USE OF HILL EQUATION FOR KINETIC ANALYSIS OF Ca2+-TRANSPORTING SYSTEMS OF CHIRONOMUS PLUMOSUS LARVAE SECRETORY CELLS

Hill equation was used for kinetic analysis of functioning plasma membrane Ca2+-transporting systems in salivary gland's secretory cells of Chironomus plumosus larvae. For the calculation of Hill equation parameters, modified Eadie-Hofstee plot was used. Hill equation was applied for description of Na+-Ca2+ exchange trans-membrane current dependence on appropriate ligand concentration. However, when the value of Hill coefficient n > 1 for Na+-Ca2+ exchange, inward current dependence on [Ca2+]e was found. It testifies to exchanger allosteric regulation by cytosolic Ca2+ but not to transported cations cooperativity. It is also postulated that Hill equation may be used to describe such heterogenic system as ligand-inducted changing of Ca2+ content in the gland tissue. In this case it should be abstracted away from specific Ca2+-transporting systems properties; in case of n = 1 it specific ligand linking to only one Ca2+-transporting system is evident at given concentration mag-nitude (for example, eosin Y with plasma membrane Ca2+-pump). In case of n > 1, one can propose several Ca2+-transporting systems to take part in single-directed Ca2+ content changes (for example, Na+-Ca2+ exchanger activation in reverse mode and plasma membrane Ca2+-pump inhibition) under the condition, when the system sensitivity to ligand is in the same range. Vice versa, if n < 1, single-directed changes in different-directed Ca2+-transporting systems can be observed (for example, voltage operated Ca2+-channels and Ca2+-pump of plasma membrane inhibition).