Evaluation of the Role of the Sympathetic Nervous System in the Glucoregulatory Response to Insulin-Induced Hypoglycemia in the Rat

Abstract

<p>We perform differential potentiometric titration measurements for the binding of Ca<sup>2+</sup> ions to micelles composed of the carboxylic acid end-standing Pluronic P85 block copolymer (i.e., CAE-85 (COOH-(EO) <sub>26</sub>-(PO)<sub>39</sub>-(EO)<sub>26</sub>-COOH)). Two different ion-selective electrodes (ISEs) are used to detect the free calcium concentration; the first ISE is an indicator electrode, and the second is a reference electrode. The titration is done by adding the block copolymers to a known solution of Ca<sup>2+</sup> at neutral pH and high enough temperature (above the critical micellization temperature CMT) and various amount of added monovalent salt. By measuring the difference in the electromotive force between the two ISEs, the amount of Ca<sup>2+</sup> that is bound by the micelles is calculated. This is then used to determine the binding constant of Ca <sup>2+</sup> with the micelles, which is a missing parameter needed to perform molecular realistic self-consistent-field (SCF) calculations. It turns out that the micelles from block copolymer CAE-85 bind Ca<sup>2+</sup> ions both electrostatically and specifically. The specific binding between Ca<sup>2+</sup> and carboxylic groups in the corona of the micelles is modeled through the reaction equilibrium -COOCa<sup>+</sup> ⇄ -COO<sup>-</sup> + Ca <sup>2+</sup> with pK<sub>Ca</sub> = 1.7 ± 0.06.</p>