Activation of purinergic receptors triggers oscillatory contractions in adult rat ventricular myocytes

Extracellular ATP is an important neurotransmitter that modulates cardiac function by activation of purinergic receptors. In this study, the effect of P₂ purinergic receptor activation on contractions and on [Ca²⁺](i) was investigated in adult rat ventricular myocytes. Fura 2 was used to measure [Ca²⁺](i), and video edge detection was used to measure contraction. Superfusion of 2-methylthio-adenosine-5'-triphosphate (2-M-S-ATP) over quiescent myocytes induced oscillations in contraction and in [Ca²⁺](i). The frequency of the oscillatory contractions increased with increasing concentrations of 2-M-S-ATP, but the amplitude of contractions varied from cell to cell and was independent of the concentration of 2-M-S-ATP. During electrical stimulation, activation of purinergic receptors in myocytes potentiated the amplitude of contraction and induced arrhythmias. In populations of quiescent myocytes, the plateau phase of the [Ca²⁺](i) signal evoked by 2-M-S-ATP could be shown to represent summed oscillations in [Ca²⁺](i) in individual cells. Pretreatment of quiescent myocytes with thapsigargin or caffeine reduced or abolished the oscillations in contractions and in [Ca²⁺](i) triggered by 2-M-S-ATP, indicating a dependence of the oscillations on uptake and release of Ca²⁺ by the sarcoplasmic reticulum. These data demonstrate the novel phenomenon that activation of purinergic receptors in quiescent myocytes stimulates oscillations in [Ca²⁺](i) and contraction. In electrically stimulated myocytes, activation of purinergic receptors triggers oscillatory contractions and potentiates the amplitude of electrically triggered contractions.