Figure 3. Evidence for the independence of Ca²⁺ influx from receptor activation.

In both experiments, Ca²⁺ stores were discharged by addition of epinephrine in the absence of extracellular Ca²⁺, the α-adrenergic receptors were then blocked by phentolamine (Phentol.), and the status of Ca²⁺ stores assessed by addition of carbachol (Carb.). In the experiment with open circles, Ca²⁺ was added before phentolamine, so Ca²⁺ could flow into the cell through presumed receptor activated channels. In the experiment with closed circles, phentolamine was added before Ca²⁺ such that receptor-activated channels would presumably be closed. Nonetheless, the stores were refilled with similar efficiency in both cases, indicating that it is not receptor activation per se that is responsible for Ca²⁺ entry and refilling of intracellular stores. Redrawn from data originally presented in [25].