Interaction of I_Ca with I_Na/Ca2 [Ca⁺⁺]_i and Ca⁺⁺ Binding Proteins in Frog Atrium: A Theoretical Study

Abstract

The sarcoplasmic reticulum (S-R) of cells from the atrium of the bullfrog (Rana catesbeiana) is not involved in the cyclic uptake and release of calcium under normal physiological conditions. Consequently, bullfrog atrial cells provide a system in which the interactions between the “L”-type calcium current (I_Ca), sodium-calcium exchange current (I_Na/Ca) and intracellular Ca⁺⁺ binding proteins (calmodulin and two different classes of Ca⁺⁺ binding sites on troponin: T_Ca, a calcium-specific binding site thought to be responsible for phasic tension generation, and T_Mg, a Ca⁺⁺−Mg⁺⁺ competitive binding site) may be examined in isolation from complications arising from S−R uptake and release. A Hodgkin and Huxley type model¹ incorporating I_Na/Ca and intracellular Ca⁺⁺ binding sites has been used to examine the potential interactions between these different mechanisms during the cardiac action potential.