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

Abstract

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).