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

摘要

β-Adrenergic signalling induces positive inotropic effects on the heart that associate with pro-arrhythmic spontaneous Ca²⁺ waves. A threshold level of sarcoplasmic reticulum (SR) Ca²⁺ ([Ca²⁺]SR) is necessary to trigger Ca²⁺ waves, and whether the increased incidence of Ca²⁺ waves during β-adrenergic stimulation is due to an alteration in this threshold remains controversial. Using the low-affinity Ca²⁺ indicator fluo-5N entrapped within the SR of rabbit ventricular myocytes, we addressed this controversy by directly monitoring [Ca²⁺]SR and Ca²⁺ waves during β-adrenergic stimulation. Electrical pacing in elevated extracellular Ca²⁺ ([Ca²⁺]o= 7 mm) was used to increase [Ca²⁺]SR to the threshold where Ca²⁺ waves were consistently observed. The β-adrenergic agonist isoproterenol (ISO; 1 μm) increased [Ca²⁺]SR well above the control threshold and consistently triggered Ca²⁺ waves. However, when [Ca²⁺]SR was subsequently lowered in the presence of ISO (by lowering [Ca²⁺]o to 1 mm and partially inhibiting sarcoplasmic/endoplasmic reticulum calcium ATPase with cyclopiazonic acid or thapsigargin), Ca²⁺ waves ceased to occur at a [Ca²⁺]SR that was higher than the control threshold. Furthermore, for a set [Ca²⁺]SR level the refractoriness of wave occurrence (Ca²⁺ wave latency) was prolonged during β-adrenergic stimulation, and was highly dependent on the extent that [Ca]SR exceeded the wave threshold. These data show that acute β-adrenergic stimulation increases the [Ca²⁺]SR threshold for Ca²⁺ waves, and therefore the primary cause of Ca²⁺ waves is the robust increase in [Ca²⁺]SR above this higher threshold level. Elevation of the [Ca²⁺]SR wave threshold and prolongation of wave latency represent potentially protective mechanisms against pro-arrhythmogenic Ca²⁺ release during β-adrenergic stimulation.