title: Some effects of removal of external calcium on pig striated muscle.
abstract: 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.
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D008564:Membrane Potentials; D008954:Models, Biological; D009119:Muscle Contraction / Q000187:drug effects*; D009132:Muscles / Q000502:physiology; D011188:Potassium / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D013552:Swine
chemical_list: D002110:Caffeine; D011188:Potassium; D002118:Calcium