High Levels of Calsequestrin in Some Snake Muscles. Why?

Calsequestrin (CSQ) is the most abundant Ca2+ binding protein in sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. It is characterized by a high capacity and a moderate affinity for Ca2+ and thus it maintains luminal free Ca2+ in the junctional SR cisternae (jSR) at the physiological concentration of 1mM, while storing up to 20mM total calcium. Thin-section TEMs of superficial epaxial muscles from Nerodia sipedon and Boa constrictor show at least two structurally distinguishable types of fibers, probably twitch and tonic. Twitch fibers have unusually wide jSR cisternae completely filled with a finely granular matrix of CSQ polymers. The visible luminal content also extends far from the triads into the longitudinal SR. This is also the case in anterior intermandibular muscles from the same snakes. The percentage of fiber volume occupied by SR in twitch fibers from superficial epaxial muscles was calculated, from morphometry of thin sections, to be 9.5% in Nerodia, 10.6% in Boa and 7.5% in leg muscles from the lizard Anolis carolinensis, another lepidosaur. Comparable data from the literature for mouse indicate 5.5% in EDL and 2.5% in soleus. So, overall, the snake muscles have considerably higher Ca2+ storage capacity than lizard and mouse, two species that move frequently. We suggest that this may be related to prolonged periods of inactivity of these snakes, interrupted by the very infrequent use of muscles during prey capture and locomotion.