Interstitial pressure and lung oedema in chronic hypoxia.

We evaluated how the increase in lung interstitial pressure correlates with the pulmonary vascular response to chronic hypoxia. In control and hypoxic (30 days; 10% O2) Wistar male rats, we measured: pulmonary interstitial pressure (P(ip)), cardiac and haemodynamic parameters by echocardiography, and performed lung morphometry on tissue specimens fixed in situ. In control animals, mean ± sd P(ip), air/tissue volume ratio and capillary vascularity index in the air-blood barrier were -12 ± 2.03 cmH2O, 3.9 and 0.43, respectively. After hypoxia exposure, the corresponding values of these indices in apparently normal lung regions were 2.6 ± 1.7 cmH2O, 3.6, and 0.5, respectively. In oedematous regions, the corresponding values were 12 ± 4 cmH2O, 0.4 and 0.3, respectively. Furthermore, in normal regions, the density of pre-capillary vessels (diameter ~50-200 µm) increased and their thickness/internal diameter ratio decreased, while opposite results were found in oedematous regions. Pulmonary artery pressure increased in chronic hypoxia relative to the control (39.8 ± 5.9 versus 26.2 ± 2.2 mmHg). Heterogeneity in local lung vascular response contributes to developing pulmonary hypertension in chronic hypoxia. In oedematous regions, the decrease in capillary vascularity correlated with the remarkable increase in interstitial pressure and morphometry of the pre-capillary vessels suggested an increase in vascular resistance; the opposite was true in apparently normal regions.

Efficacy of diltiazem in two experimental feline models of sudden cardiac death.

The potential role of calcium entry blockers in the prevention of life-threatening arrhythmias associated with acute myocardial ischemia and reperfusion is still controversial. In 98 anesthetized cats, the effect of diltiazem was examined in two experimental models. In protocol I, ventricular tachycardia or fibrillation was consistently induced by the interaction between a 2 minute coronary artery occlusion and a 30 second left stellate ganglion stimulation. After three trials under control conditions, if the same pattern of arrhythmia was induced, the drug under study was administered and three additional trials were performed. In 16 animals the administration of saline solution did not modify the pattern of arrhythmias. In contrast, diltiazem (0.1 mg/kg body weight plus 0.2 mg/kg per h) abolished both ventricular tachycardia and fibrillation that had occurred in 64 and 36%, respectively, of the cats in the control state. In protocol II, a 20 minute coronary artery occlusion was released in three groups; one served as the control group, one received diltiazem 15 minutes before occlusion and one received diltiazem 3 minutes before reperfusion. The incidence of reperfusion ventricular fibrillation was 62% (16 of 26) in the control group. It was significantly (p less than 0.05) reduced by diltiazem administered before the occlusion to 25% (4 of 16), whereas it was not affected when diltiazem was administered just before reperfusion (7 [47%] of 15). These results indicate that diltiazem exerts a striking protective effect against the malignant arrhythmias induced by the combination of acute myocardial ischemia and sympathetic hyperactivity. Diltiazem was also effective in reducing the incidence of life-threatening reperfusion arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Baroreflex sensitivity and its evolution during the first year after myocardial infarction.

Experimental data have indicated that baroreflex sensitivity is often depressed in dogs after myocardial infarction and that this depression correlates strongly with subsequent mortality during episodes of acute myocardial ischemia. This finding has several clinical implications. The present study was undertaken with the objectives of assessing the potential existence of differences in baroreflex sensitivity between men with and without myocardial infarction and the time course during the 1st year after infarction of these potential changes in baroreflex sensitivity. Fifty-three subjects entered the study: 32 postinfarction patients and 21 control subjects. Baroreflex sensitivity was assessed by increasing mean blood pressure by aphenylephrine infusion (70 micrograms/ml) and recording the consequent RR interval changes. Baroreflex sensitivity, expressed as the slope of the regression line relating mean blood pressure to RR interval changes, was evaluated 18 days (n = 32), 3 months (n = 17) and 13 months (n = 10) after infarction. Baroreflex sensitivity was lower in the patients than in the control subjects (8.2 +/- 3.7 versus 12.3 +/- 2.9 ms/mm Hg, p = 0.0001). Moreover, 13 (41%) of 32 patients had a baroreflex slope less than 6.5 ms/mm Hg, which was 2 SD below the mean value of the control subjects. The internal control follow-up study showed that baroreflex sensitivity increased 3 months after infarction to values quite similar to those observed in the control subjects (11.1 +/- 5.3 versus 8.7 +/- 3.5 ms/mm Hg, p = 0.02). No further change was observed between 3 and 13 months after myocardial infarction. These data indicate that baroreflex sensitivity is lower in a proportion of postinfarction patients than in control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Autonomic indexes based on the analysis of heart rate variability: a view from the sinus node.

OBJECTIVE: Clinical indexes of autonomic activity are based on the analysis of sinus cycle length and of its variability. A common assumption underlying this practice is that neural activity and cycle length may be linearly related. Recent experimental evidence suggests that such an assumption may not be correct; indeed, the relation linking autonomic agonist concentration to cycle length was found to be strongly non-linear in single sinoatrial myocytes. In the present work, we (i) test compatibility of non-linearity of neural modulation of cycle length (CL) with previous experimental and clinical observations; (ii) evaluate its implications for what concerns the interpretation of time- and frequency-domain parameters of heart rate variability (HRV) and baroreflex sensitivity (BRS). CONCLUSION: Non-linearity of neural modulation of CL may result in an intrinsic rate-dependency of autonomic indexes, with the exception of normalised frequency-domain indexes (e.g. the low frequency/high frequency (LF/HF) ratio), which appear to be devoid of intrinsic rate-dependency. This characteristic may not limit the value of HRV indexes and BRS in risk stratification, but has to be taken into account in their pathophysiological interpretation.

Parasympathetic control of cycle length dependence of endocardial ventricular repolarisation in the intact feline heart during steady state conditions.

OBJECTIVE: Parasympathetic modulation of the rate dependence of ventricular repolarisation in vivo during steady state conditions was investigated by analysing the effects of removing vagal activity in three groups of anaesthetised cats. METHODS: Bilateral vagotomy was performed while different levels of background sympathetic tone were present in the control periods of each group: in group 1 (n = 6), cardiac sympathetic nerves were intact; in group 2 (n = 7), bilateral stellate ganglionectomy was performed; and in group 3 (n = 7), beta adrenergic blockade (propranolol, 0.5 mg.kg-1) was performed after stellate ganglionectomy. The duration of a left ventricular endocardial monophasic action potential (APD) was measured during atrial pacing at 7-10 cycle lengths (CL). The APD/CL relation was fitted to a hyperbolic function: APD = CL/[(axCL)+b]. Two parameters were considered: APDmax (1/a, ie, APD extrapolated at infinite cycle length, a rate-independent measure of APD) and CL50 (bxAPDmax, ie, the cycle length at which 50% of APDmax is reached). RESULTS: In control conditions, APDmax and CL50 were longer in groups 2 and 3, when cardiac sympathetic effects were reduced or absent. Vagotomy reduced APDmax and CL50 similarly in groups 1 and 2 (APDmax, -24% and -18%; CL50, -36% and -27%) (p < 0.05 v control). In group 3, vagotomy did not affect APDmax and CL50. CONCLUSIONS: No direct parasympathetic influence on the rate dependence of endocardial ventricular repolarisation was observed. The vagal modulation of sympathetic effects may take place either through vago-sympathetic reflexes or via the antagonism of circulating catecholamines distal to the beta receptor.

Sulfonylureas blockade of neural and cardiac HERG channels.

The human ether-a-go-go-related gene (herg) encodes a K+ current (I(HERG)) which plays a fundamental role in heart excitability and in neurons by contributing to action potential repolarization and to spike-frequency adaptation, respectively. In this paper we show that I(HERG), recorded in neuroblastoma cells and guinea-pig ventricular myocytes, was reversibly inhibited by the K(ATP) channel blocker glibenclamide (IC50 = 74 microM). The voltage and use dependence of glibenclamide blockade were also evaluated. Another sulfonylurea, glimepiride, had less effective results in blocking I(HERG). The findings of this study are relevant to the interpretation of glibenclamide effects on cellular electrophysiology and suggest that oral antidiabetic therapy with sulfonylureas may contribute to iatrogenic QT prolongation and related arrhythmias.

Effect of ventricular fibrillation complicating acute myocardial infarction on long-term prognosis: importance of the site of infarction.

Identification of high-risk subgroups of patients after acute myocardial infarction (AMI) is essential for evaluation of targeted preventive strategies. A case-control study was performed in 250 post-AMI patients to examine whether an episode of ventricular fibrillation (VF) during the in-hospital period modifies the long-term prognosis for patients with either an anterior or an inferior AMI. After identification of 70 patients with an anterior AMI and 55 patients with an inferior AMI, all complicated by VF and discharged alive, we selected 125 additional patients who had an AMI not complicated by VF (control subjects). To minimize the potential sources of differences in outcome, cases and controls were matched for the following variables: sex (all men), age (same +/- 2 years), coronary care unit (same), epoch of AMI (same +/- 3 months), and site of AMI (same). Left ventricular dysfunction and prior AMI were present in only a few patients. Patients receiving either acute or long-term treatment with beta-adrenergic blocking agents were not included. The average follow up was 59 months (range 12 to 120). The cumulative mortality during the first 5 years for the patients with inferior AMI without VF (6%, 11%, 13%, 13% and 13%) was modest and not significantly different from that of inferior AMI complicated by VF (6%, 11%, 20%, 20%, and 26%). In contrast, a striking difference appeared when the cumulative mortality of patients with anterior AMI without VF (9%, 13%, 17%, 27%, and 29%) was compared with that of patients with anterior AMI complicated by VF (32%, 40%, 46%, 49% and 54%) (p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Selective cardiodepressant activity of fluodipine, a fluorenone-1,4-dihydropyridine derivative.

The effect of the dihydropyridine derivative, 1,4-dihydro-2,6-dimethyl-4-(fluorenon-4-yl)pyridine-3,5-dicarboxyl ic acid diallyl ester (fluodipine) was studied in vitro in different rabbit, rat and guinea pig preparations and in vivo in the rabbit in order to characterize its pharmacological profile at cardiac and at vascular sites. Compared to nifedipine, fluodipine showed a similar cardiodepressant activity, and a much lower inhibitory activity on vascular contraction. The highest tissue selectivity was observed in guinea pig preparations: fluodipine was about 2-3 times more effective than nifedipine on chronotropism and inotropism in isolated atria, and about 150 times less effective on aortic strip contraction. Accordingly, fluodipine (i) showed high-affinity binding to guinea pig ventricular L-type cardiac Ca2+ channels (Ki=2.57 nM), (ii) was about 80 times less effective than nifedipine to inhibit Ca2+ influx in vascular smooth muscle cells and (iii) induced a significant reduction of heart rate in the anesthetized rabbit (ID25=8.5 mg kg(-1), i.v.) without affecting the blood pressure up to 20 mg kg(-1), whereas nifedipine showed a significant hypotensive effect at very low doses (ID25=0.18 mg kg(-1), i.v.). The pacemaker current If of rabbit sino-atrial node myocytes was not affected by fluodipine. These findings demonstrate that fluodipine exerts selective cardiodepressant activity, likely due to a higher affinity for cardiac than for vascular Ca2+ channels.

Post-natal cardiomyocytes can generate iPS cells with an enhanced capacity toward cardiomyogenic re-differentation.

Adult mammalian cells can be reprogrammed to a pluripotent state by forcing the expression of a few embryonic transcription factors. The resulting induced pluripotent stem (iPS) cells can differentiate into cells of all three germ layers. It is well known that post-natal cardiomyocytes (CMs) lack the capacity to proliferate. Here, we report that neonatal CMs can be reprogrammed to generate iPS cells that express embryonic-specific markers and feature gene-expression profiles similar to those of mouse embryonic stem (mES) cell and cardiac fibroblast (CF)-derived iPS cell populations. CM-derived iPS cells are able to generate chimeric mice and, moreover, re-differentiate toward CMs more efficiently then either CF-derived iPS cells or mES cells. The increased differentiation capacity is possibly related to CM-derived iPS cells retaining an epigenetic memory of the phenotype of their founder cell. CM-derived iPS cells may thus lead to new information on differentiation processes underlying cardiac differentiation and proliferation.

Mechanism of reverse rate-dependent action of cardioactive agents.

Class 3 antiarrhythmic agents exhibit reverse rate-dependent lengthening of the action potential duration (APD), i.e. changes in APD are greater at longer than at shorter cycle lengths. In spite of the several theories developed to explain this reverse rate-dependency, its mechanism has been clarified only recently. The aim of the present study is to elucidate the mechanisms responsible for reverse rate-dependency in mammalian ventricular myocardium. Action potentials were recorded using conventional sharp microelectrodes from human, canine, rabbit, guinea pig, and rat ventricular myocardium in a rate-dependent manner. Rate-dependent drug-effects of various origin were studied using agents known to lengthen or shorten action potentials allowing thus to determine the drug-induced changes in APD as a function of the cycle length. Both drug-induced lengthening and shortening of action potentials displayed reverse rate-dependency in human, canine, and guinea pig preparations, but not in rabbit and rat myocardium. Similar results were obtained when repolarization was modified by injection of inward or outward current pulses in isolated canine cardiomyocytes. In contrast to reverse rate-dependence, drug-induced changes in APD well correlated with baseline APD values (i.e. that measured before the superfusion of drug or injection of current) in all of the preparations studied. Since the net membrane current (I(net)), determined from the action potential waveform at the middle of the plateau, was inversely proportional to APD, and consequently to cycle length, it is concluded that that reverse rate-dependency may simply reflect the inverse relationship linking I(net) to APD. In summary, reverse rate-dependency is an intrinsic property of drug action in the hearts of species showing positive APD - cycle length relationship, including humans. This implies that development of a pure K(+) channel blocking agent without reverse rate-dependent effects is not likely to be successful.

Rate dependency of beta-adrenergic modulation of repolarizing currents in the guinea-pig ventricle.

Beta-adrenergic stimulation modulates ventricular currents and sinus cycle length (CL). We investigated how changes in CL affect the current induced by isoprenaline (Iso) during the action potential (AP) of guinea-pig ventricular myocytes. Action-potential clamp was applied at CLs of 250 and 1000 ms to measure: (1) the net current induced by 0.1 microm Iso (I(Iso)); (2) the L-type Ca2+ current I(CaL) and slow delayed rectifier current I(Ks) components of I(Iso) (I(IsoCa) and I(IsoK)), identified as the Iso-induced current sensitive to nifedipine and HMR1556, respectively; and (3) I(Iso) persisting after inhibition of both I(Ca) and I(Ks) (I(isoR)). The pause dependency of I(Ks) and its modulation were evaluated in voltage-clamp experiments. The rate dependency of the duration of the action potential at 90% repolarization (APD90) and its modulation by isoprenaline were tested in current-clamp experiments. At a CL of 250 ms I(Iso) was inward during initial repolarization and reversed at 59% of APD90. At a CL of 1000 ms I(Iso) became mostly inward in all cells. Switching to shorter CL did not change I(IsoCa) and I(IsoK) amplitudes, but moved their peak amplitudes to earlier repolarization; I(IsoR) was independent of CL. Acceleration of I(IsoK) at shorter CL was based on faster pause dependency of I(Ks) activation rate. The 'restitution' of activation rates was modulated by isoprenaline. The APD90-CL relation was rotated anticlockwise by isoprenaline and crossed the control curve at a CL of 150 ms (400 beats min(-1)). We conclude that: (1) isoprenaline induced markedly different current profiles according to pacing rate, involving CL-dependent I(Ca) and I(Ks) modulation; (2) the effect of isoprenaline on APD90 was CL dependent, and negligible during tachycardia; and (3) during sympathetic activation, repolarization stability may involve matched modulation of sinus rate and repolarizing currents.

Role of the autonomic nervous system in the genesis of early ischemic arrhythmias.

The first minutes of myocardial ischemia are accompanied by a transient phase of cardiac electrical instability with special features, known as "phase 1a." The activation of the autonomic nervous system, associated with the onset of ischemia, seems to play a major role in the genesis of phase 1a arrhythmias and is potentially responsible for their unique characteristics. In this article, the relative influence of the sympathetic and parasympathetic components of cardiac neural activity on the ventricular electrical properties during early ischemia will be discussed. To study the influence of neural activity on acutely ischemic myocardium, and to evaluate the efficacy of antiarrhythmic interventions in conditions of sympathetic hyperactivity, we developed two different experimental models.

Characterization of the non-linear rate-dependency of QT interval in humans.

AIMS: Repolarization has rate-dependent and rate-independent components. A function considering such components separately was validated in canine Purkinje fibres and applied to the QT/RR relation in humans. METHODS AND RESULTS: Action potential duration (APD) was measured in Purkinje fibres during steady-state pacing at different cycle lengths (CL) and after prolonged quiescence (APD(0)). The APD/CL relationship was expressed by this function: APD=APD(max)(*)CL(S)/(CL(50)(S)+CL(S)), where APD(max) (APD extrapolated at infinite CL) is a rate-independent measure of repolarization, CL(50) (CL at which 50% of APD(max) is achieved) and S evaluates the rate dependency of APD. The same function was used to fit the QT/RR relation in 46 normal subjects (20 males, 26 females) and in 7 amiodarone-treated subjects undergoing a bicycle stress test. RR and QT (V(5)) were measured at the end of each load step; QT(c) (Bazett's formula) was obtained at rest. The APD/CL and QT/RR relations were equally well expressed by the function with high correlation coefficients (R>or=0.90). In Purkinje fibres, APD(max) was 461+/-37 ms, CL(50) was 394+/-54 ms and S was 0.98+/-0.11. APD(max) and APD(0) correlated (R=0.96) and were similar. The corresponding values in humans were: QT(max) 432+/-63 ms, RR(50) 345+/-60 ms and S 2.6+/-0.8. While QT(c) and QT(max) were longer in females, RR(50) and S were similar between genders. Amiodarone increased QT(c), QT(max) and RR(50) and decreased S. In QT(max) and QT(c) distributions generated by pooling data from treated and untreated subjects, 86% of treated subjects were correctly identified by QT(max) and 28% by QT(c). CONCLUSIONS: Canine and human repolarization showed a saturating dependency on cycle length, described by the proposed function. Gender and amiodarone independently affected QT(max), RR(50) and S: therefore they might reflect specific ionic mechanisms. Finally, QT(max) identified drug-induced repolarization abnormalities in individual subjects better than QT(c).

Modulation of the hyperpolarization-activated current (I(f)) by adenosine in rabbit sinoatrial myocytes.

BACKGROUND: Modulation of sinoatrial pacemaking by adenosine (Ado) in the absence of concomitant adrenergic stimulation (direct modulation) has been attributed to activation of a K+ conductance. In the present study, we evaluated the direct effects of Ado on the pacemaking current I(f) and tested their interaction with those of acetylcholine (ACh). METHODS AND RESULTS: Rabbit sinoatrial myocytes were patch-clamped at 35 degrees C in the presence of 1 mmol/L BaCl2 and 2 mmol/ L MnCl2, Ado (1 mumol/L) reversibly reduced I(f) by 33.1 +/- 5.7% of control (n = 5; P < .05). Ado (1 mumol/L) reversibly shifted I(f) midactivation potential by -6.63 +/- 1.18 mV (n = 4; P < .05). Fully activated I(f) conductance (0.262 +/- 0.037 versus 0.254 +/- 0.036 nS/ pF; n = 6, NS) and reversal potential (-17.35 +/- 0.99 versus -18.01 +/- 1.42 mV; n = 6, NS) were not changed by 10 mumol/L Ado. The Ado receptor antagonist 8-PST (10 mumol/L) reversed the effect of 0.3 mumol/L Ado by 64.9 +/- 4.2% (n = 6; P < .05). Ado maximally shifted the I(f) activation curve by -5.85 mV, with a half-maximal concentration of 0.0796 mumol/L (n = 93). The shifts in I(f) activation induced by Ado (0.3 mumol/L) and ACh (1 mumol/ L) separately were -4.89 +/- 0.05 and -8.84 +/- 0.51 mV, respectively; concomitant Ado and ACh superfusion shifted activation by -9.7 +/- 0.45 mV (NS versus ACh alone; n = 9). Threshold Ado concentrations dose-dependently reduced the rate of spontaneous pacemaker activity (eg, -18.8 +/- 3.4% at Ado 0.03 mumol/L). CONCLUSIONS: Submicromolar Ado directly inhibits I(f) and slows pacemaking in sinoatrial myocytes; the mode of I(f) inhibition is similar to that previously described for ACh. Thus, Ado may exert local modulation of sinus rate through signaling pathways similar to those used by ACh.

Effects of alpha-adrenergic stimulation on intracellular sodium activity and automaticity in canine Purkinje fibers.

Approximately 60% of adult canine Purkinje fibers respond to alpha 1-adrenergic stimulation with a decrease in automaticity. Recent studies of disaggregated Purkinje myocytes have suggested that this negative chronotropic effect results from alpha 1-adrenergic activation of the Na-K pump. In this study we evaluated 1) whether Na-K pump activation is associated with the negative chronotropic effect of alpha 1-adrenergic stimulation in adult canine Purkinje fibers and 2) if the effect of alpha-agonists on the pump is direct or mediated by an increase in intracellular sodium activity (aNai). We used sodium selective microelectrodes to determine the effects of 5 x 10(-9) and 5 x 10(-8) M phenylephrine on aNai. Phenylephrine decreased automaticity in five of eight Purkinje fibers while an increase occurred in the other three. The rate decrease was always accompanied by a decrease in aNai (-3.9 mM; p less than 0.05), whereas in fibers showing an increase in rate, aNai was unchanged. To evaluate the effect of phenylephrine in the absence of changes in automaticity, 10 Purkinje fibers were studied during pacing. A clear-cut reduction in aNai (-2.8 mM) was present in six fibers; no change was seen in the other four. The effect of phenylephrine was blocked by prazosin but not by propranolol. We conclude that the effect of alpha 1-adrenergic stimulation to reduce aNai is consistent with activation of the Na-K pump. Moreover, this action of alpha 1-adrenergic stimulation is closely linked to its negative chronotropic effect.

Cadmium-induced blockade of the cardiac fast Na channels in calf Purkinje fibres.

The fast transient inward current elicited by depolarizations above about -60 mV in calf Purkinje fibres was found to be depressed by Cd in concentrations less than 1 mM. The Cd-sensitive current, which strongly depended on external Na, was recorded in the presence of 2 mM MnCl2 and was blocked by TTX, indicating that a contamination from slow Ca-dependent currents could be discounted. The current reduction caused by Cd was also observed in nominally Ca-free solutions. The Cd-induced depression of the fast Na current was not accompanied by changes in the current kinetic parameters, as revealed by comparing inactivation curves and peak current voltage relations at different Cd concentrations, and could be attributed to a voltage-independent channel blocking action. Half-blockade occurred at 0.182 +/- 0.06 mM (n = 4). Plots of peak current amplitude as a function of the Cd concentration showed that the cooperation of two Cd ions was required to block a single channel.

The effect of antiarrhythmic drugs on life-threatening arrhythmias induced by the interaction between acute myocardial ischemia and sympathetic hyperactivity.

Transient myocardial ischemia, with attendant sympathetic hyperactivity, seems to play a major role in sudden cardiac death among patients with ischemic heart disease. Ventricular tachycardia (VT) and fibrillation (VF) are consistently and repeatedly elicited in cats by the interaction between a 2-minute occlusion of the left descending coronary artery and a 30-second stimulation of the left stellate ganglion. When three consecutive trials yield almost identical results, time alone will not modify the response and a given drug can be injected to test its efficacy with an internal control analysis. In 90 cats the efficacy of the following drugs was assessed: lidocaine (n = 11), mexiletine (n = 12), propafenone (n = 12), propranolol (n = 19), prazosin (n = 10), amiodarone (n = 14), and verapamil (n = 12). Class I antiarrhythmic drugs completely failed to afford protection and worsening of arrhythmia was observed in several instances. Propranolol and prazosin showed efficacy in approximately 80% and 60% of the animals, respectively. Amiodarone and verapamil completely prevented the onset of VT and VF. Protection from arrhythmias seems to be related to the combined presence of a noncompetitive adrenergic blockade associated with salutary effects on coronary circulation. These findings correlate with and help to explain the results of clinical trials in postmyocardial infarction patients. This model may help to provide a rational choice of antiarrhythmic drugs to be tested in clinical trials.

Intracellular calcium does not directly modulate cardiac pacemaker (if) channels.

A study on the cardiac pacemaker current if in inside-out macro-patches of sino-atrial (SA) node cells has recently demonstrated that if channels are directly activated by intracellular cAMP. Using the same preparation, here we investigate the possibility that internal Ca2+ ions play a role in the modulation of if channels. Our results indicate that Ca2+ ions do not have a direct activating effect on if.

Muscarinic effects on action potential duration and its rate dependence in canine Purkinje fibers.

Studies of the autonomic influence on action potential duration (APD) in the ventricles show direct effects of muscarinic stimulation on epicardial, but not endocardial, APD and conflicting results regarding direct vagal effects on the conduction system. In canine Purkinje fibers, we analyzed the action of the M2 agonist oxotremorine (OXO, 0.1 microM) on APD and on its cycle length (CL) dependence. Fibers were impaled with glass microelectrodes and superfused with Tyrode's solution. APD90 was measured after 3 minutes of drive at CL between 0.3 and 5 seconds. The best fit for the APD/CL relationship at steady state was a hyperbole: APD = APDmax*CL/(CL+CL50), where APDmax (APD at infinite CL) is a rate independent measure of APD, and CL50 (CL at which 50% APDmax is reached) is an index of the rate dependence of APD. In five fibers, OXO reduced APD at all CL (P < 0.05), APDmax was also reduced to 377 +/- 41 ms from 447 +/- 34 ms (P < 0.05), while CL50 was unchanged (405 +/- 46 ms from 437 +/- 28 ms). No effects of OXO on APD and APDmax were seen in two fibers obtained from dogs pretreated with pertussis toxin (PTX). In conclusion, stimulation of M2 receptors in intact, and not PTX treated, Purkinje fibers affects APD but not its CL dependence. This may reflect the activation of a rate independent, background current through a GTP binding protein-linked pathway, such as, IK,ACh. These data differ from those obtained in endocardial and epicardial muscle, stressing the regional differences in vagal modulation of ventricular electrophysiological properties.