β‐Adrenergic stimulation increases the intra‐sarcoplasmic reticulum Ca 2+ threshold for Ca 2+ wave generation

Key points•  In the heart, Ca 2+ waves are arrhythmogenic spontaneous sarcoplasmic reticulum (SR) Ca 2+ release events that arise when the Ca 2+ content in the SR reaches a critical threshold level. •  β‐Adrenergic signalling induces Ca 2+ waves in cardiac myocytes, but it remains unclear if this is due to a decrease in the Ca 2+ wave threshold or more simply due to an increase in SR Ca 2+ content. •  We used direct, dynamic measurement of SR Ca 2+ levels to show that the Ca 2+ wave threshold is unexpectedly increased during β‐adrenergic stimulation. •  Our data show that the primary cause of Ca 2+ waves following acute β‐adrenergic stimulation is the increase in SR Ca 2+ content and not a decrease in the Ca 2+ wave threshold. •  We propose that the elevation of the Ca 2+ 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 Ca 2+ waves. A threshold level of sarcoplasmic reticulum (SR) Ca 2+ ([Ca 2+ ] SR ) 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+ ] SR and Ca 2+ waves during β‐adrenergic stimulation. Electrical pacing in elevated extracellular Ca 2+ ([Ca 2+ ] o = 7 m m ) was used to increase [Ca 2+ ] SR to the threshold where Ca 2+ waves were consistently observed. The β‐adrenergic agonist isoproterenol (ISO; 1 μ m ) increased [Ca 2+ ] SR well above the control threshold and consistently triggered Ca 2+ waves. However, when [Ca 2+ ] SR was subsequently lowered in the presence of ISO (by lowering [Ca 2+ ] o to 1 m m and partially inhibiting sarcoplasmic/endoplasmic reticulum calcium ATPase with cyclopiazonic acid or thapsigargin), Ca 2+ waves ceased to occur at a [Ca 2+ ] SR that was higher than the control threshold. Furthermore, for a set [Ca 2+ ] SR 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] SR exceeded the wave threshold. These data show that acute β‐adrenergic stimulation increases the [Ca 2+ ] SR threshold for Ca 2+ waves, and therefore the primary cause of Ca 2+ waves is the robust increase in [Ca 2+ ] SR above this higher threshold level. Elevation of the [Ca 2+ ] SR wave threshold and prolongation of wave latency represent potentially protective mechanisms against pro‐arrhythmogenic Ca 2+ release during β‐adrenergic stimulation.