The Functional Complex Composed of the Sodium/Bicarbonate Cotransporter and the Soluble Adenylate Cyclase (sAC) Modulates Basal Cardiac Contractility

Abstract

In addition to the adenylate cyclase (AC) associated to the plasma membrane, a new source of cyclic AMP (cAMP) was identified, the soluble AC (sAC). However, the physiological function of sAC in the heart is unknown. The cardiac Na+/HCO3- cotransporter (NBC) promotes the cellular co-influx of HCO3- and Na+. Since sAC is a HCO3--dependent enzyme, we aimed to investigate the potential impact of the relationship between the activity of NBC and sAC on cAMP-dependent cardiac contractility. Rat ventricular myocytes were loaded with Fura-2 or Fluo-3 in order to measure Ca2+ transient amplitude (CaT) by epifluorescence or Ca2+ sparks frequency (CaSF) by confocal microscopy, respectively. Sarcomere shortening as contractility index was measured simultaneously with epifluorescence. The NBC blocker S0859 (10μM) induced a negative inotropic effect (NIE) in the presence of HCO3- (Control: 19.1±3.2% vs. S0859: 14.6±2.6%; n=9, P<0.05) which was associated with a decrease of 18.5±2.6% in CaT. S0859 failed to induce a NIE in the absence of HCO3-. The selective inhibitor of sAC, KH7 (1μM) decreased contractility (Control: 15.7±0.7% vs. KH7: 11.3±0.9%, n=5, P<0.05) and CaT (15.7±4.9%) only in HCO3-. KH7 also prevented the NIE of S0859 (KH7+S0859: 11.1±0.9%, n=5). Since cAMP activates the kinase PKA, which in turn increases Ca2+ release through sarcoplasmic reticulum RyR channels, CaSF was measured as an index of RyR open probability. The increase in CaSF observed when field stimulation frequency was increased from 0.5 to 3 Hz (Control variation ratio: 1.23±0.1) was reversed in the presence of S0859 (0.62±0.2, n=5, P<0.05) only when HCO3- was present in the extracellular medium. In summary, these results demonstrate for the first time that the complex NBC-sAC plays a relevant role in Ca2+ management and basal cardiac contractility.