Interdependence of H+ and K+ fluxes during the Ca(2+)-pumping activity of sarcoplasmic reticulum vesicles

Sarcoplasmic reticulum K(+) (TRIC) channel does not carry essential countercurrent during Ca(2+) release

The charge translocation associated with sarcoplasmic reticulum (SR) Ca(2+) efflux is compensated for by a simultaneous SR K(+) influx. This influx is essential because, with no countercurrent, the SR membrane potential (Vm) would quickly (<1 ms) reach the Ca(2+) equilibrium potential and SR Ca(2+) release would cease. The SR K(+) trimeric intracellular cation (TRIC) channel has been proposed to carry the essential countercurrent. However, the ryanodine receptor (RyR) itself also carries a substantial K(+) countercurrent during release. To better define the physiological role of the SR K(+) channel, we compared SR Ca(2+) transport in saponin-permeabilized cardiomyocytes before and after limiting SR K(+) channel function. Specifically, we reduced SR K(+) channel conduction 35 and 88% by replacing cytosolic K(+) for Na(+) or Cs(+) (respectively), changes that have little effect on RyR function. Calcium sparks, SR Ca(2+) reloading, and caffeine-evoked Ca(2+) release amplitude (and rate) were unaffected by these ionic changes. Our results show that countercurrent carried by SR K(+) (TRIC) channels is not required to support SR Ca(2+) release (or uptake). Because K(+) enters the SR through RyRs during release, the SR K(+) (TRIC) channel most likely is needed to restore trans-SR K(+) balance after RyRs close, assuring SR Vm stays near 0 mV.

Evidence for direct involvement of the sarcoplasmic reticulum Ca(2+)-ATPase in a passive monovalent cation (K+/Na+) exchange

A specific inhibitor of SERCA-pumps, thapsigargin (TG) was used to demonstrate the direct involvement of the SR Ca(2+)-ATPase in passive K+/Na+ exchange. The K(+)-potential variations across vesicle membranes were measured in the absence of ATP with a fluorescent probe: 3,3'-dipropylthiodicarbocyanine iodide. Addition of EGTA dissipates the K(+)-potential whereas the presence of TG abolishes this effect. Our data prove that the Ca(2+)-ATPase translocates monovalent cations at a rate similar to the E2-->E1 conformational change.