Some effects of removal of external calcium on pig striated muscle.

Bundles of about 800 cells from the m. thyreopharyngicus of pigs were used to measure activation and inactivation during contracture by K+ depolarization. When [Ca2+] in the medium was lowered to less than 5 X 10(-10) M for 3 min (replacing Ca2+ by Mg2+) the activation occurred at the same [K+] in the medium as in normal solution (3 mM-Ca2+) but inactivation was shifted to lower external [K+]. The absolute value of this shift in terms of membrane potential is uncertain, because [K+] at the cell surface is unknown. Exposure for 4 min to Ca2+-free medium (Ca2+ being replaced by Mg2+) had no effect on contractility tested after a subsequent rest of 22-25 min in normal solution ( [Ca2+] = 2 mM). However, if the muscle underwent one maximal K+ contracture in Ca2+-free medium the response (tetanus or K+ contracture) after the same interval in normal solution was strongly reduced, although the membrane potential recovered fully. K+ contractures in normal solution could be repeated without loss of contractile force. A K+ contracture in Ca2+-free medium had very little effect on the response to caffeine, tested after 25 min in normal solution. It seems that Ca2+ is lost into Ca2+-free medium only during depolarization, from a site which is not accessible to Ca2+ from outside at the resting membrane potential, or from inside at any membrane potential. This site might be located inside the transverse-tubular membrane and, when loaded with Ca2+, might represent the positive group of the model of Chandler, Rakowski & Schneider ( 1976b ), the movement of which during depolarization activates and inactivates the Ca2+ release from the sarcoplasmic reticulum.