Figure 2

Guanosine 5'-tetraphosphate acts subsequent to InsP ₃ -mediated Ca²⁺ release during excitation. Intracellular pressure injection from a microelectrode containing 25 mM GtetP decreased both the responses to a test flash and intracellular pressure injection of 1 mM 3dInsP₃. Brackets and numbers match sets of five consecutive responses to light (1, 3, 5) or 3dInsP₃ (2, 4, 6) averaged to produce the respective trace in the inset. 3dInsP₃ injections were interspersed between test flashes during the periods indicated by brackets (2, 4, 6). GtetP was injected during the period indicated by a solid bar. Averaged responses are shown before (1, 2), at the end of drug application (3, 4), and late in recovery (5, 6).