Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB021

Poster Communications

Different Intrinsic Heart Rate Resetting Responses and the Associated Changes in Cardiac mRNA Expression with Warm Acclimation in Rainbow Trout

R. L. Sutcliffe1, S. Li2, K. M. Miller2, A. P. Farrell1

1. Zoology, University of British Columbia, Vancouver, British Columbia, Canada. 2. Pacific Biological Station Fisheries and Oceans, Nanaimo, British Columbia, Canada.

Intrinsic heart rate (IHR) resetting is hypothesised to maintain cardiac scope with warm temperature acclimation in rainbow trout. However, little is understood about the timescale of this process, and the mechanisms causing this change. The original aim was to follow the timescale of IHR resetting of rainbow trout. Two groups of rainbow trout, acclimated to 4°C, were transferred 12°C to follow the timescale of IHR resetting (measured in vitro). While one group reset IHR within 1 h at 12°C (Figure 1); the second group did not. In fact, they showed no IHR difference after 10 weeks of acclimation to either 4°C or 12°C (Figure 2). This variation in intrinsic heart rate resetting was unexpected. However, this serendipitous discovery created an opportunity to better link changes in mRNA expression with IHR resetting responses, rather than more general warm acclimation responses, in different cardiac tissues. Therefore, a Fluidigm qRT-PCR system was used to compare mRNA expression of 30 cardiac function proteins, after 3 weeks of warm acclimation, in three cardiac tissues (sino-atrial node [SAN], atrium and ventricle), for fish with, and without, IHR resetting, and 1 h after acclimation with IHR resetting (n=7). Under control conditions at 4°C, higher HCN1, Cav1.3 and col1α1 expression was unique to the SAN; while higher RYR3 and NKAα3 expression was unique to the ventricle. Warm acclimation to 12°C for > 3 weeks, both with and without IHR resetting, was associated with downregulation of RYR3 in the atrium and HCN2a2 in the SAN, and upregulation of HCN3 in the ventricle. Downregulation of NKAα1c in all three tissue types and S100 in the SAN, and upregulation HCN1, Nav1.6, and NKAα3 in only the ventricle were unique to the fish that reset IHR. Downregulation of HCN2a1 in the atrium and HCN1 in the ventricle, and upregulation of HCN3 in the SAN, and TGF-β1 and col1α1 in the atrium were unique to fish that didn't reset IHR. However, no mRNA expression changes were observed after 1 h of 12°C acclimation. While the initial IHR resetting does not involve changes in mRNA expression, changes in mRNA expression are apparent over the longer term. Moreover, these results suggest that while IHR resetting can be fast, it cannot be assumed to always occur in rainbow trout. Statistical significance = p<0.05.

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