Table 1.

Ligand binding properties and IP accumulation in COS-7 cells expressing wild-type α_IB-AR or mutant receptors carrying different amino acids at position 142.

[125I] HEAT binding			3H-IP accumulation
Receptor	Bmax, pmol/mg	EPI, Ki (μM)	Rmax, % increase	Basal IP, % control	Normalized basal
αIB-AR (9)	1.17	12.80	375 ± 55	0	0.00
D142N (6)	0.55	9.42	320 ± 79	23 ± 11	0.42
D142E (6)	0.78	11.30	297 ± 27	39 ± 15	0.50
D142F (4)	0.47	3.45	200 ± 71	82 ± 14	1.74
D142R (6)	0.14	0.24	50 ± 14	27 ± 9	1.93
D142W (4)	0.25	1.81	30 ± 1	49 ± 2	1.96
D142P (4)	0.05	3.50	59 ± 13	18 ± 1	3.60
D142Y (5)	0.23	2.73	41 ± 14	104 ± 26	4.52
D142G (5)	0.24	3.20	144 ± 33	126 ± 14	5.25
D142H (4)	0.13	1.11	66 ± 21	73 ± 14	5.62
D142K (4)	0.13	0.30	22 ± 16	115 ± 19	8.85
D142S (3)	0.38	0.55	25 ± 16	361 ± 70	9.50
D142A (3)	0.34	0.45	36 ± 16	329 ± 42	9.68
D142C (5)	0.37	0.73	13 ± 8	422 ± 58	11.41
D142L (4)	0.35	1.10	48 ± 27	407 ± 26	11.63
D142M (4)	0.19	1.70	57 ± 21	302 ± 41	15.89
D142Q (4)	0.37	0.37	38 ± 24	599 ± 43	15.89
D142V (5)	0.23	0.37	14 ± 4	382 ± 21	16.61
D142I (3)	0.22	0.52	8 ± 1	427 ± 49	19.41
D142T (5)	0.28	0.80	10 ± 4	698 ± 74	24.93

The amount of transfected DNA/10⁶ cells was calibrated for each receptor mutant to achieve expression levels ranging 0.7–0.2 pmol/mg of protein (0.17–0.05 pmol/10⁶ cells) for most receptor mutants. [¹²⁵I]HEAT and epinephrine (EPI) binding as well as IP accumulation were measured as described. R_max indicates the EPI-induced IP accumulation expressed as percentage increase above basal levels in the absence of EPI. Basal IP indicates the IP levels in the absence of EPI expressed as percentage increase above those of mock-transfected cells (control). Normalized basal indicates the ratio between the basal IP values divided by 100 and receptor number (pmol/mg of protein). Results are the mean ± SE of several independent experiments whose numbers are indicated in parentheses. The K_i values were from two to three experiments that agreed within 20%.