The Pulmonary veins (PVs) are a widely recognised source of ectopic electrical activity that can lead to atrial fibrillation. While this activity likely originates from the external sleeve of cardiomyocytes that surround the PV, the underlying mechanisms are currently unknown. Changes in intracellular Ca2+ signalling are thought to influence the electrical properties of PV cardiomyocytes; therefore the PV was isolated from the rat and intracellular Ca2+ was monitored with the fluorescent indicator fluo-4. Spontaneous Ca2+ transients were present under resting conditions and typically manifested as waves occurring asynchronously between neighbouring cells. Immediately following a brief period of electrical field stimulation (EFS) at 3 Hz or greater, the frequency of the spontaneous Ca2+ transients was increased. Furthermore, this effect was more pronounced after the extracellular Ca2+ concentration was increased. Further analysis showed that the spontaneous Ca2+ transients occurred more synchronously in the initial few seconds following electrical stimulation at 3 to 9 Hz. As spontaneous Ca2+ transients are due to Ca2+ released from the sarcoplasmic reticulum through the ryanodine receptors, immunocytochemistry was used to determine their distribution. The ryanodine receptors were arranged in a striated pattern in the cell interior and also along the periphery of cell, which was similar to atria myocytes. However, unlike atrial cells, labelling of the membrane with Di-4 ANEPPS revealed the presence of t-tubules in PV cardiomyocytes. When the contractile response of the PV was studied in vitro, contractions that were ~50–80 ms in duration, which is typical of cardiac tissue, could be evoked by electrical stimulation. The α and β adrenoceptor agonist noradrenaline induced periodic bursts of contractions that occurred independently of electrical stimulation and this arrhythmogenic activity was supressed by inhibiting the Na+/Ca2+ exchanger (NCX) with ORM-10103, suggesting an important role for the NCX in noradrenaline induced automaticity.
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