Comparison of 8-bromo (8-Br)-cGMP-mediated relaxations in the cGKI+/+ and cGKI−/− aorta and IAS. Sample traces showing relaxations elicited with 8-Br-cGMP in the cGKI+/+ aorta (Aa) and IAS (Ba) and in the cGKI−/− aorta (Ab) and IAS (Bb). 8-Br-cGMP responses were absent at concentrations up to 300 μM in the cGKI−/− aorta (Ab, D) whereas some relaxation persisted with ≥1 μM in the cGKI−/− IAS (Bb, E). Both differences were significant (*P < 0.05). The decline in spontaneous contractions in the IAS of cGKI+/+ (Ca) and cGKI−/− (Cb) mice during drug addition was not due to rundown since activity returned to the control level following washout (W, arrows). IC50 values for the relaxation in cGKI+/+ were 3.2 × 10−4 M and 8.1 × 10−6 M for aorta and IAS, respectively; n = 4 cGKI+/+, n = 5 cGKI−/− aorta; n = 4 cGKI+/+, n = 5 cGKI−/− IAS. NE, norepinephrine.