RESUMO
We examined cardiac pacemaker rate resetting in rainbow trout following a reciprocal temperature transfer. In the original experiment, performed in winter, 4°C-acclimated fish transferred to 12°C reset intrinsic heart rate after just 1â h (from 56.8±1.2 to 50.8±1.5â beatsâ min-1); 12°C-acclimated fish transferred to 4°C reset intrinsic heart rate after 8â h (from 33.4±0.7 to 37.7±1.2â beatsâ min-1). However, in a replicate experiment, performed in the summer using a different brood year, intrinsic heart rate was not reset, even after 10â weeks at a new temperature. Using this serendipitous opportunity, we compared mRNA expression changes of a suite of proteins in sinoatrial node (SAN), atrial and ventricular tissues after both 1â h and longer than 3â weeks for both experimental acclimation groups to identify those changes only associated with pacemaker rate resetting. Of the changes in mRNA expression occurring after more than 3â weeks of warm acclimation and associated with pacemaker rate resetting, we observed downregulation of NKA α1c in the atrium and ventricle, and upregulation of HCN1 in the ventricle. However, in the SAN there were no mRNA expression changes unique to the fish with pacemaker rate resetting after either 1â h or 3â weeks of warm acclimation. Thus, despite identifying changes in mRNA expression of contractile cardiac tissues, there was an absence of changes in mRNA expression directly involved with the initial, rapid pacemaker rate resetting with warm acclimation. Importantly, pacemaker rate resetting with thermal acclimation does not always occur in rainbow trout.