What is the source of the calcium that activates contraction of barnacle muscles under physiological conditions?

Dear Sir: The source of the calcium necessary to activate the contractile response of muscle fibers may be either internal stores or a Ca ++ ions that enter during the change in membrane potential initiating the contraction. In a recently published study of contractile activation in barnacle by voltage clamp, the magni tude of the tension produced by depolarizing pulses was found to reach a saturating level at a membrane potential of about +10 mV and to be unchanged at potentials up to 230 mV (Caputo and DiPolo, 1978). Since the equilibrium potential for Ca is between 157 and 186 mV, the constancy of the tension response for membrane potentials between 10 and 230 mV argues strongly against a significant contribution of entering calcium in activating tension under the conditions of the experiment. The authors conclude: "These results suggest that under physiological conditions the contractile activator is probably released from an internal store . . . . " The extrapolation from the conditions used in the experiment to physiological conditions may not be completely justified. The physiological activation of the barnacle muscle is initiated by endplate potentials of up to 40 mV in size, which may set up graded local responses. The magnitude of these is reported never to exceed that of the resting potential (Hoyle and Smyth, 1963), so that under physiological conditions, contraction is activated at negative membrane potentials. Therefore, the range of potentials used by Caputo and DiPolo (1978) to demonstrate the absence of a relationship between tension and membrane potential, i.e., 10-230 mV, may not be physiological. Caputo and DiPolo (1978) found with voltage clamp that depolarizations of less than the magnitude of the resting potential produced tension with only an outward current. However, the amount of inward current necessary for sufficient Ca ++ to enter to initiate contraction would likely not be detectable in their experiments. The Ca ++ threshold for contraction is about 5-9 • 10 -7 M (Ashley, 1967). For a fiber of 2-mm diameter, the current density to move sufficient Ca ++ across the membrane to reach threshold in 50 ms is 1-2 • 10 -4 A / c m 2. Tet rae thylammonium, which inhibits the outward K § current, was found by Caputo and DiPolo (1978) to reduce the outward current during voltage clamp by something of the order of this amount. Therefore, the inward Ca ++ current necessary to initiate contraction could have been masked by a larger outward K § current.