Enhanced activity of multiple TRIC-B channels: an endoplasmic reticulum/sarcoplasmic reticulum mechanism to boost counterion currents.

Abstract

Key points There are two subtypes of trimeric intracellular cation (TRIC) channels but their distinct single‐channel properties and physiological regulation have not been characterized. We examined the differences in function between native skeletal muscle sarcoplasmic reticulum (SR) K+‐channels from wild‐type (WT) mice (where TRIC‐A is the principal subtype) and from Tric‐a knockout (KO) mice that only express TRIC‐B.We find that lone SR K+‐channels from Tric‐a KO mice have a lower open probability and gate more frequently in subconducting states than channels from WT mice but, unlike channels from WT mice, multiple channels gate with high open probability with a more than six‐fold increase in activity when four channels are present in the bilayer.No evidence was found for a direct gating interaction between ryanodine receptor and SR K+‐channels in Tric‐a KO SR, suggesting that TRIC‐B–TRIC‐B interactions are highly specific and may be important for meeting counterion requirements during excitation–contraction coupling in tissues where TRIC‐A is sparse or absent. The trimeric intracellular cation channels, TRIC‐A and TRIC‐B, represent two subtypes of sarcoplasmic reticulum (SR) K+‐channel but their individual functional roles are unknown. We therefore compared the biophysical properties of SR K+‐channels derived from the skeletal muscle of wild‐type (WT) or Tric‐a knockout (KO) mice. Because TRIC‐A is the major TRIC‐subtype in skeletal muscle, WT SR will predominantly contain TRIC‐A channels, whereas Tric‐a KO SR will only contain TRIC‐B channels. When lone SR K+‐channels were incorporated into bilayers, the open probability (Po) of channels from Tric‐a KO mice was markedly lower than that of channels from WT mice; gating was characterized by shorter opening bursts and more frequent brief subconductance openings. However, unlike channels from WT mice, the Po of SR K+‐channels from Tric‐a KO mice increased as increasing channel numbers were present in the bilayer, driving the channels into long sojourns in the fully open state. When co‐incorporated into bilayers, ryanodine receptor channels did not directly affect the gating of SR K+‐channels, nor did the presence or absence of SR K+‐channels influence ryanodine receptor activity. We suggest that because of high expression levels in striated muscle, TRIC‐A produces most of the counterion flux required during excitation‐contraction coupling. TRIC‐B, in contrast, is sparsely expressed in most cells and, although lone TRIC‐B channels exhibit low Po, the high Po levels reached by multiple TRIC‐B channels may provide a compensatory mechanism to rapidly restore K+ gradients and charge differences across the SR of tissues containing few TRIC‐A channels.