Basic functional properties of the cardiac muscle of the common shrew (Sorex araneus) and some other small mammals.

The resting heartbeat frequency of all the studied wild small mammals (body mass 3-20 g) was lower than that predicted by the allometric equation for a typical mammal. The heart rate of the laboratory mouse was a little higher than the expected value. The ventricular mass of the small wild mammals was higher than predicted for their size, but that of the laboratory mouse was below the expected value. Thus, adequate cardiac output in the wild small mammals is achieved by compensating the low heartbeat frequency with greater stroke volume. The shrew species are notable exceptions, which, despite having a metabolic rate 2-3 times higher than the mammalian average, neither have exceptionally high heart rates nor larger hearts than other wild small mammals. The adaptation of the shrew heart to high metabolic rate may reside in the shape of heart. The ventricular myocardium of shrews is characteristically long and narrow with a tapered apex, whereas other small mammals have rounder hearts. The duration of the ventricular action potential was short and inversely proportional to the resting heart rate of the mammalian species. Caffeine (5 mmol l-1) strongly decreased the isometric contractile force of right ventricular strips in all the studied mammals. These findings suggest that in the small mammals intracellular stores are the main source of activating Ca2+, whereas transsarcolemmal Ca2+ movement may only serve the triggering function.