Dependence on ionic strength of the calcium-induced contraction of glycerinated stalk of the peritrich ciliate, Vorticella convallaria

Abstract

1. 1. The dependence on ionic strength of the calcium-induced contraction of glycerinated stalks of the protozooan peritrich ciliate, Vorticella convallaria, has been investigated. 2. 2. With increasing ionic strength (KC1) in the reaction solution, the calcium concentration (Ca 2+) m, at which the fractional length of the stalk becomes 0.5, increases monotonically. 3. 3. The logarithm of (Ca 2+) m, can be expressed as a linear function of the logarithm of the activity coefficient of KC1 over a wide range of KC1 concentrations. 4. 4. The Hill constant, n, which is the number of calcium ions bound simultaneously and cooperatively to a contractile element for spasmoneme contraction, is about 2, independent of ionic strength. 5. 5. Our observations are consistent with the suggestion that the calcium binding site in a contractile element of the spasmoneme is negatively charged and is probably composed of carboxylic amino acid residues of an acidic calcium-binding protein.