On the Ca2+-dependence of inositol-phospholipid-specific phospholipase C of microvillar photoreceptors from Sepia officinalis

Abstract

The breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) by the action of a phosphoinositide-specific phospholipase C is a key event in the signal transduction mechanism of microvillar photoreceptors. The enzymatic activity of phospholipase C from photoreceptor cell membranes of the cuttlefish Sepia officinalis was measured at different concentrations of free calcium. A bell-shaped curve with a maximum of enzyme activity at about 0·2 to 0·7 μM of free calcium was obtained. In the range of 10 to 90 μM free Ca2+, the activity of the phospholipase C decreased to about 30%.The activity of the phospholipase was also modulated by strontium ions. The presence of 10 μM Sr2+ strongly reduced the dependence of the phospholipase from a given calcium concentration. Similarly to 10 mM EGTA, the presence of 10 μM La3+ reduced the enzymatic activity more than ten-fold.A comparison of these values with the changes of the calcium concentration measured in ventral photoreceptor cells of Limulus polyphemus in the dark (∼ 0·5 μM Ca2+) and under illumination (up to 30-100 μM Ca2+) indicates that in microvillar photoreceptor cells an elevation of the calcium concentration in the physiological range reduces the activity of the phospholipase C. The IP3 produced by the action of phospholipase C is known to elevate the intracellular Ca2+ concentration transiently, which in turn will be able to reduce the further IP3 formation. A negative feedback in this early part of the enzyme cascade of vision in invertebrates may substantially contribute to the process of adaptation.