Table 1. Effects of IP₃ analogues on Ca²⁺ release by subtypes of IP₃ receptor.

	IP3R1					IP3R2					IP3R3
	EC50 (nM)	pEC50	h	Ca2+release (%)	n	EC50 (nM)	pEC50	h	Ca2+release (%)	n	EC50 (nM)	pEC50	h	Ca2+release (%)	n
(1,4,5)IP3	87	7.06±0.05	0.99±0.05	75±1	31	145	6.84±0.06	1.26±0.09	61±2	34	417	6.38±0.05	1.26±0.07	64±2	30
(4,5)IP2	8128	5.09±0.12	1.11±0.15	69±4	4	6310	5.20±0.20	1.67±0.27	56±4	5	11482	4.94±0.07	1.28±0.03*	83±2	3
(1,3,4,5)IP4	11749	4.93±0.05	1.09±0.16	66±6	3	5495	5.26±0.09	2.26±0.18	61±5	3	114815b	3.94±0.03	1.08±0.22	36±1a	3
2-deoxy(1,4,5)IP3	123	6.91±0.15	0.84±0.07	82±3	4	115	6.94±0.11	1.71±0.37	65±2	6	324	6.49±0.02	1.36±0.10	74±3	3
(1,4,6)IP3	4365	5.36±0.19	0.98±0.10	81±4	6	12589	4.90±0.16	1.06±0.17	68±2	6	15849	4.80±0.12	2.58±0.61*	46±1	6
3-deoxy(1,4,5)IP3	3311	5.48±0.09	1.75±0.24*	61±2	6	5888	5.23±0.07	1.78±0.34	57±6	6	15849	4.80±0.13	2.59±0.68	53±7	6
MG(1,4,5)IP3	447	6.35±0.12	0.95±0.10	73±4	9	708	6.15±0.09	1.04±0.08	57±3	9	1738	5.76±0.11	1.18±0.11	64±4	8
(1,3,4)IP3	ND	ND	ND	39±8a	3	ND	ND	ND	18±6a	3	ND	ND	ND	2±1a	3

The EC₅₀, pEC₅₀, Hill coefficient (h) and fraction of the intracellular Ca²⁺ stores released by a maximally effective concentration of each analogue are shown for each IP₃R subtype. All results (except EC₅₀) are shown as means ± SEM from n independent experiments. The (1,3,4,5)IP₄ was provided by Calbiochem. ND, not determined.

^aCa²⁺ release evoked with 100 µM of the analogue (where the highest attainable concentrations of ligand failed to saturate the response).

^bEC₅₀ estimated by assuming that maximally effective concentrations of (1,3,4,5)IP₄ and (1,4,5)IP₃ release the same fraction of the Ca²⁺ stores.

^*Denotes Hill slopes that are significantly different (P<0.05) from 1. Statistical comparisons of maximal Ca²⁺ release and pEC₅₀ values used paired comparisons (Δmax or ΔpEC₅₀), which are presented in Table 2.