Seasonal adaptation of crucian carp (Carassius carassius L.) heart: glycogen stores and lactate dehydrogenase activity

Abstract

The glycogen content of the crucian carp heart followed a clear annual cycle, while the size of the heart remained constant throughout the year. Glycogen stores were very abundant at the beginning of the winter (8% of heart mass) and were consumed during the winter and spring, so the stores were at their minimum in early summer (1.4% in May). New glycogen depositions accumulated in the heart during summer and autumn, the maximum glycogen content being attained at the end of October. Glycogen occurred in two distinct granular forms: small β particles about 25 nm in diameter and large α particles about 100 nm in diameter. In late autumn glycogen was mainly in the form of small β particles, which were gathered in vast "glycogen seas" up to 45 μm in length and 10 μm in width. In spring and especially in summer the larger α particles were more numerous than β particles. Lactate dehydrogenase (LDH) activity of the crucian carp heart was relatively high and constant throughout the year. The kinetic properties of cardiac LDH were intermediate between pure cardiac and pure muscle type LDH with a low Km value (0.1 mM) for pyruvate, but only moderately (30%) inhibited by high pyruvate concentrations. The isoenzyme composition and kinetics of LDH did not change seasonally. Crucian carp showed an immediate and strong reduction in heart rate when exposed to hypoxic water. These findings suggest that the crucian carp heart tolerates a long hypoxic winter period by suppressing energy consumption with strong bradycardic reflex and utilizing the massive glycogen stores of the tissues through anaerobic metabolism.