In vitro studies of the gain and exchange of calcium in frog skeletal muscle.

(1) The Ca(++), Na(+), and K(+) contents of frog sartorius muscles were found analytically after exposure to various media including some containing labeled Ca(++). (2) During storage in media with 100 to 120 mM Na(+) and 1 mM Ca(++) both Na(+) and Ca(++) are gained while K(+) is lost; there is a high correlation between Na(+) and Ca(++) gains. (3) When Ca(++) gain occurs from a solution containing labeled Ca(++) there is also some exchange of the original Ca(++) with the labeled Ca(++). The amount exchanged is considerably less (e.g. 50 per cent) than the total amount of labeled Ca(++) taken up by the tissue. (4) When the external Na(+) concentration is reduced to 30 mM the amount of labeled Ca(++) taken up is increased. Part of the increase is attributable to a greater net gain and part to a greater degree of exchange. (5) It is pointed out that muscles which have been loaded in vitro with labeled Ca(++) will not provide a valid measure of the exchangeability of the normal Ca(++) content present at the time of dissection. (6) Comparison is made between results obtained using Sr(89) and Ca(45) as labels for the Ca(++). Little, if any, difference is perceptible.