The pulmonary veins are widely recognised as a source of ectopic electrical activity that can cause atrial fibrillation. While the ectopic activity likely originates in the cardiomyocytes that form an external sleeve around the veins, the underlying mechanisms are unknown. Changes in intracellular Ca2+ signalling have been proposed to play an important role in the arrhythmogenic properties of the pulmonary vein. Therefore, the aim of this thesis was to study factors that might influence Ca2+ signalling in the cardiomyocytes. This involved determining the localisation of Ca2+ handling proteins in the cardiomyocytes, examining interventions that might alter the characteristics of intracellular Ca2+ signalling, as well as looking at the arrhythmogenic effect of adrenergic stimulation.In the rat pulmonary vein, the cardiomyocytes displayed spontaneous Ca2+ transients that were usually manifest as waves, and were asynchronous in neighbouring cardiomyocytes. The frequency of spontaneous Ca2+ transients was increased following a brief period of electrical stimulation at 3 Hz or greater, and this effect was enhanced in the presence of isoprenaline, or when the external Ca2+ concentration was raised.Noradrenaline also increased the frequency of the spontaneous Ca2+ transients; however, synchronous Ca2+ transients, like those that could be evoked by electrical field stimulation, were not observed. 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 with some distribution at the periphery of the cells, which was similar to myocytes from the atria. When the sarcolemma of the pulmonary vein cardiomyocytes was labelled with Di-4 ANEPPS, they were shown to possess transverse (T)-tubules, which are involved in co-ordinating the intracellular Ca2+ transient in response to depolarisation. This differed from atrial myocytes where a T-tubule system was not observed. Furthermore, the L-type Ca2+ channels and Na+/Ca2+ exchanger (NCX), which are involved in Ca2+ influx and removal during excitation-contraction coupling, were arranged in a more striated manner in the pulmonary vein cardiomyocytes, compared to those of the atria. This could have important consequences for the contractile activity of the cardiomyocytes, as well as their ability to generate abnormal electrical activity, as the NCX is known to cause depolarisation in response to an increase in intracellular Ca2+.The contractile properties of the pulmonary vein were studied in vitro using myography techniques, where it was shown to display a negative force-frequency relationship, whereby increasing the frequency of electrical stimulation reduced the contractile amplitude. In the presence of noradrenaline, the amplitude of the electrically evoked contractions was increased and the negative force-frequency relationship was only evident at the higher stimulation frequencies (5 to 7 Hz). Noradrenaline also induced periodic bursts of contractions that occurred independently of electrical stimulation, suggesting that it had an arrhythmogenic effect. Such activity was partially inhibited by blocking the NCX with ORM-10103. This suggests a potential target for future research into selective pharmacological intervention for catecholamine based arrhythmias.
|Date of Award||1 Oct 2014|
- University Of Strathclyde
|Sponsors||British Heart Foundation BHF|
|Supervisor||Robert Drummond (Supervisor) & Edward Rowan (Supervisor)|