Faculty Bibliography

OBJECTIVES/OBJECTIVE:This study was designed to determine the effects of glucose-insulin infusion on ischemia-induced changes in extracellular potassium ([K+]o) accumulation and the associated electrophysiologic abnormalities in the canine heart. BACKGROUND:Although glucose-insulin-potassium infusion has been shown to limit myocardial injury in acute ischemia, its effect on ischemia-induced electrophysiologic alterations has not been investigated. METHODS:Recordings of [K+]o and local electrograms from the normal, border and ischemic zones were obtained during serial (10-min) left anterior descending coronary artery occlusions in the control state and after infusion of glucose-insulin (eight dogs), glucose alone (six dogs) or insulin alone (eight dogs). RESULTS:Glucose-insulin infusion caused significant reduction in the rise of [K+]o during the entire period of ischemia in both ischemic and border zones associated with significant improvement in the degree of intramyocardial conduction delay. At 10 min of ischemia, [K+]o was reduced from a mean control level of 15.9 +/- 3.7 to 10.1 +/- 4.3 mmol/liter (p < 0.005) in the ischemic zone and from 6.8 +/- 1.9 to 5.5 +/- 1.1 mmol/liter (p < 0.05) in the border zone. The electrogram duration was shortened from a mean control value of 102 +/- 13 to 78 +/- 12 ms in the ischemic zone and from 79.2 +/- 7.8 to 58.1 +/- 6.6 ms in the border zone (p < 0.005). Glucose alone caused significant reduction in [K+]o during the initial 6 min of ischemia, only in the ischemic zone. Conversely, insulin caused no changes in [K+]o accumulation during ischemia. Neither glucose nor insulin alone had any effect on ischemia-induced intramyocardial conduction delay. CONCLUSIONS:The present study demonstrated that the combination of glucose and insulin is essential for the salutary effect of reducing [K+]o accumulation during ischemia and improving the associated intramyocardial conduction delay. It could be postulated that glucose in the presence of insulin increases the glycolytic flux, thereby providing adequate adenosine triphosphate for suppressing the cardiac adenosine triphosphate-sensitive potassium ion channels. The latter are, at least partially, responsible for the [K+]o rise in the early phase of ischemia. This study highlights the antiarrhythmic potential of interventions that modulate the metabolic consequences of ischemia.

We investigated the mechanism by which alpha 1-adrenergic activation regulates basal and stimulated whole cell L-type Ca current (ICa) in rat ventricular myocytes using the physiological neurotransmitter, norepinephrine (NE, 10 microM). Stimulation of alpha 1-adrenoceptors, achieved by NE + 10 microM esmolol (a beta-receptor antagonist), had no significant effect on basal ICa. alpha 1-adrenergic activation had a marked inhibitory effect on ICa elevated by beta activation (NE + 1 microM) prazosin, an alpha 1-receptor antagonist) or activation of adenylyl cyclase by forskolin (25 microM); the inhibitory effect was reversible upon washout. However, alpha 1-adrenergic stimulation had no significant effect on ICa previously increased by intracellular application of cAMP (25 microM). The inhibitory effect seen on ICa elevated by NE showed no significant shift of either I-V or inactivation curves. It is unlikely that the inhibitory effect of alpha 1-adrenergic stimulation on NE or forskolin-elevated ICa is mediated through activation of Ca-dependent protein kinase C or changes in intracellular free Ca (pCa = 8.5, EGTA 5 mM) or cAMP-dependent phosphodiesterase. We conclude that alpha 1-adrenergic inhibition of beta-adrenergic stimulated-ICa is probably mediated through an as yet unknown G-protein. This inhibitory effect could serve as a regulatory feedback mechanism in physiological and pathophysiological settings.

STUDY OBJECTIVE/OBJECTIVE:ATXII is a polypeptide toxin isolated from the sea anemone, Anemonia sulcata, known to delay sodium inactivation markedly and to induce early afterdepolarisations. The aim was to investigate the mechanism of its action. DESIGN AND MATERIALS/METHODS:The mechanism of ATXII induced early afterdepolarisations was investigated in vitro in canine endocardial preparations using standard microelectrode techniques. MEASUREMENTS AND MAIN RESULTS/RESULTS:ATXII (2 x 10(-7) M) induced cycle length dependent prolongation of plateau, more marked in Purkinje than in muscle fibres, and early afterdepolarisations in Purkinje fibres only. The calcium channel antagonists verapamil (10(-6) M, 10(-5) M) and cobalt (2-4 mM), and drugs that block calcium release from the sarcoplasmic reticulum, ryanodine (10(-6) M, 10(-5) M) and caffeine (10 mM), did not antagonise the ATXII effects. However, tetrodotoxin (5 x 10(-6) M) and lignocaine (4 x 10(-5) M) shortened the action potential and suppressed early afterdepolarisations. The effects of lignocaine were seen at concentrations that did not significantly affect Vmax. CONCLUSIONS:ATXII induced early after depolarisations are due to the effects of ATXII on Na+ entry, probably via a slowly inactivated Na+ channel population. Calcium entry through the sarcolemmal Ca2+ channels and cyclic Ca2+ release from the sarcoplasmic reticulum are not required for the genesis of early afterdepolarisations in this model.

The reality of sinoatrial Wenckebach periods (WP) has been suggested, but not proven, in the literature. We report experimental and clinical data showing WP in sinoatrial blocks. Experimental sinoatrial blocks were induced by superfusion of bepridil (10(-5) M) in 15 preparations of isolated rabbit right atria. Different types of block were observed, including Blumberger I block, i.e., sinoatrial WP. The recordings showed that the typical pattern of Blumberger type IA block and sinoatrial WP may be due to transient acceleration of the sinus rate, without change in the increment. We also observed sinoatrial WP in a 72-year-old patient on direct recordings of the sinus node (SN) electrical activity. In this case, transient acceleration of the sinus rate also seemed to be involved in the genesis of sinoatrial WP. Analysis of these clinical and experimental data showed similarities that may explain the mechanism of the WP. Usually type IA Blumberger block is said to involve a decrease in the sinoatrial increment to explain the atrial sequence (decrease in PP interval followed by a pause shorter than twice the value of the preceding cycle, then a cycle longer than the one preceding the pause). In fact, this pattern can be observed when the acceleration of the higher structure, the SN, induces block within the lower structure, i.e., the atrium.

Alpha 1-adrenoceptor agonists were shown to induce delayed afterdepolarizations (DADs) and triggered activity in the presence of elevated extracellular Ca2+. We investigated the effects of alpha 1-adrenoceptor stimulation on DADs and triggered activity in canine Purkinje fibers that survived 1-day of myocardial infarction. Endocardial preparations were studied using standard microelectrode techniques. In quiescent preparations showing no DADs and in presence of propranolol (2 x 10(-7) M), phenylephrine (10(-6) M), an alpha 1-adrenoceptor agonist induced DADs (n = 6) and differentially induced triggered activity in ischemic but not in normal Purkinje fibers (n = 4). In 8 preparations that showed subthreshold DADs, phenylephrine increased the DAD amplitude from 4.0 +/- 2.5 mV to 8.0 +/- 3.3 mV (P less than 0.03) and from 3.2 +/- 1.5 mV to 6.5 +/- 3.7 mV (P less than 0.05) at paced cycle lengths of 800 and 400 ms, respectively. Phenylephrine caused subthreshold DADs to reach threshold and result in triggered activity (n = 6). The effects of phenylephrine were abolished by 10(-6) M prazosin, an alpha 1-adrenoceptor blocker. Our results suggest that alpha 1-adrenoceptor stimulation regulates DADs and triggered activity seen in subendocardial Purkinje fibers surviving 1 day of myocardial infarction and may contribute to the spontaneous ventricular tachycardia seen in vivo at this stage.

The diterpene forskolin is widely known for its ability to directly activate adenylyl cyclase and consequently increase intracellular cAMP. In cardiac cells, one result is a cAMP-mediated increase in the L-type Ca2(+)-channel current (ICa). However, forskolin was also shown recently to affect a number of ionic channels in noncardiac cells by mechanisms that do not involve activation of adenylyl cyclase. The present study reveals such an effect of forskolin on cardiac Ca2+ channels. Indeed, under appropriate conditions, forskolin was found to cause an inhibition of ICa. Although the stimulation of adenylyl cyclase and ICa requires micromolar concentrations of forskolin, the inhibitory effect of forskolin was observed in the nanomolar range of concentrations, i.e., 2-3 orders of magnitude lower. This high affinity forskolin inhibition of ICa was observed when ICa was previously enhanced via a cAMP-dependent pathway, but not when ICa was at its basal level or when the current was elevated by the dihydropyridine Bay K 8644. The inhibitory effect occurred at a site of action remote from adenylyl cyclase, because forskolin similarly inhibited ICa that had been previously elevated by isoprenaline (a beta-adrenergic agonist) or directly by intracellular perfusion with cAMP. Under these conditions, forskolin was inhibitory when applied to either side of the cell membrane, but only in its lipid-soluble form. The inhibitory effect of forskolin appeared to be independent of membrane potential and was not accompanied by a change in the time constants of ICa activation and inactivation. This may indicate that forskolin mainly reduces the number of functional Ca2+ channels without changing the gating of individual channels. However, the reduction in ICa amplitude was not equally distributed among the different exponential components that constitute ICa, which suggests that forskolin also modifies the resting state of the channels. This novel high affinity forskolin inhibition of ICa may take place at some step in the pathway between cAMP and Ca2+ channel phosphorylation and/or at Ca2+ channels only after they have been phosphorylated.

New information about the pathophysiology of the sinus node and sino-atrial conduction has been published in the last few years. The sinus node consists of cells separated by a network of collagen fibres. This anatomical disparity explains the different electrophysiological characteristics of the node; the morphology of cellular action potentials depends on the site of recording. The dominant and most automatic pacemaker cells are situated in the cephalic region and the latent pacemaker cells in the caudal region. However, synchronisation of these different cellular activities is possible and results in a coherent signal. This complex synchronisation has been the object of several recent papers. The phenomenon of intrasinusal pacemaker shift and the stimuli which induce it have been studied in depth. In general, positive chronotropic stimuli tend to shift the dominant pacemaker towards the cephalic part and negative chronotropic stimuli towards the caudal part of the node. It is possible to assess pacemaker shift clinically and this phenomenon must be taken into consideration when studying sinus node function. Intercellular conduction and especially electrotonic conduction does not play a role in the genesis of the flux, which represents spontaneous cellular automatism, but in its mode of expression, that is to say the sinus rhythm. The pathophysiology of sinoatrial block is complex because it may be situated within and/or around the sinus node. The extrinsic or intrinsic mechanisms of these blocks may be interrelated. Variations in sinus rhythm must be taken into account in the genesis of sinoatrial block; an acceleration in rhythm may block conduction in the perisinusal region. Finally, our knowledge of the ionic fluxes underlying sinus automatism has also improved with individualization of the pacemaker current (if).(ABSTRACT TRUNCATED AT 250 WORDS)

The increase in extracellular potassium [K+]o levels during the early phase of myocardial ischemia may result in part from activation of adenosine triphosphate-sensitive K+ channels. Glyburide, a second-generation hypoglycemic sulfonylurea, is a potent blocker of these channels. We studied the effects of glyburide on [K+]o and on intramyocardial conduction delay during a 10-minute occlusion of the left anterior descending artery in the dog. K(+)-sensitive electrodes and bipolar plunge electrodes were introduced to record, respectively, [K+]o and local electrograms from close sites in midmyocardial regions in normal, border, and ischemic zones. Recordings were obtained before (control ischemia [CI]) and 20 minutes after intravenous administration of 0.15 mg/kg of glyburide (glyburide plus ischemia [G + I]). During G + I the extent of the increase in [K+]o was less compared to that during CI, and the difference was statistically significant during the first 7 minutes of ischemia in the ischemic zone and during the first 4 minutes of ischemia in the border zone. On the other hand, the degree of local intramyocardial conduction delay was significantly reduced during G + I compared to CI during the entire 10 minutes of ischemia in both the ischemic and border zones. In summary, our results have shown that glyburide significantly reduced the rise of [K+]o and intramyocardial delay during the early phase of acute ischemia and could thus attenuate the electrophysiologic consequences of ischemia that underlie the initial phase of malignant tachyarrhythmias. Although the effects of glyburide may result in part from a direct action of the drug on cardiac adenosine triphosphate-sensitive K+ channels, other metabolic antiischemic effects cannot be ruled out.

Caffeine and ryanodine are known to modulate oscillatory release of Ca2+ from the sarcoplasmic reticulum. The effects of caffeine and ryanodine on delayed afterdepolarizations (DADs) and sustained rhythmic activity in subendocardial Purkinje fibers surviving 1-day-old myocardial infarction in the dog were studied with standard microelectrode techniques. In preparations that showed sustained rhythmic activity, a high concentration of caffeine (10 mM) and ryanodine (10(-7) and 10(-6) M) slowed and terminated the sustained rhythmic activity and markedly suppressed DADs. An increase in the temperature of the tissue bath from 37 degrees to 39 degrees C did not change these results. In quiescent normal and infarcted preparations, a low concentration of caffeine (0.5 mM) differentially induced DADs in ischemic but not in normal Purkinje fibers, increased the amplitude of existing DADs, and brought subthreshold DADs to threshold potential that caused triggered activity. Our results are consistent with the hypothesis that triggered activity arising from DADs characterizes the sustained rhythmic activity in endocardial preparations 1 day after infarction and indicate an important role for the sarcoplasmic reticulum in the genesis of DADs and triggered activity in this model.