External Sodium Affects SOCE Activation In Mouse FDB Muscle Fibers

Abstract

We report experiments carried out to study the possible contribution of the Na+/Ca2+ exchanger activity to [Ca2+]myo levels during SOCE activation in mouse muscle fibers. Enzymatically dissociated FDB fibers, loaded with FURA-2 AM, and treated with 5 μM CPA, were submitted to a SR Ca2+ store depletion procedure, in the absence of external Ca2+. Exposure to external Ca2+ (2mM) activated SOCE under two different modalities. In 9 fibers, [Ca2+]myo increased at a rate of 0.9 ± 0.1 nM/s showing saturation. In 7 other fibers the rate of [Ca2+]myo increase was 1.5 ± 0.3 (p<0.05), with no sign of saturation at 115 ± 11 s after SOCE activation. Exposure to a 0Na+ solution caused a fast, partial reversal of the SOCE dependent [Ca2+]myo increase. Activation of SOCE was much depressed in the absence of external Na+; subsequent re-exposure to Na+ greatly increased [Ca2+]myo and its rate of rise. TRIS was about twice more effective than Li+ as Na+ substitute. Thus, [Ca2+]myo decreased by 76.6 ± 6.4 or 33.8 ± 21.4 nM, (p<0.05) in the presence of TRIS or Li+ respectively. In both cases however, the effect occurred with similar time constants: 32.3 ± 2.2 vs 29.9 ± 3.6 s-1. When TRIS was used, Na+ dependent SOCE activation showed a Kd of about 25 mM. Preliminary experiments with the Na+ sensitive compound benzofuran isophtalate (SBFI) indicate that simultaneously with SOCE activation there is an increase in the myoplasmic Na+ concentration. If so, the possibility arise that high myoplasmic Na+ could activate the sarcolemmal Na+/Ca2+ exchanger in its reverse mode, or even the mitochondrial Na+/Ca2+ exchanger. An alternate possibility could be a direct modulatory effect of Na+ on SOCE activation machinery (FONACIT G-2001000637).