Differential effects of hypoxia with age on the chick embryonic heart. Changes in membrane potential, intracellular K and Na, K efflux and glycogen

Properties and functions of KATP during mouse perinatal development

BACKGROUND

Prevailing data suggest that ATP-sensitive potassium channels (K(ATP)) contribute to a surprising resistance to hypoxia in mammalian embryos, thus we aimed to characterize the developmental changes of K(ATP) channels in murine fetal ventricular cardiomyocytes.

METHODS

Patch clamp was applied to investigate the functions of K(ATP). RT-PCR, Western blot were used to further characterize the molecular properties of K(ATP) channels.

RESULTS

Similar K(ATP) current density was detected in ventricular cardiomyocytes of late development stage (LDS) and early development stage (EDS). Molecular-biological study revealed the upregulation of Kir6.1/SUR2A in membrane and Kir6.2 remained constant during development. Kir6.1, Kir6.2, and SUR1 were detectable in the mitochondria without marked difference between EDS and LDS. Acute hypoxia-ischemia led to cessation of APs in 62.5% of tested EDS cells and no APs cessation was observed in LDS cells. SarcK(ATP) blocker glibenclamide rescued 47% of EDS cells but converted 42.8% of LDS cells to APs cessations under hypoxia-ischemic condition. MitoK(ATP) blocker 5-HD did not significantly influence the response to acute hypoxia-ischemia at either EDS or LDS. In summary, sarcK(ATP) played distinct functional roles under acute hypoxia-ischemic condition in EDS and LDS fetal ventricular cardiomyocytes, with developmental changes in sarcK(ATP) subunits. MitoK(ATP) were not significantly involved in the response of fetal cardiomyocytes to acute hypoxia-ischemia and no developmental changes of K(ATP) subunits were found in mitochondria.

Expression of ATP-sensitive K+ channel subunits during perinatal maturation in the mouse heart

Prevailing data suggest that sarcolemmal ATP-sensitive (K(ATP)) channels in the adult heart consist of Kir6.2 and SUR2A subunits, but the expression of other K(ATP) channel subunits (including SUR1, SUR2B, and Kir6.1) is poorly defined. The situation is even less clear for the immature heart, which shows a remarkable resistance to hypoxia and metabolic stress. The hypoxia-induced action potential shortening and opening of sarcolemmal K(ATP) channels that occurs in adults is less prominent in the immature heart. This might be due in part to the different biophysical and pharmacological properties of K(ATP) channels of immature and adult K(ATP) channels. Because these properties are largely conferred by subunit composition, it is important to examine the relative expression levels of the various K(ATP) channel subunits during maturation. We therefore used RNAse protection assays, reverse transcription-PCR approaches, and Western blotting to characterize the mRNA and protein expression profiles of K(ATP) channel subunits in fetal, neonatal, and adult mouse heart. Our data indicate that each of the K(ATP) channel subunits (Kir6.1, Kir6.2, SUR1, SUR2A, and SUR2B) is expressed in the mouse heart at all of the developmental time points studied. However, the expression level of each of the subunits is low in the fetal heart and progressively increases with maturation. Each of the subunits seems to be expressed in ventricular myocytes with a subcellular expression pattern matching that found in the adult. Our data suggest that the K(ATP) channel composition may change during maturation, which has important implications for K(ATP) channel function in the developing heart.

Glucose is arrhythmogenic in the anoxic-reoxygenated embryonic chick heart

Unlike in adult heart, embryonic myocardium works at low PO2 and depends preferentially on glucose. Therefore, activity of the embryonic heart during anoxia and reoxygenation should be particularly affected by changes in glucose availability. Hearts excised from 4-d-old chick embryos were submitted in vitro to strictly controlled anoxia-reoxygenation transitions at glucose concentrations varying from 0 to 20 mmol/L. Spontaneous and regular heart contractions were detected optically as movements of the ventricle wall and instantaneous heart rate, amplitude of contraction, and velocities of contraction and relaxation were determined. Anoxia induced transient tachycardia and rapidly depressed contractile activity, whereas reoxygenation provoked a temporary and complete cardioplegia (oxygen paradox). In the presence of glucose, atrial rhythm became irregular during anoxia and chaotic-periodic during reoxygenation. The incidence of these arrhythmias depended on duration of anoxia, and no ventricular ectopic beats were observed. Removal of glucose or blockade of glycolysis suppressed arrhythmias. These results show similarities but also differences with respect to the adult heart. Indeed, glucose 1) delayed and anoxic contractile failure, shortened the reoxygenation-induced cardiac arrest, and improved the recovery of contractile activity; 2) attenuated stunning at 20 mmol/L but worsened it at 8 mmol/L; and 3) paradoxically, was arrhythmogenic during anoxia and reoxygenation, especially when present at the physiologic concentration of 8 mmol/L. The last named phenomenon seems to be characteristic of the young embryonic heart, and our findings underscore that fluctuations of glycolytic activity may play a role in the reactivity of the embryonic myocardium to anoxiareoxygenation transitions.

Developmental changes in membrane Ca2+ and K+ currents in fetal, neonatal, and adult rabbit ventricular myocytes

Whole-cell calcium current (ICa) and inwardly rectifying potassium current (IK1) were studied in 21-day fetal, 28-day fetal (total gestation, 31 days), 2-5-day neonatal, and adult rabbit ventricular myocytes isolated by enzymatic dissociation. Whole-cell peak ICa and IK1 at -100 mV increased significantly after birth. Cell size approximated from cell membrane capacitance also increased with age, with the most significant increase occurring after birth. When normalized to cell surface area, peak ICa density increased from day 21 of gestation to the neonate and then increased again from neonate to adult. In all age groups, peak ICa occurred at a test potential of +10 mV, and the shape of the Ca2+ current-voltage relation did not change with age. These findings suggest that there are no significant developmental changes in the voltage dependence of ICa. Therefore, the measured age-related increase in Ca2+ current density may result from increased channel expression. IK1 also exhibited a pattern of increasing current density with age. For IK1, the increase in current density was most rapid between day 21 and the perinatal period and much slower after birth. These results demonstrate that ICa and IK1 undergo significant changes during late fetal and postnatal development.

The force-frequency relationship in rat myocardium. The influence of muscle dimensions

Peak force and membrane potential were recorded from papillary muscles and trabeculae excised from the ventricles of adult rat hearts. Experiments were performed at 2.5 mM Ca2+ and 26 degrees C. In thick preparations (diameter 0.2-1.2 mm) an increase of stimulation frequency caused a reduction of peak force and action potential duration as has been found in many studies previously. In thin preparations (diameter less than 0.2 mm) both peak force and action potential duration were almost independent of stimulation frequency. When the flow of Tyrode solution through the muscle bath was reduced an increase of stimulation frequency caused a reduction of peak force and action potential duration in thin preparations. We conclude that the reduced peak force and action potential duration in papillary muscles at high stimulation frequencies is due to insufficient exchange of metabolites and oxygen between the medium and the core of the muscle. The results indicate that the critical diameter for the preparations is about 0.2 mm.

Effects of l-carnitine on action potential of canine papillary muscle during hypoxic perfusion

Under hypoxic (95% N2 + 5% CO2) perfusion, electrophysiological effects of L-carnitine on canine papillary muscles were studied using standard microelectrode techniques. During hypoxic perfusion for 60 min, resting membrane potential (RMP), action potential amplitude (APA) and maximum upstroke velocity of phase 0 were decreased, and action potential duration (APD) and effective refractory period (ERP) were shortened. Application of L-carnitine 25 mM under hypoxic perfusion increased RMP and APA and prolonged APD and ERP significantly. As effects of L-carnitine during hypoxic perfusion might be that of hypertonicity, effects of sucrose of the same tonicity as L-carnitine were studied under hypoxia. Sucrose did not cause significant changes on various parameters of action potential compared with hypoxic perfusion. It was suggested that the increase in RMP, and the prolongation of APD and ERP might be caused by an increase in intracellular ATP content. The findings in this study could be an explanation of possible antiarrhythmic effects of L-carnitine.

Effects of hypoxia on passive electrical properties of canine ventricular muscle

The effect of hypoxia, either in the presence or in the absence of glucose, on the passive electrical properties of canine ventricular muscle fibers was examined, employing a single sucrose gap method. The significant changes after 30 min of hypoxia (PO2 = 35-45 mm Hg) were an increase in the specific internal longitudinal resistance (Ri) and a decrease in the space constant (lambda). The values during the control (PO2 greater than 450 mm Hg) were 198 +/- 77 omega cm for Ri and 0.81 +/- 0.15 mm for lambda, and they changed to 245 +/- 90 omega cm and 0.70 +/- 0.10 mm, respectively, after 30 min of hypoxia. Hypoxia decreased the specific membrane resistance (Rm), but the changes were not statistically significant. The membrane time constant (tau m) and capacity (Cm) were not affected significantly. The absence of glucose during hypoxia was found to cause more profound changes than hypoxia alone in the passive electrical properties, especially Ri and lambda, suggesting that glucose might counteract the effects of hypoxia on these parameters of ventricular muscles.

Ionic currents during hypoxia in voltage-clamped cat ventricular muscle

To explore the mechanisms underlying the shortening of the cardiac action potential in hypoxia, we studied the effect of hypoxia on the ionic currents in cat papillary and trabecular muscles using the single sucrose gap-voltage clamp technique. For potentials positive to -70 mV, hypoxia induces an increase in time-independent outward current. The changes in the tail current suggest that time-dependent outward current is not increased but, rather, reduced. Because the time course of ik remains unchanged, we concluded that the shortening of the action potential is not a result of a change in the time-dependent outward current. In the potential range of the plateau, the amplitude of the slow inward current is not affected by hpoxia. Its time constant of inactivation appears slightly decreased. The prolongation of the action potential by epinephrine during hypoxia is accompanied by an increase in the slow inward current. As a result of these studies, we conclude that the shortening of the cardiac action potential in the early stage of hypoxia results from an increase in K+ outward background current.

Ontogenetic increase of the maximal rate of rise of the chick embryonic heart action potential. Relationship to voltage, time, and tetrodotoxin

The maximal rate of rise (Vmax) of the embryonic chick ventricular action potential increased about 2-fold from 112 +/- 6 V/sec and 113 +/- 5 V/sec on the 4th and 6th incubation days, respectively, to 217 +/- 7 V/sec on the 12th incubation day. Neither the steady state inactivation (hinfinity) of Vmax, the temperature-dependent shift of hinfinity, nor the time constant for recovery (tauRec) of Vmax changed significantly when Vmax had doubled. Inhibition of Vmax by tetrodotoxin (TTX) was the same in hearts from the 4th, 6th, and 18th incubation days. The relation between Vmax and TTX concentration could be described by a one-to-one binding curve with an apparent dissociation constant of 2 X 10(-8) M. The increase of Vmax, a valid measure of the early inward Na+ current (iNa) during the rising phase of the cardiac action potential, can be attributed to an elevated maximal Na+ conductance (-gNa) during ontogenesis. These results indicate that the physicochemical properties of the gNa unit responsible for the rapid rising phase of the action potential and of the TTX binding site associated with it may remain constant during a period of embryonic development when -gNa increased significantly.