Activation of the Na+/K+/2 Cl−-Cotransporter in Mammalian Skeletal Muscle

Abstract

Skeletal muscle expresses a functional active isoform of the Na+/K+/2 Cl--Cotransporter (NKCC). This cotransporter is activated by an increase of extracellular osmolality. Previous studies show a linear relationship between the NKCC activity and osmolality [1]. This is contradictory to the fact, that the transport rate should saturate, when the NKCC is activated to its maximum. The aim of this study was to determine the activation curve of the NKCC activity of mammalian skeletal muscle. Methods and results: Activation of the NKCC has an impact on the resting membrane potential. Therefore, we measured membrane potentials of rat diaphragm and flexor digitorum brevis muscles at different extracellular osmolalities. Histogram plots of the data revealed a bimodal distribution of membrane potentials - one fraction with high (HP) and one with low (LP) membrane potentials. The means of the LP fractions at different osmolalities were depolarized to values between −50 and −60 mV, those of the HP fractions represented a sigmoidal shaped curve. A computer model of an excitable cell, in which a volume-dependent NKCC activity was incorporated, was fitted to the HP data. In rat diaphragm the NKCC is maximal activated at an extracellular osmolality of 340 mOsmol, in rat FDB this maximum occurs at an osmolality of 320 mOsmol. Conclusion: The apparent linear relationship between osmolality and membrane depolarization caused by NKCC activation is based on the invalid calculation of means under the assumption of a monomodal distribution. Accounting for the real bimodal distribution, we successfully revealed the assumed sigmoidal activation curve.[1] van Mil HG et al., Br J Pharmacol. 1997, 120(1):39-44.