β-Adrenergic stimulation increases the intra-sarcoplasmic reticulum Ca2+ threshold for Ca2+ wave generation.

β-Adrenergic signalling induces positive inotropic effects on the heart that associate with pro-arrhythmic spontaneous Ca(2+) waves. A threshold level of sarcoplasmic reticulum (SR) Ca(2+) (Ca(2+)) is necessary to trigger Ca(2+) waves, and whether the increased incidence of Ca(2+) waves during β-adrenergic stimulation is due to an alteration in this threshold remains controversial. Using the low-affinity Ca(2+) indicator fluo-5N entrapped within the SR of rabbit ventricular myocytes, we addressed this controversy by directly monitoring Ca(2+) and Ca(2+) waves during β-adrenergic stimulation. Electrical pacing in elevated extracellular Ca(2+) (Ca(2+) = 7 mM) was used to increase Ca(2+) to the threshold where Ca(2+) waves were consistently observed. The β-adrenergic agonist isoproterenol (ISO; 1 μM) increased Ca(2+) well above the control threshold and consistently triggered Ca(2+) waves. However, when Ca(2+) was subsequently lowered in the presence of ISO (by lowering Ca(2+) to 1 mM and partially inhibiting sarcoplasmic/endoplasmic reticulum calcium ATPase with cyclopiazonic acid or thapsigargin), Ca(2+) waves ceased to occur at a Ca(2+) that was higher than the control threshold. Furthermore, for a set Ca(2+) level the refractoriness of wave occurrence (Ca(2+) wave latency) was prolonged during β-adrenergic stimulation, and was highly dependent on the extent that Ca exceeded the wave threshold. These data show that acute β-adrenergic stimulation increases the Ca(2+) threshold for Ca(2+) waves, and therefore the primary cause of Ca(2+) waves is the robust increase in Ca(2+) above this higher threshold level. Elevation of the Ca(2+) wave threshold and prolongation of wave latency represent potentially protective mechanisms against pro-arrhythmogenic Ca(2+) release during β-adrenergic stimulation.