Thermal Acclimation Alters Both Adrenergic Sensitivity and Adrenoceptor Density in Cardiac Tissue of Rainbow Trout

We examined the effect of temperature acclimation on the sensitivity of the rainbow trout heart to adrenaline and on the density of b-adrenoceptors. The sensitivity of the heart was assessed using in situ working perfused heart and in vitro isometric ventricular strip preparations. When tested in situ and at acclimation temperature, hearts from fish acclimated to 8 ̊C were approximately 10-fold more sensitive to adrenalinesupplemented perfusate than were hearts from fish acclimated to 18 ̊C. The concentrations required for half-maximal stimulation (EC50) of myocardial power output were 1.931028 mol l21 adrenaline and 1.731027 mol l21 adrenaline for hearts acclimated to 8 ̊C and 18 ̊C, respectively. In vitro, isometric ventricular strip preparations demonstrated a similar increase in adrenergic sensitivity with coldacclimation. The EC50 values for maximal tension development were 2.731027 mol l21 adrenaline (8 ̊C-acclimated) and 1.131026 mol l21 adrenaline (18 ̊C-acclimated) when tested at acclimation temperature. This shift in adrenergic sensitivity was a function of the temperature acclimation because changes in bath temperature per se, either from 8 ̊C to 18 ̊C for 8 ̊C-acclimated hearts or from 18 ̊C to 8 ̊C for 18 ̊C-acclimated hearts, had no significant effect on the concentration–response curve for adrenaline. We conducted radioligand binding studies with [125I]iodocyanopindolol and propranolol to quantify the b-adrenoceptor density (Bmax) of both homogenates and isolated sarcolemmal fractions of ventricles from rainbow trout acclimated to either 8 ̊C or 18 ̊C. The Bmax for isolated sarcolemmal fractions was significantly higher in the 8 ̊Cacclimated group, but the Bmax of ventricular homogenates was not significantly different in the two acclimation groups. No significant differences in dissociation constant (Kd) were apparent in either the homogenates or sarcolemmal fractions. These results suggest that cardiac tissue from rainbow trout acclimated to 8 ̊C has a greater cell surface