Activation of phosphoinositide-specific PLC increase free intracellular calcium in rabbit retinal cultures

Mobilization of cytosolic calcium was studied in 3-5 and 25-30 day old primary rabbit retinal cultures. Agonist stimulation of receptors linked to phospholipase C (PLC) and the production of inositol phosphates lead to an instantaneous (< 3 s) increase in free intracellular calcium as measured by the fluorescence of Quin2. Agents which had no effect on inositol phosphates (IPs) accumulation were ineflective in mobilizing calcium. The magnitude of calcium increase cuased by the agonists tested (carbachol, noradrenaline and serotonin) was dose-dependent and correlated closely with their respective potency in stimulating the accumulating of inositol phosphates. Agonist-induced effects on both Ca(2+) responses and hydrolysis of polyphosphoinositides were inhibited by the same specific antagonists. The two responses appear therefore to be mediated through the same receptors. The transmitter-induced Ca(2+) mobilization was independent of extracellular calcium and was not affected by either inorganic or organic calcium channels blockers. Studies on young and old retinal cultures provide further evidence that calcium mobilization is mediated by receptors linked to PLC. Serotonin increases the accumulation of IPs only in the young cultures which is in agreement with the calcium mobilization studies where serotonin induced a Ca(2+) responses in the young but not in the old non-neuronal (Müller) cell cultures.