Table 3.

Analysis of amounts of ATP-ion species in EC₅₀ concentrations

	rP2Y4				RP2Y2
	pH 8.0	pH 7.5	pH 6.5	pH 5.5	pH 8.0	pH 7.5	pH 6.5	pH 5.5
Mean EC50 (nM)	1332	1458	529	145	5801	2511	4566	5955
	ATP species (nM)				ATP species (nM)
ATP4−	106	115	38	6	460	197	329	230
HATP3−	4	13	43	63	16	22	373	2594
H2ATP2−	0	0	0	3	0	0	2	130
NaATP3−	182	198	66	10	794	340	568	395
KATP3−	3	4	1	0	15	6	10	7
CaATP2−	849	922	307	45	3698	1590	2648	1842
Ca2ATP	187	203	68	10	817	352	585	407
CaHATP−	1	2	6	9	2	3	50	351
Total	1332	1457	529	146	5802	2510	4565	5956

Computational determinations of the amounts (in nM) of each ATP-ion species present in the oocyte bathing solution (Ringer's solution; composition in Methods, temperature 18°C) for the mean concentration of ATP (in nM) evoking the 50% maximum response (EC₅₀) at rP2Y₄ receptors (left) and rP2Y₂ receptors (right) examined at four extracellular pH levels (pH 8.0−5.5). The amounts of each ATP-ion species were determined by the programme “Bound and Determined” (Brooks & Storey, 1992).