Morphology and fine structure of the heart of the burbot, a cold stenothermal fish

The ventricle of the burbot Lota lota heart comprised 0·148 ± 0·006% of the body mass which is nearly two‐fold heavier than the relative ventricular mass (MV) of other similarly sized teleosts. The shape of the ventricle is pyramidal and the wall is exclusively composed of spongious muscle without a distinct compact layer. The atrium forms 0·017 ± 0·002% of the body mass. Length, width, sarcolemmal surface area and volume of enzymatically isolated myocytes from burbot ventricle were 147·2 ± 10·2 μm, 6·3 ± 0·4 μm, 2440·8 · 251·5 μm2 and 2356·8 ± 316·6 μm3, respectively. The myofibrils were peripherally located and their volume density was remarkably high: 65 ± 2 and 68 ± 3% in ventricle and atrium, respectively (P>0·05). Although not particularly conspicuous, some nonjunctional and junctional sarcoplasmic reticulum (SR) was present in both atrial and ventricular myocytes. The SR formed peripheral couplings with the sarcolemma and the junctional clefts were frequently occupied by foot processes. These findings suggest that cold‐adaptation is achieved by cardiac enlargement, high volume density of myofibrils and well‐developed peripheral couplings in the SR in the heart of stenothermal burbot.