During ischemia, intracellular Na+ (Nai) increased from 15 to 64 mumol/g dry weight (dwt). During reperfusion, Nai declined rapidly (at 10 min of reperfusion: 48 mumol/g dwt, at 30 min: 25 mumol/g dwt) and regular rhythm was recovered within 10 min in hearts without any intervention during reperfusion. 45Ca2+ uptake increased from 0.8 to 7.5 mumol/g dwt after 30 min of reperfusion. Ventricular function recovered by 45%. A 10-min perfusion with 10 or 50 microM of ouabain increased Nai (17 to 21 or 27 mumol/g dwt) with increased left-ventricular (LV) contractile function, but these effects were reversed by combination of high perfusate [K+] (20 mM) in non-ischemic hearts. A 10-min reperfusion with ouabain retarded or stopped the decline in Nai (at 10 min of reperfusion: 54 or 63 mumol/g dwt, at 30 min: 32 or 40 mumol/g dwt). These amounts of ouabain also increased the incidence of ventricular tachyarrhythmias during reperfusion to 30% or 50%, and increased the duration of ventricular fibrillation from 6.5 to 11.5 or 18.0 min. 45Ca2+ uptake reached to 8.8 or 10.0 mumol/g dwt, and function recovered only 35% or 28%. When high perfusate [K+] was combined with ouabain during reperfusion, the retarded decline in Nai, augmented 45Ca2+ uptake, and reduced recovery of function caused by ouabain alone were attenuated. These results suggest that digitalis has toxic effects on reperfused ischemic hearts by inhibition of rapid active outward transport of previously elevated Nai and potentiation of Ca2+ overload.