A G-PROTEIN ACTIVATOR, NAF, INDUCES [CA-2+]0-DEPENDENT [CA-2+]C OSCILLATION AND SECRETORY RESPONSE IN RAT PANCREATIC ACINI

Sodium fluoride (NaF), which is known to directly activate various guanine nucleotide binding proteins (G-proteins) in the plasma membrane, induced a dosedependent secretory response in isolated rat pancreatic acini. Effective concentrations to induce maximum response and half maximum response (EC50) were 10 mM and 5mM NaF, respectively. Stimulation with NaF at 15 mM or higher caused smaller response. Typical compound exocytosis was demonstrated by electron microscopy when the pancreas was stimulated with 10mM NaF, whereas it was rarely seen when 100 mM NaF was used. Cell damage was not detected by estimation of LDH leakage or by electron microscopy in any experimental procedures in the present study. Using a Ca2+-sensitive fluorescent dye, fura-2-AM, oscillatory changes in the cytosolic concentration of Ca2+, [Ca2+]c, were detected by microspectrofluorometry during continuous stimulation with 10mM NaF. Both NaF-induced secretory response and [Ca2+]c oscillation were abolished in [Ca2+]o-free environment. The finding that NaF mimics physiological stimulation suggests that activation of G-proteins is an essential physiological step in stimulus-secretion coupling. On these results, we propose a view that the G-protein activation induces [Ca2+]c oscillating originated from an influx of extracellular calcium ions and thereby initiates and maintains final cellular responses.