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

Key points •  In the heart, Ca2+ waves are arrhythmogenic spontaneous sarcoplasmic reticulum (SR) Ca2+ release events that arise when the Ca2+ content in the SR reaches a critical threshold level. •  β-Adrenergic signalling induces Ca2+ waves in cardiac myocytes, but it remains unclear if this is due to a decrease in the Ca2+ wave threshold or more simply due to an increase in SR Ca2+ content. •  We used direct, dynamic measurement of SR Ca2+ levels to show that the Ca2+ wave threshold is unexpectedly increased during β-adrenergic stimulation. •  Our data show that the primary cause of Ca2+ waves following acute β-adrenergic stimulation is the increase in SR Ca2+ content and not a decrease in the Ca2+ wave threshold. •  We propose that the elevation of the Ca2+ wave threshold represents a protective mechanism against arrhythmogenic events during periods of β-adrenergic stimulation. Abstract  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.