Fig. 2. Resting and CAP_2b-stimulated inositol (1,4,5)-trisphosphate (IP₃) levels in itpr mutants. (A) Resting IP₃ levels are shown for tubules from the following lines: Oregon R (control), itpr^XR12/+, itpr^90B.0/+, itpr¹⁶⁶⁴/+, itpr¹⁶⁶⁴/itpr^XR12, itpr¹⁶⁶⁴/itpr^90B.0, itpr¹⁶⁶⁴/itpr¹⁶⁶⁴ and itpr^WC361/itpr^UG3. In order to aid comparison between experiments, data are shown as the % difference between IP₃ levels in itpr mutants compared with wild type (100%) ± S.E.M. (N=4). Typical IP₃ content of wild-type tubules was as described in Table 1. (B) CAP_2b stimulates IP₃ production in itpr lines. Stimulated IP₃ levels were measured in CAP_2b-stimulated tubules (10^-7 moll^-1, 5s). Data are expressed as the % increase of unstimulated IP₃ levels (calculated as [stimulated IP₃]/[resting IP₃]x100%; [IP³] measured in pmol µg protein^-1) ± S.E.M. (N=3-4). Significant differences between IP₃ content in wild-type and itpr lines are denoted by ^* (P<0.05, Student's t-test, unpaired samples).