Calcium carrying system in the giant muscle fibre of the barnacle species, Balanus nubilus.

1. Single barnacle muscle fibres from Balanus nubilus were internally perfused with an isotonic solution containing 180 mM-tetraethylammonium acetate and the effects of Ca concentration in the external solution on the voltage-clamp currents, especially the initial inward current, were examined.2. Muscle fibre in external solution containing no added Ca (concentration of Ca < 10(-5)M) gave a current-voltage curve that showed a small inward current followed by a small outward current. An identical curve was obtained when the chloride in the external solution was replaced by acetate.3. The peak inward current increased with increase in Ca concentration in the external solution; but the increase in peak current for equal increments of Ca concentration was reduced and attained saturation conforming to an adsorption regime which can be described by the Langmuir equation.4. A double reciprocal plot of peak inward current due to Ca as a function of Ca concentration gave values of 20.7 mM and 200% (the current due to 20 mM-Ca normalized to 100%) for the dissociation constant and the maximum current respectively. These values were found to depend on the concentration of Mg used in the external solutions.5. The peak inward Ca currents measured at two Ca concentrations as a function of pH were unaffected in the pH range 6.5-8.8; however when the pH was reduced below 6.5, the currents were depressed.