The effect of temperature acclimation on the force-frequency relationship and adrenergic sensitivity of the ventricle of two populations of juvenile sockeye salmon.

We tested the hypothesis that cardiorespiratory differences known to exist among adult sockeye salmon populations also exist in the juveniles. To test this hypothesis, we compared cardiac contractility and adrenergic responsiveness of juvenile sockeye salmon from two geographically isolated populations that were reared from eggs under common garden conditions and at two acclimation temperatures (5 °C and 14 °C). However, we found no substantive differences in the force-frequency response (FFR) and the cardiac pumping capacity of juveniles from Weaver Creek and Chilko River populations, even when we considered wild-reared juveniles from one of the populations. An unexpected discovery for all fish groups at 5 °C was a rather flat FFR during tonic ß-adrenergic stimulation (ßAR) stimulation. Curiously, while active tension nearly doubled with maximum ßAR stimulation at low pacing frequencies for all fish groups, a negative FFR with maximum ßAR stimulation meant that this inotropic benefit was lost at the highest pacing frequency (0.8 Hz). Active tension with tonic ßAR stimulation was similar at 14 °C, but maximum pacing frequency doubled and all fish groups displayed a modest negative FFR. Maximum ßAR stimulation again doubled active tension and this benefit was retained even at the highest pacing frequency (1.6 Hz) at 14 °C. Even though subtle population differences were apparent for the FFR and pumping capacity, their biological significance is unclear. What is clear, however, is that the cardiac pumping capacity of juvenile sockeye would benefit more from ßAR stimulation swimming at 15 °C than when swimming at 5 °C.

Quantification of ventricular ß2 -adrenoceptor density and ligand binding affinity in wild sockeye salmon Oncorhynchus nerka smolts using a novel modification to the tritiated ligand technique.

A new, image-based, tritiated ligand technique for measuring cardiac ß2 -adrenoceptor (ß2 -AR) binding characteristics was developed and validated with adult rainbow trout Oncorhynchus mykiss hearts so that the tissue limitation of traditional receptor binding techniques could be overcome and measurements could be made in hearts nearly 14-times smaller than previously used. The myocardial cell-surface (functional) ß2 -AR density of O. nerka smolts sampled at the headwaters of the Chilko River was 54·2 fmol mg protein(-1) and about half of that previously found in return migrating adults of the same population, but still more than twice that of adult hatchery O. mykiss (21·1 fmol mg protein(-1) ). This technique now opens the possibility of investigating cardiac receptor density in a much wider range of fish species and life stages.