The E. coli toxin α-haemolysin (HlyA) lyses erythrocytes by inserting as pores in the plasma membrane. Recently, we demonstrated that HlyA-induced haemolysis is a sequential process with an initial volume reduction that precedes swelling and lysis. The initial volume reduction is triggered by an abrupt increase in [Ca²⁺]_i that activates Ca²⁺-dependent (K_Ca3.1) and Cl- (TMEM16A) channels. Moreover, we have shown that HlyA-induced haemolysis requires extracellular ATP and P2 receptor activation (1, 2). The present study investigates the underlying mechanism for ATP release and the [Ca²⁺]_i increase that precede the volume changes in erythrocytes.

By the luciferin/luciferase assay, we show that HlyA leads to ATP release within 5 minutes after the toxin is added. ATP release was independent of pannexin- and volume-dependent anion channels; two suggested ATP exit pathways. Thus, ATP may simply be released via the toxin pore.

Through a combination of flow cytometry and live cell imaging, we show that the HlyA-induced [Ca²⁺]_i response consists of an abrupt increase followed by at slower, sustained increment. Various P2 antagonists do not interfere with the initial acute [Ca²⁺]_i increase, but significantly reduces the slow phase of the HlyA-induced influx of [Ca²⁺]_i and cause a marked delay in the swelling of the cells.

In conclusion, we suggest that ATP may be released via the toxin pore, which activates P2 receptors in the erythrocyte membrane. The influx of cations through P2 receptors contributes to net ion influx and thereby cell swelling and lysis.