INTRACELLULAR INJECTION OF HEPARIN AND POLYAMINES - EFFECTS ON PHOTOTRANSDUCTION IN LIMULUS VENTRAL PHOTORECEPTORS

Heparin is thought to inhibit InsP3 binding to receptors involved in t he intracellular release of Ca2+. Injection of heparin into Limulus ve ntral photoreceptors to high intracellular concentrations reduces the amplitude and slows the rate of rise of voltage-clamp currents induced by brief flashes, tends to make the responses to long flashes more '' square,'' and tends to block the light-induced rise in [Ca2+]i detecte d by arsenazo III. In these ways, intracellular heparin mimics the eff ects of high concentrations of intracellular BAPTA or EGTA. In additio n, the effects of heparin are attenuated by prior injection of BAPTA t o high intracellular concentrations. Neomycin and spermine are thought to inhibit phospholipase C activity. Injections of spermine or neomyc in to low intracellular concentrations largely mimic the effects of in tracellular heparin. These findings suggest that the predominant effec t of polyamines is to inhibit light-induced production of InsP3 by pho spholipase C activity and thereby reduce the light-induced increase in [Ca2+]i. Our findings suggest that excitation can proceed in the abse nce of InSP3-induced increases in [Ca2+]i, but (a) the gain and speed of transduction are reduced and (b) adaptation is largely blocked.