Poster

Comparative cardiac ultrastructural morphometry in mammals with distinctive cardiac activity has revealed structural differences reflecting functional adaptations. Quantitative differences have been noted in sizes of T-tubules and in volumes of mitochondria, myofibrils, lipid bodies and specific heart granules (SHG) that contain atrial natriuretic peptide (ANP). In general, smaller mammals with higher heart rates have higher concentrations of mitochondria than larger ones with slower heart rates. Wider T-tubules are observed in the hamster and hedgehog which hibernate and in the gray seal which is capable of diving. During both hibernation and diving, cardiac rate is reduced to as low as 4 beats per minute. However, little is known about the variation in SHG concentrations between species. Few studies have focused on the ultrastructure of pinniped myocardium to determine what structural adaptations may occur for diving and electrolyte balance. This study examines the ultrastructure of myocardial cells in harbor and northern elephant seals (Phoca vitulina and Mirounga angustirostris).

Hearts from 3 harbor seal and 3 elephant seal pups that were euthanised following stranding due to incurable disease were used for this study. Within 30 minutes of death, hearts were perfused via the coronary arteries with normal saline followed by 3% glutaraldehyde in 0.1M phosphate buffer at pH 7.3. Samples of atrial and ventricular tissue were post-fixed in 1% osmium in 0.1M phosphate buffer, dehydrated through graded concentrations of ethanol and embedded in epon. Thin sections of longitudinally orientated tissue were stained with uranyl acetate and lead citrate were examined in a JEOL 100CX II electron microscope.

The phocid myocytes were similar to those of other mammals having alternating rows of myofibrils and mitochondria, the latter having an open arrangement of cristae with moderate numbers of matrix granules. The free cytoplasm contained abundant glycogen granules but few lipid droplets. SHG granules were sparse in atrial myocytes of both phocid species, with clusters around the Golgi body. On average there were 7 granules per cell profile, and rarely were cells with more than 15 SHG observed. SHGs consisted of 0.32 (± 0.046) and 0.23 (± 0.012) per cent of cytoplasmic volume for harbor seal and elephant seal atrial myocytes respectively. Mean diameter of SHGs was 173 nm in the harbor seal and 165 in the elephant seal. The transverse tubular system was well characterized and abundant, particularly in ventricular cells, and occupied 2% of cytoplasmic volume. T-tubules were observed in all fields examined and were coupled with elements of the sarcoplasmic reticulum, the two structures being separated by a gap of 15-30 nm.

The low concentration of SHG suggests that these phocids have little need for ANP in their osmoregulatory system. This may be a consequence of the large blood reservoirs present in these species, resulting in little change in atrial volume associated with changes in blood sodium concentrations. Atrial stretch triggering ANP release as occurs in some terrestrial mammals would thus be an insensitive homeostatic mechanism. The well developed T-tubule system may be an adaptation to low heart rates, as it increases the total surface area for metabolic exchange. Further studies are required to determine the extent to which the structural myocyte adaptations observed in these two phocid species occur in other diving mammals, such as the cetacea.