The electrophysiological properties of human coronary endothelial cells (HCEC) of macro- and microvascular origin were studied using the whole-cell configuration of the patch-clamp technique. The membrane potential of confluent HCEC (-41.9 +/- 3.9 mV (mean +/- SEM, n = 32) for macro- and -33.6 +/- 2.6 mV (n = 64) for microvascular cells, respectively) was less negative than the K+ equilibrium potential. Inward currents of isolated cells at potentials below the K+ equilibrium potential were blocked by external Ba2+ (1 mM), inactivated due to time- and voltage-dependent block caused by external Na+, and their amplitudes were enhanced by increasing extracellular [K+]; these currents were identified as inwardly rectifying K+ currents. Some isolated cells displayed outwardly directed K+ currents which were abolished after replacement of Cs+ for K+ on both sides of the membrane. Voltage-dependent Ca2+ currents could not be observed in isolated HCEC. Hyperpolarizations induced by vasoactive agonists have been observed in some endothelial cells from different species. In contrast, extracellularly applied ATP (adenosine-5'-triphosphate) and ADP (adenosine-5'-diphosphate) at micromolar concentrations depolarized confluent HCEC, whereas adenosine had no effect on resting potentials (RP), indicating that the nucleotide-induced depolarizations were mediated via P2- purinoceptors. These depolarizations occurred even after replacement of N-methyl-D-glucamine for extracellular Na+, indicating that Ca(2+)-influx was involved. There were no marked differences in the electrophysiological properties between cells of macro and microvascular origin.
The effects of changing the time course of action potential repolarization on the amplitude and coupling interval of delayed afterdepolarizations were studied in small preparations of coronary sinus atrial fibers exposed to catecholamines. Repolarization was accelerated or retarded by current pulses passed through an intracellular microelectrode in the depolarizing or repolarizing direction. Acceleration of repolarization decreased the amplitude of delayed afterdepolarizations, prolonged their coupling interval to the action potential upstroke, and prevented triggered activity. Prolonging the time for repolarization increased afterdepolarization amplitude, shortened the coupling interval, and caused triggered activity. The afterdepolarization amplitude and coupling interval had a linear relationship to the duration of the action potential plateau. In some preparations, the action potential plateau increased spontaneously at stimulation rates that caused afterdepolarization amplitude to increase and triggering to occur, and this may have contributed to the occurrence of triggering. The effects of action potential repolarization on delayed afterdepolarizations suggest that pharmacological agents such as antiarrhythmic drugs which alter action potential duration should influence afterdepolarizations. Drugs which shorten action potential duration might prevent triggered activity from occurring, whereas drugs which prolong duration might cause triggering.
Vasoactive agonists like adenosine-5'-triphosphate (ATP) increase intracellular Ca2+ ([Ca"]i) in vascular endothelial cells with an initial peak due to inositol 1,4,5-triphosphate-mediated Ca2+ release from intracellular stores followed by a sustained plateau that is dependent on the presence of extracellular Ca2+, thus leading to an increased synthesis and release of prostacyclin and nitric oxide. We studied the effects of nucleotides on membrane potential and [Ca2+Ii in confluent human microvascular cardiac endothelial cells obtained from patients with dilated cardiomyopathy. The whole-cell configuration of the patch-clamp technique and a confocal laser scanning microscope employing fluo-3 as a Ca2+ indicator were used. Both uridine-S'hphosphate (UTP) and 2-methylthioadenosine-5'-triphosphate (2MeSATP) induced depolarizations in human microvascular cardiac endothelial cells and increased [Ca2+Ii with a rank order of potency 2MeSATP>ATP= UTP (ECSo values (in pM) were 0.084 2MeSATP, 0.67 ATP and 1.1 UTP). This suggests that both PZu and PZy purinoceptors are present on human microvascular cardiac endothelial cells. Maximal [Ca2+Ii responses of confluent human microvascular cardiac endothelial cell monolayers to UTP were lower when compared to 2MeSATP. Nucleotide-induced increases in [Caz+Ii consisted of a transient peak, which was also observed in the absence of extracellular Ca2+, and a sustained [Ca"], plateau. This plateau, which was not observed in all monolayers studied, was not markedly influenced by increasing extracellular [K+]. Previous incubation with thapsigargin abolished ATP-induced increases of [Ca"],. It is concluded that human microvascular cardiac endothelial cells express both PZy and P2,, purinoceptors. P2 purinoceptor agonists release Ca2+ from intracellular thapsigargin-sensitive stores and stimulate capacitative Ca2+ influx pathways. K+ efflux through Ca'+-dependent K+ (K,-*) channels does not play a major role in the regulation of nucleotide-induced Ca2+ influx in human microvascular cardiac endothelial cells, which might be related to an impaired function of the cells.
Our results suggest an independent association between OSA and impaired renal function. Further prospective studies will have to be done to elucidate the pathophysiological mechanisms.
1. Bursts of triggered activity can be induced in atrial fibres of the canine coronary sinus exposed to catecholamines. During a triggered burst there is an initial acceleration of rate accompanied by depolarization of the maximum diastolic potential (m.d.p.) followed by slowing of the rate and termination accompanied by hyperpolarization. 2. We have used extracellular K+-sensitive micro-electrodes (potassium ISE) to monitor extracellular K+ concentration ([K+]o) during and following triggered activity, while simultaneously measuring membrane potential with conventional intracellular micro-electrodes. 3. We found that the initial increase in rate during triggered activity is accompanied by increased [K+]o and depolarization. Later rate slowing and m.d.p. hyperpolarization is accompanied by decline of extracellular K+ accumulation. Following termination of triggered activity, extracellular K+ depletion occurred. 4. The decline of [K+]o and slowing of rate are known responses to enhanced Na+-K+ pump activation, as is the post-triggering depletion of extracellular K+. 5. Strophanthidin, which blocks the Na+-K+ pump, also blocks the [K+]o decline, the slowing of rate seen towards the end of the triggered episode, and the post-triggering depletion of extracellular K+. 6. Separate experiments studying the effects of elevated bath K+ and depolarizing current on triggering rate and delayed after-depolarization amplitude support our hypothesis that the rate profile of the triggered episode is to a large extent controlled by variations in m.d.p. subsequent to extracellular K+ accumulation and Na+-K+ pump activation.
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