UBC Theses and Dissertations
Intrinsic heart rate resetting and associated changes in cardiac mRNA expression with thermal acclimation in rainbow trout, oncorhynchus mykiss Sutcliffe, Rachel Louise
The original aim of my thesis was to follow the timescale of intrinsic heart rate resetting in rainbow trout. Groups of rainbow trout acclimated to either 4°C or 12°C were reciprocally transferred to follow intrinsic heart rate resetting. However, while one group did reset intrinsic heart rate after 1 h at 12°C from 56.8 ± 1.2 to 50.8 ± 1.5 bpm, and after 8 h at 4°C from 33.4 ± 0.7 to 37.7 ± 1.2 bpm; another group did not reset intrinsic heart rate, even after 10 weeks of either warm or cold acclimation. Even though this variation in intrinsic heart rate resetting was unexpected, this serendipitous discovery created an opportunity to better associate changes in mRNA expression specifically with intrinsic heart rate resetting responses, as opposed to more general responses to warm acclimation. Therefore, I used a Fluidigm qRT-PCR system to compare mRNA expression of 28 cardiac function proteins after warm acclimation in three different cardiac tissues (sino-atrial node [SAN], atrium and ventricle) for fish with, and without, an intrinsic heart rate resetting response. When mRNA expression of three cardiac tissues was compared under control conditions at 4°C, the SAN had a uniquely higher HCN1, Cav1.3 and collagen 1α1 expression, while the ventricle had a uniquely higher RYR3 and NKA α3 expression. After > 3 weeks of warm acclimation, downregulation of NKA α1c in the atrium and ventricle, and upregulation of HCN1 in the ventricle were discovered in fish with an intrinsic heart rate resetting response, whereas upregulation of HCN3 in the SAN and atrium was discovered in fish without an intrinsic heart rate resetting response. However, no mRNA expression changes were observed after just 1 h of warm acclimation. In conclusion, while initial intrinsic heart rate resetting does not involve changes in mRNA expression, changes in mRNA expression are apparent over the longer term and may be associated with the different intrinsic heart rate resetting responses. Moreover, my results suggest that while intrinsic heart rate resetting can be fast, it cannot be assumed to always occur in rainbow trout.
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