Guanosine 5'-tetraphosphate acts subsequent to InsP
-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).